From 8e5298caee30769740557fc3785f6596341c0537 Mon Sep 17 00:00:00 2001
From: "Peter Helcmanovsky (Ped)" <ped@7gods.org>
Date: Mon, 19 Sep 2022 01:09:21 +0200
Subject: [PATCH] z80_unpacker: optimisation: -1B in decode_number = 172B (but
 +4T per length)

---
 z80_unpacker/example/example.sna | Bin 49179 -> 49179 bytes
 z80_unpacker/unpack.asm          |   9 ++++-----
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/z80_unpacker/example/example.sna b/z80_unpacker/example/example.sna
index f9d8ac6a21f3660ab798d06a222eb4ffeaf3405a..0bb80da49a0c00415f7f501071bc24f56ac9cb94 100644
GIT binary patch
delta 222
zcmbQ;z&yKwd4u>u#&eS;7FzfhKC}?vIa~Pfw4lMy%9}#MHv=SIR<P{4Quxq-`|Re2
z(p8&JybVr!$R`ivT)ml;&B(y;*6HBe4L1V>4qiQ7qaeW1uE2J;j^VCy|J$rj<&zgI
zH0I#|8z^;J8c0vRzfjI!<`KvenMY7dq+yoGJOWx0_XuK%&?BHFagU&ua2|ZS18Bz~
h6g#v*b|^iXyl|lr$P%D|U`u2+cQ1M=Gr4I0N&rZMXd3_k

delta 224
zcmbQ;z&yKwd4u>u#`BXU7Fq-pJ+u(uIa~Dbw4lMy%9}#MHv=SIR<P{4QuNS(hv)2;
zhtgG>PrMCIf5@i*<XpX(l+DP%@Yd<z+YL7Z1P)w1U85kt(XPODw~pbia{t?`PZg8r
zFEr-i1e+*zS{g`CzPC^=K<*L97P&`ITclyO$UOqtlJE#(i^wCOEeVgHws0PNy8~#+
iK@>~0L6)dIn!I445y%#xiC|mgHg_(1DKoib|8fAh4QNmR

diff --git a/z80_unpacker/unpack.asm b/z80_unpacker/unpack.asm
index cb458e4..e925d07 100644
--- a/z80_unpacker/unpack.asm
+++ b/z80_unpacker/unpack.asm
@@ -122,8 +122,7 @@ unpack:
     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
+    call    decode_number       ; context_index is already 257-1 as needed by decode_number
     dec     de                  ; offset = upkr_decode_length(257) - 1;
     ld      a,d
     or      e
@@ -136,7 +135,7 @@ unpack:
         ;                 ++write_ptr;
         ;             }
         ;             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);
     push    de
     exx
@@ -288,7 +287,7 @@ int upkr_decode_length(int context_index) {
 decode_number:
   ; HL = upkr_state
   ; IX = upkr_data_ptr
-  ; BC = probs+context_index
+  ; BC = probs+context_index-1
   ; 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
@@ -298,8 +297,8 @@ decode_number:
     call    decode_bit
     rr      d
     rr      e                   ; DE = length = (length >> 1) | (bit << 15);
-    inc     c                   ; context_index += 2
 .loop_entry:
+    inc     c                   ; context_index += 2
     call    decode_bit
     jr      c,.loop
 .fix_bit_pos: