inertia

_custom_jump.asm

@@ -1,6 +1,9 @@
 _custom_jump:
 if CUSTOM_JUMP_BANK == PLAYER_UPDATE_BANK
     jsr rts_if_cutscene
+    if INERTIA != 0
+        ERROR INERTIA must be 0 if PLAYER_UPDATE_BANK=E
+    endif
     JSR zero_hspfra
 else
     ; return if in cutscene
@@ -11,9 +14,12 @@ else
     rts
 __continue
 
+if INERTIA == 0
     ; zero out fractional hspeed
     lda #$00
     sta hspfra
+endif
+
 endif
     LDA joypad_down
     AND #$03 ; 1=right, 2=left
@@ -27,14 +33,22 @@ air_control_right:
     BCS hcancel
     
     ; standard right
+    if INERTIA == 0
+        LDX #$01
+        STX hspint
+    else
+        jsr apply_inertia_rightward_full
+    endif
     LDY #$00
-    LDX #$01
-    STX hspint
-    BPL __jumping_contd ; guaranteed
+    BEQ __jumping_contd ; guaranteed
 air_control_left:
+    if INERTIA == 0
+        LDX #$FF
+        STX hspint
+    else
+        jsr apply_inertia_leftward_full
+    endif
     LDY #$01
-    LDX #$FF
-    STX hspint
 __jumping_contd
 
     ; decide whether or not to set facing
@@ -50,7 +64,31 @@ set_facing:
     BNE check_vcancel ; guaranteed
     
 hcancel:
+if INERTIA == 0
     STA hspint
+else
+
+    ; apply positive or negative inertia as necessary
+    ; zero out hspfra *if* within INERTIA of #$0
+    lda hspint
+    bne standard_hcancel:
+    ldy #$0
+    lda hspfra
+    sty hspfra
+    cmp #INERTIA
+    bcc check_vcancel
+    CMP #($100-INERTIA)
+    bcs check_vcancel
+    sta hspfra
+standard_hcancel
+    bit hspint
+    bmi +
+    jsr apply_inertia_leftward_full
+    jmp check_vcancel
++   
+    jsr apply_inertia_rightward_full
+    
+endif
     
 check_vcancel:
     ifdef VCANCEL
@@ -66,4 +104,45 @@ check_vcancel:
         STA vspint
     endif
 __vcancel_rts:
-    RTS
+    RTS
+    
+if INERTIA != 0
+apply_inertia_rightward_full:
+    clc
+    lda #INERTIA
+    adc hspfra
+    sta hspfra
+    lda #$0
+    adc hspint
+    
+    ; clamp hspint
+    cmp #$01
+    bpl __apply_inertia_rts_r
+    sta hspint
+    rts
+
+__apply_inertia_rts_r
+    lda #$01
+__apply_inertia_rts
+    sta hspint
+zero_hspfra_custom_bank:
+    lda #$0
+    sta hspfra
+    rts
+
+apply_inertia_leftward_full:
+    sec
+    lda hspfra
+    sbc #INERTIA
+    sta hspfra
+    lda hspint
+    sbc #0
+    cmp #$FF
+    bmi __apply_inertia_l_rts
+    sta hspint
+    rts
+    
+__apply_inertia_l_rts:
+    lda #$FF
+    bne __apply_inertia_rts ; guaranteed
+endif

offsets-jp.asm

@@ -13,8 +13,13 @@ standard_stair_walk=$9AAB
 stair_walk_resume=$9A43
 sypha_jumptable=$9C19
 alucard_jumptable=$A59B
-empty_bank_e=$BF39
+empty_bank_e=$BF90 ; actually $BF39
 empty_bank_jump=$BED0 ; actually can go a bit earlier
 setAndSaveLowerBank=$E2D0
 PLAYER_UPDATE_BANK=$E
-CUSTOM_JUMP_BANK=$1
+CUSTOM_JUMP_BANK=$1
+
+; if $0, disabled
+; otherwise, between $1 and $255 enabled
+; higher values are actually less inertia, with $1 being the weightiest.
+INERTIA=$40

offsets-us.asm

@@ -17,4 +17,6 @@ empty_bank_e=$BFAB
 empty_bank_jump=$BED0 ; we actually can go a bit earlier
 setAndSaveLowerBank=$E2E6
 PLAYER_UPDATE_BANK=$E
-CUSTOM_JUMP_BANK=$0
+CUSTOM_JUMP_BANK=$0
+
+INERTIA=$40

pkg_readme.md

@@ -41,6 +41,9 @@ Similar controls hacks are available for [Castlevania](https://www.romhacking.ne
 
 ASM hacking:
     Sodium
+
+Testing:
+    AmarthDokuro
 
 Tools:
     `fceux` and `asm6f`