homebrew/lzss.baku.asm

// a decompressor for the variant of LZSS used by Bomberman 64.
// a matching compressor is available here:
// https://github.com/notwa/baku/blob/master/compressor.c

LzDecomp:
    // a0: pointer to compressed data (must be RDRAM, cart is unsupported)
    // a1: compressed size
    // a2: output pointer
    // a3: maximum uncompressed size
    // v0: error code (0 is OK)

    // t0: current pointer for reading
    // t1: end pointer (exclusive)
    // t2: current pointer for writing
    // t3: end pointer (exclusive)
    // t4: current code, shifted as necessary
    // t5: 1 means raw, 0 means copy
    // t6: either raw byte or part of copy command
    // t7: current pseudo-position
    // t8: match length
    // t9: match position

    beqz    a1, LzExit // nothing to decompress? nothing to do!
    nop

    blez    a3, LzNoSpace
    nop

    or      t0, a0, r0
    addu    t1, a0, a1
    or      t2, a2, r0
    addu    t3, a2, a3

    lli     t7, 0x3BE

LzNextCode:
    // READ
    lbu     t4, 0(t0)
    addiu   t0, 1

    ori     t4, t4, 0x100 // add end marker

    andi    t5, t4, 1
LzReadCode:
    beq     t2, t3, LzReiterate
    nop

    // READ
    beq     t0, t1, LzBadEnd
    nop
    lbu     t6, 0(t0)
    addiu   t0, 1

    beqz    t5, LzCopy
    nop

LzRaw:
    // WRITE
    beq     t2, t3, LzNoSpace
    addiu   t7, 1
    andi    t7, t7, 0x3FF
    sb      t6, 0(t2)
    addiu   t2, 1

    // SHIFT
    srl     t4, 1
    xori    at, t4, 1
    bnez    at, LzReadCode // branch if t4 != 1
    andi    t5, t4, 1

    b       LzReiterate
    nop

LzCopy:
    // READ
    beq     t0, t1, LzBadEnd
    nop
    lbu     t8, 0(t0)
    addiu   t0, 1

    // extract match position
    sll     t9, t8, 2
    andi    t9, 0x0300
    or      t9, t6

    // extract and prepare match length
    andi    t8, 0x003F
    addiu   t8, 3

    // prepare absolute match position
    subu    t9, t7, t9
    subiu   t9, 1
    andi    t9, 0x03FF
    addiu   t9, 1
    subu    t9, t2, t9

    // pre-emptively move psuedo-position ahead (don't need it for the loop)
    addu    t7, t8
    andi    t7, t7, 0x3FF

LzCopyLoop:
    // repurposing t6: byte being copied
    slt     at, t9, a2
    bnez    at,+ // don't copy bytes before the output pointer
    lli     t6, 0 // use nulls instead
    lbu     t6, 0(t9)
+

    // WRITE
    beq     t2, t3, LzNoSpace
    subiu   t8, 1
    sb      t6, 0(t2)
    addiu   t2, 1

    bnez    t8, LzCopyLoop
    addiu   t9, 1

    // SHIFT
    srl     t4, 1
    xori    at, t4, 1
    bnez    at, LzReadCode // branch if t4 != 1
    andi    t5, t4, 1

LzReiterate:
    bne     t0, t1, LzNextCode
    nop

LzExit:
    jr      ra
    lli     v0, 0

LzNoSpace:
    jr      ra
    lli     v0, 1

LzBadEnd: // aka Unexpected End Of File
    jr      ra
    lli     v0, 2