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huffman stuff

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This commit is contained in:
Ryan Oldenburg 2022-05-12 22:46:57 -05:00
parent c280cd7864
commit f03b2e2ef3
1 changed files with 21 additions and 18 deletions
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39
src/pixie/fileformats/jpeg.nim
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@ -143,6 +143,7 @@ proc decodeDQT(state: var DecoderState) =
proc buildHuffman(huffman: var Huffman, counts: array[16, uint8]) =
## Builds the huffman data structure.
block:
# JPEG spec page 51
var k: int
for i in 0.uint8 ..< 16:
for j in 0.uint8 ..< counts[i]:
@ -152,14 +153,15 @@ proc buildHuffman(huffman: var Huffman, counts: array[16, uint8]) =
inc k
huffman.sizes[k] = 0
var code, j: int
# JPEG spec page 52
var code, k: int
for i in 1.uint8 .. 16:
huffman.deltas[i] = j - code
if huffman.sizes[j] == i:
while huffman.sizes[j] == i:
huffman.codes[j] = code.uint16
huffman.deltas[i] = k - code
if huffman.sizes[k] == i:
while huffman.sizes[k] == i:
huffman.codes[k] = code.uint16
inc code
inc j
inc k
if code - 1 >= 1 shl i:
failInvalid()
huffman.maxCodes[i] = code shl (16 - i)
@ -169,12 +171,12 @@ proc buildHuffman(huffman: var Huffman, counts: array[16, uint8]) =
for i in 0 ..< huffman.fast.len:
huffman.fast[i] = 255
for i in 0 ..< j:
for i in 0 ..< k:
let size = huffman.sizes[i]
if size <= fastBits:
let fast = huffman.codes[i].int shl (fastBits - size)
for k in 0 ..< 1 shl (fastBits - size):
huffman.fast[fast + k] = i.uint8
for j in 0 ..< 1 shl (fastBits - size):
huffman.fast[fast + j] = i.uint8
proc decodeDHT(state: var DecoderState) =
## Decode Define Huffman Table
@ -405,34 +407,35 @@ proc huffmanDecode(state: var DecoderState, tableCurrent, table: int): uint8 =
if state.bitCount < 16:
state.fillBits()
var huffman {.byaddr.} = state.huffmanTables[tableCurrent][table]
let
fastId = (state.bits shr (32 - fastBits)) and ((1 shl fastBits) - 1)
fast = state.huffmanTables[tableCurrent][table].fast[fastId]
fast = huffman.fast[fastId]
if fast < 255:
let size = state.huffmanTables[tableCurrent][table].sizes[fast].int
let size = huffman.sizes[fast].int
if size > state.bitCount:
failInvalid()
state.bits = state.bits shl size
state.bitCount -= size
return state.huffmanTables[tableCurrent][table].symbols[fast]
return huffman.symbols[fast]
var
tmp = (state.bits shr 16).int
i = fastBits + 1
while i < state.huffmanTables[tableCurrent][table].maxCodes.len:
if tmp < state.huffmanTables[tableCurrent][table].maxCodes[i]:
while i < huffman.maxCodes.len:
if tmp < huffman.maxCodes[i]:
break
inc i
if i == 17 or i > state.bitCount:
failInvalid()
let symbolId = (state.bits shr (32 - i)).int +
state.huffmanTables[tableCurrent][table].deltas[i]
let symbolId = (state.bits shr (32 - i)).int + huffman.deltas[i]
state.bits = state.bits shl i
state.bitCount -= i
return state.huffmanTables[tableCurrent][table].symbols[symbolId]
huffman.symbols[symbolId]
template lrot(value: uint32, shift: int): uint32 =
## Left rotate - used for huffman decoding.