133 lines
2.7 KiB
NASM
133 lines
2.7 KiB
NASM
// a decompressor for the variant of LZSS used by Bomberman 64.
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// a matching compressor is available here:
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// https://github.com/notwa/baku/blob/master/compressor.c
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// this unsafe variant assumes that:
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// * the 0x400 bytes before the destination are readable and null
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// * the input data ends exactly where it is expected to
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// * the input data writes exactly as many bytes as expected
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// the performance gain over the safe variant is around 38%.
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LzDecomp:
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// a0: pointer to compressed data (must be RDRAM, cart is unsupported)
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// a1: compressed size
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// a2: output pointer
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// a3: maximum uncompressed size
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// v0: error code (0 is OK)
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// t0: current pointer for reading
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// t1: end pointer (exclusive)
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// t2: current pointer for writing
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// t3: end pointer (exclusive)
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// t4: current code, shifted as necessary
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// t5: 1 means raw, 0 means copy
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// t6: either raw byte or part of copy command
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// t7: current pseudo-position
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// t8: match length
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// t9: match position
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beqz a1, LzExit // nothing to decompress? nothing to do!
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lli v0, 0
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blez a3, LzExit
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lli v0, 1
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or t0, a0, r0
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addu t1, a0, a1
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or t2, a2, r0
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addu t3, a2, a3
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lli t7, 0x3BE
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LzNextCode:
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// READ
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lbu t4, 0(t0)
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addiu t0, 1
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ori t4, t4, 0x100 // add end marker
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andi t5, t4, 1
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LzReadCode:
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beq t2, t3, LzExit
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lli v0, 0
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// READ
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lbu t6, 0(t0)
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addiu t0, 1
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beqz t5, LzCopy
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nop
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LzRaw:
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// WRITE
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addiu t7, 1
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andi t7, t7, 0x3FF
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sb t6, 0(t2)
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addiu t2, 1
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// SHIFT
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srl t4, 1
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xori at, t4, 1
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bnez at, LzReadCode // branch if t4 != 1
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andi t5, t4, 1
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// reiterate
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bne t0, t1, LzNextCode
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nop
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b LzExit
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lli v0, 0
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LzCopy:
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// READ
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lbu t8, 0(t0)
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addiu t0, 1
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// extract match position
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sll t9, t8, 2
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andi t9, 0x0300
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or t9, t6
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// extract and prepare match length
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andi t8, 0x003F
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addiu t8, 3
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// prepare absolute match position
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subu t9, t7, t9
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subiu t9, 1
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andi t9, 0x03FF
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addiu t9, 1
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subu t9, t2, t9
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// preemptively move pseudo-position ahead (don't need it for the loop)
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addu t7, t8
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andi t7, t7, 0x3FF
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LzCopyLoop:
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// repurposing t6: byte being copied
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lbu t6, 0(t9)
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// WRITE
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subiu t8, 1
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sb t6, 0(t2)
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addiu t2, 1
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bnez t8, LzCopyLoop
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addiu t9, 1
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// SHIFT
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srl t4, 1
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xori at, t4, 1
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bnez at, LzReadCode // branch if t4 != 1
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andi t5, t4, 1
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// reiterate
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bne t0, t1, LzNextCode
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nop
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lli v0, 0
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LzExit:
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jr ra
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nop
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