;license:MIT
;(c) 2018-2020 by 4am
;
; hi-res font drawing routines
;
; Public functions
; - Draw40Chars
; - DrawCenteredString
; - DrawString
;

Draw40Chars
; A/Y contains address of character buffer
; carry bit clear -> draw on page 1
; carry bit set   -> draw on page 2
; $25 contains textpage line (0..23) (this is the standard VTAB address)
; drawing starts at HTAB 0
; increments VTAB on exit
; sets HTAB to 0 on exit
; Z=1 on exit
; clobbers A/X/Y
         jsr   +
         ldx   #40
         jsr   .DrawBufferInternal
+        ldx   #0
         stx   HTAB
         rts

DrawCenteredString
; A/Y contains address of length-prefixed string
; carry bit clear -> draw on page 1
; carry bit set   -> draw on page 2
; $25 contains textpage line (0..23) (this is the standard VTAB address)
; clobbers A/X/Y
         +ST16 PTR
         php
         ldy   #0
         lda   #40
         sec
         sbc   (PTR),y
         lsr
         sta   HTAB
         plp
         jmp   DrawString_PTR_is_already_set

DrawString
; A/Y contains address of length-prefixed string
; length can be 0
; carry bit clear -> draw on page 1
; carry bit set   -> draw on page 2
; $24 contains starting column (0..39) (this is the standard HTAB address)
; $25 contains textpage line (0..23) (this is the standard VTAB address)
; clobbers A/X/Y
         +ST16 PTR
DrawString_PTR_is_already_set
         ldy   #0
         lda   (PTR),y
         inc   PTR
         bne   +
         inc   PTR+1
+        tax
         bpl   +
         rts
+
         +LD16 PTR
         ; /!\ execution falls through here to .DrawBufferInternal

.DrawBufferInternal
; [private]
; A/Y contains address of character buffer
; X contains buffer length (0..40)
; carry bit clear -> draw on page 1
; carry bit set   -> draw on page 2
; characters MUST have high bit off (0x00..0x7F)
; special characters (0x00..0x1F) will be drawn
; $24 contains starting column (0..39) (this is the standard HTAB address)
; $25 contains textpage line (0..23) (this is the standard VTAB address)
; all characters are drawn on the same line
; HTAB is incremented for each character
; clobbers X,Y
; increments VTAB on exit
; A=buffer length on exit
; N=0,Z=0 on exit
         +ST16 DBISrc+1
         php
         lda   VTAB
         asl
         asl
         asl
; routine to calculate memory address within HGR page
; and self-modify addresses within draw loop that follows
; (routine clobbers A and Y but preserves X)
         asl
         tay
         and   #$F0
         bpl   @calc1
         ora   #$05
@calc1   bcc   @calc2
         ora   #$0A
@calc2   asl
         asl
         sta   @hgrlo+1
         tya
         and   #$0E
         adc   #$10
         asl   @hgrlo+1
         rol
         plp
         bcc   +
         eor   #$60
         clc
+
         sta   DBIRow0+2
         adc   #$04
         sta   DBIRow1+2
         adc   #$04
         sta   DBIRow2+2
         adc   #$04
         sta   DBIRow3+2
         adc   #$04
         sta   DBIRow4+2
         adc   #$04
         sta   DBIRow5+2
         adc   #$04
         sta   DBIRow6+2
         adc   #$04
         sta   DBIRow7+2
@hgrlo   lda   #$FD
         adc   HTAB
         sta   DBIRow0+1
         sta   DBIRow1+1
         sta   DBIRow2+1
         sta   DBIRow3+1
         sta   DBIRow4+1
         sta   DBIRow5+1
         sta   DBIRow6+1
         sta   DBIRow7+1
         txa
         pha
         bpl   +                     ; always branches because X is 0..40
DBILoop
DBISrc   ldy   $FDFD,x
         lda   FontDataRow0,y
DBIRow0  sta   $FDFD,x
         lda   FontDataRow1,y
DBIRow1  sta   $FDFD,x
         lda   FontDataRow2,y
DBIRow2  sta   $FDFD,x
         lda   FontDataRow3,y
DBIRow3  sta   $FDFD,x
         lda   FontDataRow4,y
DBIRow4  sta   $FDFD,x
         lda   FontDataRow5,y
DBIRow5  sta   $FDFD,x
         lda   FontDataRow6,y
DBIRow6  sta   $FDFD,x
         lda   FontDataRow7,y
DBIRow7  sta   $FDFD,x
         inc   HTAB
+        dex
         bpl   DBILoop
         pla                         ; A = buffer length (passed in in X)
         inc   VTAB
         rts