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#!/usr/bin/ruby
# Author: Trizen
# Date: 12 July 2023
# Edit: 06 February 2024
# https://github.com/trizen
# The Arithmetic Coding algorithm, implemented using native integers.
# References:
# Data Compression (Summer 2023) - Lecture 15 - Infinite Precision in Finite Bits
# https://youtube.com/watch?v=EqKbT3QdtOI
#
# Basic arithmetic coder in C++
# https://github.com/billbird/arith32
define (
BITS = 32, # can be modified
MAX = (1 << BITS)-1,
EOF_SYMBOL = 256,
)
func create_cfreq(freq) {
var c_low = Hash()
var c_high = Hash()
var T = 0
for i in (0 .. EOF_SYMBOL) {
freq{i} \\ next
c_low{i} = T
T += freq{i}
c_high{i} = T
}
return (c_low, c_high, T)
}
func encode (String str) {
var bytes = str.bytes+[EOF_SYMBOL]
var enc = FileHandle.new_buf(:raw)
var freq = bytes.freq
var(cf_low, cf_high, T) = create_cfreq(freq)
if (T > MAX) {
die ("Too few bits: #{T} > ", MAX)
}
var low = 0
var high = MAX
var uf_count = 0
for c in (bytes) {
var w = (high - low + 1)
high = (low + idiv(w * cf_high{c}, T) - 1)&MAX
low = (low + idiv(w * cf_low{c}, T))&MAX
if (high > MAX) {
die "high > MAX: #{high} > #{MAX}"
}
if (low >= high) { die "#{low} >= #{high}" }
loop {
if ((high>>(BITS-1)) == (low>>(BITS-1))) {
var bit = high>>(BITS-1)
enc << bit
if (uf_count > 0) {
enc << Str(1 - bit)*uf_count
uf_count = 0
}
low <<= 1
high <<= 1 |= 1
}
elsif ((((low>>(BITS-2))&0x1) == 1) && (((high>>(BITS-2))&0x1) == 0)) {
high <<= 1 |= (1 << (BITS-1)) |= 1
low <<= 1 &= ((1 << (BITS-1)) - 1)
++uf_count
}
else {
break
}
low &= MAX
high &= MAX
}
}
enc.print(0)
enc.print(1)
while (enc.parent.len % 8 != 0) {
enc.print(1)
}
return (enc.parent, freq)
}
func decode (bits, freq) {
var fh = bits.open_r(:raw)
var(cf_low, cf_high, T) = create_cfreq(freq)
if (T > MAX) {
die "Too large value: #{T} > #{MAX}"
}
var dec = FileHandle.new_buf(:raw)
var low = 0
var high = MAX
var enc = Num(BITS.of { fh.getc \\ 1 }.join, 2)
var table = []
for i in (0..EOF_SYMBOL) {
cf_low{i} \\ next
for j in (cf_low{i} ..^ cf_high{i}) {
table[j] = i
}
}
loop {
var w = (high - low + 1)
var ss = idiv(T*(enc - low + 1) - 1, w)
var i = table[ss] \\ break
break if (i == EOF_SYMBOL)
dec << i.chr
high = (low + idiv(w * cf_high{i}, T) - 1)&MAX
low = (low + idiv(w * cf_low{i}, T))&MAX
if (high > MAX) {
die "high > MAX: #{high} > #{MAX}"
}
if (low >= high) { die "#{low} >= #{high}" }
loop {
if ((high>>(BITS-1)) == (low>>(BITS-1))) {
high <<= 1 |= 1
low <<= 1
enc <<= 1 |= (fh.getc \\ 1)
}
elsif ((((low>>(BITS-2))&0x1) == 1) && (((high>>(BITS-2))&0x1) == 0)) {
high <<= 1 |= (1 << (BITS-1)) |= 1
low <<= 1 &= ((1 << (BITS-1)) - 1)
enc = ((enc>>(BITS-1))<<(BITS-1))|((enc&((1<<(BITS-2))-1))<<1)|(fh.getc \\ 1)
}
else {
break
}
low &= MAX
high &= MAX
enc &= MAX
}
}
return dec.parent
}
var str = "ABRACADABRA AND A VERY SAD SALAD"
if (ARGV) {
if (File(ARGV[0]).exists) {
str = File(ARGV[0]).read(:raw)
}
else {
str = ARGV[0]
}
}
var (enc, freq) = encode(str)
say enc
say ("String length: ", str.len)
say ("Encoded bytes length: ", length(enc)/8)
say ("Ratio: ", (enc.len/8)/str.len * 100)
var dec = decode(enc, freq)
assert_eq(str.len, dec.len)
assert_eq(dec, str)
__END__
0100110110111110100000000100000111110000110110011111000010110011011001000101100011011101001110000000010001111111
String length: 32
Encoded bytes length: 14
Ratio: 43.75