optimise lzss prefix lookup with custom hashmap

3 files changed, 66 insertions, 30 deletions

diff --git a/src/lzss/lzssencoder.nim b/src/lzss/lzssencoder.nim
index 36e0c7e..72f5081 100644
--- a/src/lzss/lzssencoder.nim
+++ b/src/lzss/lzssencoder.nim
@@ -14,36 +14,37 @@
 # 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 matchtable, lzssnode, lzsschain
+import matchring, matchtable, lzssnode, lzsschain
 
 const matchGroupLength* = 3
 const maxRefByteLength = high(uint8).int + matchGroupLength
 
-proc commonPrefixLength*(a, b: openArray[uint8], skipFirst, maxLength: int): int =
-  result = skipFirst
+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: openArray[int], searchBuf, lookAheadBuf: openArray[uint8]): tuple[length, pos: int] =
-  for startIndex in candidatePos:
+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, matchGroupLength, maxRefByteLength)
+      searchBuf.toOpenArray(startIndex, searchBuf.len - 1), lookAheadBuf, 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) =
+proc addGroups*(matchTable: var MatchTable, buf: openArray[uint8], fromPosIncl, toPosExcl: int) =
   for cursor in fromPosIncl..(toPosExcl - matchGroupLength):
-    let group = buffer[cursor..<(cursor + matchGroupLength)]
-    matchTable.addMatch(group, cursor)
+    matchTable.addMatch(buf.matchGroup(cursor), cursor)
 
 proc lzssEncode*(buf: openArray[uint8]): LzssChain =
   result = newSeqOfCap[LzssNode](buf.len)
-  let matchTable = initMatchTable(seq[uint8], int)
+  var matchTable = initMatchTable()
   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:
+    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:
diff --git a/src/lzss/matchring.nim b/src/lzss/matchring.nim
new file mode 100644
index 0000000..a2d45f7
--- /dev/null
+++ b/src/lzss/matchring.nim
@@ -0,0 +1,37 @@
+# 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/>.
+
+const matchLimit* = 4
+
+type MatchRing* = object
+  offset, size: int
+  indices: array[matchLimit, int]
+
+proc initMatchRing*(): MatchRing =
+  MatchRing()
+
+proc addMatch*(ring: var MatchRing, index: int) =
+  if ring.size < matchLimit:
+    ring.indices[ring.size] = index
+    ring.size += 1
+  else:
+    let ringIndex = (ring.offset + ring.size) mod matchLimit
+    ring.indices[ringIndex] = index
+    ring.offset = (ring.offset + 1) mod ring.indices.len
+
+iterator items*(ring: MatchRing): int {.closure.} =
+  for i in countdown(ring.size - 1, 0):
+    yield ring.indices[(ring.offset + i) mod ring.indices.len]
diff --git a/src/lzss/matchtable.nim b/src/lzss/matchtable.nim
index cc04f49..879f47d 100644
--- a/src/lzss/matchtable.nim
+++ b/src/lzss/matchtable.nim
@@ -15,25 +15,23 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 import tables
+import matchring
 
-type MatchTable*[K, V] = ref object
-  matchLimit: int
-  table: TableRef[K, seq[V]]
+const matchGroupLength = 3
+const hashShift = 5
+const tableHeight = 0b1 shl 15
 
-proc initMatchTable*[K, V](keyType: typedesc[K], valueType: typedesc[V], matchLimit = 5): MatchTable[K, V] =
-  MatchTable[K, V](matchLimit: matchLimit, table: newTable[K, seq[V]]())
+type MatchTable* = object
+  table: array[tableHeight, MatchRing]
 
-proc len*[K, V](matchTable: MatchTable[K, V]): int =
-  matchTable.table.len
+proc initMatchTable*(): MatchTable =
+  result = MatchTable()
 
-proc matchList*[K, V](matchTable: MatchTable[K, V], pattern: K): seq[V] =
-  if matchTable.table.hasKey(pattern):
-    matchTable.table[pattern]
-  else:
-    newSeqOfCap[V](matchTable.matchLimit)
+proc hash(pattern: array[matchGroupLength, uint8]): int =
+  ((pattern[0].int shl (hashShift * 2)) xor (pattern[1].int shl hashShift) xor pattern[2].int) mod tableHeight
 
-proc addMatch*[K, V](matchTable: MatchTable[K, V], pattern: K, value: V) =
-  var matchList = matchTable.matchList(pattern)
-  if matchList.len >= matchTable.matchLimit: matchList.del(matchList.len - 1)
-  matchList.insert(value)
-  matchTable.table[pattern] = matchList
+proc addMatch*(matchTable: var MatchTable, pattern: array[matchGroupLength, uint8], index: int) =
+  matchTable.table[hash(pattern)].addMatch(index)
+
+proc candidates*(matchTable: MatchTable, pattern: array[matchGroupLength, uint8]): MatchRing =
+  matchTable.table[hash(pattern)]