UpdateScrollColumn

.proc UpdateScrollColumn
  WhichMetaColumn = 1
  LevelBufIndex = 14
  NewColumn = StatusRow2

; calculate PPU address to write the 28 byte buffer to
; (repurposed from being a status bar)
  lda #$20
  sta Update28Address+0
  lda ScrollX+1
  lsr
  lsr
  lsr
  sta 0   ; nametable column used to determine what level column to update
  add #64 ; start one block down
  sta Update28Address+1

  ldy #0 ; y will be Update28 index
  sty WhichMetaColumn
  lda 0
  lsr    ; get the LevelBuf index
  sta LevelBufIndex

; we split the tasks for scrolling into different parts per frame
; partly to leave more CPU for other stuff and partly
ChooseTask:
  lda ScrollX+1
  and #15
  cmp #8
  jeq RenderRight
  bcc :+
    rts
: asl
  tax
  lda ScrollTasks+1,x
  pha
  lda ScrollTasks+0,x
  pha
  rts

RenderLeftGenerate:
; generate new level stuff here, if left column
  ldx #15
  lda #0
  ldy #SCROLLCOL_MIDGROUND*16
: lda ScrollColumnTemplates,y
  sta NewColumn,x
  iny
  dex
  bpl :-

; just put a random block on here for now
  jsr huge_rand
  and #15
  tax
  lda #METATILE_GRASSM
  sta NewColumn,x

  ldy #0
RenderLeft:
  tya
  lsr
  tax
  lda NewColumn,x
  sta TempVal

  lda LevelBufIndex ; step to next metatile
  tax               ; but for this iteration, use the previous value
  add #16
  sta LevelBufIndex

  lda TempVal
  sta LevelBuf+16,x ; find index into MetatileData
  asl
  asl
  tax

  lda MetatileTiledata,x
  sta Update28+0,y
  lda MetatileTiledata+2,x
  sta Update28+1,y

  iny
  iny
  cpy #28
  bne RenderLeft
  rts

RenderRight:
  lda LevelBufIndex ; step to next metatile
  tax               ; but for this iteration, use the previous value
  add #16
  sta LevelBufIndex

  lda LevelBuf+16,x ; find index into MetatileData
  asl
  asl
  tax

  lda MetatileTiledata+1,x
  sta Update28+0,y
  lda MetatileTiledata+3,x
  sta Update28+1,y

  iny
  iny
  cpy #28
  bne RenderRight
  rts

FixColors1:
  ldx #0
  ldy #FixColorSet2-FixColorSet1
  bne FixColors
FixColors2:
  ldx #FixColorSet2-FixColorSet1
  ldy #FixColorSet3-FixColorSet2
  bne FixColors
FixColors3:
  ldx #FixColorSet3-FixColorSet1
  ldy #FixColorSetEnd-FixColorSet3

FixColors:
  ColorSetIndex = 12
  ColorSetLoopCount = 13
  stx ColorSetIndex
  sty ColorSetLoopCount
FixColorsLoop:
  ldx ColorSetIndex
  inc ColorSetIndex

  lda FixColorSet1,x   ; what block to do currently
  add LevelBufIndex
  tay
  sty 0
  lda LevelBuf,y       ; get tile we're going to be coloring
  tay
  lda MetatileFlags,y  ; get the flags for this tile
  and #3               ; mask off just the palette number
  ldy 0
  jsr ChangeBlockColor ; A = new palette, Y = LevelBuf index

  dec ColorSetLoopCount
  bne FixColorsLoop
  rts

; two consecutive rows are going to share the same byte
; so I spread it out so we're not rewriting the same byte twice in one NMI
FixColorSet1: .byt 1*16,  3*16,  5*16,  7*16, 9*16
FixColorSet2: .byt 11*16, 13*16, 2*16,  4*16
FixColorSet3: .byt 6*16,  8*16,  10*16, 12*16, 14*16
FixColorSetEnd:

ScrollTasks:
  .raddr RenderLeftGenerate
  .raddr FixColors1
  .raddr FixColors2
  .raddr FixColors3
  .raddr EmptyObject
  .raddr EmptyObject
  .raddr EmptyObject
  .raddr EmptyObject
.endproc