hacked parity context version for arm code

c_unpacker/.gitignore

@@ -1,5 +0,0 @@
-unpack
-unpack_bitstream
-unpack_debug
-*.upk
-

c_unpacker/Makefile

@@ -1,10 +0,0 @@
-all: unpack unpack_bitstream
-
-unpack: main.c unpack.c
-	cc -O2 -o unpack main.c unpack.c
-
-unpack_bitstream: main.c unpack.c
-	cc -O2 -D UPKR_BITSTREAM -o unpack_bitstream main.c unpack.c
-	
-unpack_debug: main.c unpack.c
-	cc -g -o unpack_debug main.c unpack.c

c_unpacker/main.c

@@ -1,25 +0,0 @@
-#include <stdio.h>
-#include <stdlib.h>
-
-int upkr_unpack(void* destination, void* compressed_data);
-
-int main(int argn, char** argv) {
-  void* input_buffer = malloc(1024*1024);
-  void* output_buffer = malloc(1024*1024);
-  
-  FILE* in_file = fopen(argv[1], "rb");
-  int in_size = fread(input_buffer, 1, 1024*1024, in_file);
-  fclose(in_file);
-  
-  printf("Compressed size: %d\n", in_size);
-  
-  int out_size = upkr_unpack(output_buffer, input_buffer);
-  
-  printf("Uncompressed size: %d\n", out_size);
-  
-  FILE* out_file = fopen(argv[2], "wb");
-  fwrite(output_buffer, 1, out_size, out_file);
-  fclose(out_file);
-  
-  return 0;
-}

c_unpacker/readme.txt

@@ -1,4 +0,0 @@
-a very simple unpacker in c, as a reference for people wanting to implement their own unpacker.
-absolutely not production ready, it makes no effort to ensure the output buffer can actually
-hold the uncompressed data.
-!!! Never run on untrusted input !!!

c_unpacker/unpack.c

@@ -1,98 +0,0 @@
-typedef unsigned char u8;
-typedef unsigned short u16;
-typedef unsigned long u32;
-
-u8* upkr_data_ptr;
-u8 upkr_probs[1 + 255 + 1 + 2*32 + 2*32]; 
-#ifdef UPKR_BITSTREAM
-u16 upkr_state;
-u8 upkr_current_byte;
-int upkr_bits_left;
-#else
-u32 upkr_state;
-#endif
-
-int upkr_decode_bit(int context_index) {
-#ifdef UPKR_BITSTREAM
-    while(upkr_state < 32768) {
-        if(upkr_bits_left == 0) {
-            upkr_current_byte = *upkr_data_ptr++;
-            upkr_bits_left = 8;
-        }
-        upkr_state = (upkr_state << 1) + (upkr_current_byte >> 7);
-        upkr_current_byte <<= 1;
-        --upkr_bits_left;
-    }
-#else
-    while(upkr_state < 4096) {
-        upkr_state = (upkr_state << 8) | *upkr_data_ptr++;
-    }
-#endif
-   
-    int prob = upkr_probs[context_index];
-    int bit = (upkr_state & 255) >= prob ? 1 : 0;
-    
-    int prob_offset = 16;
-    int state_offset = 0;
-    int state_scale = prob;
-    if(bit) {
-        state_offset = -prob;
-        state_scale = 256 - prob;
-        prob_offset = 0;
-    }
-    upkr_state = state_offset + state_scale * (upkr_state >> 8) + (upkr_state & 255);
-    upkr_probs[context_index] = prob_offset + prob - ((prob + 8) >> 4); 
-
-    return bit;
-}
-
-int upkr_decode_length(int context_index) {
-    int length = 0;
-    int bit_pos = 0;
-    while(upkr_decode_bit(context_index)) {
-        length |= upkr_decode_bit(context_index + 1) << bit_pos++;
-        context_index += 2;
-    }
-    return length | (1 << bit_pos);
-}
-
-int upkr_unpack(void* destination, void* compressed_data) {
-    upkr_data_ptr = (u8*)compressed_data;
-    upkr_state = 0;
-#ifdef UPKR_BITSTREAM
-    upkr_bits_left = 0;
-#endif
-    for(int i = 0; i < sizeof(upkr_probs); ++i)
-        upkr_probs[i] = 128;
-    
-    u8* write_ptr = (u8*)destination;
-    
-    int prev_was_match = 0;
-    int offset = 0;
-    for(;;) {
-        if(upkr_decode_bit(0)) {
-            if(prev_was_match || upkr_decode_bit(256)) {
-                offset = upkr_decode_length(257) - 1;
-                if(offset == 0) {
-                    break;
-                }
-            }
-            int length = upkr_decode_length(257 + 64);
-            while(length--) {
-                *write_ptr = write_ptr[-offset];
-                ++write_ptr;
-            }
-            prev_was_match = 1;
-        } else {
-            int byte = 1;
-            while(byte < 256) {
-                int bit = upkr_decode_bit(byte);
-                byte = (byte << 1) + bit;
-            }
-            *write_ptr++ = byte;
-            prev_was_match = 0;
-        }
-    }
-    
-    return write_ptr - (u8*)destination;
-}

compare-variants

@@ -1,50 +0,0 @@
-#!/bin/env ruby
-
-configs = [
-    [:master, '-b'],
-    [:z80, '-b'],
-    [:z80, ['-b', '-r']],
-    ['old-prob-update', '-b']
-]
-
-files = Dir[ARGV[0] + '/*'].select {|f| !(f =~ /\.txt$/) }
-short_names = files.map {|f| File.basename(f)[..16] }
-results = []
-
-def print_results(configs, names, results)
-    configs.each_with_index do |config, i|
-        printf "%d: %s\n", i + 1, config
-    end
-    
-    print '                '
-    configs.each_index do |i|
-        printf "  %-4d", i + 1
-    end
-    puts
-    names.each_with_index do |name, i|
-        printf "%16s", name
-        for res in results
-            res = res[i]
-            printf "  %-4s", res if res
-        end
-        puts
-    end
-end
-
-for config in configs
-    raise unless system('git', 'checkout', config[0].to_s)
-    config_results = []
-    results << config_results
-    for file in files
-        if system(*['cargo', 'run', '--release', 'pack', '-l', '9', config[1], file, '/tmp/out.upk'].flatten) &&
-            system(*['cargo', 'run', '--release', 'unpack', config[1], '/tmp/out.upk', '/tmp/out.bin'].flatten) &&
-            File.read(file) == File.read('/tmp/out.bin')
-            size = File.size('/tmp/out.upk')
-            config_results << size
-        else
-            config_results << 'ERR'
-        end
-        print_results(configs, short_names, results)
-    end
-end
-

src/context_state.rs

@@ -1,8 +1,8 @@
-use crate::rans::{ONE_PROB, PROB_BITS};
+use crate::rans::{PROB_BITS, ONE_PROB};
 
 const INIT_PROB: u16 = 1 << (PROB_BITS - 1);
-const UPDATE_RATE: i32 = 4;
-const UPDATE_ADD: i32 = 8;
+const UPDATE_RATE: u32 = 4;
+const UPDATE_ADD: u32 = 8;
 
 #[derive(Clone)]
 pub struct ContextState {
@@ -33,12 +33,10 @@ impl<'a> Context<'a> {
 
     pub fn update(&mut self, bit: bool) {
         let old = self.state.contexts[self.index];
-        if bit {
-            self.state.contexts[self.index] =
-                old - ((old as i32 + UPDATE_ADD) >> UPDATE_RATE) as u8;
+        self.state.contexts[self.index] = if bit {
+            old + ((ONE_PROB - old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u8
         } else {
-            self.state.contexts[self.index] =
-                old + (((ONE_PROB as i32 - old as i32) + UPDATE_ADD) >> UPDATE_RATE) as u8;
-        }
+            old - ((old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u8
+        };
     }
 }

src/lib.rs

@@ -21,12 +21,3 @@ pub fn pack(
         parsing_packer::pack(data, level, use_bitstream, progress_callback)
     }
 }
-
-pub fn compressed_size(mut data: &[u8]) -> f32 {
-    let mut state = 0;
-    while state < 4096 {
-        state = (state << 8) | data[0] as u32;
-        data = &data[1..];
-    }
-    data.len() as f32 + (state as f32).log2() / 8.
-}

src/lz.rs

@@ -9,29 +9,32 @@ pub enum Op {
 
 impl Op {
     pub fn encode(&self, coder: &mut dyn EntropyCoder, state: &mut CoderState) {
+        let base_context = 256 * (state.pos & 3);
         match self {
             &Op::Literal(lit) => {
-                encode_bit(coder, state, 0, false);
+                encode_bit(coder, state, base_context, false);
                 let mut context_index = 1;
                 for i in (0..8).rev() {
                     let bit = (lit >> i) & 1 != 0;
-                    encode_bit(coder, state, context_index, bit);
+                    encode_bit(coder, state, base_context + context_index, bit);
                     context_index = (context_index << 1) | bit as usize;
                 }
+                state.pos += 1;
                 state.prev_was_match = false;
             }
             &Op::Match { offset, len } => {
-                encode_bit(coder, state, 0, true);
+                encode_bit(coder, state, base_context, true);
                 if !state.prev_was_match {
-                    encode_bit(coder, state, 256, offset != state.last_offset);
+                    encode_bit(coder, state, 1024, offset != state.last_offset);
                 } else {
                     assert!(offset != state.last_offset);
                 }
                 if offset != state.last_offset {
-                    encode_length(coder, state, 257, offset + 1);
+                    encode_length(coder, state, 1025, offset + 1);
                     state.last_offset = offset;
                 }
-                encode_length(coder, state, 257 + 64, len);
+                encode_length(coder, state, 1025 + 64, len);
+                state.pos += len as usize;
                 state.prev_was_match = true;
             }
         }
@@ -39,11 +42,11 @@ impl Op {
 }
 
 pub fn encode_eof(coder: &mut dyn EntropyCoder, state: &mut CoderState) {
-    encode_bit(coder, state, 0, true);
+    encode_bit(coder, state, 256 * (state.pos & 3), true);
     if !state.prev_was_match {
-        encode_bit(coder, state, 256, true);
+        encode_bit(coder, state, 1024, true);
     }
-    encode_length(coder, state, 257, 1);
+    encode_length(coder, state, 1025, 1);
 }
 
 fn encode_bit(
@@ -77,14 +80,16 @@ fn encode_length(
 pub struct CoderState {
     contexts: ContextState,
     last_offset: u32,
+    pos: usize,
     prev_was_match: bool,
 }
 
 impl CoderState {
     pub fn new() -> CoderState {
         CoderState {
-            contexts: ContextState::new(1 + 255 + 1 + 64 + 64),
+            contexts: ContextState::new((1 + 255) * 4 + 1 + 64 + 64),
             last_offset: 0,
+            pos: 0,
             prev_was_match: false,
         }
     }
@@ -96,7 +101,7 @@ impl CoderState {
 
 pub fn unpack(packed_data: &[u8], use_bitstream: bool) -> Vec<u8> {
     let mut decoder = RansDecoder::new(packed_data, use_bitstream);
-    let mut contexts = ContextState::new(1 + 255 + 1 + 64 + 64);
+    let mut contexts = ContextState::new((1 + 255) * 4 + 1 + 64 + 64);
     let mut result = vec![];
     let mut offset = 0;
     let mut prev_was_match = false;
@@ -119,14 +124,15 @@ pub fn unpack(packed_data: &[u8], use_bitstream: bool) -> Vec<u8> {
     }
 
     loop {
-        if decoder.decode_with_context(&mut contexts.context_mut(0)) {
-            if prev_was_match || decoder.decode_with_context(&mut contexts.context_mut(256)) {
-                offset = decode_length(&mut decoder, &mut contexts, 257) - 1;
+        let base_context = 256 * (result.len() & 3);
+        if decoder.decode_with_context(&mut contexts.context_mut(base_context)) {
+            if prev_was_match || decoder.decode_with_context(&mut contexts.context_mut(1024)) {
+                offset = decode_length(&mut decoder, &mut contexts, 1025) - 1;
                 if offset == 0 {
                     break;
                 }
             }
-            let length = decode_length(&mut decoder, &mut contexts, 257 + 64);
+            let length = decode_length(&mut decoder, &mut contexts, 1025 + 64);
             for _ in 0..length {
                 result.push(result[result.len() - offset]);
             }
@@ -135,7 +141,7 @@ pub fn unpack(packed_data: &[u8], use_bitstream: bool) -> Vec<u8> {
             let mut context_index = 1;
             let mut byte = 0;
             for i in (0..8).rev() {
-                let bit = decoder.decode_with_context(&mut contexts.context_mut(context_index));
+                let bit = decoder.decode_with_context(&mut contexts.context_mut(base_context + context_index));
                 context_index = (context_index << 1) | bit as usize;
                 byte |= (bit as u8) << i;
             }

src/main.rs

@@ -58,10 +58,7 @@ fn main() -> Result<()> {
 
 fn print_help() {
     eprintln!("Usage:");
-    eprintln!("  upkr pack [-b] [-l level(0-9)] <infile> <outfile>");
-    eprintln!("  upkr unpack [-b] <infile> <outfile>");
-    eprintln!();
-    eprintln!(" -b, --bitstream     bitstream mode");
-    eprintln!(" -l, --level N       compression level 0-9");
+    eprintln!("  upkr pack [-l level(0-9)] <infile> <outfile>");
+    eprintln!("  upkr unpack <infile> <outfile>");
     std::process::exit(1);
 }

src/rans.rs

@@ -38,15 +38,15 @@ impl RansCoder {
         let mut state = 1 << l_bits;
 
         let mut byte = 0u8;
-        let mut bit = 0;
+        let mut bit = 8;
         let mut flush_state: Box<dyn FnMut(&mut u32)> = if self.use_bitstream {
             Box::new(|state: &mut u32| {
+                bit -= 1;
                 byte |= ((*state & 1) as u8) << bit;
-                bit += 1;
-                if bit == 8 {
+                if bit == 0 {
                     buffer.push(byte);
                     byte = 0;
-                    bit = 0;
+                    bit = 8;
                 }
                 *state >>= 1;
             })
@@ -61,7 +61,7 @@ impl RansCoder {
         let max_state_factor: u32 = 1 << (l_bits + num_flush_bits - PROB_BITS);
         for step in self.bits.into_iter().rev() {
             let prob = step as u32 & 32767;
-            let (start, prob) = if step & 32768 == 0 {
+            let (start, prob) = if step & 32768 != 0 {
                 (0, prob)
             } else {
                 (prob, ONE_PROB - prob)
@@ -118,7 +118,7 @@ impl CostCounter {
 
 impl EntropyCoder for CostCounter {
     fn encode_bit(&mut self, bit: bool, prob: u16) {
-        let prob = if !bit {
+        let prob = if bit {
             prob as u32
         } else {
             ONE_PROB - prob as u32
@@ -163,8 +163,8 @@ impl<'a> RansDecoder<'a> {
                     self.data = &self.data[1..];
                     self.bits_left = 8;
                 }
-                self.state = (self.state << 1) | (self.byte >> 7) as u32;
-                self.byte <<= 1;
+                self.state = (self.state << 1) | (self.byte & 1) as u32;
+                self.byte >>= 1;
                 self.bits_left -= 1;
             }
         } else {
@@ -174,12 +174,12 @@ impl<'a> RansDecoder<'a> {
             }
         }
 
-        let bit = (self.state & PROB_MASK) >= prob;
+        let bit = (self.state & PROB_MASK) < prob;
 
         let (start, prob) = if bit {
-            (prob, ONE_PROB - prob)
-        } else {
             (0, prob)
+        } else {
+            (prob, ONE_PROB - prob)
         };
         self.state = prob * (self.state >> PROB_BITS) + (self.state & PROB_MASK) - start;
 

z80_unpacker/.gitignore

@@ -1,4 +0,0 @@
-*.bin
-*.tap
-*.sna
-*.lst

z80_unpacker/Makefile

@@ -1,11 +0,0 @@
-all: unpack.bin example/example.sna
-
-# binary is positioned from ORG 0, not usable, just assembling to verify the syntax
-unpack.bin: unpack.asm
-	sjasmplus --msg=war --lst --lstlab=sort --raw=unpack.bin unpack.asm
-
-example/example.sna: unpack.asm example/example.asm
-	cd example && sjasmplus --msg=war --lst --lstlab=sort example.asm
-
-clean:
-	$(RM) unpack.bin unpack.lst example/example.sna example/example.lst

z80_unpacker/example/example.asm

@@ -1,49 +0,0 @@
-;; Example using upkr depacker for screens slideshow
-    OPT --syntax=abf
-    DEVICE ZXSPECTRUM48,$8FFF
-
-    ORG     $9000
-compressed_scr_files:       ; border color byte + upkr-packed .scr file
-    DB      1
-    INCBIN  "screens/Grongy - ZX Spectrum (2022).scr.upk"
-    DB      7
-    INCBIN  "screens/Schafft - Poison (2017).scr.upk"
-    DB      0
-    INCBIN  "screens/diver - Mercenary 4. The Heaven's Devil (2014) (Forever 2014 Olympic Edition, 1).scr.upk"
-    DB      6
-    INCBIN  "screens/diver - Back to Bjork (2015).scr.upk"
-.e:
-
-start:
-    di
-;     OPT --zxnext
-;     nextreg 7,3                 ; ZX Next: switch to 28Mhz
-    ld      ix,compressed_scr_files
-.slideshow_loop
-  ; set BORDER for next image
-    ldi     a,(ix)              ; fake: ld a,(ix) : inc ix
-    out     (254),a
-  ; call unpack of next image directly into VRAM
-    ld      de,$4000            ; target VRAM
-    exx
-  ; IX = packed data, DE' = destination ($4000)
-  ; returned IX will point right after the packed data
-    call    upkr.unpack
-  ; do some busy loop with CPU to delay between images
-    ld      bc,$AA00
-.delay:
-    .8 ex      (sp),ix
-    dec     c
-    jr      nz,.delay
-    djnz    .delay
-  ; check if all images were displayed, loop around from first one then
-    ld      a,ixl
-    cp      low compressed_scr_files.e
-    jr      z,start
-    jr      .slideshow_loop
-
-  ; include the depacker library, optionally putting probs array buffer near end of RAM
-    DEFINE  UPKR_PROBS_ORIGIN $FA00   ; if not defined, array will be put after unpack code
-    INCLUDE "../unpack.asm"
-
-    SAVESNA "example.sna",start

z80_unpacker/readme.txt

@@ -1,19 +0,0 @@
-Z80 asm implementation of C unpacker, code-size focused (not performance).
-
-**ONLY BITSTREAM** variant is currently supported, make sure to use "-b" in packer.
-
-The project is expected to further evolve, including possible changes to binary format, this is
-initial version of Z80 unpacker to explore if/how it works and how it can be improved further.
-
-(copy full packer+depacker source to your project if you plan to use it, as future revisions
-may be incompatible with files you will produce with current version)
-
-Asm syntax is z00m's sjasmplus: https://github.com/z00m128/sjasmplus
-
-TODO:
-- build base corpus of test data to benchmark future changes in algorithm/format
-- review first implementation to identify weak spots where the implementation can be shorter+faster
-with acceptable small changes to the format
-- review non-bitstream variant, if it's feasible to try to implement it with Z80
-- (@ped7g) Z80N version of unpacker for ZX Next devs
-- (@exoticorn) add Z80 specific packer (to avoid confusion with original MicroW8 variant), and land it all to master branch, maybe in "z80" directory or something? (and overall decide how to organise+merge this upstream into main repo)

z80_unpacker/unpack.asm

@@ -1,301 +0,0 @@
-;; https://github.com/exoticorn/upkr/blob/z80/c_unpacker/unpack.c - original C implementation
-;; C source in comments ahead of asm - the C macros are removed to keep only bitstream variant
-;;
-;; initial version by Peter "Ped" Helcmanovsky (C) 2022, licensed same as upkr project ("unlicensed")
-;; to assemble use z00m's sjasmplus: https://github.com/z00m128/sjasmplus
-;;
-;; you can define UPKR_PROBS_ORIGIN to specific 256 byte aligned address for probs array (386 bytes),
-;; otherwise it will be positioned after the unpacker code (256 aligned)
-;;
-;; public API:
-;;
-;;     upkr.unpack
-;;         IN: IX = packed data, DE' (shadow DE) = destination
-;;         OUT: IX = after packed data
-;;         modifies: all registers except IY, requires 14 bytes of stack space
-;;
-
-    OPT push reset --syntax=abf
-    MODULE upkr
-
-/*
-u8* upkr_data_ptr;
-u8 upkr_probs[1 + 255 + 1 + 2*32 + 2*32];
-u16 upkr_state;
-u8 upkr_current_byte;
-int upkr_bits_left;
-
-int upkr_unpack(void* destination, void* compressed_data) {
-    upkr_data_ptr = (u8*)compressed_data;
-    upkr_state = 0;
-    upkr_bits_left = 0;
-    for(int i = 0; i < sizeof(upkr_probs); ++i)
-        upkr_probs[i] = 128;
-
-    u8* write_ptr = (u8*)destination;
-
-    int prev_was_match = 0;
-    int offset = 0;
-    for(;;) {
-        if(upkr_decode_bit(0)) {
-            if(prev_was_match || upkr_decode_bit(256)) {
-                offset = upkr_decode_length(257) - 1;
-                if(offset == 0) {
-                    break;
-                }
-            }
-            int length = upkr_decode_length(257 + 64);
-            while(length--) {
-                *write_ptr = write_ptr[-offset];
-                ++write_ptr;
-            }
-            prev_was_match = 1;
-        } else {
-            int byte = 1;
-            while(byte < 256) {
-                int bit = upkr_decode_bit(byte);
-                byte = (byte << 1) + bit;
-            }
-            *write_ptr++ = byte;
-            prev_was_match = 0;
-        }
-    }
-
-    return write_ptr - (u8*)destination;
-}
-*/
-; IN: IX = compressed_data, DE' = destination
-unpack:
-  ; ** reset probs to 0x80, also reset HL (state) to zero, and set BC to probs+context 0
-    ld      hl,probs.c>>1
-    ld      bc,probs.e
-    ld      a,$80
-.reset_probs:
-    dec     bc
-    ld      (bc),a              ; will overwrite one extra byte after the array because of odd length
-    dec     bc
-    ld      (bc),a
-    dec     l
-    jr      nz,.reset_probs
-    exa
-    ; BC = probs (context_index 0), state HL = 0, A' = 0x80 (no source bits left in upkr_current_byte)
-
-  ; ** main loop to decompress data
-.decompress_data_reset_match:
-    ld      d,0                 ; prev_was_match = 0;
-.decompress_data:
-    ld      c,0
-    call    decode_bit          ; if(upkr_decode_bit(0))
-    jr      c,.copy_chunk
-
-  ; * extract byte from compressed data (literal)
-    inc     c                   ; C = byte = 1 (and also context_index)
-.decode_byte:
-    call    decode_bit          ; bit = upkr_decode_bit(byte);
-    rl      c                   ; byte = (byte << 1) + bit;
-    jr      nc,.decode_byte     ; while(byte < 256)
-    ld      a,c
-    exx
-    ld      (de),a              ; *write_ptr++ = byte;
-    inc     de
-    exx
-    jr      .decompress_data_reset_match
-
-  ; * copy chunk of already decompressed data (match)
-.copy_chunk:
-    inc     b                   ; context_index = 256
-        ;             if(prev_was_match || upkr_decode_bit(256)) {
-        ;                 offset = upkr_decode_length(257) - 1;
-        ;                 if (0 == offset) break;
-        ;             }
-    xor     a
-    cp      d                   ; CF = prev_was_match
-    call    nc,decode_bit       ; if not prev_was_match, then upkr_decode_bit(256)
-    jr      nc,.keep_offset     ; if neither, keep old offset
-    inc     c
-    call    decode_length
-    dec     de                  ; offset = upkr_decode_length(257) - 1;
-    ld      a,d
-    or      e
-    ret     z                   ; if(offset == 0) break
-    ld      (.offset),de
-.keep_offset:
-        ;             int length = upkr_decode_length(257 + 64);
-        ;             while(length--) {
-        ;                 *write_ptr = write_ptr[-offset];
-        ;                 ++write_ptr;
-        ;             }
-        ;             prev_was_match = 1;
-    ld      c,low(257+64)       ; context_index = 257+64
-    call    decode_length       ; length = upkr_decode_length(257 + 64);
-    push    de
-    exx
-    ld      h,d                 ; DE = write_ptr
-    ld      l,e
-.offset+*:  ld      bc,0
-    sbc     hl,bc               ; CF=0 from decode_length ; HL = write_ptr - offset
-    pop     bc                  ; BC = length
-    ldir
-    exx
-    ld      d,b                 ; prev_was_match = non-zero
-    djnz    .decompress_data    ; adjust context_index back to 0..255 range, go to main loop
-
-/*
-int upkr_decode_bit(int context_index) {
-    while(upkr_state < 32768) {
-        if(upkr_bits_left == 0) {
-            upkr_current_byte = *upkr_data_ptr++;
-            upkr_bits_left = 8;
-        }
-        upkr_state = (upkr_state << 1) + (upkr_current_byte >> 7);
-        upkr_current_byte <<= 1;
-        --upkr_bits_left;
-    }
-
-    int prob = upkr_probs[context_index];
-    int bit = (upkr_state & 255) >= prob ? 1 : 0;
-
-    int prob_offset = 16;
-    int state_offset = 0;
-    int state_scale = prob;
-    if(bit) {
-        state_offset = -prob;
-        state_scale = 256 - prob;
-        prob_offset = 0;
-    }
-    upkr_state = state_offset + state_scale * (upkr_state >> 8) + (upkr_state & 255);
-    upkr_probs[context_index] = prob_offset + prob - ((prob + 8) >> 4);
-
-    return bit;
-}
-*/
-decode_bit:
-  ; HL = upkr_state
-  ; IX = upkr_data_ptr
-  ; BC = probs+context_index
-  ; A' = upkr_current_byte (!!! init to 0x80 at start, not 0x00)
-  ; preserves DE
-  ; ** while (state < 32768) - initial check
-    push    de
-    bit     7,h
-    jr      nz,.state_b15_set
-    exa
-  ; ** while body
-.state_b15_zero:
-  ; HL = upkr_state
-  ; IX = upkr_data_ptr
-  ; A = upkr_current_byte (init to 0x80 at start, not 0x00)
-    add     a,a                     ; upkr_current_byte <<= 1; // and testing if(upkr_bits_left == 0)
-    jr      nz,.has_bit             ; CF=data, ZF=0 -> some bits + stop bit still available
-  ; CF=1 (by stop bit)
-    ld      a,(ix)
-    inc     ix                      ; upkr_current_byte = *upkr_data_ptr++;
-    adc     a,a                     ; CF=data, b0=1 as new stop bit
-.has_bit:
-    adc     hl,hl                   ; upkr_state = (upkr_state << 1) + (upkr_current_byte >> 7);
-    jp      p,.state_b15_zero       ; while (state < 32768)
-    exa
-  ; ** set "bit"
-.state_b15_set:
-    ld      a,(bc)                  ; A = upkr_probs[context_index]
-    dec     a                       ; prob is in ~7..249 range, never zero, safe to -1
-    cp      l                       ; CF = bit = prob-1 < (upkr_state & 255) <=> prob <= (upkr_state & 255)
-    inc     a
-  ; ** adjust state
-    push    af
-    push    af
-    push    hl
-    push    af
-    jr      nc,.bit_is_0
-    neg                             ; A = -prob == (256-prob), CF=1 preserved
-.bit_is_0:
-    ld      d,0
-    ld      e,a                     ; DE = state_scale ; prob || (256-prob)
-    ld      l,d                     ; H:L = (upkr_state>>8) : 0
-    ld      a,8                     ; counter
-.mulLoop:
-    add     hl,hl
-    jr      nc,.mul0
-    add     hl,de
-.mul0:
-    dec     a
-    jr      nz,.mulLoop             ; until HL = state_scale * (upkr_state>>8)
-    pop     af
-    jr      nc,.bit_is_0_2
-    dec     d                       ; D = 0xFF (DE = -prob)
-    add     hl,de                   ; HL += -prob
-.bit_is_0_2:                        ; HL = state_offset + state_scale * (upkr_state >> 8)
-    pop     de
-    ld      d,0                     ; DE = (upkr_state & 255)
-    add     hl,de                   ; HL = state_offset + state_scale * (upkr_state >> 8) + (upkr_state & 255) ; new upkr_state
- ; *** adjust probs[context_index]
-    pop     af                      ; restore prob and bit
-    ld      e,a
-    jr      c,.bit_is_1
-    ld      d,-16                   ; 0xF0
-.bit_is_1:                          ; D:E = -prob_offset:prob, A = prob
-    and     $F8
-    rra
-    rra
-    rra
-    rra
-    adc     a,d                     ; A = -prob_offset + ((prob + 8) >> 4)
-    neg
-    add     a,e                     ; A = prob_offset + prob - ((prob + 8) >> 4)
-    ld      (bc),a                  ; update probs[context_index]
-    pop     af                      ; restore resulting CF = bit
-    pop     de
-    ret
-
-/*
-int upkr_decode_length(int context_index) {
-    int length = 0;
-    int bit_pos = 0;
-    while(upkr_decode_bit(context_index)) {
-        length |= upkr_decode_bit(context_index + 1) << bit_pos++;
-        context_index += 2;
-    }
-    return length | (1 << bit_pos);
-}
-*/
-decode_length:
-  ; HL = upkr_state
-  ; IX = upkr_data_ptr
-  ; BC = probs+context_index
-  ; A' = upkr_current_byte (!!! init to 0x80 at start, not 0x00)
-  ; return length in DE, CF=0
-    ld      de,$7FFF            ; length = 0 with positional-stop-bit
-    jr      .loop_entry
-.loop:
-    inc     c                   ; context_index + 1
-    call    decode_bit
-    rr      d
-    rr      e                   ; DE = length = (length >> 1) | (bit << 15);
-    inc     c                   ; context_index += 2
-.loop_entry:
-    call    decode_bit
-    jr      c,.loop
-.fix_bit_pos:
-    ccf                         ; NC will become this final `| (1 << bit_pos)` bit
-    rr      d
-    rr      e
-    jr      c,.fix_bit_pos      ; until stop bit is reached (all bits did land to correct position)
-    ret                         ; return with CF=0 (important for unpack routine)
-
-    DISPLAY "upkr.unpack total size: ",/D,$-unpack
-
-    ; reserve space for probs array without emitting any machine code (using only EQU)
-
-    IFDEF UPKR_PROBS_ORIGIN     ; if specific address is defined by user, move probs array there
-    ORG UPKR_PROBS_ORIGIN
-    ENDIF
-
-probs:      EQU ($+255) & -$100                 ; probs array aligned to 256
-.real_c:    EQU 1 + 255 + 1 + 2*32 + 2*32       ; real size of probs array
-.c:         EQU (.real_c + 1) & -2              ; padding to even size (required by init code)
-.e:         EQU probs + .c
-
-    DISPLAY "upkr.unpack probs array placed at: ",/A,probs,",\tsize: ",/A,probs.c
-
-    ENDMODULE
-    OPT pop