lib/core: Added new `append_substring` service to avoid creating ephemeral instances

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

# All the array-based text representations
module flat

intrude import abstract_text
intrude import native

`{
#include <stdio.h>
#include <string.h>
`}

private class FlatSubstringsIter
        super Iterator[FlatText]

        var tgt: nullable FlatText

        redef fun item do
                assert is_ok
                return tgt.as(not null)
        end

        redef fun is_ok do return tgt != null

        redef fun next do tgt = null
end

redef class FlatText

        # First byte of the NativeString
        protected fun first_byte: Int do return 0

        # Last byte of the NativeString
        protected fun last_byte: Int do return first_byte + _bytelen - 1

        # Cache of the latest position (char) explored in the string
        var position: Int = 0

        # Cached position (bytes) in the NativeString underlying the String
        var bytepos: Int = 0

        # Index of the character `index` in `_items`
        fun char_to_byte_index(index: Int): Int do
                var dpos = index - _position
                var b = _bytepos
                var its = _items

                if dpos == 1 then
                        if its[b] & 0x80u8 == 0x00u8 then
                                b += 1
                        else
                                b += its.length_of_char_at(b)
                        end
                        _bytepos = b
                        _position = index
                        return b
                end
                if dpos == -1 then
                        b = its.find_beginning_of_char_at(b - 1)
                        _bytepos = b
                        _position = index
                        return b
                end
                if dpos == 0 then return b

                var ln = _length
                var pos = _position
                # Find best insertion point
                var delta_begin = index
                var delta_end = (ln - 1) - index
                var delta_cache = (pos - index).abs
                var min = delta_begin

                if delta_cache < min then min = delta_cache
                if delta_end < min then min = delta_end

                var ns_i: Int
                var my_i: Int

                if min == delta_cache then
                        ns_i = _bytepos
                        my_i = pos
                else if min == delta_begin then
                        ns_i = first_byte
                        my_i = 0
                else
                        ns_i = its.find_beginning_of_char_at(last_byte)
                        my_i = _length - 1
                end

                ns_i = its.char_to_byte_index_cached(index, my_i, ns_i)

                _position = index
                _bytepos = ns_i

                return ns_i
        end

        # By escaping `self` to HTML, how many more bytes will be needed ?
        fun chars_to_html_escape: Int do
                var its = _items
                var max = last_byte
                var pos = first_byte
                var endlen = 0
                while pos <= max do
                        var c = its[pos]
                        if c == 0x3Cu8 then
                                endlen += 3
                        else if c == 0x3Eu8 then
                                endlen += 3
                        else if c == 0x26u8 then
                                endlen += 4
                        else if c == 0x22u8 then
                                endlen += 4
                        else if c == 0x27u8 then
                                endlen += 4
                        else if c == 0x2Fu8 then
                                endlen += 4
                        end
                        pos += 1
                end
                return endlen
        end

        redef fun html_escape
        do
                var extra = chars_to_html_escape
                if extra == 0 then return to_s
                var its = _items
                var max = last_byte
                var pos = first_byte
                var nlen = extra + _bytelen
                var nits = new NativeString(nlen)
                var outpos = 0
                while pos <= max do
                        var c = its[pos]
                        # Special codes:
                        # Some HTML characters are used as meta-data, they need
                        # to be replaced by an HTML-Escaped equivalent
                        #
                        # * 0x3C (<) => &lt;
                        # * 0x3E (>) => &gt;
                        # * 0x26 (&) => &amp;
                        # * 0x22 (") => &#34;
                        # * 0x27 (') => &#39;
                        # * 0x2F (/) => &#47;
                        if c == 0x3Cu8 then
                                nits[outpos] = 0x26u8
                                nits[outpos + 1] = 0x6Cu8
                                nits[outpos + 2] = 0x74u8
                                nits[outpos + 3] = 0x3Bu8
                                outpos += 4
                        else if c == 0x3Eu8 then
                                nits[outpos] = 0x26u8
                                nits[outpos + 1] = 0x67u8
                                nits[outpos + 2] = 0x74u8
                                nits[outpos + 3] = 0x3Bu8
                                outpos += 4
                        else if c == 0x26u8 then
                                nits[outpos] = 0x26u8
                                nits[outpos + 1] = 0x61u8
                                nits[outpos + 2] = 0x6Du8
                                nits[outpos + 3] = 0x70u8
                                nits[outpos + 4] = 0x3Bu8
                                outpos += 5
                        else if c == 0x22u8 then
                                nits[outpos] = 0x26u8
                                nits[outpos + 1] = 0x23u8
                                nits[outpos + 2] = 0x33u8
                                nits[outpos + 3] = 0x34u8
                                nits[outpos + 4] = 0x3Bu8
                                outpos += 5
                        else if c == 0x27u8 then
                                nits[outpos] = 0x26u8
                                nits[outpos + 1] = 0x23u8
                                nits[outpos + 2] = 0x33u8
                                nits[outpos + 3] = 0x39u8
                                nits[outpos + 4] = 0x3Bu8
                                outpos += 5
                        else if c == 0x2Fu8 then
                                nits[outpos] = 0x26u8
                                nits[outpos + 1] = 0x23u8
                                nits[outpos + 2] = 0x34u8
                                nits[outpos + 3] = 0x37u8
                                nits[outpos + 4] = 0x3Bu8
                                outpos += 5
                        else
                                nits[outpos] = c
                                outpos += 1
                        end
                        pos += 1
                end
                var s = new FlatString.with_infos(nits, nlen, 0)
                return s
        end

        # By escaping `self` to C, how many more bytes will be needed ?
        #
        # This enables a double-optimization in `escape_to_c` since if this
        # method returns 0, then `self` does not need escaping and can be
        # returned as-is
        fun chars_to_escape_to_c: Int do
                var its = _items
                var max = last_byte
                var pos = first_byte
                var req_esc = 0
                while pos <= max do
                        var c = its[pos]
                        if c == 0x0Au8 then
                                req_esc += 1
                        else if c == 0x09u8 then
                                req_esc += 1
                        else if c == 0x22u8 then
                                req_esc += 1
                        else if c == 0x27u8 then
                                req_esc += 1
                        else if c == 0x5Cu8 then
                                req_esc += 1
                        else if c < 32u8 then
                                req_esc += 3
                        end
                        pos += 1
                end
                return req_esc
        end

        redef fun escape_to_c do
                var ln_extra = chars_to_escape_to_c
                if ln_extra == 0 then return self.to_s
                var its = _items
                var max = last_byte
                var nlen = _bytelen + ln_extra
                var nns = new NativeString(nlen)
                var pos = first_byte
                var opos = 0
                while pos <= max do
                        var c = its[pos]
                        # Special codes:
                        #
                        # Any byte with value < 32 is a control character
                        # All their uses will be replaced by their octal
                        # value in C.
                        #
                        # There are two exceptions however:
                        #
                        # * 0x09 => \t
                        # * 0x0A => \n
                        #
                        # Aside from the code points above, the following are:
                        #
                        # * 0x22 => \"
                        # * 0x27 => \'
                        # * 0x5C => \\
                        if c == 0x09u8 then
                                nns[opos] = 0x5Cu8
                                nns[opos + 1] = 0x74u8
                                opos += 2
                        else if c == 0x0Au8 then
                                nns[opos] = 0x5Cu8
                                nns[opos + 1] = 0x6Eu8
                                opos += 2
                        else if c == 0x22u8 then
                                nns[opos] = 0x5Cu8
                                nns[opos + 1] = 0x22u8
                                opos += 2
                        else if c == 0x27u8 then
                                nns[opos] = 0x5Cu8
                                nns[opos + 1] = 0x27u8
                                opos += 2
                        else if c == 0x5Cu8 then
                                nns[opos] = 0x5Cu8
                                nns[opos + 1] = 0x5Cu8
                                opos += 2
                        else if c < 32u8 then
                                nns[opos] = 0x5Cu8
                                nns[opos + 1] = 0x30u8
                                nns[opos + 2] = ((c & 0x38u8) >> 3) + 0x30u8
                                nns[opos + 3] = (c & 0x07u8) + 0x30u8
                                opos += 4
                        else
                                nns[opos] = c
                                opos += 1
                        end
                        pos += 1
                end
                return nns.to_s_unsafe(nlen)
        end

        redef fun [](index) do
                var len = _length

                # Statistically:
                # * ~70% want the next char
                # * ~23% want the previous
                # * ~7% want the same char
                #
                # So it makes sense to shortcut early. And early is here.
                var dpos = index - _position
                var b = _bytepos
                if dpos == 1 and index < len - 1 then
                        var its = _items
                        var c = its[b]
                        if c & 0x80u8 == 0x00u8 then
                                # We want the next, and current is easy.
                                # So next is easy to find!
                                b += 1
                                _position = index
                                _bytepos = b
                                # The rest will be done by `dpos==0` bellow.
                                dpos = 0
                        end
                else if dpos == -1 and index > 1 then
                        var its = _items
                        var c = its[b-1]
                        if c & 0x80u8 == 0x00u8 then
                                # We want the previous, and it is easy.
                                b -= 1
                                dpos = 0
                                _position = index
                                _bytepos = b
                                return c.ascii
                        end
                end
                if dpos == 0 then
                        # We know what we want (+0 or +1) just get it now!
                        var its = _items
                        var c = its[b]
                        if c & 0x80u8 == 0x00u8 then return c.ascii
                        return items.char_at(b)
                end

                assert index >= 0 and index < len
                return fetch_char_at(index)
        end

        # Gets a `Char` at `index` in `self`
        #
        # WARNING: Use at your own risks as no bound-checking is done
        fun fetch_char_at(index: Int): Char do
                var i = char_to_byte_index(index)
                var items = _items
                var b = items[i]
                if b & 0x80u8 == 0x00u8 then return b.ascii
                return items.char_at(i)
        end

        # If `self` contains only digits and alpha <= 'f', return the corresponding integer.
        #
        #     assert "ff".to_hex == 255
        redef fun to_hex(pos, ln) do
                var res = 0
                if pos == null then pos = 0
                if ln == null then ln = length - pos
                pos = char_to_byte_index(pos)
                var its = _items
                var max = pos + ln
                for i in [pos .. max[ do
                        res <<= 4
                        res += its[i].ascii.from_hex
                end
                return res
        end
end

# Immutable strings of characters.
abstract class FlatString
        super FlatText
        super String

        # Index at which `self` begins in `_items`, inclusively
        redef var first_byte is noinit

        redef var chars = new FlatStringCharView(self) is lazy

        redef var bytes = new FlatStringByteView(self) is lazy

        redef var to_cstring is lazy do
                var blen = _bytelen
                var new_items = new NativeString(blen + 1)
                _items.copy_to(new_items, blen, _first_byte, 0)
                new_items[blen] = 0u8
                return new_items
        end

        redef fun reversed do
                var b = new FlatBuffer.with_capacity(_bytelen + 1)
                var i = _length - 1
                while i >= 0 do
                        b.add self.fetch_char_at(i)
                        i -= 1
                end
                var s = b.to_s.as(FlatString)
                s._length = self._length
                return s
        end

        redef fun fast_cstring do return _items.fast_cstring(_first_byte)

        redef fun substring(from, count)
        do
                if count <= 0 then return ""

                if from < 0 then
                        count += from
                        if count <= 0 then return ""
                        from = 0
                end

                var ln = _length
                if (count + from) > ln then count = ln - from
                if count <= 0 then return ""
                var end_index = from + count - 1
                return substring_impl(from, count, end_index)
        end

        private fun substring_impl(from, count, end_index: Int): String do
                var cache = _position
                var dfrom = (cache - from).abs
                var dend = (end_index - from).abs

                var bytefrom: Int
                var byteto: Int
                if dfrom < dend then
                        bytefrom = char_to_byte_index(from)
                        byteto = char_to_byte_index(end_index)
                else
                        byteto = char_to_byte_index(end_index)
                        bytefrom = char_to_byte_index(from)
                end

                var its = _items
                byteto += its.length_of_char_at(byteto) - 1

                var s = new FlatString.full(its, byteto - bytefrom + 1, bytefrom, count)
                return s
        end

        redef fun empty do return "".as(FlatString)

        redef fun to_upper
        do
                var outstr = new FlatBuffer.with_capacity(self._bytelen + 1)

                var mylen = _length
                var pos = 0

                while pos < mylen do
                        outstr.add(chars[pos].to_upper)
                        pos += 1
                end

                return outstr.to_s
        end

        redef fun to_lower
        do
                var outstr = new FlatBuffer.with_capacity(self._bytelen + 1)

                var mylen = _length
                var pos = 0

                while pos < mylen do
                        outstr.add(chars[pos].to_lower)
                        pos += 1
                end

                return outstr.to_s
        end

        redef fun output
        do
                for i in chars do i.output
        end

        ##################################################
        #              String Specific Methods           #
        ##################################################

        # Low-level creation of a new string with minimal data.
        #
        # `_items` will be used as is, without copy, to retrieve the characters of the string.
        # Aliasing issues is the responsibility of the caller.
        private new with_infos(items: NativeString, bytelen, from: Int)
        do
                var len = items.utf8_length(from, bytelen)
                if bytelen == len then return new ASCIIFlatString.full_data(items, bytelen, from, len)
                return new UnicodeFlatString.full_data(items, bytelen, from, len)
        end

        # Low-level creation of a new string with all the data.
        #
        # `_items` will be used as is, without copy, to retrieve the characters of the string.
        # Aliasing issues is the responsibility of the caller.
        private new full(items: NativeString, bytelen, from, length: Int)
        do
                if bytelen == length then return new ASCIIFlatString.full_data(items, bytelen, from, length)
                return new UnicodeFlatString.full_data(items, bytelen, from, length)
        end

        redef fun ==(other)
        do
                if not other isa FlatText then return super

                if self.object_id == other.object_id then return true

                var my_length = _bytelen

                if other._bytelen != my_length then return false

                var my_index = _first_byte
                var its_index = other.first_byte

                var last_iteration = my_index + my_length

                var its_items = other._items
                var my_items = self._items

                while my_index < last_iteration do
                        if my_items[my_index] != its_items[its_index] then return false
                        my_index += 1
                        its_index += 1
                end

                return true
        end

        redef fun <(other)
        do
                if not other isa FlatText then return super

                if self.object_id == other.object_id then return false

                var myits = _items
                var itsits = other._items

                var mbt = _bytelen
                var obt = other.bytelen

                var minln = if mbt < obt then mbt else obt
                var mst = _first_byte
                var ost = other.first_byte

                for i in [0 .. minln[ do
                        var my_curr_char = myits[mst]
                        var its_curr_char = itsits[ost]

                        if my_curr_char > its_curr_char then return false
                        if my_curr_char < its_curr_char then return true

                        mst += 1
                        ost += 1
                end

                return mbt < obt
        end

        redef fun +(o) do
                var s = o.to_s
                var slen = s.bytelen
                var mlen = _bytelen
                var nlen = mlen + slen
                var mits = _items
                var mifrom = _first_byte
                if s isa FlatText then
                        var sits = s._items
                        var sifrom = s.first_byte
                        var ns = new NativeString(nlen + 1)
                        mits.copy_to(ns, mlen, mifrom, 0)
                        sits.copy_to(ns, slen, sifrom, mlen)
                        return new FlatString.full(ns, nlen, 0, _length + o.length)
                else
                        abort
                end
        end

        redef fun *(i) do
                var mybtlen = _bytelen
                var new_bytelen = mybtlen * i
                var mylen = _length
                var newlen = mylen * i
                var its = _items
                var fb = _first_byte
                var ns = new NativeString(new_bytelen + 1)
                ns[new_bytelen] = 0u8
                var offset = 0
                while i > 0 do
                        its.copy_to(ns, mybtlen, fb, offset)
                        offset += mybtlen
                        i -= 1
                end
                return new FlatString.full(ns, new_bytelen, 0, newlen)
        end

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

                        var my_items = _items
                        var max = last_byte

                        while i <= max do
                                h = (h << 5) + h + my_items[i].to_i
                                i += 1
                        end

                        hash_cache = h
                end

                return hash_cache.as(not null)
        end

        redef fun substrings do return new FlatSubstringsIter(self)
end

# Regular Nit UTF-8 strings
private class UnicodeFlatString
        super FlatString

        init full_data(items: NativeString, bytelen, from, length: Int) do
                self._items = items
                self._length = length
                self._bytelen = bytelen
                _first_byte = from
                _bytepos = from
        end

        redef fun substring_from(from) do
                if from >= self._length then return empty
                if from <= 0 then return self
                var c = char_to_byte_index(from)
                var st = c - _first_byte
                var fln = bytelen - st
                return new FlatString.full(items, fln, c, _length - from)
        end
end

# Special cases of String where all the characters are ASCII-based
#
# Optimizes access operations to O(1) complexity.
private class ASCIIFlatString
        super FlatString

        init full_data(items: NativeString, bytelen, from, length: Int) do
                self._items = items
                self._length = length
                self._bytelen = bytelen
                _first_byte = from
                _bytepos = from
        end

        redef fun [](idx) do
                assert idx < _bytelen and idx >= 0
                return _items[idx + _first_byte].ascii
        end

        redef fun substring(from, count) do
                if count <= 0 then return ""

                if from < 0 then
                        count += from
                        if count < 0 then return ""
                        from = 0
                end
                var ln = _length
                if (count + from) > ln then count = ln - from
                return new ASCIIFlatString.full_data(_items, count, from + _first_byte, count)
        end

        redef fun reversed do
                var b = new FlatBuffer.with_capacity(_bytelen + 1)
                var i = _length - 1
                while i >= 0 do
                        b.add self[i]
                        i -= 1
                end
                var s = b.to_s.as(FlatString)
                return s
        end

        redef fun char_to_byte_index(index) do return index + _first_byte

        redef fun substring_impl(from, count, end_index) do
                return new ASCIIFlatString.full_data(_items, count, from + _first_byte, count)
        end

        redef fun fetch_char_at(i) do return _items[i + _first_byte].ascii
end

private class FlatStringCharReverseIterator
        super IndexedIterator[Char]

        var target: FlatString

        var curr_pos: Int

        redef fun is_ok do return curr_pos >= 0

        redef fun item do return target[curr_pos]

        redef fun next do curr_pos -= 1

        redef fun index do return curr_pos

end

private class FlatStringCharIterator
        super IndexedIterator[Char]

        var target: FlatString

        var max: Int is noautoinit

        var curr_pos: Int

        init do max = target._length - 1

        redef fun is_ok do return curr_pos <= max

        redef fun item do return target[curr_pos]

        redef fun next do curr_pos += 1

        redef fun index do return curr_pos

end

private class FlatStringCharView
        super StringCharView

        redef type SELFTYPE: FlatString

        redef fun [](index) do return target[index]

        redef fun iterator_from(start) do return new FlatStringCharIterator(target, start)

        redef fun reverse_iterator_from(start) do return new FlatStringCharReverseIterator(target, start)

end

private class FlatStringByteReverseIterator
        super IndexedIterator[Byte]

        var target: FlatString

        var target_items: NativeString is noautoinit

        var curr_pos: Int

        init
        do
                var tgt = target
                target_items = tgt._items
                curr_pos += tgt._first_byte
        end

        redef fun is_ok do return curr_pos >= target._first_byte

        redef fun item do return target_items[curr_pos]

        redef fun next do curr_pos -= 1

        redef fun index do return curr_pos - target._first_byte

end

private class FlatStringByteIterator
        super IndexedIterator[Byte]

        var target: FlatString

        var target_items: NativeString is noautoinit

        var curr_pos: Int

        init
        do
                var tgt = target
                target_items = tgt._items
                curr_pos += tgt._first_byte
        end

        redef fun is_ok do return curr_pos <= target.last_byte

        redef fun item do return target_items[curr_pos]

        redef fun next do curr_pos += 1

        redef fun index do return curr_pos - target._first_byte

end

private class FlatStringByteView
        super StringByteView

        redef type SELFTYPE: FlatString

        redef fun [](index)
        do
                # Check that the index (+ _first_byte) is not larger than last_byte
                # In other terms, if the index is valid
                var target = _target
                assert index >= 0 and index < target._bytelen
                var ind = index + target._first_byte
                return target._items[ind]
        end

        redef fun iterator_from(start) do return new FlatStringByteIterator(target, start)

        redef fun reverse_iterator_from(start) do return new FlatStringByteReverseIterator(target, start)

end

redef class Buffer
        redef new do return new FlatBuffer

        redef new with_cap(i) do return new FlatBuffer.with_capacity(i)
end

# Mutable strings of characters.
class FlatBuffer
        super FlatText
        super Buffer

        redef var chars: Sequence[Char] = new FlatBufferCharView(self) is lazy

        redef var bytes = new FlatBufferByteView(self) is lazy

        private var char_cache: Int = -1

        private var byte_cache: Int = -1

        private var capacity = 0

        # Real items, used as cache for when to_cstring is called
        private var real_items: NativeString is noinit

        redef fun fast_cstring do return _items.fast_cstring(0)

        redef fun substrings do return new FlatSubstringsIter(self)

        # Re-copies the `NativeString` into a new one and sets it as the new `Buffer`
        #
        # This happens when an operation modifies the current `Buffer` and
        # the Copy-On-Write flag `written` is set at true.
        private fun reset do
                var nns = new NativeString(capacity)
                if _bytelen != 0 then _items.copy_to(nns, _bytelen, 0, 0)
                _items = nns
                written = false
        end

        # Shifts the content of the buffer by `len` bytes to the right, starting at byte `from`
        #
        # Internal only, does not modify _bytelen or length, this is the caller's responsability
        private fun rshift_bytes(from: Int, len: Int) do
                var oit = _items
                var nit = _items
                var bt = _bytelen
                if bt + len > capacity then
                        capacity = capacity * 2 + 2
                        nit = new NativeString(capacity)
                        oit.copy_to(nit, 0, 0, from)
                end
                oit.copy_to(nit, bt - from, from, from + len)
        end

        # Shifts the content of the buffer by `len` bytes to the left, starting at `from`
        #
        # Internal only, does not modify _bytelen or length, this is the caller's responsability
        private fun lshift_bytes(from: Int, len: Int) do
                var it = _items
                it.copy_to(it, _bytelen - from, from, from - len)
        end

        redef fun []=(index, item)
        do
                assert index >= 0 and index <= _length
                if written then reset
                is_dirty = true
                if index == _length then
                        add item
                        return
                end
                var it = _items
                var ip = it.char_to_byte_index(index)
                var c = it.char_at(ip)
                var clen = c.u8char_len
                var itemlen = item.u8char_len
                var size_diff = itemlen - clen
                if size_diff > 0 then
                        rshift_bytes(ip + clen, size_diff)
                else if size_diff < 0 then
                        lshift_bytes(ip + clen, -size_diff)
                end
                _bytelen += size_diff
                it.set_char_at(ip, item)
        end

        redef fun add(c)
        do
                if written then reset
                is_dirty = true
                var clen = c.u8char_len
                var bt = _bytelen
                enlarge(bt + clen)
                _items.set_char_at(bt, c)
                _bytelen += clen
                _length += 1
        end

        redef fun clear do
                is_dirty = true
                _bytelen = 0
                _length = 0
                if written then reset
        end

        redef fun empty do return new Buffer

        redef fun enlarge(cap)
        do
                var c = capacity
                if cap <= c then return
                if c <= 16 then c = 16
                while c <= cap do c = c * 2
                # The COW flag can be set at false here, since
                # it does a copy of the current `Buffer`
                written = false
                var bln = _bytelen
                var a = new NativeString(c)
                if bln > 0 then
                        var it = _items
                        if bln > 0 then it.copy_to(a, bln, 0, 0)
                end
                _items = a
                capacity = c
        end

        redef fun to_s
        do
                written = true
                var bln = _bytelen
                if bln == 0 then _items = new NativeString(1)
                return new FlatString.full(_items, bln, 0, _length)
        end

        redef fun to_cstring
        do
                if is_dirty then
                        var bln = _bytelen
                        var new_native = new NativeString(bln + 1)
                        new_native[bln] = 0u8
                        if _length > 0 then _items.copy_to(new_native, bln, 0, 0)
                        real_items = new_native
                        is_dirty = false
                end
                return real_items
        end

        # Create a new empty string.
        init do end

        # Low-level creation a new buffer with given data.
        #
        # `_items` will be used as is, without copy, to store the characters of the buffer.
        # Aliasing issues is the responsibility of the caller.
        #
        # If `_items` is shared, `written` should be set to true after the creation
        # so that a modification will do a copy-on-write.
        private init with_infos(items: NativeString, capacity, bytelen, length: Int)
        do
                self._items = items
                self.capacity = capacity
                self._bytelen = bytelen
                self._length = length
        end

        # Create a new string copied from `s`.
        init from(s: Text)
        do
                _items = new NativeString(s.bytelen)
                if s isa FlatText then
                        _items = s._items
                else
                        for i in substrings do i.as(FlatString)._items.copy_to(_items, i._bytelen, 0, 0)
                end
                _bytelen = s.bytelen
                _length = s.length
                _capacity = _bytelen
                written = true
        end

        # Create a new empty string with a given capacity.
        init with_capacity(cap: Int)
        do
                assert cap >= 0
                _items = new NativeString(cap)
                capacity = cap
                _bytelen = 0
        end

        redef fun append(s)
        do
                if s.is_empty then return
                is_dirty = true
                var sl = s.bytelen
                var nln = _bytelen + sl
                enlarge(nln)
                if s isa FlatText then
                        s._items.copy_to(_items, sl, s.first_byte, _bytelen)
                else
                        for i in s.substrings do append i
                        return
                end
                _bytelen = nln
                _length += s.length
        end

        # Copies the content of self in `dest`
        fun copy(start: Int, len: Int, dest: Buffer, new_start: Int)
        do
                var self_chars = self.chars
                var dest_chars = dest.chars
                for i in [0..len-1] do
                        dest_chars[new_start+i] = self_chars[start+i]
                end
        end

        redef fun substring(from, count)
        do
                assert count >= 0
                if from < 0 then from = 0
                if (from + count) > _length then count = _length - from
                if count <= 0 then return new Buffer
                var its = _items
                var bytefrom = its.char_to_byte_index(from)
                var byteto = its.char_to_byte_index(count + from - 1)
                byteto += its.char_at(byteto).u8char_len - 1
                var byte_length = byteto - bytefrom + 1
                var r_items = new NativeString(byte_length)
                its.copy_to(r_items, byte_length, bytefrom, 0)
                return new FlatBuffer.with_infos(r_items, byte_length, byte_length, count)
        end

        redef fun append_substring_impl(s, from, length) do
                if length <= 0 then return
                if not s isa FlatText then
                        super
                        return
                end
                var bytest = s.char_to_byte_index(from)
                var bytend = s.char_to_byte_index(from + length - 1)
                var btln = bytend - bytest + 1
                enlarge(btln + _bytelen)
                s._items.copy_to(_items, btln, bytest, _bytelen)
                _bytelen += btln
                _length += length
        end

        redef fun reverse
        do
                written = false
                var ns = new FlatBuffer.with_capacity(capacity)
                for i in chars.reverse_iterator do ns.add i
                _items = ns._items
        end

        redef fun times(repeats)
        do
                var bln = _bytelen
                var x = new FlatString.full(_items, bln, 0, _length)
                for i in [1 .. repeats[ do
                        append(x)
                end
        end

        redef fun upper
        do
                if written then reset
                for i in [0 .. _length[ do self[i] = self[i].to_upper
        end

        redef fun lower
        do
                if written then reset
                for i in [0 .. _length[ do self[i] = self[i].to_lower
        end
end

private class FlatBufferByteReverseIterator
        super IndexedIterator[Byte]

        var target: FlatBuffer

        var target_items: NativeString is noautoinit

        var curr_pos: Int

        init do target_items = target._items

        redef fun index do return curr_pos

        redef fun is_ok do return curr_pos >= 0

        redef fun item do return target_items[curr_pos]

        redef fun next do curr_pos -= 1

end

private class FlatBufferByteView
        super BufferByteView

        redef type SELFTYPE: FlatBuffer

        redef fun [](index) do return target._items[index]

        redef fun iterator_from(pos) do return new FlatBufferByteIterator(target, pos)

        redef fun reverse_iterator_from(pos) do return new FlatBufferByteReverseIterator(target, pos)

end

private class FlatBufferByteIterator
        super IndexedIterator[Byte]

        var target: FlatBuffer

        var target_items: NativeString is noautoinit

        var curr_pos: Int

        init do target_items = target._items

        redef fun index do return curr_pos

        redef fun is_ok do return curr_pos < target._bytelen

        redef fun item do return target_items[curr_pos]

        redef fun next do curr_pos += 1

end

private class FlatBufferCharReverseIterator
        super IndexedIterator[Char]

        var target: FlatBuffer

        var curr_pos: Int

        redef fun index do return curr_pos

        redef fun is_ok do return curr_pos >= 0

        redef fun item do return target[curr_pos]

        redef fun next do curr_pos -= 1

end

private class FlatBufferCharView
        super BufferCharView

        redef type SELFTYPE: FlatBuffer

        redef fun [](index) do return target[index]

        redef fun []=(index, item)
        do
                assert index >= 0 and index <= length
                if index == length then
                        add(item)
                        return
                end
                target[index] = item
        end

        redef fun push(c)
        do
                target.add(c)
        end

        redef fun add(c)
        do
                target.add(c)
        end

        fun enlarge(cap: Int)
        do
                target.enlarge(cap)
        end

        redef fun append(s)
        do
                var s_length = s.length
                if target.capacity < s.length then enlarge(s_length + target._length)
                for i in s do target.add i
        end

        redef fun iterator_from(pos) do return new FlatBufferCharIterator(target, pos)

        redef fun reverse_iterator_from(pos) do return new FlatBufferCharReverseIterator(target, pos)

end

private class FlatBufferCharIterator
        super IndexedIterator[Char]

        var target: FlatBuffer

        var max: Int is noautoinit

        var curr_pos: Int

        init do max = target._length - 1

        redef fun index do return curr_pos

        redef fun is_ok do return curr_pos <= max

        redef fun item do return target[curr_pos]

        redef fun next do curr_pos += 1

end

redef class NativeString
        redef fun to_s
        do
                return to_s_with_length(cstring_length)
        end

        redef fun to_s_with_length(length)
        do
                assert length >= 0
                return clean_utf8(length)
        end

        redef fun to_s_full(bytelen, unilen) do
                return new FlatString.full(self, bytelen, 0, unilen)
        end

        redef fun to_s_unsafe(len) do
                if len == null then len = cstring_length
                return new FlatString.with_infos(self, len, 0)
        end

        redef fun to_s_with_copy do return to_s_with_copy_and_length(cstring_length)

        # Get a `String` from `length` bytes at `self` copied into Nit memory
        fun to_s_with_copy_and_length(length: Int): String
        do
                var r = clean_utf8(length)
                if r.items != self then return r
                var new_self = new NativeString(length + 1)
                copy_to(new_self, length, 0, 0)
                var str = new FlatString.with_infos(new_self, length, 0)
                new_self[length] = 0u8
                str.to_cstring = new_self
                return str
        end

        # Cleans a NativeString if necessary
        fun clean_utf8(len: Int): FlatString do
                var replacements: nullable Array[Int] = null
                var end_length = len
                var pos = 0
                var chr_ln = 0
                var rem = len
                while rem > 0 do
                        while rem >= 4 do
                                var i = fetch_4_chars(pos)
                                if i & 0x80808080 != 0 then break
                                pos += 4
                                chr_ln += 4
                                rem -= 4
                        end
                        if rem == 0 then break
                        var b = self[pos]
                        if b & 0x80u8 == 0x00u8 then
                                pos += 1
                                chr_ln += 1
                                rem -= 1
                                continue
                        end
                        var nxst = length_of_char_at(pos)
                        var ok_st: Bool
                        if nxst == 1 then
                                ok_st = b & 0x80u8 == 0u8
                        else if nxst == 2 then
                                ok_st = b & 0xE0u8 == 0xC0u8
                        else if nxst == 3 then
                                ok_st = b & 0xF0u8 == 0xE0u8
                        else
                                ok_st = b & 0xF8u8 == 0xF0u8
                        end
                        if not ok_st then
                                if replacements == null then replacements = new Array[Int]
                                replacements.add pos
                                end_length += 2
                                pos += 1
                                rem -= 1
                                chr_ln += 1
                                continue
                        end
                        var ok_c: Bool
                        var c = char_at(pos)
                        var cp = c.code_point
                        if nxst == 1 then
                                ok_c = cp >= 0 and cp <= 0x7F
                        else if nxst == 2 then
                                ok_c = cp >= 0x80 and cp <= 0x7FF
                        else if nxst == 3 then
                                ok_c = cp >= 0x800 and cp <= 0xFFFF
                                ok_c = ok_c and not (cp >= 0xD800 and cp <= 0xDFFF) and cp != 0xFFFE and cp != 0xFFFF
                        else
                                ok_c = cp >= 0x10000 and cp <= 0x10FFFF
                        end
                        if not ok_c then
                                if replacements == null then replacements = new Array[Int]
                                replacements.add pos
                                end_length += 2
                                pos += 1
                                chr_ln += 1
                                rem -= 1
                                continue
                        end
                        var clen = c.u8char_len
                        pos += clen
                        rem -= clen
                        chr_ln += 1
                end
                var ret = self
                if end_length != len then
                        ret = new NativeString(end_length)
                        var old_repl = 0
                        var off = 0
                        var repls = replacements.as(not null)
                        var r = repls.items.as(not null)
                        var imax = repls.length
                        for i in [0 .. imax[ do
                                var repl_pos = r[i]
                                var chkln = repl_pos - old_repl
                                copy_to(ret, chkln, old_repl, off)
                                off += chkln
                                ret[off] = 0xEFu8
                                ret[off + 1] = 0xBFu8
                                ret[off + 2] = 0xBDu8
                                old_repl = repl_pos + 1
                                off += 3
                        end
                        copy_to(ret, len - old_repl, old_repl, off)
                end
                return new FlatString.full(ret, end_length, 0, chr_ln)
        end

        # Sets the next bytes at position `pos` to the value of `c`, encoded in UTF-8
        #
        # Very unsafe, make sure to have room for this char prior to calling this function.
        private fun set_char_at(pos: Int, c: Char) do
                if c.code_point < 128 then
                        self[pos] = c.code_point.to_b
                        return
                end
                var ln = c.u8char_len
                native_set_char(pos, c, ln)
        end

        private fun native_set_char(pos: Int, c: Char, ln: Int) `{
                char* dst = self + pos;
                switch(ln){
                        case 1:
                                dst[0] = c;
                                break;
                        case 2:
                                dst[0] = 0xC0 | ((c & 0x7C0) >> 6);
                                dst[1] = 0x80 | (c & 0x3F);
                                break;
                        case 3:
                                dst[0] = 0xE0 | ((c & 0xF000) >> 12);
                                dst[1] = 0x80 | ((c & 0xFC0) >> 6);
                                dst[2] = 0x80 | (c & 0x3F);
                                break;
                        case 4:
                                dst[0] = 0xF0 | ((c & 0x1C0000) >> 18);
                                dst[1] = 0x80 | ((c & 0x3F000) >> 12);
                                dst[2] = 0x80 | ((c & 0xFC0) >> 6);
                                dst[3] = 0x80 | (c & 0x3F);
                                break;
                }
        `}
end

redef class Int
        # return displayable int in base 10 and signed
        #
        #     assert 1.to_s            == "1"
        #     assert (-123).to_s       == "-123"
        redef fun to_s do
                # Fast case for common numbers
                if self == 0 then return "0"
                if self == 1 then return "1"

                var nslen = int_to_s_len
                var ns = new NativeString(nslen + 1)
                ns[nslen] = 0u8
                native_int_to_s(ns, nslen + 1)
                return new FlatString.full(ns, nslen, 0, nslen)
        end
end

redef class Array[E]

        # Fast implementation
        redef fun plain_to_s
        do
                var l = _length
                if l == 0 then return ""
                var its = _items.as(not null)
                var first = its[0]
                if l == 1 then if first == null then return "" else return first.to_s
                var na = new NativeArray[String](l)
                var i = 0
                var sl = 0
                var mypos = 0
                while i < l do
                        var itsi = its[i]
                        if itsi == null then
                                i += 1
                                continue
                        end
                        var tmp = itsi.to_s
                        sl += tmp.bytelen
                        na[mypos] = tmp
                        i += 1
                        mypos += 1
                end
                var ns = new NativeString(sl + 1)
                ns[sl] = 0u8
                i = 0
                var off = 0
                while i < mypos do
                        var tmp = na[i]
                        if tmp isa FlatString then
                                var tpl = tmp._bytelen
                                tmp._items.copy_to(ns, tpl, tmp._first_byte, off)
                                off += tpl
                        else
                                for j in tmp.substrings do
                                        var s = j.as(FlatString)
                                        var slen = s._bytelen
                                        s._items.copy_to(ns, slen, s._first_byte, off)
                                        off += slen
                                end
                        end
                        i += 1
                end
                return new FlatString.with_infos(ns, sl, 0)
        end
end

redef class NativeArray[E]
        redef fun native_to_s do
                assert self isa NativeArray[String]
                var l = length
                var na = self
                var i = 0
                var sl = 0
                var mypos = 0
                while i < l do
                        sl += na[i].bytelen
                        i += 1
                        mypos += 1
                end
                var ns = new NativeString(sl + 1)
                ns[sl] = 0u8
                i = 0
                var off = 0
                while i < mypos do
                        var tmp = na[i]
                        if tmp isa FlatString then
                                var tpl = tmp._bytelen
                                tmp._items.copy_to(ns, tpl, tmp._first_byte, off)
                                off += tpl
                        else
                                for j in tmp.substrings do
                                        var s = j.as(FlatString)
                                        var slen = s._bytelen
                                        s._items.copy_to(ns, slen, s._first_byte, off)
                                        off += slen
                                end
                        end
                        i += 1
                end
                return new FlatString.with_infos(ns, sl, 0)
        end
end

redef class Map[K,V]
        redef fun join(sep, couple_sep)
        do
                if is_empty then return ""

                var s = new Buffer # Result

                # Concat first item
                var i = iterator
                var k = i.key
                var e = i.item
                s.append("{k or else "<null>"}{couple_sep}{e or else "<null>"}")

                # Concat other _items
                i.next
                while i.is_ok do
                        s.append(sep)
                        k = i.key
                        e = i.item
                        s.append("{k or else "<null>"}{couple_sep}{e or else "<null>"}")
                        i.next
                end
                return s.to_s
        end
end