; Rainbow Bars - GameBoy Color Demo
; Scrolling rainbow colored bars

INCLUDE "hardware.inc"

SECTION "Header", ROM0[$100]
    jp EntryPoint
    ds $143 - @, 0
    db $C0          ; GBC only flag
    ds $150 - @, 0

SECTION "Variables", WRAM0
wScrollY: db
wFrameCounter: db
wColorCycle: db

SECTION "EntryPoint", ROM0[$150]
EntryPoint:
    ; Wait for VBlank
.waitVBlank:
    ld a, [rLY]
    cp 144
    jp c, .waitVBlank

    ; Turn off LCD
    xor a
    ld [rLCDC], a
    
    ; Copy tiles
    ld de, Tiles
    ld hl, $9000
    ld bc, TilesEnd - Tiles
.copyTiles:
    ld a, [de]
    ld [hl+], a
    inc de
    dec bc
    ld a, b
    or c
    jp nz, .copyTiles
    
    ; Fill tilemap with horizontal bars
    ld hl, $9800
    ld b, 18        ; 18 rows
    xor a           ; Tile index
.rowLoop:
    ld c, 32        ; 32 columns
.colLoop:
    ld [hl+], a
    dec c
    jr nz, .colLoop
    inc a           ; Next tile for next row
    and $07         ; Cycle 0-7
    dec b
    jr nz, .rowLoop
    
    ; Set up GBC palettes
    call SetupPalettes
    
    ; Set tilemap attributes for colors
    ld a, 1
    ld [rVBK], a    ; Switch to VRAM bank 1 (attributes)
    
    ld hl, $9800
    ld b, 18
    xor a
.attrRowLoop:
    ld c, 32
.attrColLoop:
    ld [hl+], a
    dec c
    jr nz, .attrColLoop
    inc a
    and $07         ; Cycle through palettes 0-7
    dec b
    jr nz, .attrRowLoop
    
    xor a
    ld [rVBK], a    ; Back to VRAM bank 0
    
    ; Set DMG palette fallback
    ld a, %11100100
    ld [rBGP], a
    
    ; Initialize variables
    xor a
    ld [wScrollY], a
    ld [wFrameCounter], a
    ld [wColorCycle], a
    
    ; Turn on LCD
    ld a, LCDC_ON | LCDC_BG_ON
    ld [rLCDC], a

Main:
    ; Wait for VBlank
    ld a, [rLY]
    cp 144
    jp nc, Main
.waitVBlank2:
    ld a, [rLY]
    cp 144
    jp c, .waitVBlank2
    
    ; Update every frame for smooth scroll
    ld a, [wScrollY]
    inc a
    ld [wScrollY], a
    ld [rSCY], a
    
    ; Cycle colors every 30 frames
    ld a, [wFrameCounter]
    inc a
    ld [wFrameCounter], a
    cp 30
    jp nz, Main
    
    xor a
    ld [wFrameCounter], a
    
    ld a, [wColorCycle]
    inc a
    and $1F
    ld [wColorCycle], a
    call UpdatePalettes
    
    jp Main

SetupPalettes:
    ; Set up 8 different colored palettes
    ; Palette 0 - Red shades
    ld a, $80       ; Auto-increment, palette 0
    ld [rBCPS], a
    ld a, $00       ; Black
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $15       ; Dark red
    ld [rBCPD], a
    ld a, $00
    ld [rBCPD], a
    ld a, $1F       ; Red
    ld [rBCPD], a
    ld a, $00
    ld [rBCPD], a
    ld a, $3F       ; Bright red
    ld [rBCPD], a
    ld a, $00
    ld [rBCPD], a
    
    ; Palette 1 - Green shades
    ld a, $88
    ld [rBCPS], a
    ld a, $00
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $A0       ; Dark green
    ld [rBCPD], a
    ld a, $02
    ld [rBCPD], a
    ld a, $E0       ; Green
    ld [rBCPD], a
    ld a, $03
    ld [rBCPD], a
    ld a, $E0       ; Bright green
    ld [rBCPD], a
    ld a, $07
    ld [rBCPD], a
    
    ; Palette 2 - Blue shades
    ld a, $90
    ld [rBCPS], a
    ld a, $00
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $00       ; Dark blue
    ld [rBCPD], a
    ld a, $0A
    ld [rBCPD], a
    ld a, $00       ; Blue
    ld [rBCPD], a
    ld a, $1F
    ld [rBCPD], a
    ld a, $00       ; Bright blue
    ld [rBCPD], a
    ld a, $3F
    ld [rBCPD], a
    
    ; Palette 3 - Yellow shades
    ld a, $98
    ld [rBCPS], a
    ld a, $00
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $B5       ; Dark yellow
    ld [rBCPD], a
    ld a, $02
    ld [rBCPD], a
    ld a, $DF       ; Yellow
    ld [rBCPD], a
    ld a, $03
    ld [rBCPD], a
    ld a, $FF       ; Bright yellow
    ld [rBCPD], a
    ld a, $03
    ld [rBCPD], a
    
    ; Palette 4 - Cyan shades
    ld a, $A0
    ld [rBCPS], a
    ld a, $00
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $A0
    ld [rBCPD], a
    ld a, $0A
    ld [rBCPD], a
    ld a, $E0
    ld [rBCPD], a
    ld a, $1F
    ld [rBCPD], a
    ld a, $E0
    ld [rBCPD], a
    ld a, $3F
    ld [rBCPD], a
    
    ; Palette 5 - Magenta shades
    ld a, $A8
    ld [rBCPS], a
    ld a, $00
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $15
    ld [rBCPD], a
    ld a, $0A
    ld [rBCPD], a
    ld a, $1F
    ld [rBCPD], a
    ld a, $1F
    ld [rBCPD], a
    ld a, $3F
    ld [rBCPD], a
    ld a, $3F
    ld [rBCPD], a
    
    ; Palette 6 - Orange shades
    ld a, $B0
    ld [rBCPS], a
    ld a, $00
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $95
    ld [rBCPD], a
    ld a, $01
    ld [rBCPD], a
    ld a, $1F
    ld [rBCPD], a
    ld a, $02
    ld [rBCPD], a
    ld a, $9F
    ld [rBCPD], a
    ld a, $03
    ld [rBCPD], a
    
    ; Palette 7 - Purple shades
    ld a, $B8
    ld [rBCPS], a
    ld a, $00
    ld [rBCPD], a
    ld [rBCPD], a
    ld a, $0A
    ld [rBCPD], a
    ld a, $05
    ld [rBCPD], a
    ld a, $1F
    ld [rBCPD], a
    ld a, $0F
    ld [rBCPD], a
    ld a, $3F
    ld [rBCPD], a
    ld a, $1F
    ld [rBCPD], a
    ret

UpdatePalettes:
    ; Rotate palette colors dynamically
    ; Each palette shifts through the color spectrum
    
    ld a, [wColorCycle]
    ld b, a
    
    ; Update all 8 palettes with shifted colors
    ld c, 0         ; Palette counter
.paletteLoop:
    ; Calculate palette address
    ld a, c
    sla a
    sla a
    sla a           ; Multiply by 8
    or $80          ; Auto-increment
    ld [rBCPS], a
    
    ; Calculate color offset for this palette
    ld a, b         ; wColorCycle
    ld d, a
    ld a, c         ; Palette number
    sla a
    sla a           ; Multiply by 4 for offset
    add d           ; Add cycle offset
    and $1F         ; Keep in 0-31 range
    
    ; Generate colors based on position in cycle
    call GenerateShiftingColor
    
    ; Next palette
    inc c
    ld a, c
    cp 8
    jr nz, .paletteLoop
    ret

GenerateShiftingColor:
    ; Generate a color based on value in A (0-31)
    ; Creates a smooth rainbow effect
    push bc
    ld b, a
    
    ; Color 0 - always black
    xor a
    ld [rBCPD], a
    ld [rBCPD], a
    
    ; Color 1 - Dark shade
    ld a, b
    call .calcRGB
    srl d           ; Halve brightness
    srl e
    srl c
    ld a, d
    ld [rBCPD], a
    ld a, e
    or c
    ld [rBCPD], a
    
    ; Color 2 - Medium shade
    ld a, b
    add 5           ; Offset for variation
    and $1F
    call .calcRGB
    ld a, d
    ld [rBCPD], a
    ld a, e
    or c
    ld [rBCPD], a
    
    ; Color 3 - Bright shade
    ld a, b
    add 10          ; More offset
    and $1F
    call .calcRGB
    ; Boost brightness
    ld a, d
    or $1F
    ld [rBCPD], a
    ld a, e
    or c
    or $1F
    ld [rBCPD], a
    
    pop bc
    ret

.calcRGB:
    ; Calculate RGB values based on position (0-31) in A
    ; Returns: D=low byte, E=high byte (bits 0-4), C=high byte (bits 5-7)
    push af
    
    ; Determine which third of spectrum (0-10=red, 11-21=green, 22-31=blue)
    cp 11
    jr c, .redPhase
    cp 22
    jr c, .greenPhase
    
.bluePhase:
    ; Blue to Red transition
    sub 22
    ld b, a
    ld a, 9
    sub b           ; Inverse for fade
    
    ; Red component (fading in)
    sla a
    and $1F
    ld d, a         ; Low byte = red (bits 0-4)
    
    ; Blue component (high, fading out)
    ld a, b
    sla a
    sla a
    and $1F
    ld c, a
    sla c
    sla c
    sla c           ; Shift to bits 5-7 of high byte
    
    xor a
    ld e, a         ; Green in middle bits
    jr .done
    
.greenPhase:
    ; Green to Blue transition
    sub 11
    ld b, a
    
    ; Green component (fading out)
    ld a, 10
    sub b
    and $1F
    sla a
    sla a
    sla a
    ld e, a         ; Green in bits 5-7 of low byte
    
    ; Blue component (fading in)
    ld a, b
    sla a
    and $1F
    ld c, a
    sla c
    sla c
    sla c           ; Blue in high byte
    
    xor a
    ld d, a         ; No red
    jr .done
    
.redPhase:
    ; Red to Green transition
    ld b, a
    
    ; Red component (high, fading out)
    ld a, 10
    sub b
    sla a
    and $1F
    ld d, a         ; Red in low byte
    
    ; Green component (fading in)
    ld a, b
    sla a
    and $1F
    sla a
    sla a
    sla a
    ld e, a         ; Green in low byte bits 5-7
    
    xor a
    ld c, a         ; No blue
    
.done:
    pop af
    ret

Tiles:
    ; Tile 0 - Solid
    dw `33333333
    dw `33333333
    dw `33333333
    dw `33333333
    dw `33333333
    dw `33333333
    dw `33333333
    dw `33333333
    
    ; Tile 1
    dw `22222222
    dw `22222222
    dw `22222222
    dw `22222222
    dw `22222222
    dw `22222222
    dw `22222222
    dw `22222222
    
    ; Tile 2
    dw `11111111
    dw `11111111
    dw `11111111
    dw `11111111
    dw `11111111
    dw `11111111
    dw `11111111
    dw `11111111
    
    ; Tile 3 - Gradient
    dw `33333333
    dw `33333333
    dw `22222222
    dw `22222222
    dw `22222222
    dw `11111111
    dw `11111111
    dw `00000000
    
    ; Tile 4
    dw `33332222
    dw `33332222
    dw `33332222
    dw `33332222
    dw `22221111
    dw `22221111
    dw `22221111
    dw `22221111
    
    ; Tile 5
    dw `22221111
    dw `22221111
    dw `22221111
    dw `22221111
    dw `11110000
    dw `11110000
    dw `11110000
    dw `11110000
    
    ; Tile 6
    dw `33333333
    dw `22222222
    dw `11111111
    dw `00000000
    dw `33333333
    dw `22222222
    dw `11111111
    dw `00000000
    
    ; Tile 7
    dw `33221100
    dw `33221100
    dw `33221100
    dw `33221100
    dw `33221100
    dw `33221100
    dw `33221100
    dw `33221100
TilesEnd: