Title: Sliding Dash fix
Author: SilentEnigma
Version: 1.1
Release Date: June 03 2019
Applies to: FF3us 1.0, FF3us 1.1
Tested on: FF3us 1.0

NOTE: These patches are for HEADERED ROMs (3146240 bytes) only!

Contents:

    SlidingDash.ips = the patch
    
    SlidingDashN.ips = the anti-patch
    
    readme.txt = this file
    
ROM Addresses:

    C0/5892 - C0/5894
    C0/5885 - C0/5887
    C0/D68D - C0/D69E
    
Free space used: 18 bytes, C0/D68D - C0/D69E

Urgency: Low. This design quirk becomes a painfully obvious bug only when using
    Master ZED's FF3DashB and FF3DashC patches. In vanilla FF3 the effect is
    only briefly noticeable with some of the slower-moving NPCs.
________________________________________________________________________________

TABLE OF CONTENTS

0. Description
1. Relevant Offsets & Disassembly
2. Other Learnings
  a. New comments for C0 bank assembly code
  b. "100+ bugs" insight
3. Revision History
4. Credits
________________________________________________________________________________

0. DESCRIPTION

When characters walk on the field, the game determines which sprite to display
at any given moment by using an offset derived from the character's current
position and direction, cycling through a set of four offsets for each 2-tile
walking animation. The routine that determines this offset works smoothly when
the character is decreasing its position value (moving up or left), but it lags
by one sprite when the character is increasing its position value (moving down
or right). This can become noticable with extra-slow NPCs, as they will often
appear to slide mid-step when changing direction. 

More notably, this causes the bug in Master ZED's FF3dashB and FF3dashC patches 
where the sprite appears to be standing still when running down or right at
maximum speed (Dash + Sprint Shoes). This occurs because two sprite instances
are thrown out of the animation cycle when running at that speed; while Up/Left
correctly omits the standing sprite, Down/Right omits the stepping sprites,
since the offsets lag by an iteration.

Final Fantasy Anthology attempted to remedy the dash issue by simply reordering
the sprite I.D. table accessed with the offset; however, this made characters
finish their walking animations in the 'stepping' pose after dashing - which is
also undesirable.

This fix instead rewrites the offset derivation function so that the flow of
walking animations is consistent in all four directions. Slow NPCs will now walk
a bit more believably, and there will be no more sliding-while-dashing nonsense
when using Master ZED's patch.

________________________________________________________________________________

1. RELEVANT OFFSETS & DISASSEMBLY

Original code for sprite offset calculation:

C0/5882:  B9 6A 08    LDA $086A,Y    ; Load horiz. position to be used for offset
C0/5885:  4A          LSR A          ; Shift offset value such that
C0/5886:  4A          LSR A          ; the least- significant bit will change 
C0/5887:  4A          LSR A          ; roughly two times in the course of one step.
C0/5888:  80 13       BRA $589D
C0/588A:  BE 73 08    LDX $0873,Y
C0/588D:  F0 1B       BEQ $58AA
C0/588F:  B9 6D 08    LDA $086D,Y    ; Load vert. position to be used for offset
C0/5892:  4A          LSR A          ; Shift offset value such that
C0/5893:  4A          LSR A          ; the least- significant bit will change 
C0/5894:  4A          LSR A          ; roughly two times in the course of one step.
C0/5895:  80 06       BRA $589D
C0/5897:  A5 46       LDA $46
C0/5899:  4A          LSR A
C0/589A:  4A          LSR A
C0/589B:  80 00       BRA $589D
C0/589D:  29 03       AND #$03       ; Zero all but the last two bits
C0/589F:  18          CLC 
C0/58A0:  65 1A       ADC $1A        ; Add offset determined by the direction
C0/58A2:  AA          TAX 
C0/58A3:  BF 1D 58 C0 LDA $C0581D,X  ; Use offset to look up the sprite I.D.


Modified code:

C0/5885: 20 8D D6       JSR $D68D      ; Jump to new subroutine $D68D

C0/5892: 20 8D D6       JSR $D68D      ; Jump to new subroutine $D68D

New Subroutine $D68D:

C0/D68D:  4A          LSR            ; Shift offset bits right
C0/D68E:  4A          LSR            ; Shift offset bits right
C0/D68F:  48          PHA            ; Push offset value to stack
C0/D690:  B9 7F 08    LDA $087F,y    ; Load direction value
C0/D693:  3A          DEA            ; Check if direction is Right
C0/D694:  F0 03       BEQ $D699      ; Branch if the direction is Right
C0/D696:  3A          DEA            ; Check if the direction is Down
C0/D697:  D0 03       BNE $D69C      ; Branch if the direction is Down
C0/D699:  68          PLA            ; Pull position off of stack
C0/D69A:  1A          INC            ; Increase the offset value by 1P
C0/D68B:  48          PHA            ; Push offset value to stack
C0/D69C:  68          PLA            ; Pull position off of stack
C0/D69D:  4A          LSR            ; Shift position bits right
C0/D69E:  60          RTS            ; Return from subroutine

________________________________________________________________________________

2. OTHER LEARNINGS

a. New comments for C0 bank assembly code

Here are a few snippets of the C0 bank that was able to decipher during the course
of this hack:


C0/4978:    A507        LDA $07        ; Get value stored from controller1 byte 2
C0/497A:    2901        AND #$01       ; AND for Right button on d-pad
C0/497C:    F018        BEQ $4996      ; Branch if button not pressed or held in
C0/497E:    A947        LDA #$47       ; Load I.D. for sprite standing, facing right
C0/4980:    997708      STA $0877,Y    ; Store the sprite I.D.
C0/4983:    A901        LDA #$01       ; Load I.D. for the Right direction
C0/4985:    997F08      STA $087F,Y    ; Store direction I.D.
C0/4988:    20514D      JSR $4D51
C0/498B:    D062        BNE $49EF
C0/498D:    A902        LDA #$02
C0/498F:    20164E      JSR $4E16
C0/4992:    F05B        BEQ $49EF
C0/4994:    806D        BRA $4A03
C0/4996:    A507        LDA $07        ; Get value stored from controller1 byte 2
C0/4998:    2902        AND #$02       ; AND for Left button on d-pad
C0/499A:    F018        BEQ $49B4      ; Branch if button not pressed or held in
C0/499C:    A907        LDA #$07       ; Load I.D. for sprite standing, facing left
C0/499E:    997708      STA $0877,Y    ; Store the sprite I.D.
C0/49A1:    A903        LDA #$03       ; Load I.D. for the Left direction
C0/49A3:    997F08      STA $087F,Y    ; Store direction I.D.
C0/49A6:    20514D      JSR $4D51
C0/49A9:    D044        BNE $49EF
C0/49AB:    A904        LDA #$04
C0/49AD:    20164E      JSR $4E16
C0/49B0:    F03D        BEQ $49EF
C0/49B2:    804F        BRA $4A03
C0/49B4:    A507        LDA $07        ; Get value stored from controller1 byte 2
C0/49B6:    2908        AND #$08       ; AND for Up button on d-pad
C0/49B8:    F017        BEQ $49D1      ; Branch if button not pressed or held in
C0/49BA:    A904        LDA #$04       ; Load I.D. for sprite standing, facing forward
C0/49BC:    997708      STA $0877,Y    ; Store the sprite I.D.
C0/49BF:    7B          TDC            ; Load I.D. for the Up direction (00h)
C0/49C0:    997F08      STA $087F,Y    ; Store direction I.D.
C0/49C3:    20514D      JSR $4D51
C0/49C6:    D027        BNE $49EF
C0/49C8:    A901        LDA #$01
C0/49CA:    20164E      JSR $4E16
C0/49CD:    F020        BEQ $49EF
C0/49CF:    8032        BRA $4A03
C0/49D1:    A507        LDA $07        ; Get value stored from controller1 byte 2
C0/49D3:    2904        AND #$04       ; AND for Down button on d-pad
C0/49D5:    F018        BEQ $49EF      ; Branch if button not pressed or held in
C0/49D7:    A901        LDA #$01       ; Load I.D. for sprite standing, facing backward
C0/49D9:    997708      STA $0877,Y    ; Store the sprite I.D.
C0/49DC:    A902        LDA #$02       ; Load I.D. for the Down direction
C0/49DE:    997F08      STA $087F,Y    ; Store direction I.D.
C0/49E1:    20514D      JSR $4D51
C0/49E4:    D009        BNE $49EF
C0/49E6:    A903        LDA #$03
C0/49E8:    20164E      JSR $4E16
C0/49EB:    F002        BEQ $49EF
C0/49ED:    8014        BRA $4A03
C0/49EF:    AC0308      LDY $0803
C0/49F2:    7B          TDC 
C0/49F3:    997E08      STA $087E,Y
C0/49F6:    9C8608      STZ $0886
C0/49F9:    20A5C8      JSR $C8A5
C0/49FC:    20F446      JSR $46F4
C0/49FF:    20834B      JSR $4B83
C0/4A02:    60          RTS
  
  
; Sprite offset table for walking animations:

C0/580D:    04 05 04 03     ; Unknown
C0/5811:    6E 6F 6E 6F     ; Unknown
C0/5815:    01 02 01 00     ; Unknown
C0/5819:    2E 2F 2E 2F     ; Unknown
C0/581D:    04 05 04 03     ; Character sprite moving Up
C0/5821:    47 48 47 46     ; Character sprite moving Right
C0/5825:    01 02 01 00     ; Character sprite moving Down
C0/5829:    07 08 07 06     ; Character sprite moving Left
  
----------------------------------------  

b. "100+ bugs" insight

In the "100+ bugs" thread, Novalia Spirit speculated that bug #103 "Stuck in a
pose" is directly related to the dash bug in FFAnthology; however, this does not
appear to be the case. This patch DOES fix the animation quirk that FFA failed
to address properly; it DOES NOT fix the "Stuck in a pose" bug in general
(which seems more like a pattern of sloppy design than a bug).

Since version 1.0 of this patch, the "Stuck in a pose" bug has been addressed by
the author's subsequent patch, aptly named "Stuck in a Pose fix."

________________________________________________________________________________

3. REVISION HISTORY

February 11, 2016 - Version 1.0 completed.
June 03, 2019     - Version 1.1 completed. 
                     > Uses 3 fewer bytes of free space.

________________________________________________________________________________

4. CREDITS

Master ZED, for writing the FF3 Dash button hack.
Imzogelmo, for his disassembly of bank C0 and for his patch allocation list.
Novalia Spirit, for compiling an excellent bugs list.
Squaresoft, for always leaving room for improvement ;)

________________________________________________________________________________

Copyright (C) 2019 David R. Thompson (SilentEnigma).
The author intends for this readme document to accompany the distributed file.
The author expects to be given credit whenever his work is re-used or modified.