119 lines
2.8 KiB
JavaScript
119 lines
2.8 KiB
JavaScript
/**
|
|
* javascript port of java LZW decompression
|
|
* Original java author url: https://gist.github.com/devunwired/4479231
|
|
*/
|
|
|
|
export const lzw = (minCodeSize, data, pixelCount) => {
|
|
const MAX_STACK_SIZE = 4096
|
|
const nullCode = -1
|
|
const npix = pixelCount
|
|
var available,
|
|
clear,
|
|
code_mask,
|
|
code_size,
|
|
end_of_information,
|
|
in_code,
|
|
old_code,
|
|
bits,
|
|
code,
|
|
i,
|
|
datum,
|
|
data_size,
|
|
first,
|
|
top,
|
|
bi,
|
|
pi
|
|
|
|
const dstPixels = new Array(pixelCount)
|
|
const prefix = new Array(MAX_STACK_SIZE)
|
|
const suffix = new Array(MAX_STACK_SIZE)
|
|
const pixelStack = new Array(MAX_STACK_SIZE + 1)
|
|
|
|
// Initialize GIF data stream decoder.
|
|
data_size = minCodeSize
|
|
clear = 1 << data_size
|
|
end_of_information = clear + 1
|
|
available = clear + 2
|
|
old_code = nullCode
|
|
code_size = data_size + 1
|
|
code_mask = (1 << code_size) - 1
|
|
for (code = 0; code < clear; code++) {
|
|
prefix[code] = 0
|
|
suffix[code] = code
|
|
}
|
|
|
|
// Decode GIF pixel stream.
|
|
var datum, bits, count, first, top, pi, bi
|
|
datum = bits = count = first = top = pi = bi = 0
|
|
for (i = 0; i < npix; ) {
|
|
if (top === 0) {
|
|
if (bits < code_size) {
|
|
// get the next byte
|
|
datum += data[bi] << bits
|
|
|
|
bits += 8
|
|
bi++
|
|
continue
|
|
}
|
|
// Get the next code.
|
|
code = datum & code_mask
|
|
datum >>= code_size
|
|
bits -= code_size
|
|
// Interpret the code
|
|
if (code > available || code == end_of_information) {
|
|
break
|
|
}
|
|
if (code == clear) {
|
|
// Reset decoder.
|
|
code_size = data_size + 1
|
|
code_mask = (1 << code_size) - 1
|
|
available = clear + 2
|
|
old_code = nullCode
|
|
continue
|
|
}
|
|
if (old_code == nullCode) {
|
|
pixelStack[top++] = suffix[code]
|
|
old_code = code
|
|
first = code
|
|
continue
|
|
}
|
|
in_code = code
|
|
if (code == available) {
|
|
pixelStack[top++] = first
|
|
code = old_code
|
|
}
|
|
while (code > clear) {
|
|
pixelStack[top++] = suffix[code]
|
|
code = prefix[code]
|
|
}
|
|
|
|
first = suffix[code] & 0xff
|
|
pixelStack[top++] = first
|
|
|
|
// add a new string to the table, but only if space is available
|
|
// if not, just continue with current table until a clear code is found
|
|
// (deferred clear code implementation as per GIF spec)
|
|
if (available < MAX_STACK_SIZE) {
|
|
prefix[available] = old_code
|
|
suffix[available] = first
|
|
available++
|
|
if ((available & code_mask) === 0 && available < MAX_STACK_SIZE) {
|
|
code_size++
|
|
code_mask += available
|
|
}
|
|
}
|
|
old_code = in_code
|
|
}
|
|
// Pop a pixel off the pixel stack.
|
|
top--
|
|
dstPixels[pi++] = pixelStack[top]
|
|
i++
|
|
}
|
|
|
|
for (i = pi; i < npix; i++) {
|
|
dstPixels[i] = 0 // clear missing pixels
|
|
}
|
|
|
|
return dstPixels
|
|
}
|