;;; -*- 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
while_x_bigger_then_y:

        JSR pixel_draw

        INC Y_pos ; y++
        INC Y_rel

        ;;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:
        DEC X_pos ; x--
        DEC X_rel
        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