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deflate_encoder.coffee
261 lines (223 loc) · 5.32 KB
/
deflate_encoder.coffee
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"use strict"
Huffman = require './huffman'
OutputStream = require './output_stream'
MinWindowSize = Math.pow(2, 15)
LengthCodes = do ->
r = {}
for i in [3..10]
r[i] = {code: i + 254, extraBits: 0, extra: 0}
extraBits = 1
incrementExtra = 1
maxOffset = 2
offset = 0
code = 265
for i in [11..257]
if offset == maxOffset
offset = 0
code++
if incrementExtra % 4 == 0
extraBits++
maxOffset *= 2
incrementExtra++
r[i] = {code: code, extraBits: extraBits, extra: offset++}
r[258] = {code: 285, extraBits: 0, extra: 0}
r
length_code = (l)->
# Just copy from the hashtable
{code, extraBits, extra} = LengthCodes[l]
{code, extraBits, extra}
DistCodes = do ->
r = {}
for i in [1..4]
r[i] = {code: i - 1, extraBits: 0, extra: 0}
extraBits = 1
incrementExtra = 1
maxOffset = 2
offset = 0
code = 4
for i in [5..32768]
if offset == maxOffset
offset = 0
code++
if incrementExtra % 2 == 0
extraBits++
maxOffset *= 2
incrementExtra++
r[i] = {code: code, extraBits: extraBits, extra: offset++}
r
dist_code = (d)->
{code, extraBits, extra} = DistCodes[d]
{code, extraBits, extra}
# ScanWindow = 32768
ScanWindow = Math.pow(2,10)
encode = (buffer)->
output = []
hashTable = {}
lengthFreq = (0 for i in [0..285])
distFreq = (0 for i in [0..29])
i = 0
l = buffer.length
while i < l - 2
hash = (buffer[i] << 16) + (buffer[i + 1] << 8) + buffer[i + 2]
ocurrences = hashTable[hash]
if ocurrences?
to_discard = 0
while to_discard < ocurrences.length and i - ocurrences[to_discard] > ScanWindow
to_discard++
if to_discard > 0
ocurrences = hashTable[hash] = ocurrences.slice(to_discard)
else
ocurrences = hashTable[hash] = []
if ocurrences.length > 0
max_str = 3
max_idx = ocurrences[0]
for j in ocurrences
k = 3
while k < 258 and k + i < l and buffer[k+i] == buffer[k+j]
k++
if k >= max_str
max_str = k
max_idx = j
code = length_code(max_str)
code.distance = dist_code(i - max_idx)
output.push code
lengthFreq[code.code]++
distFreq[code.distance.code]++
# now we update the hash table with the values we'll skip
end = max_str + i
stop = Math.min(end, l - 2)
hash = hash >> 8
while i < stop
hash = ((hash & 0xffff) << 8) | buffer[i + 2]
oc = hashTable[hash] or []
oc.push(i)
hashTable[hash] = oc
i++
i = end
else
hashTable[hash] = [i]
lengthFreq[buffer[i]]++
output.push {code: buffer[i++]}
# encode the remaining 1 or 2 bytes
while i < l
lengthFreq[buffer[i]]++
output.push {code: buffer[i++]}
output.push {code: 256}
lengthFreq[256]++
lengths = Huffman.fromFrequencies(lengthFreq)
dists = Huffman.fromFrequencies(distFreq)
# make dictionary
# find highest codes
maxLength = 256
for k, v of lengths when +k > maxLength
maxLength = +k
maxDist = 0
for k, v of dists when +k > maxDist
maxDist = +k
dictionary = []
for i in [0..maxLength]
if lengths[i]?
dictionary.push lengths[i].length
else
dictionary.push 0
for i in [0..maxDist] by 1
if dists[i]?
dictionary.push dists[i].length
else
dictionary.push 0
# group codes
gdict = []
l = dictionary.length
i = 0
while i < l
v = dictionary[i++]
count = 1
while i < l and dictionary[i] == v
count++
i++
gdict.push {value: v, count: count}
# encode dictionary
edict = []
dictFreq = (0 for i in [0..18])
for o in gdict
{value, count} = o
if value > 0
edict.push { code: value, extraBits: 0 }
dictFreq[value]++
count--
while count >= 3
ammount = if count < 6 then count else 6
edict.push
code: 16
extraBits: 2
extra: ammount - 3
dictFreq[16]++
count -= ammount
while count > 0
edict.push
code: value
extraBits: 0
dictFreq[value]++
count--
else
while count >= 11
amount = if count < 138 then count else 138
edict.push
code: 18
extraBits: 7
extra: amount - 11
dictFreq[18]++
count -= amount
while count >= 3
amount = if count < 10 then count else 10
edict.push
code: 17
extraBits: 3
extra: amount - 3
dictFreq[17]++
count -= amount
while count > 0
edict.push
code: 0
extraBits: 0
dictFreq[0]++
count--
dictCodes = Huffman.fromFrequencies(dictFreq, 7)
order = [16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15]
metaDictSize = order.length - 1
while not dictCodes[order[metaDictSize]]?
metaDictSize--
metaDictSize
metaDict = []
for i in [0..metaDictSize]
idx = order[i]
if dictCodes[idx]?
metaDict.push dictCodes[idx].length
else
metaDict.push 0
os = new OutputStream()
# write header
os.writeBits(6, 3)
# write dictionary
os.writeBits(maxLength - 256, 5)
os.writeBits(maxDist, 5)
os.writeBits(metaDictSize - 3, 4)
for symbol in metaDict
os.writeBits(symbol, 3)
for symbol in edict
{code, length} = dictCodes[symbol.code]
os.writeBits(code, length)
if symbol.extraBits
os.writeBits(symbol.extra, symbol.extraBits)
# write data
for symbol in output
{code, length} = lengths[symbol.code]
os.writeBits(code, length)
if symbol.extraBits
os.writeBits(symbol.extra, symbol.extraBits)
if symbol.distance?
{code, length} = dists[symbol.distance.code]
os.writeBits(code, length)
os.writeBits(symbol.distance.extra, symbol.distance.extraBits)
os.getBuffer()
module.exports = encode