;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-

.proc circle
        .include "circle.inc"
        ;; We use the algorithm jerkos method
        ;; https://schwarzers.com/algorithms/

        ;; X_rel = radius (share the same address)

        ;;Y_rel =0
        LDA #$00
        STA Y_rel

        ;; t1 = radius >> 4
        LDA radius
        LSR
        LSR
        LSR
        LSR
        STA t1


draw_center_px_in_circle:
        JSR  pixel_draw
        LDA btp_mem_pos
        STA btp_mem_pos_center
        LDA btp_mem_pos + 1
        STA btp_mem_pos_center + 1
        ;; btp_mem_pos_center_two = 2*btp_mem_pos_center | used later for calculating btp_mem_pos_inv
        btp_mem_pos_center_two = btp_mem_pos_center
        Mult_16 btp_mem_pos_center, btp_mem_pos_center + 1


draw_right_px_in_circle:
        CLC
        LDA X_pos
        ADC radius
        STA X_pos
        ;; We only draw the first pixel using absolute position.
        ;; After that we use relative position.
        JSR pixel_draw
        ;; This sets byte_to_paint, btp_mem_pos and Y

while_x_bigger_then_y:


draw_pixel:
        LDA byte_to_paint ;A byte containing a single 1. Coresponds to X position in the chunk.
        ORA (btp_mem_pos), Y
        STA (btp_mem_pos), Y

draw_pixel_inv:
        SEC
        LDA btp_mem_pos_center_two
        SBC btp_mem_pos
        STA btp_mem_pos_inv
        LDA btp_mem_pos_center_two + 1
        SBC btp_mem_pos + 1
        STA btp_mem_pos_inv + 1

;; calculate the inverted y value Y = 7-Y
STY temp
LDA #$07
SEC
SBC temp
TAY

;; calculate byte_to_paint_inv 00000001 --> 10000000, 00000010 --> 01000000 ... etc
;; uses a table!
        stx temp_
        ldx byte_to_paint
	LDA inverse_factor_value, X;; (see END.s)
	ldx temp_

; A = byte_to_paint_inv
        ORA (btp_mem_pos_inv), Y
        STA (btp_mem_pos_inv), Y

;;Recover the Y value (we changed it because evrything is inverted)
ldy temp

increment_y_pos:
        INC Y_rel ; y++
        INY
        CPY #$08

        BNE increment_y_pos_end
move_8px_down:
        LDY #$00
        ;; Switch to chunk bellow
        ; So we subtract  #$4000
        Add_16 btp_mem_pos, btp_mem_pos + 1, #$40, #$01 ;+320
increment_y_pos_end:

        ;;t1 += y
        CLC
        LDA t1
        ADC Y_rel
        STA t1
        ;; t2 = t1 - x
        SEC
        LDA t1
        SBC X_rel
        STA t2
        ;; if t2 < 0 then skip to endif
        CMP #$00
        BMI endif
if:
decrement_x_pos:
        DEC X_rel
        ASL byte_to_paint
        BCC decrement_x_pos_end
move_8px_left:
;; Next chunk is 8 addresses away. Look in pixel_draw for more detail.
;; -8.
Sub_16 btp_mem_pos, btp_mem_pos + 1, #$08, #$00
;; Restores byte to paint
LDA #%00000001
STA byte_to_paint
decrement_x_pos_end:
        LDA t2
        STA t1    ; t1 = t2
endif:
        ;; repeat if X > Y
        LDA X_rel
        CMP Y_rel

        BCS while_x_bigger_then_y
        RTS
.endproc