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

.proc circle
        .include "circle.inc"
        ;; We use the algorithm jerkos method
        ;; git

        ;; 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
        Mult_16 btp_mem_pos_center, btp_mem_pos_center + 1
        ;; fix offset of 8 bytes, idk why this is needed
        Sub_16 btp_mem_pos_center, btp_mem_pos_center + 1, #$40, #$01 ;-320

draw_lower_px_in_circle:
        LDA Y_pos
        STA temp
        CLC
        ADC radius
        STA Y_pos

        JSR pixel_draw

        LDA btp_mem_pos
        STA btp_mem_pos_swop
        STA btp_mem_pos_swop_inv_y
        LDA btp_mem_pos + 1
        STA btp_mem_pos_swop_inv_y + 1
        STA btp_mem_pos_swop + 1
        ;; fix offset of 8 bytes, idk why this is needed
        Sub_16 btp_mem_pos_swop,       btp_mem_pos_swop       + 1, #$40, #$01 ;-320
        Sub_16 btp_mem_pos_swop_inv_y, btp_mem_pos_swop_inv_y + 1, #$40, #$01 ;-320

        LDA temp
        STA Y_pos
draw_right_px_in_circle:
        ;; C = 0, because Mult_16
        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

        LDA btp_mem_pos
        STA btp_mem_pos_inv_y
        LDA btp_mem_pos + 1
        STA btp_mem_pos_inv_y + 1

        ;; fix offset of 8 bytes, idk why this is needed
        Sub_16 btp_mem_pos_inv_y, btp_mem_pos_inv_y + 1, #$40, #$01 ;-320



SEC ;; circle_help expect this
while_x_bigger_then_y:
        ;;Draw pixels and does the ypos incrementation logic
        JSR circle_help


        ;;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
        ;; we can skipp CMP #$00 because SBC above do the same
        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.
        ;; C = 1 because branching
        Sub_16 btp_mem_pos, btp_mem_pos + 1, #$08, #$00, !

        ;; Y is inverted
        Sub_16 btp_mem_pos_inv_y, btp_mem_pos_inv_y + 1, #$08, #$00, !
        ;; X and Y has swoped
        Sub_16 btp_mem_pos_swop, btp_mem_pos_swop + 1, #$40, #$01 ;-320
        ;; X and Y has swoped and Y has inverted
        Sub_16 btp_mem_pos_swop_inv_y, btp_mem_pos_swop_inv_y +1, #$40, #$01 ;+320

        ;; 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
        .include "routines/circle/circle_help.s"
.endproc