core: add Text::levenshtein_distance

[nit.git] / lib / core / text / abstract_text.nit

# This file is part of NIT ( http://www.nitlanguage.org ).
#
# This file is free software, which comes along with NIT.  This software is
# distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
# without  even  the implied warranty of  MERCHANTABILITY or  FITNESS FOR A
# PARTICULAR PURPOSE.  You can modify it is you want,  provided this header
# is kept unaltered, and a notification of the changes is added.
# You  are  allowed  to  redistribute it and sell it, alone or is a part of
# another product.

# Abstract class for manipulation of sequences of characters
module abstract_text

import native
import math
import collection
intrude import collection::array

in "C" `{
        #include <stdio.h>
        #include <stdlib.h>
        #include <string.h>
`}

# High-level abstraction for all text representations
abstract class Text
        super Comparable

        redef type OTHER: Text

        # Type of self (used for factorization of several methods, ex : substring_from, empty...)
        type SELFTYPE: Text

        # Gets a view on the chars of the Text object
        #
        #     assert "hello".chars.to_a == ['h', 'e', 'l', 'l', 'o']
        fun chars: SequenceRead[Char] is abstract

        # Gets a view on the bytes of the Text object
        #
        #     assert "hello".bytes.to_a == [104u8, 101u8, 108u8, 108u8, 111u8]
        fun bytes: SequenceRead[Byte] is abstract

        # Number of characters contained in self.
        #
        #     assert "12345".length == 5
        #     assert "".length == 0
        #     assert "あいうえお".length == 5
        fun length: Int is abstract

        # Number of bytes in `self`
        #
        #     assert "12345".bytelen == 5
        #     assert "あいうえお".bytelen == 15
        fun bytelen: Int is abstract

        # Create a substring.
        #
        #     assert "abcd".substring(1, 2)      ==  "bc"
        #     assert "abcd".substring(-1, 2)     ==  "a"
        #     assert "abcd".substring(1, 0)      ==  ""
        #     assert "abcd".substring(2, 5)      ==  "cd"
        #     assert "あいうえお".substring(1,3) ==  "いうえ"
        #
        # A `from` index < 0 will be replaced by 0.
        # Unless a `count` value is > 0 at the same time.
        # In this case, `from += count` and `count -= from`.
        fun substring(from: Int, count: Int): SELFTYPE is abstract

        # Iterates on the substrings of self if any
        private fun substrings: Iterator[FlatText] is abstract

        # Is the current Text empty (== "")
        #
        #     assert "".is_empty
        #     assert not "foo".is_empty
        fun is_empty: Bool do return self.length == 0

        # Returns an empty Text of the right type
        #
        # This method is used internally to get the right
        # implementation of an empty string.
        protected fun empty: SELFTYPE is abstract

        # Gets the first char of the Text
        #
        # DEPRECATED : Use self.chars.first instead
        fun first: Char do return self.chars[0]

        # Access a character at `index` in the string.
        #
        #     assert "abcd"[2]         == 'c'
        #
        # DEPRECATED : Use self.chars.[] instead
        fun [](index: Int): Char do return self.chars[index]

        # Gets the index of the first occurence of 'c'
        #
        # Returns -1 if not found
        #
        # DEPRECATED : Use self.chars.index_of instead
        fun index_of(c: Char): Int
        do
                return index_of_from(c, 0)
        end

        # Gets the last char of self
        #
        # DEPRECATED : Use self.chars.last instead
        fun last: Char do return self.chars[length-1]

        # Gets the index of the first occurence of ´c´ starting from ´pos´
        #
        # Returns -1 if not found
        #
        # DEPRECATED : Use self.chars.index_of_from instead
        fun index_of_from(c: Char, pos: Int): Int
        do
                var iter = self.chars.iterator_from(pos)
                while iter.is_ok do
                        if iter.item == c then return iter.index
                        iter.next
                end
                return -1
        end

        # Gets the last index of char ´c´
        #
        # Returns -1 if not found
        #
        # DEPRECATED : Use self.chars.last_index_of instead
        fun last_index_of(c: Char): Int
        do
                return last_index_of_from(c, length - 1)
        end

        # Return a null terminated char *
        fun to_cstring: NativeString is abstract

        # The index of the last occurrence of an element starting from pos (in reverse order).
        #
        #     var s = "/etc/bin/test/test.nit"
        #     assert s.last_index_of_from('/', s.length-1) == 13
        #     assert s.last_index_of_from('/', 12)         == 8
        #
        # Returns -1 if not found
        #
        # DEPRECATED : Use self.chars.last_index_of_from instead
        fun last_index_of_from(item: Char, pos: Int): Int do return chars.last_index_of_from(item, pos)

        # Gets an iterator on the chars of self
        #
        # DEPRECATED : Use self.chars.iterator instead
        fun iterator: Iterator[Char]
        do
                return self.chars.iterator
        end


        # Gets an Array containing the chars of self
        #
        # DEPRECATED : Use self.chars.to_a instead
        fun to_a: Array[Char] do return chars.to_a

        # Create a substring from `self` beginning at the `from` position
        #
        #     assert "abcd".substring_from(1)    ==  "bcd"
        #     assert "abcd".substring_from(-1)   ==  "abcd"
        #     assert "abcd".substring_from(2)    ==  "cd"
        #
        # As with substring, a `from` index < 0 will be replaced by 0
        fun substring_from(from: Int): SELFTYPE
        do
                if from >= self.length then return empty
                if from < 0 then from = 0
                return substring(from, length - from)
        end

        # Does self have a substring `str` starting from position `pos`?
        #
        #     assert "abcd".has_substring("bc",1)            ==  true
        #     assert "abcd".has_substring("bc",2)            ==  false
        #
        # Returns true iff all characters of `str` are presents
        # at the expected index in `self.`
        # The first character of `str` being at `pos`, the second
        # character being at `pos+1` and so on...
        #
        # This means that all characters of `str` need to be inside `self`.
        #
        #     assert "abcd".has_substring("xab", -1)         == false
        #     assert "abcd".has_substring("cdx", 2)          == false
        #
        # And that the empty string is always a valid substring.
        #
        #     assert "abcd".has_substring("", 2)             == true
        #     assert "abcd".has_substring("", 200)           == true
        fun has_substring(str: String, pos: Int): Bool
        do
                if str.is_empty then return true
                if pos < 0 or pos + str.length > length then return false
                var myiter = self.chars.iterator_from(pos)
                var itsiter = str.chars.iterator
                while myiter.is_ok and itsiter.is_ok do
                        if myiter.item != itsiter.item then return false
                        myiter.next
                        itsiter.next
                end
                if itsiter.is_ok then return false
                return true
        end

        # Is this string prefixed by `prefix`?
        #
        #     assert "abcd".has_prefix("ab")           ==  true
        #     assert "abcbc".has_prefix("bc")          ==  false
        #     assert "ab".has_prefix("abcd")           ==  false
        fun has_prefix(prefix: String): Bool do return has_substring(prefix,0)

        # Is this string suffixed by `suffix`?
        #
        #     assert "abcd".has_suffix("abc")        ==  false
        #     assert "abcd".has_suffix("bcd")        ==  true
        fun has_suffix(suffix: String): Bool do return has_substring(suffix, length - suffix.length)

        # Returns `self` as the corresponding integer
        #
        #     assert "123".to_i        == 123
        #     assert "-1".to_i         == -1
        #     assert "0x64".to_i       == 100
        #     assert "0b1100_0011".to_i== 195
        #     assert "--12".to_i       == 12
        #
        # REQUIRE: `self`.`is_int`
        fun to_i: Int is abstract

        # If `self` contains a float, return the corresponding float
        #
        #     assert "123".to_f        == 123.0
        #     assert "-1".to_f         == -1.0
        #     assert "-1.2e-3".to_f    == -0.0012
        fun to_f: Float
        do
                # Shortcut
                return to_s.to_cstring.atof
        end

        # If `self` contains only digits and alpha <= 'f', return the corresponding integer.
        #
        #     assert "ff".to_hex == 255
        fun to_hex: Int do return a_to(16)

        # If `self` contains only digits <= '7', return the corresponding integer.
        #
        #     assert "714".to_oct == 460
        fun to_oct: Int do return a_to(8)

        # If `self` contains only '0' et '1', return the corresponding integer.
        #
        #     assert "101101".to_bin == 45
        fun to_bin: Int do return a_to(2)

        # If `self` contains only digits '0' .. '9', return the corresponding integer.
        #
        #     assert "108".to_dec == 108
        fun to_dec: Int do return a_to(10)

        # If `self` contains only digits and letters, return the corresponding integer in a given base
        #
        #     assert "120".a_to(3)     == 15
        fun a_to(base: Int) : Int
        do
                var i = 0
                var neg = false

                for j in [0..length[ do
                        var c = chars[j]
                        var v = c.to_i
                        if v > base then
                                if neg then
                                        return -i
                                else
                                        return i
                                end
                        else if v < 0 then
                                neg = true
                        else
                                i = i * base + v
                        end
                end
                if neg then
                        return -i
                else
                        return i
                end
        end

        # Returns `true` if the string contains only Numeric values (and one "," or one "." character)
        #
        #     assert "123".is_numeric  == true
        #     assert "1.2".is_numeric  == true
        #     assert "1,2".is_numeric  == true
        #     assert "1..2".is_numeric == false
        fun is_numeric: Bool
        do
                var has_point_or_comma = false
                for i in [0..length[ do
                        var c = chars[i]
                        if not c.is_numeric then
                                if (c == '.' or c == ',') and not has_point_or_comma then
                                        has_point_or_comma = true
                                else
                                        return false
                                end
                        end
                end
                return true
        end

        # Returns `true` if the string contains only Hex chars
        #
        #     assert "048bf".is_hex  == true
        #     assert "ABCDEF".is_hex  == true
        #     assert "0G".is_hex == false
        fun is_hex: Bool
        do
                for i in [0..length[ do
                        var c = chars[i]
                        if not (c >= 'a' and c <= 'f') and
                           not (c >= 'A' and c <= 'F') and
                           not (c >= '0' and c <= '9') then return false
                end
                return true
        end

        # Returns `true` if the string contains only Binary digits
        #
        #     assert "1101100".is_bin  == true
        #     assert "1101020".is_bin  == false
        fun is_bin: Bool do
                for i in chars do if i != '0' and i != '1' then return false
                return true
        end

        # Returns `true` if the string contains only Octal digits
        #
        #     assert "213453".is_oct  == true
        #     assert "781".is_oct     == false
        fun is_oct: Bool do
                for i in chars do if i < '0' or i > '7' then return false
                return true
        end

        # Returns `true` if the string contains only Decimal digits
        #
        #     assert "10839".is_dec == true
        #     assert "164F".is_dec  == false
        fun is_dec: Bool do
                for i in chars do if i < '0' or i > '9' then return false
                return true
        end

        # Are all letters in `self` upper-case ?
        #
        #     assert "HELLO WORLD".is_upper == true
        #     assert "%$&%!".is_upper       == true
        #     assert "hello world".is_upper == false
        #     assert "Hello World".is_upper == false
        fun is_upper: Bool
        do
                for i in [0..length[ do
                        var char = chars[i]
                        if char.is_lower then return false
                end
                return true
        end

        # Are all letters in `self` lower-case ?
        #
        #     assert "hello world".is_lower == true
        #     assert "%$&%!".is_lower       == true
        #     assert "Hello World".is_lower == false
        fun is_lower: Bool
        do
                for i in [0..length[ do
                        var char = chars[i]
                        if char.is_upper then return false
                end
                return true
        end

        # Removes the whitespaces at the beginning of self
        #
        #     assert " \n\thello \n\t".l_trim == "hello \n\t"
        #
        # `Char::is_whitespace` determines what is a whitespace.
        fun l_trim: SELFTYPE
        do
                var iter = self.chars.iterator
                while iter.is_ok do
                        if not iter.item.is_whitespace then break
                        iter.next
                end
                if iter.index == length then return self.empty
                return self.substring_from(iter.index)
        end

        # Removes the whitespaces at the end of self
        #
        #     assert " \n\thello \n\t".r_trim == " \n\thello"
        #
        # `Char::is_whitespace` determines what is a whitespace.
        fun r_trim: SELFTYPE
        do
                var iter = self.chars.reverse_iterator
                while iter.is_ok do
                        if not iter.item.is_whitespace then break
                        iter.next
                end
                if iter.index < 0 then return self.empty
                return self.substring(0, iter.index + 1)
        end

        # Trims trailing and preceding white spaces
        #
        #     assert "  Hello  World !  ".trim   == "Hello  World !"
        #     assert "\na\nb\tc\t".trim          == "a\nb\tc"
        #
        # `Char::is_whitespace` determines what is a whitespace.
        fun trim: SELFTYPE do return (self.l_trim).r_trim

        # Is the string non-empty but only made of whitespaces?
        #
        #     assert " \n\t ".is_whitespace    == true
        #     assert "  hello  ".is_whitespace == false
        #     assert "".is_whitespace          == false
        #
        # `Char::is_whitespace` determines what is a whitespace.
        fun is_whitespace: Bool
        do
                if is_empty then return false
                for c in self.chars do
                        if not c.is_whitespace then return false
                end
                return true
        end

        # Returns `self` removed from its last line terminator (if any).
        #
        #     assert "Hello\n".chomp == "Hello"
        #     assert "Hello".chomp   == "Hello"
        #
        #     assert "\n".chomp == ""
        #     assert "".chomp   == ""
        #
        # Line terminators are `"\n"`, `"\r\n"` and `"\r"`.
        # A single line terminator, the last one, is removed.
        #
        #     assert "\r\n".chomp     == ""
        #     assert "\r\n\n".chomp   == "\r\n"
        #     assert "\r\n\r\n".chomp == "\r\n"
        #     assert "\r\n\r".chomp   == "\r\n"
        #
        # Note: unlike with most IO methods like `Reader::read_line`,
        # a single `\r` is considered here to be a line terminator and will be removed.
        fun chomp: SELFTYPE
        do
                var len = length
                if len == 0 then return self
                var l = self.chars.last
                if l == '\r' then
                        return substring(0, len-1)
                else if l != '\n' then
                        return self
                else if len > 1 and self.chars[len-2] == '\r' then
                        return substring(0, len-2)
                else
                        return substring(0, len-1)
                end
        end

        # Justify a self in a space of `length`
        #
        # `left` is the space ratio on the left side.
        # * 0.0 for left-justified (no space at the left)
        # * 1.0 for right-justified (all spaces at the left)
        # * 0.5 for centered (half the spaces at the left)
        #
        # Examples
        #
        #     assert "hello".justify(10, 0.0)  == "hello     "
        #     assert "hello".justify(10, 1.0)  == "     hello"
        #     assert "hello".justify(10, 0.5)  == "  hello   "
        #
        # If `length` is not enough, `self` is returned as is.
        #
        #     assert "hello".justify(2, 0.0)   == "hello"
        #
        # REQUIRE: `left >= 0.0 and left <= 1.0`
        # ENSURE: `self.length <= length implies result.length == length`
        # ENSURE: `self.length >= length implies result == self`
        fun justify(length: Int, left: Float): String
        do
                var diff = length - self.length
                if diff <= 0 then return to_s
                assert left >= 0.0 and left <= 1.0
                var before = (diff.to_f * left).to_i
                return " " * before + self + " " * (diff-before)
        end

        # Mangle a string to be a unique string only made of alphanumeric characters and underscores.
        #
        # This method is injective (two different inputs never produce the same
        # output) and the returned string always respect the following rules:
        #
        # * Contains only US-ASCII letters, digits and underscores.
        # * Never starts with a digit.
        # * Never ends with an underscore.
        # * Never contains two contiguous underscores.
        #
        #     assert "42_is/The answer!".to_cmangle == "_52d2_is_47dThe_32danswer_33d"
        #     assert "__".to_cmangle == "_95d_95d"
        #     assert "__d".to_cmangle == "_95d_d"
        #     assert "_d_".to_cmangle == "_d_95d"
        #     assert "_42".to_cmangle == "_95d42"
        #     assert "foo".to_cmangle == "foo"
        #     assert "".to_cmangle == ""
        fun to_cmangle: String
        do
                if is_empty then return ""
                var res = new Buffer
                var underscore = false
                var start = 0
                var c = chars[0]

                if c >= '0' and c <= '9' then
                        res.add('_')
                        res.append(c.code_point.to_s)
                        res.add('d')
                        start = 1
                end
                for i in [start..length[ do
                        c = chars[i]
                        if (c >= 'a' and c <= 'z') or (c >='A' and c <= 'Z') then
                                res.add(c)
                                underscore = false
                                continue
                        end
                        if underscore then
                                res.append('_'.code_point.to_s)
                                res.add('d')
                        end
                        if c >= '0' and c <= '9' then
                                res.add(c)
                                underscore = false
                        else if c == '_' then
                                res.add(c)
                                underscore = true
                        else
                                res.add('_')
                                res.append(c.code_point.to_s)
                                res.add('d')
                                underscore = false
                        end
                end
                if underscore then
                        res.append('_'.code_point.to_s)
                        res.add('d')
                end
                return res.to_s
        end

        # Escape " \ ' and non printable characters using the rules of literal C strings and characters
        #
        #     assert "abAB12<>&".escape_to_c         == "abAB12<>&"
        #     assert "\n\"'\\".escape_to_c         == "\\n\\\"\\'\\\\"
        #
        # Most non-printable characters (bellow ASCII 32) are escaped to an octal form `\nnn`.
        # Three digits are always used to avoid following digits to be interpreted as an element
        # of the octal sequence.
        #
        #     assert "{0.code_point}{1.code_point}{8.code_point}{31.code_point}{32.code_point}".escape_to_c == "\\000\\001\\010\\037 "
        #
        # The exceptions are the common `\t` and `\n`.
        fun escape_to_c: String
        do
                var b = new Buffer
                for i in [0..length[ do
                        var c = chars[i]
                        if c == '\n' then
                                b.append("\\n")
                        else if c == '\t' then
                                b.append("\\t")
                        else if c == '"' then
                                b.append("\\\"")
                        else if c == '\'' then
                                b.append("\\\'")
                        else if c == '\\' then
                                b.append("\\\\")
                        else if c.code_point < 32 then
                                b.add('\\')
                                var oct = c.code_point.to_base(8, false)
                                # Force 3 octal digits since it is the
                                # maximum allowed in the C specification
                                if oct.length == 1 then
                                        b.add('0')
                                        b.add('0')
                                else if oct.length == 2 then
                                        b.add('0')
                                end
                                b.append(oct)
                        else
                                b.add(c)
                        end
                end
                return b.to_s
        end

        # Escape additionnal characters
        # The result might no be legal in C but be used in other languages
        #
        #     assert "ab|\{\}".escape_more_to_c("|\{\}") == "ab\\|\\\{\\\}"
        fun escape_more_to_c(chars: String): String
        do
                var b = new Buffer
                for c in escape_to_c.chars do
                        if chars.chars.has(c) then
                                b.add('\\')
                        end
                        b.add(c)
                end
                return b.to_s
        end

        # Escape to C plus braces
        #
        #     assert "\n\"'\\\{\}".escape_to_nit      == "\\n\\\"\\'\\\\\\\{\\\}"
        fun escape_to_nit: String do return escape_more_to_c("\{\}")

        # Escape to POSIX Shell (sh).
        #
        # Abort if the text contains a null byte.
        #
        #     assert "\n\"'\\\{\}0".escape_to_sh == "'\n\"'\\''\\\{\}0'"
        fun escape_to_sh: String do
                var b = new Buffer
                b.chars.add '\''
                for i in [0..length[ do
                        var c = chars[i]
                        if c == '\'' then
                                b.append("'\\''")
                        else
                                assert without_null_byte: c != '\0'
                                b.add(c)
                        end
                end
                b.chars.add '\''
                return b.to_s
        end

        # Escape to include in a Makefile
        #
        # Unfortunately, some characters are not escapable in Makefile.
        # These characters are `;`, `|`, `\`, and the non-printable ones.
        # They will be rendered as `"?{hex}"`.
        fun escape_to_mk: String do
                var b = new Buffer
                for i in [0..length[ do
                        var c = chars[i]
                        if c == '$' then
                                b.append("$$")
                        else if c == ':' or c == ' ' or c == '#' then
                                b.add('\\')
                                b.add(c)
                        else if c.code_point < 32 or c == ';' or c == '|' or c == '\\' or c == '=' then
                                b.append("?{c.code_point.to_base(16, false)}")
                        else
                                b.add(c)
                        end
                end
                return b.to_s
        end

        # Return a string where Nit escape sequences are transformed.
        #
        #     var s = "\\n"
        #     assert s.length        ==  2
        #     var u = s.unescape_nit
        #     assert u.length        ==  1
        #     assert u.chars[0].code_point      ==  10 # (the ASCII value of the "new line" character)
        fun unescape_nit: String
        do
                var res = new Buffer.with_cap(self.length)
                var was_slash = false
                for i in [0..length[ do
                        var c = chars[i]
                        if not was_slash then
                                if c == '\\' then
                                        was_slash = true
                                else
                                        res.add(c)
                                end
                                continue
                        end
                        was_slash = false
                        if c == 'n' then
                                res.add('\n')
                        else if c == 'r' then
                                res.add('\r')
                        else if c == 't' then
                                res.add('\t')
                        else if c == '0' then
                                res.add('\0')
                        else
                                res.add(c)
                        end
                end
                return res.to_s
        end

        # Returns `self` with all characters escaped with their UTF-16 representation
        #
        #     assert "Aèあ𐏓".escape_to_utf16 == "\\u0041\\u00e8\\u3042\\ud800\\udfd3"
        fun escape_to_utf16: String do
                var buf = new Buffer
                for i in chars do buf.append i.escape_to_utf16
                return buf.to_s
        end

        # Returns the Unicode char escaped by `self`
        #
        #     assert "\\u0041".from_utf16_escape == 'A'
        #     assert "\\ud800\\udfd3".from_utf16_escape == '𐏓'
        #     assert "\\u00e8".from_utf16_escape == 'è'
        #     assert "\\u3042".from_utf16_escape == 'あ'
        fun from_utf16_escape: Char do
                var ln = length
                if ln != 6 and ln != 12 then return 0xFFFD.code_point
                var cphi = substring(2, 4).to_hex
                if cphi < 0xD800 then return cphi.code_point
                if cphi > 0xDFFF then return cphi.code_point
                if cphi > 0xDBFF then return 0xFFFD.code_point
                var cp = 0
                cp += (cphi - 0xD800) << 10
                var cplo = substring(8, 4).to_hex
                if cplo < 0xDC00 then return 0xFFFD.code_point
                if cplo > 0xDFFF then return 0xFFFD.code_point
                cp += cplo - 0xDC00
                cp += 0x10000
                return cp.code_point
        end

        # Encode `self` to percent (or URL) encoding
        #
        #     assert "aBc09-._~".to_percent_encoding == "aBc09-._~"
        #     assert "%()< >".to_percent_encoding == "%25%28%29%3c%20%3e"
        #     assert ".com/post?e=asdf&f=123".to_percent_encoding == ".com%2fpost%3fe%3dasdf%26f%3d123"
        #     assert "éあいう".to_percent_encoding == "%c3%a9%e3%81%82%e3%81%84%e3%81%86"
        fun to_percent_encoding: String
        do
                var buf = new Buffer

                for i in [0..length[ do
                        var c = chars[i]
                        if (c >= '0' and c <= '9') or
                           (c >= 'a' and c <= 'z') or
                           (c >= 'A' and c <= 'Z') or
                           c == '-' or c == '.' or
                           c == '_' or c == '~'
                        then
                                buf.add c
                        else
                                var bytes = c.to_s.bytes
                                for b in bytes do buf.append "%{b.to_i.to_hex}"
                        end
                end

                return buf.to_s
        end

        # Decode `self` from percent (or URL) encoding to a clear string
        #
        # Replace invalid use of '%' with '?'.
        #
        #     assert "aBc09-._~".from_percent_encoding == "aBc09-._~"
        #     assert "%25%28%29%3c%20%3e".from_percent_encoding == "%()< >"
        #     assert ".com%2fpost%3fe%3dasdf%26f%3d123".from_percent_encoding == ".com/post?e=asdf&f=123"
        #     assert "%25%28%29%3C%20%3E".from_percent_encoding == "%()< >"
        #     assert "incomplete %".from_percent_encoding == "incomplete ?"
        #     assert "invalid % usage".from_percent_encoding == "invalid ? usage"
        #     assert "%c3%a9%e3%81%82%e3%81%84%e3%81%86".from_percent_encoding == "éあいう"
        fun from_percent_encoding: String
        do
                var len = bytelen
                var has_percent = false
                for c in chars do
                        if c == '%' then
                                len -= 2
                                has_percent = true
                        end
                end

                # If no transformation is needed, return self as a string
                if not has_percent then return to_s

                var buf = new NativeString(len)
                var i = 0
                var l = 0
                while i < length do
                        var c = chars[i]
                        if c == '%' then
                                if i + 2 >= length then
                                        # What follows % has been cut off
                                        buf[l] = '?'.ascii
                                else
                                        i += 1
                                        var hex_s = substring(i, 2)
                                        if hex_s.is_hex then
                                                var hex_i = hex_s.to_hex
                                                buf[l] = hex_i.to_b
                                                i += 1
                                        else
                                                # What follows a % is not Hex
                                                buf[l] = '?'.ascii
                                                i -= 1
                                        end
                                end
                        else buf[l] = c.ascii

                        i += 1
                        l += 1
                end

                return buf.to_s_with_length(l)
        end

        # Escape the characters `<`, `>`, `&`, `"`, `'` and `/` as HTML/XML entity references.
        #
        #     assert "a&b-<>\"x\"/'".html_escape      ==  "a&amp;b-&lt;&gt;&#34;x&#34;&#47;&#39;"
        #
        # SEE: <https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet#RULE_.231_-_HTML_Escape_Before_Inserting_Untrusted_Data_into_HTML_Element_Content>
        fun html_escape: String
        do
                var buf = new Buffer

                for i in [0..length[ do
                        var c = chars[i]
                        if c == '&' then
                                buf.append "&amp;"
                        else if c == '<' then
                                buf.append "&lt;"
                        else if c == '>' then
                                buf.append "&gt;"
                        else if c == '"' then
                                buf.append "&#34;"
                        else if c == '\'' then
                                buf.append "&#39;"
                        else if c == '/' then
                                buf.append "&#47;"
                        else buf.add c
                end

                return buf.to_s
        end

        # Equality of text
        # Two pieces of text are equals if thez have the same characters in the same order.
        #
        #     assert "hello" == "hello"
        #     assert "hello" != "HELLO"
        #     assert "hello" == "hel"+"lo"
        #
        # Things that are not Text are not equal.
        #
        #     assert "9" != '9'
        #     assert "9" != ['9']
        #     assert "9" != 9
        #
        #     assert "9".chars.first == '9'   # equality of Char
        #     assert "9".chars       == ['9'] # equality of Sequence
        #     assert "9".to_i        == 9     # equality of Int
        redef fun ==(o)
        do
                if o == null then return false
                if not o isa Text then return false
                if self.is_same_instance(o) then return true
                if self.length != o.length then return false
                return self.chars == o.chars
        end

        # Lexicographical comparaison
        #
        #     assert "abc" < "xy"
        #     assert "ABC" < "abc"
        redef fun <(other)
        do
                var self_chars = self.chars.iterator
                var other_chars = other.chars.iterator

                while self_chars.is_ok and other_chars.is_ok do
                        if self_chars.item < other_chars.item then return true
                        if self_chars.item > other_chars.item then return false
                        self_chars.next
                        other_chars.next
                end

                if self_chars.is_ok then
                        return false
                else
                        return true
                end
        end

        # Escape string used in labels for graphviz
        #
        #   assert ">><<".escape_to_dot == "\\>\\>\\<\\<"
        fun escape_to_dot: String
        do
                return escape_more_to_c("|\{\}<>")
        end

        private var hash_cache: nullable Int = null

        redef fun hash
        do
                if hash_cache == null then
                        # djb2 hash algorithm
                        var h = 5381

                        for i in [0..length[ do
                                var char = chars[i]
                                h = (h << 5) + h + char.code_point
                        end

                        hash_cache = h
                end
                return hash_cache.as(not null)
        end

        # Gives the formatted string back as a Nit string with `args` in place
        #
        #     assert "This %1 is a %2.".format("String", "formatted String") == "This String is a formatted String."
        #     assert "\\%1 This string".format("String") == "\\%1 This string"
        fun format(args: Object...): String do
                var s = new Array[Text]
                var curr_st = 0
                var i = 0
                while i < length do
                        # Skip escaped characters
                        if self[i] == '\\' then
                                i += 1
                        # In case of format
                        else if self[i] == '%' then
                                var fmt_st = i
                                i += 1
                                var ciph_st = i
                                while i < length and self[i].is_numeric do
                                        i += 1
                                end
                                i -= 1
                                var fmt_end = i
                                var ciph_len = fmt_end - ciph_st + 1

                                var arg_index = substring(ciph_st, ciph_len).to_i - 1
                                if arg_index >= args.length then continue

                                s.push substring(curr_st, fmt_st - curr_st)
                                s.push args[arg_index].to_s
                                curr_st = i + 1
                        end
                        i += 1
                end
                s.push substring(curr_st, length - curr_st)
                return s.plain_to_s
        end

        # Return the Levenshtein distance between two strings
        #
        # ~~~
        # assert "abcd".levenshtein_distance("abcd") == 0
        # assert "".levenshtein_distance("abcd")     == 4
        # assert "abcd".levenshtein_distance("")     == 4
        # assert "abcd".levenshtein_distance("xyz")  == 4
        # assert "abcd".levenshtein_distance("xbdy") == 3
        # ~~~
        fun levenshtein_distance(other: String): Int
        do
                var slen = self.length
                var olen = other.length

                # fast cases
                if slen == 0 then return olen
                if olen == 0 then return slen
                if self == other then return 0

                # previous row of distances
                var v0 = new Array[Int].with_capacity(olen+1)

                # current row of distances
                var v1 = new Array[Int].with_capacity(olen+1)

                for j in [0..olen] do
                        # prefix insert cost
                        v0[j] = j
                end

                for i in [0..slen[ do

                        # prefix delete cost
                        v1[0] = i + 1

                        for j in [0..olen[ do
                                # delete cost
                                var cost1 = v1[j] + 1
                                # insert cost
                                var cost2 = v0[j + 1] + 1
                                # same char cost (+0)
                                var cost3 = v0[j]
                                # change cost
                                if self[i] != other[j] then cost3 += 1
                                # keep the min
                                v1[j+1] = cost1.min(cost2).min(cost3)
                        end

                        # Switch columns:
                        # * v1 become v0 in the next iteration
                        # * old v0 is reused as the new v1
                        var tmp = v1
                        v1 = v0
                        v0 = tmp
                end

                return v0[olen]
        end

        # Copies `n` bytes from `self` at `src_offset` into `dest` starting at `dest_offset`
        #
        # Basically a high-level synonym of NativeString::copy_to
        #
        # REQUIRE: `n` must be large enough to contain `len` bytes
        #
        #       var ns = new NativeString(8)
        #       "Text is String".copy_to_native(ns, 8, 2, 0)
        #       assert ns.to_s_with_length(8) == "xt is St"
        #
        fun copy_to_native(dest: NativeString, n, src_offset, dest_offset: Int) do
                var mypos = src_offset
                var itspos = dest_offset
                while n > 0 do
                        dest[itspos] = self.bytes[mypos]
                        itspos += 1
                        mypos += 1
                        n -= 1
                end
        end

end

# All kinds of array-based text representations.
abstract class FlatText
        super Text

        # Underlying C-String (`char*`)
        #
        # Warning : Might be void in some subclasses, be sure to check
        # if set before using it.
        var items: NativeString is noinit

        # Returns a char* starting at position `first_byte`
        #
        # WARNING: If you choose to use this service, be careful of the following.
        #
        # Strings and NativeString are *ideally* always allocated through a Garbage Collector.
        # Since the GC tracks the use of the pointer for the beginning of the char*, it may be
        # deallocated at any moment, rendering the pointer returned by this function invalid.
        # Any access to freed memory may very likely cause undefined behaviour or a crash.
        # (Failure to do so will most certainly result in long and painful debugging hours)
        #
        # The only safe use of this pointer is if it is ephemeral (e.g. read in a C function
        # then immediately return).
        #
        # As always, do not modify the content of the String in C code, if this is what you want
        # copy locally the char* as Nit Strings are immutable.
        fun fast_cstring: NativeString is abstract

        redef var length = 0

        redef var bytelen = 0

        redef fun output
        do
                var i = 0
                while i < length do
                        items[i].output
                        i += 1
                end
        end

        redef fun copy_to_native(dest, n, src_offset, dest_offset) do
                items.copy_to(dest, n, src_offset, dest_offset)
        end
end

# Abstract class for the SequenceRead compatible
# views on the chars of any Text
private abstract class StringCharView
        super SequenceRead[Char]

        type SELFTYPE: Text

        var target: SELFTYPE

        redef fun is_empty do return target.is_empty

        redef fun length do return target.length

        redef fun iterator: IndexedIterator[Char] do return self.iterator_from(0)

        redef fun reverse_iterator do return self.reverse_iterator_from(self.length - 1)
end

# Abstract class for the SequenceRead compatible
# views on the bytes of any Text
private abstract class StringByteView
        super SequenceRead[Byte]

        type SELFTYPE: Text

        var target: SELFTYPE

        redef fun is_empty do return target.is_empty

        redef fun length do return target.bytelen

        redef fun iterator do return self.iterator_from(0)

        redef fun reverse_iterator do return self.reverse_iterator_from(target.bytelen - 1)
end

# Immutable sequence of characters.
#
# String objects may be created using literals.
#
#     assert "Hello World!" isa String
abstract class String
        super Text

        redef type SELFTYPE: String is fixed

        redef fun to_s do return self

        # Concatenates `o` to `self`
        #
        #     assert "hello" + "world"  == "helloworld"
        #     assert "" + "hello" + ""  == "hello"
        fun +(o: Text): SELFTYPE is abstract

        # Concatenates self `i` times
        #
        #     assert "abc" * 4 == "abcabcabcabc"
        #     assert "abc" * 1 == "abc"
        #     assert "abc" * 0 == ""
        fun *(i: Int): SELFTYPE is abstract

        # Insert `s` at `pos`.
        #
        #     assert "helloworld".insert_at(" ", 5)     == "hello world"
        fun insert_at(s: String, pos: Int): SELFTYPE is abstract

        redef fun substrings is abstract

        # Returns a reversed version of self
        #
        #     assert "hello".reversed  == "olleh"
        #     assert "bob".reversed    == "bob"
        #     assert "".reversed       == ""
        fun reversed: SELFTYPE is abstract

        # A upper case version of `self`
        #
        #     assert "Hello World!".to_upper     == "HELLO WORLD!"
        fun to_upper: SELFTYPE is abstract

        # A lower case version of `self`
        #
        #     assert "Hello World!".to_lower     == "hello world!"
        fun to_lower : SELFTYPE is abstract

        # Takes a camel case `self` and converts it to snake case
        #
        #     assert "randomMethodId".to_snake_case == "random_method_id"
        #
        # The rules are the following:
        #
        # An uppercase is always converted to a lowercase
        #
        #     assert "HELLO_WORLD".to_snake_case == "hello_world"
        #
        # An uppercase that follows a lowercase is prefixed with an underscore
        #
        #     assert "HelloTheWORLD".to_snake_case == "hello_the_world"
        #
        # An uppercase that follows an uppercase and is followed by a lowercase, is prefixed with an underscore
        #
        #     assert "HelloTHEWorld".to_snake_case == "hello_the_world"
        #
        # All other characters are kept as is; `self` does not need to be a proper CamelCased string.
        #
        #     assert "=-_H3ll0Th3W0rld_-=".to_snake_case == "=-_h3ll0th3w0rld_-="
        fun to_snake_case: SELFTYPE
        do
                if self.is_lower then return self

                var new_str = new Buffer.with_cap(self.length)
                var prev_is_lower = false
                var prev_is_upper = false

                for i in [0..length[ do
                        var char = chars[i]
                        if char.is_lower then
                                new_str.add(char)
                                prev_is_lower = true
                                prev_is_upper = false
                        else if char.is_upper then
                                if prev_is_lower then
                                        new_str.add('_')
                                else if prev_is_upper and i+1 < length and chars[i+1].is_lower then
                                        new_str.add('_')
                                end
                                new_str.add(char.to_lower)
                                prev_is_lower = false
                                prev_is_upper = true
                        else
                                new_str.add(char)
                                prev_is_lower = false
                                prev_is_upper = false
                        end
                end

                return new_str.to_s
        end

        # Takes a snake case `self` and converts it to camel case
        #
        #     assert "random_method_id".to_camel_case == "randomMethodId"
        #
        # If the identifier is prefixed by an underscore, the underscore is ignored
        #
        #     assert "_private_field".to_camel_case == "_privateField"
        #
        # If `self` is upper, it is returned unchanged
        #
        #     assert "RANDOM_ID".to_camel_case == "RANDOM_ID"
        #
        # If there are several consecutive underscores, they are considered as a single one
        #
        #     assert "random__method_id".to_camel_case == "randomMethodId"
        fun to_camel_case: SELFTYPE
        do
                if self.is_upper then return self

                var new_str = new Buffer
                var is_first_char = true
                var follows_us = false

                for i in [0..length[ do
                        var char = chars[i]
                        if is_first_char then
                                new_str.add(char)
                                is_first_char = false
                        else if char == '_' then
                                follows_us = true
                        else if follows_us then
                                new_str.add(char.to_upper)
                                follows_us = false
                        else
                                new_str.add(char)
                        end
                end

                return new_str.to_s
        end

        # Returns a capitalized `self`
        #
        # Letters that follow a letter are lowercased
        # Letters that follow a non-letter are upcased.
        #
        # SEE : `Char::is_letter` for the definition of letter.
        #
        #     assert "jAVASCRIPT".capitalized == "Javascript"
        #     assert "i am root".capitalized == "I Am Root"
        #     assert "ab_c -ab0c ab\nc".capitalized == "Ab_C -Ab0C Ab\nC"
        fun capitalized: SELFTYPE do
                if length == 0 then return self

                var buf = new Buffer.with_cap(length)

                var curr = chars[0].to_upper
                var prev = curr
                buf[0] = curr

                for i in [1 .. length[ do
                        prev = curr
                        curr = self[i]
                        if prev.is_letter then
                                buf[i] = curr.to_lower
                        else
                                buf[i] = curr.to_upper
                        end
                end

                return buf.to_s
        end
end

# A mutable sequence of characters.
abstract class Buffer
        super Text

        # Returns an arbitrary subclass of `Buffer` with default parameters
        new is abstract

        # Returns an instance of a subclass of `Buffer` with `i` base capacity
        new with_cap(i: Int) is abstract

        redef type SELFTYPE: Buffer is fixed

        # Specific implementations MUST set this to `true` in order to invalidate caches
        protected var is_dirty = true

        # Copy-On-Write flag
        #
        # If the `Buffer` was to_s'd, the next in-place altering
        # operation will cause the current `Buffer` to be re-allocated.
        #
        # The flag will then be set at `false`.
        protected var written = false

        # Modifies the char contained at pos `index`
        #
        # DEPRECATED : Use self.chars.[]= instead
        fun []=(index: Int, item: Char) is abstract

        # Adds a char `c` at the end of self
        #
        # DEPRECATED : Use self.chars.add instead
        fun add(c: Char) is abstract

        # Clears the buffer
        #
        #     var b = new Buffer
        #     b.append "hello"
        #     assert not b.is_empty
        #     b.clear
        #     assert b.is_empty
        fun clear is abstract

        # Enlarges the subsequent array containing the chars of self
        fun enlarge(cap: Int) is abstract

        # Adds the content of text `s` at the end of self
        #
        #     var b = new Buffer
        #     b.append "hello"
        #     b.append "world"
        #     assert b == "helloworld"
        fun append(s: Text) is abstract

        # `self` is appended in such a way that `self` is repeated `r` times
        #
        #     var b = new Buffer
        #     b.append "hello"
        #     b.times 3
        #     assert b == "hellohellohello"
        fun times(r: Int) is abstract

        # Reverses itself in-place
        #
        #     var b = new Buffer
        #     b.append("hello")
        #     b.reverse
        #     assert b == "olleh"
        fun reverse is abstract

        # Changes each lower-case char in `self` by its upper-case variant
        #
        #     var b = new Buffer
        #     b.append("Hello World!")
        #     b.upper
        #     assert b == "HELLO WORLD!"
        fun upper is abstract

        # Changes each upper-case char in `self` by its lower-case variant
        #
        #     var b = new Buffer
        #     b.append("Hello World!")
        #     b.lower
        #     assert b == "hello world!"
        fun lower is abstract

        # Capitalizes each word in `self`
        #
        # Letters that follow a letter are lowercased
        # Letters that follow a non-letter are upcased.
        #
        # SEE: `Char::is_letter` for the definition of a letter.
        #
        #     var b = new FlatBuffer.from("jAVAsCriPt")
        #     b.capitalize
        #     assert b == "Javascript"
        #     b = new FlatBuffer.from("i am root")
        #     b.capitalize
        #     assert b == "I Am Root"
        #     b = new FlatBuffer.from("ab_c -ab0c ab\nc")
        #     b.capitalize
        #     assert b == "Ab_C -Ab0C Ab\nC"
        fun capitalize do
                if length == 0 then return
                var c = self[0].to_upper
                self[0] = c
                var prev = c
                for i in [1 .. length[ do
                        prev = c
                        c = self[i]
                        if prev.is_letter then
                                self[i] = c.to_lower
                        else
                                self[i] = c.to_upper
                        end
                end
        end

        redef fun hash
        do
                if is_dirty then hash_cache = null
                return super
        end

        # In Buffers, the internal sequence of character is mutable
        # Thus, `chars` can be used to modify the buffer.
        redef fun chars: Sequence[Char] is abstract
end

# View for chars on Buffer objects, extends Sequence
# for mutation operations
private abstract class BufferCharView
        super StringCharView
        super Sequence[Char]

        redef type SELFTYPE: Buffer

end

# View for bytes on Buffer objects, extends Sequence
# for mutation operations
private abstract class BufferByteView
        super StringByteView

        redef type SELFTYPE: Buffer
end

redef class Object
        # User readable representation of `self`.
        fun to_s: String do return inspect

        # The class name of the object in NativeString format.
        private fun native_class_name: NativeString is intern

        # The class name of the object.
        #
        #     assert 5.class_name == "Int"
        fun class_name: String do return native_class_name.to_s

        # Developer readable representation of `self`.
        # Usually, it uses the form "<CLASSNAME:#OBJECTID bla bla bla>"
        fun inspect: String
        do
                return "<{inspect_head}>"
        end

        # Return "CLASSNAME:#OBJECTID".
        # This function is mainly used with the redefinition of the inspect method
        protected fun inspect_head: String
        do
                return "{class_name}:#{object_id.to_hex}"
        end
end

redef class Bool
        #     assert true.to_s         == "true"
        #     assert false.to_s        == "false"
        redef fun to_s
        do
                if self then
                        return once "true"
                else
                        return once "false"
                end
        end
end

redef class Byte
        # C function to calculate the length of the `NativeString` to receive `self`
        private fun byte_to_s_len: Int `{
                return snprintf(NULL, 0, "0x%02x", self);
        `}

        # C function to convert an nit Int to a NativeString (char*)
        private fun native_byte_to_s(nstr: NativeString, strlen: Int) `{
                snprintf(nstr, strlen, "0x%02x", self);
        `}

        # Displayable byte in its hexadecimal form (0x..)
        #
        #     assert 1.to_b.to_s       == "0x01"
        #     assert (-123).to_b.to_s  == "0x85"
        redef fun to_s do
                var nslen = byte_to_s_len
                var ns = new NativeString(nslen + 1)
                ns[nslen] = 0u8
                native_byte_to_s(ns, nslen + 1)
                return ns.to_s_with_length(nslen)
        end
end

redef class Int

        # Wrapper of strerror C function
        private fun strerror_ext: NativeString `{ return strerror(self); `}

        # Returns a string describing error number
        fun strerror: String do return strerror_ext.to_s

        # Fill `s` with the digits in base `base` of `self` (and with the '-' sign if 'signed' and negative).
        # assume < to_c max const of char
        private fun fill_buffer(s: Buffer, base: Int, signed: Bool)
        do
                var n: Int
                # Sign
                if self < 0 then
                        n = - self
                        s.chars[0] = '-'
                else if self == 0 then
                        s.chars[0] = '0'
                        return
                else
                        n = self
                end
                # Fill digits
                var pos = digit_count(base) - 1
                while pos >= 0 and n > 0 do
                        s.chars[pos] = (n % base).to_c
                        n = n / base # /
                        pos -= 1
                end
        end

        # C function to calculate the length of the `NativeString` to receive `self`
        private fun int_to_s_len: Int `{
                return snprintf(NULL, 0, "%ld", self);
        `}

        # C function to convert an nit Int to a NativeString (char*)
        private fun native_int_to_s(nstr: NativeString, strlen: Int) `{
                snprintf(nstr, strlen, "%ld", self);
        `}

        # return displayable int in base base and signed
        fun to_base(base: Int, signed: Bool): String is abstract

        # return displayable int in hexadecimal
        #
        #     assert 1.to_hex  == "1"
        #     assert (-255).to_hex  == "-ff"
        fun to_hex: String do return to_base(16,false)
end

redef class Float
        # Pretty representation of `self`, with decimals as needed from 1 to a maximum of 3
        #
        #     assert 12.34.to_s       == "12.34"
        #     assert (-0120.030).to_s == "-120.03"
        #
        # see `to_precision` for a custom precision.
        redef fun to_s do
                var str = to_precision( 3 )
                if is_inf != 0 or is_nan then return str
                var len = str.length
                for i in [0..len-1] do
                        var j = len-1-i
                        var c = str.chars[j]
                        if c == '0' then
                                continue
                        else if c == '.' then
                                return str.substring( 0, j+2 )
                        else
                                return str.substring( 0, j+1 )
                        end
                end
                return str
        end

        # `String` representation of `self` with the given number of `decimals`
        #
        #     assert 12.345.to_precision(0)    == "12"
        #     assert 12.345.to_precision(3)    == "12.345"
        #     assert (-12.345).to_precision(3) == "-12.345"
        #     assert (-0.123).to_precision(3)  == "-0.123"
        #     assert 0.999.to_precision(2)     == "1.00"
        #     assert 0.999.to_precision(4)     == "0.9990"
        fun to_precision(decimals: Int): String
        do
                if is_nan then return "nan"

                var isinf = self.is_inf
                if isinf == 1 then
                        return "inf"
                else if isinf == -1 then
                        return  "-inf"
                end

                if decimals == 0 then return self.to_i.to_s
                var f = self
                for i in [0..decimals[ do f = f * 10.0
                if self > 0.0 then
                        f = f + 0.5
                else
                        f = f - 0.5
                end
                var i = f.to_i
                if i == 0 then return "0." + "0"*decimals

                # Prepare both parts of the float, before and after the "."
                var s = i.abs.to_s
                var sl = s.length
                var p1
                var p2
                if sl > decimals then
                        # Has something before the "."
                        p1 = s.substring(0, sl-decimals)
                        p2 = s.substring(sl-decimals, decimals)
                else
                        p1 = "0"
                        p2 = "0"*(decimals-sl) + s
                end

                if i < 0 then p1 = "-" + p1

                return p1 + "." + p2
        end
end

redef class Char

        # Returns a sequence with the UTF-8 bytes of `self`
        #
        #     assert 'a'.bytes == [0x61u8]
        #     assert 'ま'.bytes == [0xE3u8, 0x81u8, 0xBEu8]
        fun bytes: SequenceRead[Byte] do return to_s.bytes

        # Length of `self` in a UTF-8 String
        private fun u8char_len: Int do
                var c = self.code_point
                if c < 0x80 then return 1
                if c <= 0x7FF then return 2
                if c <= 0xFFFF then return 3
                if c <= 0x10FFFF then return 4
                # Bad character format
                return 1
        end

        #     assert 'x'.to_s    == "x"
        redef fun to_s do
                var ln = u8char_len
                var ns = new NativeString(ln + 1)
                u8char_tos(ns, ln)
                return ns.to_s_with_length(ln)
        end

        # Returns `self` escaped to UTF-16
        #
        # i.e. Represents `self`.`code_point` using UTF-16 codets escaped
        # with a `\u`
        #
        #     assert 'A'.escape_to_utf16 == "\\u0041"
        #     assert 'è'.escape_to_utf16 == "\\u00e8"
        #     assert 'あ'.escape_to_utf16 == "\\u3042"
        #     assert '𐏓'.escape_to_utf16 == "\\ud800\\udfd3"
        fun escape_to_utf16: String do
                var cp = code_point
                var buf: Buffer
                if cp < 0xD800 or (cp >= 0xE000 and cp <= 0xFFFF) then
                        buf = new Buffer.with_cap(6)
                        buf.append("\\u0000")
                        var hx = cp.to_hex
                        var outid = 5
                        for i in hx.chars.reverse_iterator do
                                buf[outid] = i
                                outid -= 1
                        end
                else
                        buf = new Buffer.with_cap(12)
                        buf.append("\\u0000\\u0000")
                        var lo = (((cp - 0x10000) & 0x3FF) + 0xDC00).to_hex
                        var hi = ((((cp - 0x10000) & 0xFFC00) >> 10) + 0xD800).to_hex
                        var out = 2
                        for i in hi do
                                buf[out] = i
                                out += 1
                        end
                        out = 8
                        for i in lo do
                                buf[out] = i
                                out += 1
                        end
                end
                return buf.to_s
        end

        private fun u8char_tos(r: NativeString, len: Int) `{
                r[len] = '\0';
                switch(len){
                        case 1:
                                r[0] = self;
                                break;
                        case 2:
                                r[0] = 0xC0 | ((self & 0x7C0) >> 6);
                                r[1] = 0x80 | (self & 0x3F);
                                break;
                        case 3:
                                r[0] = 0xE0 | ((self & 0xF000) >> 12);
                                r[1] = 0x80 | ((self & 0xFC0) >> 6);
                                r[2] = 0x80 | (self & 0x3F);
                                break;
                        case 4:
                                r[0] = 0xF0 | ((self & 0x1C0000) >> 18);
                                r[1] = 0x80 | ((self & 0x3F000) >> 12);
                                r[2] = 0x80 | ((self & 0xFC0) >> 6);
                                r[3] = 0x80 | (self & 0x3F);
                                break;
                }
        `}

        # Returns true if the char is a numerical digit
        #
        #     assert '0'.is_numeric
        #     assert '9'.is_numeric
        #     assert not 'a'.is_numeric
        #     assert not '?'.is_numeric
        #
        # FIXME: Works on ASCII-range only
        fun is_numeric: Bool
        do
                return self >= '0' and self <= '9'
        end

        # Returns true if the char is an alpha digit
        #
        #     assert 'a'.is_alpha
        #     assert 'Z'.is_alpha
        #     assert not '0'.is_alpha
        #     assert not '?'.is_alpha
        #
        # FIXME: Works on ASCII-range only
        fun is_alpha: Bool
        do
                return (self >= 'a' and self <= 'z') or (self >= 'A' and self <= 'Z')
        end

        # Is `self` an hexadecimal digit ?
        #
        #     assert 'A'.is_hexdigit
        #     assert not 'G'.is_hexdigit
        #     assert 'a'.is_hexdigit
        #     assert not 'g'.is_hexdigit
        #     assert '5'.is_hexdigit
        fun is_hexdigit: Bool do return (self >= '0' and self <= '9') or (self >= 'A' and self <= 'F') or
                                        (self >= 'a' and self <= 'f')

        # Returns true if the char is an alpha or a numeric digit
        #
        #     assert 'a'.is_alphanumeric
        #     assert 'Z'.is_alphanumeric
        #     assert '0'.is_alphanumeric
        #     assert '9'.is_alphanumeric
        #     assert not '?'.is_alphanumeric
        #
        # FIXME: Works on ASCII-range only
        fun is_alphanumeric: Bool
        do
                return self.is_numeric or self.is_alpha
        end
end

redef class Collection[E]
        # String representation of the content of the collection.
        #
        # The standard representation is the list of elements separated with commas.
        #
        # ~~~
        # assert [1,2,3].to_s == "[1,2,3]"
        # assert [1..3].to_s  == "[1,2,3]"
        # assert (new Array[Int]).to_s == "[]" # empty collection
        # ~~~
        #
        # Subclasses may return a more specific string representation.
        redef fun to_s
        do
                return "[" + join(",") + "]"
        end

        # Concatenate elements without separators
        #
        # ~~~
        # assert [1,2,3].plain_to_s == "123"
        # assert [11..13].plain_to_s  == "111213"
        # assert (new Array[Int]).plain_to_s == "" # empty collection
        # ~~~
        fun plain_to_s: String
        do
                var s = new Buffer
                for e in self do if e != null then s.append(e.to_s)
                return s.to_s
        end

        # Concatenate and separate each elements with `separator`.
        #
        # Only concatenate if `separator == null`.
        #
        #     assert [1, 2, 3].join(":")    == "1:2:3"
        #     assert [1..3].join(":")       == "1:2:3"
        #     assert [1..3].join            == "123"
        fun join(separator: nullable Text): String
        do
                if is_empty then return ""

                var s = new Buffer # Result

                # Concat first item
                var i = iterator
                var e = i.item
                if e != null then s.append(e.to_s)

                # Concat other items
                i.next
                while i.is_ok do
                        if separator != null then s.append(separator)
                        e = i.item
                        if e != null then s.append(e.to_s)
                        i.next
                end
                return s.to_s
        end
end

redef class Map[K,V]
        # Concatenate couples of key value.
        # Key and value are separated by `couple_sep`.
        # Couples are separated by `sep`.
        #
        #     var m = new HashMap[Int, String]
        #     m[1] = "one"
        #     m[10] = "ten"
        #     assert m.join("; ", "=") == "1=one; 10=ten"
        fun join(sep, couple_sep: String): String is abstract
end

redef class Sys
        private var args_cache: nullable Sequence[String] = null

        # The arguments of the program as given by the OS
        fun program_args: Sequence[String]
        do
                if _args_cache == null then init_args
                return _args_cache.as(not null)
        end

        # The name of the program as given by the OS
        fun program_name: String
        do
                return native_argv(0).to_s
        end

        # Initialize `program_args` with the contents of `native_argc` and `native_argv`.
        private fun init_args
        do
                var argc = native_argc
                var args = new Array[String].with_capacity(0)
                var i = 1
                while i < argc do
                        args[i-1] = native_argv(i).to_s
                        i += 1
                end
                _args_cache = args
        end

        # First argument of the main C function.
        private fun native_argc: Int is intern

        # Second argument of the main C function.
        private fun native_argv(i: Int): NativeString is intern
end

# Comparator that efficienlty use `to_s` to compare things
#
# The comparaison call `to_s` on object and use the result to order things.
#
#     var a = [1, 2, 3, 10, 20]
#     (new CachedAlphaComparator).sort(a)
#     assert a == [1, 10, 2, 20, 3]
#
# Internally the result of `to_s` is cached in a HashMap to counter
# uneficient implementation of `to_s`.
#
# Note: it caching is not usefull, see `alpha_comparator`
class CachedAlphaComparator
        super Comparator
        redef type COMPARED: Object

        private var cache = new HashMap[Object, String]

        private fun do_to_s(a: Object): String do
                if cache.has_key(a) then return cache[a]
                var res = a.to_s
                cache[a] = res
                return res
        end

        redef fun compare(a, b) do
                return do_to_s(a) <=> do_to_s(b)
        end
end

# see `alpha_comparator`
private class AlphaComparator
        super Comparator
        redef fun compare(a, b) do return a.to_s <=> b.to_s
end

# Stateless comparator that naively use `to_s` to compare things.
#
# Note: the result of `to_s` is not cached, thus can be invoked a lot
# on a single instace. See `CachedAlphaComparator` as an alternative.
#
#     var a = [1, 2, 3, 10, 20]
#     alpha_comparator.sort(a)
#     assert a == [1, 10, 2, 20, 3]
fun alpha_comparator: Comparator do return once new AlphaComparator

# The arguments of the program as given by the OS
fun args: Sequence[String]
do
        return sys.program_args
end

redef class NativeString
        # Returns `self` as a new String.
        fun to_s_with_copy: String is abstract

        # Returns `self` as a String of `length`.
        fun to_s_with_length(length: Int): String is abstract

        # Returns `self` as a String with `bytelen` and `length` set
        #
        # SEE: `abstract_text::Text` for more infos on the difference
        # between `Text::bytelen` and `Text::length`
        fun to_s_full(bytelen, unilen: Int): String is abstract
end

redef class NativeArray[E]
        # Join all the elements using `to_s`
        #
        # REQUIRE: `self isa NativeArray[String]`
        # REQUIRE: all elements are initialized
        fun native_to_s: String is abstract
end