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

;; Subroutine for drawing a filld triangle.
.proc triangle
        ;; It takes in values from A_X, A_Y, B_X, B_Y C_X, C_Y, see triangle.inc
        .include "triangle.inc"

;; Algorithm
;; (i) For height n it steps through Bresenham's line algorithm to know where the triangle starts and ends at height n.
;; (ii) It draws all pixel in this given range.
;; (iii) It change if the algorithm has hit a corner and need to change line!
;; Implementation
;; In the implementation the triangle split into 2 souch that (iii) can be redused to: check if the triangle has ended.
;; Each side of the sub-triangle can have 4 posiboul states (look at how line is implemented)

;;Lets first sort the edges such as
;; (i) A is at the left point
;; (ii) B is at the middle point
;; (iii) C is at the lowest point
;; Lets also calculate the height-differense for the new points:
;; * AB for differense betwean A and B
;; * BC for differense betwean B and C

        ;;AB
        SEC
        LDA B_Y
        SBC A_Y
        BCC AB_overflow_end
        STA AB
;; A_Y > B_Y
AB_overflow:
        EOR #$ff ; Fix byte underflow
        STA AB
        ;; swop A and B
        LDX B_Y
        LDY A_Y
        STX A_Y
        STY B_Y
        LDX B_X
        LDY A_X
        STX A_X
        STY B_X
AB_overflow_end:
        ;;BC
        SEC
        LDA C_Y
        SBC B_Y
        BCC BC_overflow_end
        STA BC
;; B_Y > C_Y
BC_overflow:
        EOR #$ff ; Fix byte underflow
        STA BC
        ;; swop A and B
        LDX C_Y
        LDY B_Y
        STX B_Y
        STY C_Y
        LDX C_X
        LDY B_X
        STX B_X
        STY C_X
        ;; If this is the case A and B may be in the wrong order. Lets redo stuff from the start
        JMP triangle
BC_overflow_end:

draw_upper_triangle:
        .include "upper_triangle.s"
draw_lower_triangle:
;;This has the same cases as upper triangle
        .include "lower_triangle.s"
        RTS
.endproc