isolate lzss chain module

1 files changed, 58 insertions, 0 deletions

diff --git a/src/lzss/lzssencoder.nim b/src/lzss/lzssencoder.nim
new file mode 100644
index 0000000..8b750fb
--- /dev/null
+++ b/src/lzss/lzssencoder.nim
@@ -0,0 +1,58 @@
+# 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 lists
+import listpolyfill, matchtable, lzssnode, lzsschain
+
+const matchGroupLength = 3
+const maxRefByteLength = high(uint8).int + matchGroupLength
+let emptySinglyLinkedList = initSinglyLinkedList[int]()
+
+proc commonPrefixLength*(a, b: openArray[uint8], skipFirst, maxLength: int): int =
+  result = skipFirst
+  let maxPrefixLength = min(min(a.len, b.len), maxLength)
+  while result < maxPrefixLength and a[result] == b[result]: result += 1
+
+proc longestPrefix*(candidatePos: SinglyLinkedList[int], searchBuf, lookAheadBuf: openArray[uint8]): tuple[length, pos: int] =
+  for startIndex in candidatePos.items:
+    let prefixLength = commonPrefixLength(
+      searchBuf.toOpenArray(startIndex, searchBuf.len - 1), lookAheadBuf, matchGroupLength, maxRefByteLength)
+    if prefixLength > result.length: result = (prefixLength, startIndex)
+    if prefixLength >= maxRefByteLength: return
+
+proc addGroups*(matchTable: MatchTable[seq[uint8], int], buffer: openArray[uint8], fromPosIncl, toPosExcl: int) =
+  for cursor in fromPosIncl..(toPosExcl - matchGroupLength):
+    let group = buffer[cursor..<(cursor + matchGroupLength)]
+    matchTable.addMatch(group, cursor)
+
+proc lzssEncode*(buf: openArray[uint8]): LzssChain =
+  result = initSinglyLinkedList[LzssNode]()
+  let matchTable = initMatchTable(seq[uint8], int)
+  var cursor = 0
+  while cursor < buf.len() - matchGroupLength:
+    let matches = matchTable.matchList(buf[cursor..<(cursor + matchGroupLength)])
+    let prefix = matches.longestPrefix(buf.toOpenArray(0, cursor - 1), buf.toOpenArray(cursor, buf.len - 1))
+    if prefix.length > 0:
+      result.append(lzssReference(prefix.length, cursor - prefix.pos))
+      cursor += prefix.length
+    else:
+      result.append(lzssCharacter(buf[cursor]))
+      cursor += 1
+    if cursor - prefix.length >= matchGroupLength:
+      matchTable.addGroups(buf, cursor - prefix.length - matchGroupLength, cursor)
+  while cursor < buf.len:
+    result.append(lzssCharacter(buf[cursor]))
+    cursor += 1