ref: ba1476058afaffc3f24ca8fbde11234def04d1bf
dir: /lib/flate/flate.myr/
use std
use bio
use "types"
/* DEFLATE https://www.ietf.org/rfc/rfc1951.txt
Terminology:
- A 'Huffman code' is one mechanism to map codes to symbols.
- A 'code' is the sequence of bits used to encode a symbol
in a Huffman code. Codes are what appears in the DEFLATE
stream.
- A 'symbol' is what a code decodes to.
- A 'length' is the length of an LZ77 reference (the number
of bytes to repeat).
- A 'distance' is the distance at which the bits referred to
by an LZ77 reference are from the current point.
*/
pkg flate =
/* state maintained during decompression */
type flatedec = struct
rdf : bio.file#
wrf : bio.file#
/* one-byte buffer */
bval : int /* in [0, 255] current input byte */
bpos : int /* in [0, 8] */
/* ring buffer window for the bytes output */
wpos : std.size
wlen : std.size
wbuf : byte[WinSz]
/* Huffman codes */
lenhc : hufc /* for lengths */
dsthc : hufc /* for distances */
;;
/* one Huffman code */
type hufc = struct
/* # of codes of length [1, MaxBits] */
count : int[MaxBits + 1]
/* symbols sorted lexicographically by
<code length, symbol value>; c.f., mkhufc()
and readcode() for usage */
symbol : int[MaxSyms]
;;
/* decode a DEFLATE stream */
const decode : (outf : bio.file#, inf : bio.file# -> std.result(void, err))
pkglocal const MaxSyms : std.size
pkglocal const MaxBits : std.size
pkglocal const WinSz : std.size
pkglocal const bits : (st : flatedec#, n : int -> std.result(int, err))
pkglocal const nocomp : (st : flatedec# -> std.result(void, err))
pkglocal const mkhufc : (lengths : int[:], hc : hufc# -> void)
pkglocal const readcode : (st : flatedec#, hc : hufc# -> std.result(int, err))
pkglocal const outb : (st : flatedec#, b : byte -> std.result(void, err))
;;
const MaxLenSyms : std.size = 288 /* max # of length symbols */
const MaxDstSyms : std.size = 32 /* max # of distance symbols */
const MaxSyms = MaxLenSyms /* max # of symbols in a Huffman code */
const MaxBits = 15 /* max length of a code (in bits) */
const WinSz = 32 * std.KiB /* max distance for a reference */
var fixedlenhc : hufc
var fixeddsthc : hufc
const __init__ = {
/* the fixed Huffman codes; c.f., RFC Section 3.2.6 */
var length : int[MaxSyms]
std.slfill(length[0:144], 8)
std.slfill(length[144:256], 9)
std.slfill(length[256:280], 7)
std.slfill(length[280:288], 8)
mkhufc(length[:], &fixedlenhc)
std.slfill(length[0:32], 5)
mkhufc(length[0:32], &fixeddsthc)
}
const outb = {st, b
/* outputs a single byte, to the output file; additionally
record the byte emitted in the output window */
var pos
match bio.putb(st.wrf, b)
| `std.Ok 0: std.die("no bytes written by bio.putb()")
| `std.Ok _:
| `std.Err e: -> `std.Err (`Io e)
;;
pos = st.wpos
if pos == WinSz
std.assert(st.wlen >= WinSz, "window should be full")
pos = 0
;;
st.wbuf[pos] = b
st.wpos = pos + 1
st.wlen++ /* don't care about it being > WinSz */
-> `std.Ok void
}
const bits = {st, n
var pos, len, bits, bval
std.assert(n >= 0 && n <= 31, "invalid argument")
pos = st.bpos
bits = st.bval >> pos
for len = 8 - pos; len < n; len += 8
match bio.getb(st.rdf)
| `std.Ok b:
bval = (b : int)
st.bval = bval
bits += bval << len
| `std.Err e: -> `std.Err (`Io e)
;;
;;
st.bpos = 8 - (len - n)
-> `std.Ok (bits & ((1 << n) - 1))
}
const nocomp = {st
/* process a non-compressed block; c.f., RFC Section 3.2.4 */
var len
st.bpos = 8 /* skip to a byte boundary */
match bits(st, 16)
| `std.Ok n: len = (n : std.size)
| `std.Err e: -> `std.Err e
;;
match bits(st, 16)
| `std.Ok nlen:
if len != (nlen : std.size) ^ 0xffff
-> `std.Err (`Format \
"non-compressed block length could " \
"not be verified")
;;
| `std.Err e: -> `std.Err e
;;
for var n = 0; n < len; n++
match bio.getb(st.rdf)
| `std.Ok b:
match outb(st, b)
| `std.Ok _:
| err: -> err
;;
| `std.Err e: -> `std.Err (`Io e)
;;
;;
-> `std.Ok void
}
const mkhufc = {length, hc
/* note: it is possible to have 0s in the length array; this means
that the symbol at this index will never be part of the input
stream */
var index : int[MaxBits + 1]
std.assert(length.len <= hc.symbol.len, "invalid argument")
std.slfill(hc.count[:], 0)
for var symb = 0; symb < length.len; symb++
hc.count[length[symb]]++
;;
std.slfill(index[:], 0)
hc.count[0] = 0
for var l = 1; l <= MaxBits; l++
index[l] = index[l - 1] + hc.count[l - 1]
;;
for var symb = 0; symb < length.len; symb++
if length[symb] != 0
hc.symbol[index[length[symb]]++] = symb
;;
;;
/* TODO: validate the Huffman code */
}
const readcode = {st, hc
/* if you want to know more about the algorithm here, you
must look for "canonical Huffman codes"; see, for example,
Managing Gigabytes: Compressing and Indexing Documents
and Images
by I. H. Witten, A. Moffat, and T. C. Bell. */
var code, first, index, count
code = 0 /* code seen so far */
first = 0 /* first code of length len */
index = 0 /* index of the first code of length len in hc.symbol[:] */
for var len = 1; len <= MaxBits; len++
match bits(st, 1)
| `std.Ok n: code += n
| err: -> err
;;
count = hc.count[len]
if code < first + count
/* the code is exactly len bits long;
return the symbol from the table */
-> `std.Ok (hc.symbol[index + (code - first)])
;;
index += count
first = (first + count) << 1 /* c.f. RFC Section 3.2.2 */
code <<= 1 /* make room for the next bit */
;;
-> `std.Err (`Format "invalid code")
}
const decomp = {st
/* process data of a compressed block; c.f., RFC Section 3.2.5 */
const basel = [
3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,
35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258
]
const extral = [
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3,
3, 4, 4, 4, 4, 5, 5, 5, 5, 0
]
const based = [
1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,
257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,
8193, 12289, 16385, 24577
]
const extrad = [
0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8,
8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13
]
var symb, len, dst, index
symb = 0
while symb != 256
match readcode(st, &st.lenhc)
| `std.Ok s: symb = s
| `std.Err e: -> `std.Err e
;;
if symb < 256
match outb(st, (symb : byte))
| `std.Ok _:
| err: -> err
;;
elif symb > 256
symb -= 257
match bits(st, extral[symb])
| `std.Ok x: len = basel[symb] + x
| `std.Err e: -> `std.Err e
;;
match readcode(st, &st.dsthc)
| `std.Ok s: symb = s
| `std.Err e: -> `std.Err e
;;
match bits(st, extrad[symb])
| `std.Ok x: dst = (based[symb] + x : std.size)
| `std.Err e: -> `std.Err e
;;
if dst > st.wlen
-> `std.Err (`Format \
"reference to a byte before file start")
;;
index = (WinSz + st.wpos - dst) % WinSz
for ; len > 0; len--
match outb(st, st.wbuf[index])
| `std.Ok _:
| err: -> err
;;
index = (index + 1) % WinSz
;;
;;
;;
-> `std.Ok void
}
const dynamic = {st
/* process a block compressed with dynamic Huffman codes;
c.f., RFC Section 3.2.7 */
const coden = [
16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2,
14, 1, 15
]
const extra = [2, 3, 7]
var length : int[MaxLenSyms + MaxDstSyms]
var nlen, ndst, ncode, symb, nbits, hufc
match bits(st, 5)
| `std.Ok n: nlen = n + 257
| `std.Err e: -> `std.Err e
;;
if nlen > 286
-> `std.Err (`Format "invalid number of length codes")
;;
match bits(st, 5)
| `std.Ok n: ndst = n + 1
| `std.Err e: -> `std.Err e
;;
if ndst > 30
-> `std.Err (`Format "invalid number of distance codes")
;;
match bits(st, 4)
| `std.Ok n: ncode = n + 4
| `std.Err e: -> `std.Err e
;;
std.slfill(length[:19], 0)
for var c = 0; c < ncode; c++
match bits(st, 3)
| `std.Ok n: length[coden[c]] = n
| `std.Err e: -> `std.Err e
;;
;;
mkhufc(length[:19], &hufc)
for var c = 0; c < nlen + ndst;
/* according to the RFC: "the code length repeat codes
can cross from HLIT + 257 to the HDIST + 1 code
lengths"; this is why we have a single loop here */
match readcode(st, &hufc)
| `std.Ok n: symb = n
| `std.Err e: -> `std.Err e
;;
if symb > 18
-> `std.Err (`Format "invalid code code")
;;
if symb < 16
length[c++] = symb
else
/* repeat some code length */
match bits(st, extra[symb - 16])
| `std.Ok n: nbits = n
| `std.Err e: -> `std.Err e
;;
if symb == 16
/* copy the previous code length 3-6 times */
nbits += 3
if c == 0 || c + nbits > nlen + ndst
-> `std.Err (`Format \
"invalid code repetition")
;;
std.slfill(length[c:c+nbits], length[c-1])
else
/* repeat a 0 length */
if symb == 17
nbits += 3
else
nbits += 11
;;
if c + nbits > nlen + ndst
-> `std.Err (`Format \
"invalid zero repetition")
;;
std.slfill(length[c:c+nbits], 0)
;;
c += nbits
;;
;;
if length[256] == 0
-> `std.Err (`Format "no code length of end-of-block symbol")
;;
mkhufc(length[:nlen], &st.lenhc)
mkhufc(length[nlen:nlen + ndst], &st.dsthc)
-> decomp(st)
}
const decode = {outf, inf
var st, err, last
st = [
.rdf = inf,
.wrf = outf,
.bpos = 8,
]
last = 0
while last != 1
match bits(&st, 1)
| `std.Ok l: last = l
| `std.Err e: -> `std.Err e
;;
match bits(&st, 2)
| `std.Ok 0:
err = nocomp(&st)
| `std.Ok 1:
st.lenhc = fixedlenhc
st.dsthc = fixeddsthc
err = decomp(&st)
| `std.Ok 2:
err = dynamic(&st)
| `std.Ok _:
-> `std.Err (`Format "invalid block type")
| `std.Err e:
-> `std.Err e
;;
match err
| `std.Ok _:
| `std.Err e: -> `std.Err e
;;
;;
-> `std.Ok void
}