-r reverses both input and output

backward unpacker + example extended

c_unpacker/unpack.c

@@ -19,8 +19,8 @@ int upkr_decode_bit(int context_index) {
             upkr_current_byte = *upkr_data_ptr++;
             upkr_bits_left = 8;
         }
-        upkr_state = (upkr_state << 1) + (upkr_current_byte & 1);
-        upkr_current_byte >>= 1;
+        upkr_state = (upkr_state << 1) + (upkr_current_byte >> 7);
+        upkr_current_byte <<= 1;
         --upkr_bits_left;
     }
 #else
@@ -30,19 +30,18 @@ int upkr_decode_bit(int context_index) {
 #endif
    
     int prob = upkr_probs[context_index];
-    int bit = (upkr_state & 255) < prob ? 1 : 0;
+    int bit = (upkr_state & 255) >= prob ? 1 : 0;
     
-    int tmp = prob;
-    if(!bit) {
-        tmp = 256 - tmp;
+    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 = tmp * (upkr_state >> 8) + (upkr_state & 255);
-    tmp += (256 - tmp + 8) >> 4;
-    if(!bit) {
-        upkr_state -= prob;
-        tmp = 256 - tmp;
-    }
-    upkr_probs[context_index] = tmp;
+    upkr_state = state_offset + state_scale * (upkr_state >> 8) + (upkr_state & 255);
+    upkr_probs[context_index] = prob_offset + prob - ((prob + 8) >> 4); 
 
     return bit;
 }

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 +33,13 @@ 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
+        let offset = if !bit {
+            ONE_PROB as i32 >> UPDATE_RATE
         } else {
-            old - ((old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u8
+            0
         };
+
+        self.state.contexts[self.index] =
+            (offset + old as i32 - ((old as i32 + UPDATE_ADD) >> UPDATE_RATE)) as u8;
     }
 }

src/main.rs

@@ -10,16 +10,20 @@ fn main() -> Result<()> {
         Some("pack") => {
             let level = args.opt_value_from_str(["-l", "--level"])?.unwrap_or(2u8);
             let use_bitstream = args.contains(["-b", "--bitstream"]);
+            let reverse = args.contains(["-r", "--reverse"]);
 
             let infile = args.free_from_os_str::<PathBuf, bool>(|s| Ok(s.into()))?;
             let outfile = args.free_from_os_str::<PathBuf, bool>(|s| Ok(s.into()))?;
 
             let mut data = vec![];
             File::open(infile)?.read_to_end(&mut data)?;
+            if reverse {
+                data.reverse();
+            }
 
             let mut pb = pbr::ProgressBar::new(data.len() as u64);
             pb.set_units(pbr::Units::Bytes);
-            let packed_data = upkr::pack(
+            let mut packed_data = upkr::pack(
                 &data,
                 level,
                 use_bitstream,
@@ -29,6 +33,10 @@ fn main() -> Result<()> {
             );
             pb.finish();
 
+            if reverse {
+                packed_data.reverse();
+            }
+
             println!(
                 "Compressed {} bytes to {} bytes ({}%)",
                 data.len(),
@@ -39,14 +47,21 @@ fn main() -> Result<()> {
         }
         Some("unpack") => {
             let use_bitstream = args.contains(["-b", "--bitstream"]);
+            let reverse = args.contains(["-r", "--reverse"]);
 
             let infile = args.free_from_os_str::<PathBuf, bool>(|s| Ok(s.into()))?;
             let outfile = args.free_from_os_str::<PathBuf, bool>(|s| Ok(s.into()))?;
 
             let mut data = vec![];
             File::open(infile)?.read_to_end(&mut data)?;
-            let packed_data = upkr::unpack(&data, use_bitstream);
-            File::create(outfile)?.write_all(&packed_data)?;
+            if reverse {
+                data.reverse();
+            }
+            let mut unpacked_data = upkr::unpack(&data, use_bitstream);
+            if reverse {
+                unpacked_data.reverse();
+            }
+            File::create(outfile)?.write_all(&unpacked_data)?;
         }
         Some(other) => {
             bail!("Unknown subcommand '{}'", other);

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 +1,3 @@
+*.bin
+*.tap
+*.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 +1,100 @@
+;; Example using upkr depacker for screens slideshow
+    OPT --syntax=abf
+    DEVICE ZXSPECTRUM48,$8FFF
+
+    ORG     $9000
+  ;; forward example data
+compressed_scr_files.fwd:               ; 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:
+  ;; backward example data (unpacker goes from the end of the data!)
+compressed_scr_files.rwd.e: EQU $-1     ; the final IX will point one byte ahead of "$" here
+    INCBIN  "screens.reversed/diver - Back to Bjork (2015).scr.upk"
+    DB      6
+    INCBIN  "screens.reversed/diver - Mercenary 4. The Heaven's Devil (2014) (Forever 2014 Olympic Edition, 1).scr.upk"
+    DB      0
+    INCBIN  "screens.reversed/Schafft - Poison (2017).scr.upk"
+    DB      7
+    INCBIN  "screens.reversed/Grongy - ZX Spectrum (2022).scr.upk"
+compressed_scr_files.rwd:               ; border color byte + upkr-packed .scr file (backward)
+    DB      1
+
+start:
+    di
+;     OPT --zxnext
+;     nextreg 7,3                       ; ZX Next: switch to 28Mhz
+
+  ;;; FORWARD packed/unpacked data demo
+    ld      ix,compressed_scr_files.fwd
+.slideshow_loop.fwd:
+  ; set BORDER for next image
+    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    fwd.upkr.unpack
+  ; do some busy loop with CPU to delay between images
+    call    delay
+  ; check if all images were displayed, loop around from first one then
+    ld      a,ixl
+    cp      low compressed_scr_files.fwd.e
+    jr      nz,.slideshow_loop.fwd
+
+  ;;; BACKWARD packed/unpacked data demo
+    ld      ix,compressed_scr_files.rwd
+.slideshow_loop.rwd:
+  ; set BORDER for next image
+    ld      a,(ix)
+    dec     ix
+    out     (254),a
+  ; call unpack of next image directly into VRAM
+    ld      de,$5AFF                    ; target VRAM
+    exx
+  ; IX = packed data, DE' = destination
+  ; returned IX will point right ahead of the packed data
+    call    rwd.upkr.unpack
+  ; do some busy loop with CPU to delay between images
+    call    delay
+  ; check if all images were displayed, loop around from first one then
+    ld      a,ixl
+    cp      low compressed_scr_files.rwd.e
+    jr      nz,.slideshow_loop.rwd
+
+    jr      start
+
+delay:
+    ld      bc,$AA00
+.delay:
+    .8 ex      (sp),ix
+    dec     c
+    jr      nz,.delay
+    djnz    .delay
+    ret
+
+  ; 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
+
+    MODULE fwd
+        INCLUDE "../unpack.asm"
+    ENDMODULE
+
+    MODULE rwd
+        DEFINE BACKWARDS_UNPACK         ; defined to build backwards unpack
+                ; initial IX points at last byte of compressed data
+                ; initial DE' points at last byte of unpacked data
+
+        INCLUDE "../unpack.asm"
+    ENDMODULE
+
+    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 +1,328 @@
+;; 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 10 bytes of stack space
+;;
+
+;     DEFINE BACKWARDS_UNPACK         ; uncomment to build backwards depacker
+            ; initial IX points at last byte of compressed data
+            ; initial DE' points at last byte of unpacked data
+
+;     DEFINE UPKR_UNPACK_SPEED        ; uncomment to get larger but faster unpack routine
+
+    OPT push reset --syntax=abf
+    MODULE upkr
+
+NUMBER_BITS     EQU     16+15       ; context-bits per offset/length (16+15 for 16bit offsets/pointers)
+    ; numbers (offsets/lengths) are encoded like: 1a1b1c1d1e0 = 0000'0000'001e'dbca
+
+/*
+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
+    ; D = prev_was_match = uninitialised, literal is expected first => will reset D to "false"
+    ; values for false/true of prev_was_match are: false = high(probs), true = 1 + high(probs)
+.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;
+    IFNDEF BACKWARDS_UNPACK : inc de : ELSE : dec de : ENDIF
+    exx
+    ld      d,b                 ; prev_was_match = false
+    jr      .decompress_data
+
+  ; * copy chunk of already decompressed data (match)
+.copy_chunk:
+    ld      a,b
+    inc     b                   ; context_index = 256
+        ;             if(prev_was_match || upkr_decode_bit(256)) {
+        ;                 offset = upkr_decode_length(257) - 1;
+        ;                 if (0 == offset) break;
+        ;             }
+    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                   ; context_index to first "number" set for offsets decoding (257)
+    call    decode_number
+    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 + NUMBER_BITS)    ; context_index to second "number" set for lengths decoding
+    call    decode_number       ; length = upkr_decode_length(257 + 64);
+    push    de
+    exx
+    ld      h,d                 ; DE = write_ptr
+    ld      l,e
+.offset+*:  ld      bc,0
+    IFNDEF BACKWARDS_UNPACK
+    sbc     hl,bc               ; CF=0 from decode_number ; HL = write_ptr - offset
+    ELSE
+    add     hl,bc               ; HL = write_ptr + offset
+    ENDIF
+    pop     bc                  ; BC = length
+    IFNDEF BACKWARDS_UNPACK : ldir : ELSE : lddr : ENDIF
+    exx
+    ld      d,b                 ; prev_was_match = true
+    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)
+    IFNDEF BACKWARDS_UNPACK : inc ix : ELSE : dec ix : ENDIF    ; 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    bc
+    ld      c,l                     ; C = (upkr_state & 255); (preserving the value)
+    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
+
+  IFNDEF UPKR_UNPACK_SPEED
+
+    ;; looped MUL for minimum unpack size
+    ld      b,8                     ; counter
+.mulLoop:
+    add     hl,hl
+    jr      nc,.mul0
+    add     hl,de
+.mul0:
+    djnz    .mulLoop                ; until HL = state_scale * (upkr_state>>8), also BC becomes (upkr_state & 255)
+
+  ELSE
+
+    ;;; unrolled MUL for better performance, +25 bytes unpack size
+    ld      b,d
+    DUP     8
+        add     hl,hl
+        jr      nc,0_f
+        add     hl,de
+0:
+    EDUP
+
+  ENDIF
+
+    add     hl,bc                   ; HL = state_scale * (upkr_state >> 8) + (upkr_state & 255)
+    pop     af                      ; restore prob and CF=bit
+    jr      nc,.bit_is_0_2
+    dec     d                       ; DE = -prob (also D = bit ? $FF : $00)
+    add     hl,de                   ; HL += -prob
+    ; ^ this always preserves CF=1, because (state>>8) >= 128, state_scale: 7..250, prob: 7..250,
+    ; so 7*128 > 250 and thus edge case `ADD hl=(7*128+0),de=(-250)` => CF=1
+.bit_is_0_2:
+ ; *** adjust probs[context_index]
+    ld      e,a                     ; preserve prob
+    rra                             ; + (bit<<4) ; part of -prob_offset, needs another -16
+    and     $FC                     ; clear/keep correct bits to get desired (prob>>4) + extras, CF=0
+    rra
+    rra
+    rra                             ; A = (bit<<4) + (prob>>4), CF=(prob & 8)
+    adc     a,-16                   ; A = (bit<<4) - 16 + ((prob + 8)>>4) ; -prob_offset = (bit<<4) - 16
+    sub     e                       ; A = (bit<<4) - 16 + ((prob + 8)>>4) - prob ; = ((prob + 8)>>4) - prob_offset - prob
+    neg                             ; A = prob_offset + prob - ((prob + 8)>>4)
+    pop     bc
+    ld      (bc),a                  ; probs[context_index] = prob_offset + prob - ((prob + 8) >> 4);
+    add     a,d                     ; restore CF = bit (D = bit ? $FF : $00 && A > 0)
+    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_number:
+  ; 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
+probs:      EQU ((UPKR_PROBS_ORIGIN) + 255) & -$100     ; probs array aligned to 256
+    ELSE
+probs:      EQU ($ + 255) & -$100                       ; probs array aligned to 256
+    ENDIF
+.real_c:    EQU 1 + 255 + 1 + 2*NUMBER_BITS             ; 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