dzx7.asm

; -----------------------------------------------------------------------------
; ZX7 Backwards by Einar Saukas, Antonio Villena
; "Standard" version (64 bytes only)
; -----------------------------------------------------------------------------
; Parameters:
;   HL: source address (compressed data)
;   DE: destination address (decompressing)
; -----------------------------------------------------------------------------
dzx7:   ld      bc, $8000       ; marker bit at MSB of B
        ld      a, b            ; move marker bit to A
copyby: inc     c               ; to keep BC value after LDD
        ldd                     ; copy literal byte
mainlo: call    getbit          ; read next bit
        jr      nc, copyby      ; next bit indicates either literal or sequence

; determine number of bits used for length (Elias gamma coding)
        push    de              ; store destination on stack
        ld      d, c            ; D= 0, assume BC=$0000 here (or $8000)
lenval: call    nc, getbit      ; don't call first time (to save bytes)
        rl      c
        rl      b               ; insert bit on BC
        call    getbit          ; more bits?
        jr      nc, lenval      ; repeat, final length on BC

; check escape sequence
        inc     c               ; detect escape sequence
        jr      z, exitdz       ; end of algorithm

; determine offset
        ld      e, (hl)         ; load offset flag (1 bit) + offset value (7 bits)
        dec     hl
        sll     e
        jr      nc, offend      ; if offset flag is set, load 4 extra bits
        ld      d, $10          ; bit marker to load 4 bits
nexbit: call    getbit
        rl      d               ; insert next bit into D
        jr      nc, nexbit      ; repeat 4 times, until bit marker is out
        inc     d               ; add 128 to DE
        srl     d               ; retrieve fourth bit from D
offend: rr      e               ; insert fourth bit into E
        ex      (sp), hl        ; store source, restore destination
        ex      de, hl          ; destination from HL to DE
        adc     hl, de          ; HL = destination + offset + 1
        lddr
exitdz: pop     hl              ; restore source address (compressed data)
        jr      nc, mainlo      ; end of loop. exit with getbit instead ret (-1 byte)

getbit: add     a, a            ; check next bit
        ret     nz              ; no more bits left?
        ld      a, (hl)         ; load another group of 8 bits
        dec     hl
        adc     a, a
        ret