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# gzip-like LZSS compressor
# Copyright (C) 2018 Pacien TRAN-GIRARD
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# This program 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. See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program. If not, see <https://www.gnu.org/licenses/>.
import matchring, matchtable, lzssnode, lzsschain
const matchGroupLength* = 3
const maxRefByteLength = high(uint8).int + matchGroupLength
proc matchGroup(buf: openArray[uint8], startIndex: int): array[matchGroupLength, uint8] =
[buf[startIndex], buf[startIndex + 1], buf[startIndex + 2]]
proc commonPrefixLength*(a, b: openArray[uint8], maxLength: int): int =
let maxPrefixLength = min(min(a.len, b.len), maxLength)
while result < maxPrefixLength and a[result] == b[result]: result += 1
proc longestPrefix*(candidatePos: MatchRing, searchBuf, lookAheadBuf: openArray[uint8]): tuple[length, pos: int] =
for startIndex in candidatePos.items:
let prefixLength = commonPrefixLength(
searchBuf.toOpenArray(startIndex, searchBuf.len - 1), lookAheadBuf, maxRefByteLength)
if prefixLength > result.length: result = (prefixLength, startIndex)
if prefixLength >= maxRefByteLength: return
proc addGroups*(matchTable: var MatchTable, buf: openArray[uint8], fromPosIncl, toPosExcl: int) =
for cursor in fromPosIncl..(toPosExcl - matchGroupLength):
matchTable.addMatch(buf.matchGroup(cursor), cursor)
proc lzssEncode*(buf: openArray[uint8]): LzssChain =
result = newSeqOfCap[LzssNode](buf.len)
var matchTable = initMatchTable()
var cursor = 0
while cursor < buf.len() - matchGroupLength:
let probableMatches = matchTable.candidates(buf.matchGroup(cursor))
let prefix = probableMatches.longestPrefix(buf.toOpenArray(0, cursor - 1), buf.toOpenArray(cursor, buf.len - 1))
if prefix.length >= matchGroupLength:
result.add(lzssReference(prefix.length, cursor - prefix.pos))
cursor += prefix.length
else:
result.add(lzssCharacter(buf[cursor]))
cursor += 1
if cursor - prefix.length >= matchGroupLength:
matchTable.addGroups(buf, cursor - prefix.length - matchGroupLength, cursor)
while cursor < buf.len:
result.add(lzssCharacter(buf[cursor]))
cursor += 1
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