lib/core: Improved cache mechanism in `ropes` for `substring` and `[]`

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

# This file is part of NIT (http://www.nitlanguage.org).
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Tree-based representation of a String.
#
# Ropes are a data structure introduced in a 1995 paper from
# Hans J. Boehm, Russ Atkinson and Michael Plass.
#
# See:
#
# > Ropes: an Alternative to Strings,
# > *Software - Practice and Experience*,
# > Vol. 25(12), 1315-1330 (December 1995).
#
# The implementation developed here provides an automatic change
# of data structure depending on the length of the leaves.
#
# The length from which a `Rope` is built from a `flat` string
# if defined at top-level (see `maxlen`) and can be redefined by clients
# depending on their needs (e.g. if you need to bench the speed of
# the creation of concat nodes, lower the size of maxlen).
#
# A Rope as defined in the original paper is a Tree made of concatenation
# nodes and containing `Flat` (that is Array-based) strings as Leaves.
#
# Example :
#
# ~~~raw
#                   Concat
#                 /        \
#           Concat          Concat
#          /      \        /      \
#       "My"     " Name"  " is"   " Simon."
# ~~~
#
# Note that the above example is not representative of the actual implementation
# of `Ropes`, since short leaves are merged to keep the rope at an acceptable
# height (hence, this rope here might actually be a `FlatString` !).
module ropes

intrude import flat

# Maxlen is the maximum length of a Leaf node
#
# When concatenating two leaves, if `new_length` > `maxlen`,
# A `Concat` node is created instead
#
# Its purpose is to limit the depth of the `Rope` (this
# improves performance when accessing/iterating).
fun maxlen: Int do return 64

# String using a tree-based representation with leaves as `FlatStrings`
private abstract class Rope
        super Text
end

# Node that represents a concatenation between two `String`
private class Concat
        super Rope
        super String

        redef var chars is lazy do return new RopeChars(self)

        redef var bytes is lazy do return new RopeBytes(self)

        redef var length is noinit

        redef var bytelen is noinit

        redef fun substrings do return new RopeSubstrings.from(self, 0)

        redef fun empty do return ""

        # Cache for the latest accessed FlatString in `self`
        var flat_cache: FlatString is noinit

        # Position of the beginning of `flat_cache` in `self`
        var flat_last_pos_start: Int = -1

        var flat_last_pos_end: Int = -1

        redef var to_cstring is lazy do
                var len = _bytelen
                var ns = new NativeString(len + 1)
                ns[len] = 0u8
                var off = 0
                for i in substrings do
                        var ilen = i._bytelen
                        i.as(FlatString)._items.copy_to(ns, ilen, i.as(FlatString)._first_byte, off)
                        off += ilen
                end
                return ns
        end

        # Left child of the node
        var left: String
        # Right child of the node
        var right: String

        init do
                var l = _left
                var r = _right
                length = l.length + r.length
                _bytelen = l.bytelen + r.bytelen
        end

        redef fun output do
                _left.output
                _right.output
        end

        redef fun iterator do return new RopeCharIterator.from(self, 0)

        redef fun *(i) do
                var x: String = self
                for j in [1 .. i[ do x += self
                return x
        end

        redef fun [](i) do
                assert i >= 0 and i <= _length
                var flps = _flat_last_pos_start
                if flps != -1 and i >= flps and i <= _flat_last_pos_end then
                        return _flat_cache.fetch_char_at(i - flps)
                end
                var lf = get_leaf_at(i)
                return lf.fetch_char_at(i - _flat_last_pos_start)
        end

        fun get_leaf_at(pos: Int): FlatString do
                var flps = _flat_last_pos_start
                if flps != -1 and pos >= flps and pos <= _flat_last_pos_end then
                        return _flat_cache
                end
                var s: String = self
                var st = pos
                loop
                        if s isa FlatString then break
                        s = s.as(Concat)
                        var lft = s._left
                        var llen = lft.length
                        if pos >= llen then
                                s = s._right
                                pos -= llen
                        else
                                s = lft
                        end
                end
                _flat_last_pos_start = st - pos
                _flat_last_pos_end = st - pos + s.length - 1
                _flat_cache = s
                return s
        end

        redef fun substring(from, count) do
                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

                var flps = _flat_last_pos_start
                if flps != -1 and from >= flps and end_index <= _flat_last_pos_end then
                        return _flat_cache.substring_impl(from - flps, count, end_index - flps)
                end

                var lft = _left
                var llen = lft.length
                if from < llen then
                        if from + count < llen then return lft.substring(from, count)
                        var lsublen = llen - from
                        return lft.substring_from(from) + _right.substring(0, count - lsublen)
                else
                        return _right.substring(from - llen, count)
                end
        end

        redef fun reversed do return new Concat(_right.reversed, _left.reversed)

        redef fun insert_at(s, pos) do
                var lft = _left
                if pos > lft.length then
                        return lft + _right.insert_at(s, pos - lft.length)
                end
                return lft.insert_at(s, pos) + _right
        end

        redef fun to_upper do return new Concat(_left.to_upper, _right.to_upper)

        redef fun to_lower do return new Concat(_left.to_lower, _right.to_lower)

        redef fun +(o) do
                var s = o.to_s
                var slen = s.bytelen
                if s isa Concat then
                        return new Concat(self, s)
                else
                        var r = _right
                        var rlen = r.bytelen
                        if rlen + slen > maxlen then return new Concat(self, s)
                        return new Concat(_left, r + s)
                end
        end

        redef fun copy_to_native(dest, n, src_offset, dest_offset) do
                var subs = new RopeSubstrings.from(self, src_offset)
                var st = src_offset - subs.pos
                var off = dest_offset
                while n > 0 do
                        var it = subs.item
                        if n > it.length then
                                var cplen = it.length - st
                                it._items.copy_to(dest, cplen, st, off)
                                off += cplen
                                n -= cplen
                        else
                                it._items.copy_to(dest, n, st, off)
                                n = 0
                        end
                        subs.next
                        st = 0
                end
        end

        # Returns a balanced version of `self`
        fun balance: String do
                var children = new Array[String]
                var rnod: String
                var iter: nullable RopeCharIteratorPiece = new RopeCharIteratorPiece(self, false, false, null)
                loop
                        if iter == null then break
                        rnod = iter.node
                        if not rnod isa Concat then
                                children.push rnod
                                iter = iter.prev
                                continue
                        end
                        if not iter.ldone then
                                iter.ldone = true
                                iter = new RopeCharIteratorPiece(rnod._left, false, false, iter)
                        else if not iter.rdone then
                                iter.rdone = true
                                iter = new RopeCharIteratorPiece(rnod._right, false, false, iter)
                        else
                                iter = iter.prev
                        end

                end
                return recurse_balance(children, children.length)
        end

        fun recurse_balance(nodes: Array[String], len: Int): String do
                var finpos = 0
                var stpos = 0
                while stpos < len do
                        if len - stpos > 1 then
                                nodes[finpos] = new Concat(nodes[stpos], nodes[stpos + 1])
                                stpos += 2
                        else
                                nodes[finpos] = nodes[stpos]
                                stpos += 1
                        end
                        finpos += 1
                end
                if finpos == 1 then return nodes[0]
                return recurse_balance(nodes, finpos)
        end
end

# Mutable `Rope`, optimized for concatenation operations
#
# A `RopeBuffer` is an efficient way of building a `String` when
# concatenating small strings.
#
# It does concatenations in an optimized way by having a
# mutable part and an immutable part built by efficiently
# concatenating strings in chain.
#
# Every concatenation operation is done by copying a string to
# the mutable part and flushing it when full.
#
# However, when a long string is appended to the `Buffer`,
# the concatenation is done at it would be in a `Rope`.
class RopeBuffer
        super Rope
        super Buffer

        redef var chars: Sequence[Char] is lazy do return new RopeBufferChars(self)

        redef var bytes is lazy do return new RopeBufferBytes(self)

        # The final string being built on the fly
        private var str: String = ""

        # Current concatenation buffer
        private var ns: NativeString is noinit

        # Next available (e.g. unset) character in the `Buffer`
        private var rpos = 0

        # Length (in chars) of the buffered part
        private var nslen = 0

        # Keeps track of the buffer's currently dumped part
        #
        # This might happen if for instance, a String was being
        # built by concatenating small parts of string and suddenly
        # a long string (length > maxlen) is appended.
        private var dumped: Int is noinit

        # Length of the complete rope in chars (0)
        redef fun length do
                var st = dumped
                var len = str.length
                while st < rpos do
                        st += ns[st].u8len
                        len += 1
                end
                return len
        end

        # Length of the complete rope in bytes
        redef var bytelen = 0

        # Length of the mutable part (in bytes)
        #
        # Is also used as base to compute the size of the next
        # mutable native string (`ns`)
        private var buf_size: Int is noinit

        redef fun substrings do return new RopeBufSubstringIterator.from(self)

        # Builds an empty `RopeBuffer`
        init do
                ns = new NativeString(maxlen)
                buf_size = maxlen
                dumped = 0
        end

        # Builds a new `RopeBuffer` with `str` in it.
        init from(str: String) do
                self.str = str
                ns = new NativeString(maxlen)
                buf_size = maxlen
                _bytelen = str.length
                dumped = 0
        end

        # Resets the informations of the `Buffer`
        #
        # This is called when doing in-place modifications
        # on a previously to_s'd `RopeBuffer`
        private fun reset do
                var nns = new NativeString(buf_size)
                var blen = rpos - dumped
                ns.copy_to(nns, blen, dumped, 0)
                ns = nns
                dumped = 0
                rpos = blen
                written = false
        end

        redef fun [](i) do
                if i < str.length then
                        return str[i]
                else
                        var index = ns.char_to_byte_index_cached(i - str.length, 0, dumped)
                        return ns.char_at(index)
                end
        end

        redef fun []=(i, c) do
                assert i >= 0 and i <= length
                if i == length then add c
                if i < str.length then
                        _bytelen += c.u8char_len - str[i].u8char_len
                        var s = str
                        var l = s.substring(0, i)
                        var r = s.substring_from(i + 1)
                        str = l + c.to_s + r
                else
                        var reali = i - str.length
                        var index = ns.char_to_byte_index_cached(reali, 0, dumped)
                        var st_nxt = ns.char_to_byte_index_cached(reali + 1, reali, index)
                        var loc_c = ns.char_at(index)
                        if loc_c.u8char_len != c.u8char_len then
                                var delta = c.u8char_len - loc_c.u8char_len
                                var remsp = buf_size - rpos
                                if remsp < delta then
                                        buf_size *= 2
                                        var nns = new NativeString(buf_size)
                                        ns.copy_to(nns, index - dumped, dumped, 0)
                                        ns.copy_to(nns, rpos - index - loc_c.u8char_len, index + loc_c.u8char_len, index - dumped + delta)
                                        ns = nns
                                        index = index - dumped
                                else
                                        ns.copy_to(ns, rpos - st_nxt, st_nxt, st_nxt + delta)
                                end
                                _bytelen += delta
                                rpos += delta
                        end
                        ns.set_char_at(index, c)
                end
        end

        redef fun empty do return new RopeBuffer

        redef fun clear do
                str = ""
                _bytelen = 0
                rpos = 0
                dumped = 0
                if written then
                        ns = new NativeString(buf_size)
                        written = false
                end
        end

        redef fun substring(from, count) do
                var strlen = str.length

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

                if count > length then count = length - from

                if count == 0 then return empty

                if from < strlen then
                        var subpos = strlen - from
                        if count <= subpos then
                                return new RopeBuffer.from(str.substring(from, count))
                        else
                                var l = str.substring_from(from)
                                var rem = count - subpos
                                var nns = new NativeString(rem)
                                ns.copy_to(nns, rem, dumped, 0)
                                return new RopeBuffer.from(l + nns.to_s_unsafe(rem))
                        end
                else
                        var nns = new NativeString(count)
                        ns.copy_to(nns, count, dumped, 0)
                        return new RopeBuffer.from(nns.to_s_unsafe(count))
                end
        end

        redef fun append(s) do
                var slen = s.bytelen
                if slen >= maxlen then
                        persist_buffer
                        str += s.to_s
                        return
                end
                if s isa FlatText then
                        var oits = s._items
                        var from = s.first_byte
                        var remsp = buf_size - rpos
                        if slen <= remsp then
                                oits.copy_to(ns, slen, from, rpos)
                                rpos += slen
                                return
                        end
                        var brk = oits.find_beginning_of_char_at(from + remsp)
                        oits.copy_to(ns, brk, from, rpos)
                        rpos += brk
                        dump_buffer
                        oits.copy_to(ns, slen - remsp, brk, 0)
                        rpos = slen - remsp
                else
                        for i in s.substrings do append i
                end
        end

        redef fun add(c) do
                var rp = rpos
                var remsp = buf_size - rp
                var cln = c.u8char_len
                if cln > remsp then
                        dump_buffer
                        rp = 0
                end
                ns.set_char_at(rp, c)
                rp += cln
                _bytelen += cln
                rpos = rp
        end

        # Converts the Buffer to a FlatString, appends it to
        # the final String and re-allocates a new larger Buffer.
        private fun dump_buffer do
                written = false
                var nstr = new FlatString.with_infos(ns, rpos - dumped, dumped)
                str += nstr
                var bs = buf_size
                bs = bs * 2
                ns = new NativeString(bs)
                buf_size = bs
                dumped = 0
                rpos = 0
        end

        # Similar to dump_buffer, but does not reallocate a new NativeString
        private fun persist_buffer do
                if rpos == dumped then return
                var nstr = new FlatString.with_infos(ns, rpos - dumped, dumped)
                str += nstr
                dumped = rpos
        end

        redef fun output do
                str.output
                new FlatString.with_infos(ns, rpos - dumped, dumped).output
        end

        # Enlarge is useless here since the `Buffer`
        # part is automatically dumped when necessary.
        #
        # Also, since the buffer can not be overused by a
        # single string, there is no need for manual
        # resizing.
        #
        # "You have no power here !"
        redef fun enlarge(i) do end

        redef fun to_s do
                persist_buffer
                written = true
                return str
        end

        redef fun reverse do
                # Flush the buffer in order to only have to reverse `str`.
                if rpos > 0 and dumped != rpos then
                        str += new FlatString.with_infos(ns, rpos - dumped, dumped)
                        dumped = rpos
                end
                str = str.reversed
        end

        redef fun upper do
                if written then reset
                persist_buffer
                str = str.to_upper
        end

        redef fun lower do
                if written then reset
                persist_buffer
                str = str.to_lower
        end
end

redef class FlatString

        redef fun insert_at(s, pos) do
                var l = substring(0, pos)
                var r = substring_from(pos)
                return l + s + r
        end

        redef fun +(o) do
                var s = o.to_s
                var slen = s.bytelen
                var mlen = _bytelen
                if slen == 0 then return self
                if mlen == 0 then return s
                var nlen = slen + mlen
                if s isa FlatString then
                        if nlen > maxlen then return new Concat(self, s)
                        var mits = _items
                        var sifrom = s._first_byte
                        var mifrom = _first_byte
                        var sits = s._items
                        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 + s.length)
                else if s isa Concat then
                        var sl = s._left
                        var sllen = sl.bytelen
                        if sllen + mlen > maxlen then return new Concat(self, s)
                        return new Concat(self + sl, s._right)
                else
                        abort
                end
        end
end

# A simple linked list for use with iterators
private class RopeCharIteratorPiece
        # The encapsulated node of the `Rope`
        var node: String
        # Was its _left child (if any) visited ?
        var ldone: Bool
        # Was its _right child (if any) visited ?
        var rdone: Bool
        # The previous node in the list.
        var prev: nullable RopeCharIteratorPiece
end

# A reverse iterator capable of working with `Rope` objects
private class RopeByteReverseIterator
        super IndexedIterator[Byte]

        # Current NativeString
        var ns: NativeString is noautoinit
        # Current position in NativeString
        var pns: Int is noautoinit
        # Position in the Rope (0-indexed)
        var pos: Int is noautoinit
        # Iterator on the substrings, does the Postfix part of
        # the Rope traversal.
        var subs: IndexedIterator[FlatString] is noautoinit

        init from(root: Concat, pos: Int) do
                self.pos = pos
                subs = new ReverseRopeSubstrings.from(root, pos)
                var s = subs.item
                ns = s._items
                pns = pos - subs.index
        end

        redef fun index do return pos

        redef fun is_ok do return pos >= 0

        redef fun item do return ns[pns]

        redef fun next do
                pns -= 1
                pos -= 1
                if pns >= 0 then return
                if not subs.is_ok then return
                subs.next
                if not subs.is_ok then return
                var s = subs.item
                ns = s._items
                pns = s.last_byte
        end
end

# Forward iterator on the bytes of a `Rope`
private class RopeByteIterator
        super IndexedIterator[Byte]

        # Position in current `String`
        var pns: Int is noautoinit
        # Current `String` being iterated on
        var ns: NativeString is noautoinit
        # Substrings of the Rope
        var subs: IndexedIterator[FlatString] is noautoinit
        # Maximum position to iterate on (e.g. Rope.length)
        var max: Int is noautoinit
        # Position (char) in the Rope (0-indexed)
        var pos: Int is noautoinit

        init from(root: Concat, pos: Int) do
                subs = new RopeSubstrings.from(root, pos)
                pns = pos - subs.index
                self.pos = pos
                ns = subs.item._items
                max = root.length - 1
        end

        redef fun item do return ns[pns]

        redef fun is_ok do return pos <= max

        redef fun index do return pos

        redef fun next do
                pns += 1
                pos += 1
                if pns < subs.item._bytelen then return
                if not subs.is_ok then return
                subs.next
                if not subs.is_ok then return
                ns = subs.item._items
                pns = 0
        end
end


# A reverse iterator capable of working with `Rope` objects
private class RopeCharReverseIterator
        super IndexedIterator[Char]

        # Current NativeString
        var ns: String is noautoinit
        # Current position in NativeString
        var pns: Int is noautoinit
        # Position in the Rope (0-indexed)
        var pos: Int is noautoinit
        # Iterator on the substrings, does the Postfix part of
        # the Rope traversal.
        var subs: IndexedIterator[String] is noautoinit

        init from(root: Concat, pos: Int) do
                self.pos = pos
                subs = new ReverseRopeSubstrings.from(root, pos)
                ns = subs.item
                pns = pos - subs.index
        end

        redef fun index do return pos

        redef fun is_ok do return pos >= 0

        redef fun item do return ns[pns]

        redef fun next do
                pns -= 1
                pos -= 1
                if pns >= 0 then return
                if not subs.is_ok then return
                subs.next
                if not subs.is_ok then return
                ns = subs.item
                pns = ns.length - 1
        end
end

# Forward iterator on the chars of a `Rope`
private class RopeCharIterator
        super IndexedIterator[Char]

        # Position in current `String`
        var pns: Int is noautoinit
        # Current `String` being iterated on
        var str: String is noautoinit
        # Substrings of the Rope
        var subs: IndexedIterator[String] is noautoinit
        # Maximum position to iterate on (e.g. Rope.length)
        var max: Int is noautoinit
        # Position (char) in the Rope (0-indexed)
        var pos: Int is noautoinit

        init from(root: Concat, pos: Int) do
                subs = new RopeSubstrings.from(root, pos)
                pns = pos - subs.index
                self.pos = pos
                str = subs.item
                max = root.length - 1
        end

        redef fun item do return str[pns]

        redef fun is_ok do return pos <= max

        redef fun index do return pos

        redef fun next do
                pns += 1
                pos += 1
                if pns < subs.item.length then return
                if not subs.is_ok then return
                subs.next
                if not subs.is_ok then return
                str = subs.item
                pns = 0
        end
end

# Substrings of a Rope (i.e. Reverse postfix iterator on leaves)
private class ReverseRopeSubstrings
        super IndexedIterator[FlatString]

        # Visit Stack
        var iter: RopeCharIteratorPiece is noinit
        # Position in `Rope`
        var pos: Int is noinit

        # Current leaf
        var str: FlatString is noinit

        init from(root: Concat, pos: Int) do
                var r = new RopeCharIteratorPiece(root, false, true, null)
                var rnod: String = root
                var off = pos
                loop
                        if rnod isa Concat then
                                if off >= rnod._left.length then
                                        off -= rnod._left.length
                                        rnod = rnod._right
                                        r = new RopeCharIteratorPiece(rnod, false, true, r)
                                else
                                        r.ldone = true
                                        rnod = rnod._left
                                        r = new RopeCharIteratorPiece(rnod, false, true, r)
                                end
                        else
                                str = rnod.as(FlatString)
                                r.ldone = true
                                iter = r
                                self.pos = pos - off
                                break
                        end
                end
        end

        redef fun item do return str

        redef fun index do return pos

        redef fun is_ok do return pos >= 0

        redef fun next do
                if pos < 0 then return
                var curr = iter.prev
                var currit = curr.node
                while curr != null do
                        currit = curr.node
                        if not currit isa Concat then
                                str = currit.as(FlatString)
                                pos -= str.length
                                iter = curr
                                return
                        end
                        if not curr.rdone then
                                curr.rdone = true
                                curr = new RopeCharIteratorPiece(currit._right, false, false, curr)
                                continue
                        end
                        if not curr.ldone then
                                curr.ldone = true
                                curr = new RopeCharIteratorPiece(currit._left, false, false, curr)
                                continue
                        end
                        curr = curr.prev
                end
                pos = -1
        end
end

private class RopeBufSubstringIterator
        super Iterator[FlatText]

        # Iterator on the substrings of the building string
        var iter: Iterator[FlatText] is noautoinit
        # Makes a String out of the buffered part of the Ropebuffer
        var nsstr: FlatString is noautoinit
        # Did we attain the buffered part ?
        var nsstr_done = false

        init from(str: RopeBuffer) do
                iter = str.str.substrings
                nsstr = new FlatString.with_infos(str.ns, str.rpos - str.dumped, str.dumped)
                if str.length == 0 then nsstr_done = true
        end

        redef fun is_ok do return iter.is_ok or not nsstr_done

        redef fun item do
                assert is_ok
                if iter.is_ok then return iter.item
                return nsstr
        end

        redef fun next do
                if iter.is_ok then
                        iter.next
                        return
                end
                nsstr_done = true
        end
end

# Substrings of a Rope (i.e. Postfix iterator on leaves)
private class RopeSubstrings
        super IndexedIterator[FlatString]

        # Visit Stack
        var iter: RopeCharIteratorPiece is noinit
        # Position in `Rope`
        var pos: Int is noinit
        # Maximum position in `Rope` (i.e. length - 1)
        var max: Int is noinit

        # Current leaf
        var str: FlatString is noinit

        init from(root: Concat, pos: Int) do
                var r = new RopeCharIteratorPiece(root, true, false, null)
                max = root.length - 1
                var rnod: String = root
                var off = pos
                loop
                        if rnod isa Concat then
                                if off >= rnod._left.length then
                                        r.rdone = true
                                        off -= rnod._left.length
                                        rnod = rnod._right
                                        r = new RopeCharIteratorPiece(rnod, true, false, r)
                                else
                                        rnod = rnod._left
                                        r = new RopeCharIteratorPiece(rnod, true, false, r)
                                end
                        else
                                str = rnod.as(FlatString)
                                r.rdone = true
                                iter = r
                                self.pos = pos - off
                                break
                        end
                end
        end

        redef fun item do return str

        redef fun is_ok do return pos <= max

        redef fun index do return pos

        redef fun next do
                pos += str.length
                if pos > max then return
                var it = iter.prev
                var rnod = it.node
                loop
                        if not rnod isa Concat then
                                it.ldone = true
                                it.rdone = true
                                str = rnod.as(FlatString)
                                iter = it.as(not null)
                                break
                        end
                        if not it.ldone then
                                rnod = rnod._left
                                it.ldone = true
                                it = new RopeCharIteratorPiece(rnod, false, false, it)
                        else if not it.rdone then
                                it.rdone = true
                                rnod = rnod._right
                                it = new RopeCharIteratorPiece(rnod, false, false, it)
                        else
                                it = it.prev
                                rnod = it.node
                                continue
                        end
                end
        end
end

# Implementation of a `StringCharView` for `Concat` objects
private class RopeChars
        super StringCharView

        redef type SELFTYPE: Concat

        redef fun [](i) do
                return target[i]
        end

        redef fun iterator_from(i) do return new RopeCharIterator.from(target, i)

        redef fun reverse_iterator_from(i) do return new RopeCharReverseIterator.from(target, i)

end

# Implementation of a `StringCharView` for `Concat` objects
private class RopeBytes
        super StringByteView

        redef type SELFTYPE: Concat

        redef fun [](i) do
                var nod: String = target
                loop
                        if nod isa FlatString then return nod._items[i]
                        if not nod isa Concat then abort
                        var lft = nod._left
                        if lft.bytelen >= i then
                                nod = nod._right
                        else
                                nod = lft
                        end
                end
        end

        redef fun iterator_from(i) do return new RopeByteIterator.from(target, i)

        redef fun reverse_iterator_from(i) do return new RopeByteReverseIterator.from(target, i)

end

# An Iterator over a RopeBuffer.
class RopeBufferCharIterator
        super IndexedIterator[Char]

        # Subiterator.
        var sit: IndexedIterator[Char] is noautoinit

        redef fun index do return sit.index

        # Init the iterator from a RopeBuffer starting from `pos`.
        init from(t: RopeBuffer, pos: Int) do
                t.persist_buffer
                sit = t.str.chars.iterator_from(pos)
        end

        redef fun is_ok do return sit.is_ok

        redef fun item do
                assert is_ok
                return sit.item
        end

        redef fun next do sit.next
end

# Reverse iterator over a RopeBuffer.
class RopeBufferCharReverseIterator
        super IndexedIterator[Char]

        # Subiterator.
        var sit: IndexedIterator[Char] is noautoinit

        redef fun index do return sit.index

        # Init the iterator from a RopeBuffer starting from `pos`.
        init from(tgt: RopeBuffer, pos: Int) do
                tgt.persist_buffer
                sit = tgt.str.chars.reverse_iterator_from(pos)
        end

        redef fun is_ok do return sit.is_ok

        redef fun item do
                assert is_ok
                return sit.item
        end

        redef fun next do sit.next
end

# View on the chars of a `RopeBuffer`
class RopeBufferChars
        super BufferCharView

        redef type SELFTYPE: RopeBuffer

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

        redef fun []=(i,c) do target[i] = c

        redef fun add(c) do target.add c

        redef fun push(c) do target.add c

        redef fun iterator_from(i) do return new RopeBufferCharIterator.from(target, i)

        redef fun reverse_iterator_from(i) do return new RopeBufferCharReverseIterator.from(target, i)
end

# An Iterator over a RopeBuffer.
class RopeBufferByteIterator
        super IndexedIterator[Byte]

        # Subiterator.
        var sit: IndexedIterator[Byte] is noautoinit

        # Native string iterated over.
        var ns: NativeString is noautoinit

        # Current position in `ns`.
        var pns: Int is noautoinit

        # Maximum position iterable.
        var maxpos: Int is noautoinit

        redef var index is noautoinit

        # Init the iterator from a RopeBuffer starting from `pos`.
        init from(t: RopeBuffer, pos: Int) do
                ns = t.ns
                maxpos = t._bytelen
                sit = t.str.bytes.iterator_from(pos)
                pns = pos - t.str.length
                index = pos
        end

        redef fun is_ok do return index < maxpos

        redef fun item do
                if sit.is_ok then return sit.item
                return ns[pns]
        end

        redef fun next do
                index += 1
                if sit.is_ok then
                        sit.next
                else
                        pns += 1
                end
        end
end

# Reverse iterator over a RopeBuffer.
class RopeBufferByteReverseIterator
        super IndexedIterator[Byte]

        # Subiterator.
        var sit: IndexedIterator[Byte] is noautoinit

        # Native string iterated over.
        var ns: NativeString is noautoinit

        # Current position in `ns`.
        var pns: Int is noautoinit

        redef var index is noautoinit

        # Init the iterator from a RopeBuffer starting from `pos`.
        init from(tgt: RopeBuffer, pos: Int) do
                sit = tgt.str.bytes.reverse_iterator_from(pos - (tgt.rpos - tgt.dumped))
                pns = pos - tgt.str.bytelen + tgt.rpos
                index = pos
                ns = tgt.ns
        end

        redef fun is_ok do return index >= 0

        redef fun item do
                if pns >= 0 then return ns[pns]
                return sit.item
        end

        redef fun next do
                index -= 1
                if pns >= 0 then
                        pns -= 1
                else
                        sit.next
                end
        end
end

# View on the chars of a `RopeBuffer`
class RopeBufferBytes
        super BufferByteView

        redef type SELFTYPE: RopeBuffer

        redef fun [](i) do
                if i < target.str.bytelen then
                        return target.str.bytes[i]
                else
                        return target.ns[i - target.str.bytelen]
                end
        end

        redef fun iterator_from(i) do return new RopeBufferByteIterator.from(target, i)

        redef fun reverse_iterator_from(i) do return new RopeBufferByteReverseIterator.from(target, i)
end