z80 branch, but with original (slightly more accurate) prob update

making the 256-alignment of probs array even more baked-in, but there
was no real chance to get rid of that any way

c_unpacker/.gitignore

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

c_unpacker/Makefile

@@ -0,0 +1,10 @@
+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

@@ -0,0 +1,25 @@
+#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

@@ -0,0 +1,4 @@
+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

@@ -0,0 +1,98 @@
+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

@@ -0,0 +1,50 @@
+#!/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::{PROB_BITS, ONE_PROB};
+use crate::rans::{ONE_PROB, PROB_BITS};
 
 const INIT_PROB: u16 = 1 << (PROB_BITS - 1);
-const UPDATE_RATE: u32 = 4;
-const UPDATE_ADD: u32 = 8;
+const UPDATE_RATE: i32 = 4;
+const UPDATE_ADD: i32 = 8;
 
 #[derive(Clone)]
 pub struct ContextState {
@@ -33,10 +33,12 @@ impl<'a> Context<'a> {
 
     pub fn update(&mut self, bit: bool) {
         let old = self.state.contexts[self.index];
-        self.state.contexts[self.index] = if bit {
-            old + ((ONE_PROB - old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u8
+        if bit {
+            self.state.contexts[self.index] =
+                old - ((old as i32 + UPDATE_ADD) >> UPDATE_RATE) as u8;
         } else {
-            old - ((old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u8
-        };
+            self.state.contexts[self.index] =
+                old + (((ONE_PROB as i32 - old as i32) + UPDATE_ADD) >> UPDATE_RATE) as u8;
+        }
     }
 }

src/lib.rs

@@ -21,3 +21,12 @@ 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/main.rs

@@ -16,7 +16,7 @@ fn main() -> Result<()> {
 
             let mut data = vec![];
             File::open(infile)?.read_to_end(&mut data)?;
-            
+
             let mut pb = pbr::ProgressBar::new(data.len() as u64);
             pb.set_units(pbr::Units::Bytes);
             let packed_data = upkr::pack(
@@ -58,7 +58,10 @@ fn main() -> Result<()> {
 
 fn print_help() {
     eprintln!("Usage:");
-    eprintln!("  upkr pack [-l level(0-9)] <infile> <outfile>");
-    eprintln!("  upkr unpack <infile> <outfile>");
+    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");
     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 = 8;
+        let mut bit = 0;
         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;
-                if bit == 0 {
+                bit += 1;
+                if bit == 8 {
                     buffer.push(byte);
                     byte = 0;
-                    bit = 8;
+                    bit = 0;
                 }
                 *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 & 1) as u32;
-                self.byte >>= 1;
+                self.state = (self.state << 1) | (self.byte >> 7) 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 {
-            (0, prob)
-        } else {
             (prob, ONE_PROB - prob)
+        } else {
+            (0, prob)
         };
         self.state = prob * (self.state >> PROB_BITS) + (self.state & PROB_MASK) - start;
 

z80_unpacker/.gitignore

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

z80_unpacker/Makefile

@@ -0,0 +1,11 @@
+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

@@ -0,0 +1,49 @@
+;; 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

@@ -0,0 +1,19 @@
+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

@@ -0,0 +1,301 @@
+;; 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