;License:MIT
;(c) 2018-9 by 4am
;
; functions for managing which HGR page is showing, and doing things on the other one
;
; - GetOffscreenAddress
; - LoadTitleOffscreen
; - LoadCoverOffscreen
; - LoadHelpOffscreen
; - LoadGameTitleOffscreen
; - ResyncPage
; - ShowOtherPage
; - ToggleOffscreenPage
; - ClearOffscreen
; - ClearHGR1
;
; Public variables
; - OffscreenPage
;

OffscreenPage
         !byte 0                     ; 0 = currently showing HGR page 2
                                     ;     (so offscreen is page 1 @ $2000)
                                     ; 1 = currently showing HGR page 1
                                     ;     (so offscreen is page 2 @ $4000)

;------------------------------------------------------------------------------
; GetOffscreenAddress
; get high byte of HGR page that is currently not showing
;
; in:    none
; out:   A = high byte of offscreen HGR page (#$20 or #$40)
;        preserves X/Y
;------------------------------------------------------------------------------
GetOffscreenAddress
         lda   OffscreenPage
         beq   +
         lda   #$40
         rts
+        lda   #$20
         rts

;------------------------------------------------------------------------------
; LoadTitleOffscreen
; load title screen in the HGR page that is currently not showing
;
; in:    none
; out:   all flags and registers clobbered
;------------------------------------------------------------------------------
LoadTitleOffscreen
!if >kTitleFile = >kCoverFile {
         lda   #<kTitleFile
         +HIDE_NEXT_2_BYTES
} else {
         +LDADDR kTitleFile
         bne   +                     ; Always branches, because Y is loaded
                                     ; last with the high byte of the address,
                                     ; which is never 0. I miss my 65c02.
}
;------------------------------------------------------------------------------
; LoadHelpOffscreen
; load help screen in the HGR page that is currently not showing
;
; in:    none
; out:   all flags and registers clobbered
;------------------------------------------------------------------------------
LoadHelpOffscreen
         +LDADDR kHelpFile
         bne   LoadOffscreenFromAY   ; always branches

;------------------------------------------------------------------------------
; LoadCoverOffscreen
; load cover screen in the HGR page that is currently not showing
;
; in:    none
; out:   all flags and registers clobbered
;------------------------------------------------------------------------------
LoadCoverOffscreen
         +LDADDR kCoverFile
LoadOffscreenFromAY
         +STAY @fname
         jsr   GetOffscreenAddress
         sta   +
         jsr   LoadFile
         !word kRootDirectory
@fname   !word $FDFD                 ; SMC
         !byte $00
+        !byte $FD                   ; SMC
         rts

LoadGameTitleOffscreen
; in:    X = game index
         +LDADDR gGamesListStore
         jsr   okvs_nth
         +STAY @fname
         jsr   GetOffscreenAddress
         sta   +
         jsr   LoadFile
         !word kHGRTitleDirectory
@fname   !word $FDFD                 ; SMC
         !byte $00
+        !byte $FD                   ; SMC
         rts

;------------------------------------------------------------------------------
; ClearOffscreen
; clear $2000..$3FFF or $4000..$5FFF, depending on which HGR page is not
; visible right now
; does not change HGR mode
;
; in:    none
; out:   $2000..$3FFF or $4000..$5FFF cleared
;        A = 0
;        X = 0
;        Y = 0
;        Z = 1
;------------------------------------------------------------------------------
ClearOffscreen
         jsr   GetOffscreenAddress
         +HIDE_NEXT_2_BYTES
ClearHGR1
         lda   #$20                  ; note to self: LDX #$20 can't move here
         sta   @a+2
         ldx   #$20
         lda   #$80
         ldy   #0
@a       sta   $2000,y
         iny
         bne   @a
         inc   @a+2
         dex
         bne   @a
         rts

ResyncPage
         jsr   ToggleOffscreenPage
         ; /!\ execution falls through here to ShowOtherPage
;------------------------------------------------------------------------------
; ShowOtherPage
; switch to the HGR page that is not currently showing
;
; in:    none
; out:   A = new value of OffscreenPage
;        preserves X/Y
;------------------------------------------------------------------------------
ShowOtherPage
         jsr   ToggleOffscreenPage
         bne   +
         bit   PAGE2                 ; show page 2
         rts
+        bit   PAGE1                 ; show page 1
         rts

;------------------------------------------------------------------------------
; ToggleOffscreenPage
; switch the internal variable that tracks which HGR page is showing
; (does not affect screen)
;
; in:    none
; out:   A = new value of OffscreenPage
;        preserves X/Y
;------------------------------------------------------------------------------
ToggleOffscreenPage
         lda   OffscreenPage
         eor   #$01
         sta   OffscreenPage
         rts