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z80_unpacker: optimisation: -1B in decode_number = 172B (but +4T per length)

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Peter Helcmanovsky (Ped)
2022-09-19 01:09:21 +02:00
parent 1fb29f3a1b
commit 8e5298caee
2 changed files with 4 additions and 5 deletions
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z80_unpacker/example/example.sna
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z80_unpacker/unpack.asm
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@@ -122,8 +122,7 @@ unpack:
cp d ; CF = prev_was_match cp d ; CF = prev_was_match
call nc,decode_bit ; if not prev_was_match, then upkr_decode_bit(256) call nc,decode_bit ; if not prev_was_match, then upkr_decode_bit(256)
jr nc,.keep_offset ; if neither, keep old offset jr nc,.keep_offset ; if neither, keep old offset
inc c ; context_index to first "number" set for offsets decoding (257) call decode_number ; context_index is already 257-1 as needed by decode_number
call decode_number
dec de ; offset = upkr_decode_length(257) - 1; dec de ; offset = upkr_decode_length(257) - 1;
ld a,d ld a,d
or e or e
@@ -136,7 +135,7 @@ unpack:
; ++write_ptr; ; ++write_ptr;
; } ; }
; prev_was_match = 1; ; prev_was_match = 1;
ld c,low(257 + NUMBER_BITS) ; context_index to second "number" set for lengths decoding ld c,low(257 + NUMBER_BITS - 1) ; context_index to second "number" set for lengths decoding
call decode_number ; length = upkr_decode_length(257 + 64); call decode_number ; length = upkr_decode_length(257 + 64);
push de push de
exx exx
@@ -288,7 +287,7 @@ int upkr_decode_length(int context_index) {
decode_number: decode_number:
; HL = upkr_state ; HL = upkr_state
; IX = upkr_data_ptr ; IX = upkr_data_ptr
; BC = probs+context_index ; BC = probs+context_index-1
; A' = upkr_current_byte (!!! init to 0x80 at start, not 0x00) ; A' = upkr_current_byte (!!! init to 0x80 at start, not 0x00)
; return length in DE, CF=0 ; return length in DE, CF=0
ld de,$7FFF ; length = 0 with positional-stop-bit ld de,$7FFF ; length = 0 with positional-stop-bit
@@ -298,8 +297,8 @@ decode_number:
call decode_bit call decode_bit
rr d rr d
rr e ; DE = length = (length >> 1) | (bit << 15); rr e ; DE = length = (length >> 1) | (bit << 15);
inc c ; context_index += 2
.loop_entry: .loop_entry:
inc c ; context_index += 2
call decode_bit call decode_bit
jr c,.loop jr c,.loop
.fix_bit_pos: .fix_bit_pos: