.proc triangle
;; Subroutine for drawing a filld 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 highest 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
        STA AB
        BCC AB_overflow_end
;; A_Y > B_Y
AB_overflow:
        EOR #$ff ; Fix byte underflow
        ;; swop A and B
        LDX B_Y
        LDY A_Y
        STX A_Y
        STY B_Y
        LDX B_X
        LDY A_X
        STX A_X
        STY B_X
AB_overflow_end:
;;BC
SEC
LDA C_Y
SBC B_Y
STA BC
BCC BC_overflow_end
;; B_Y > C_Y
BC_overflow:
        EOR #$ff ; Fix byte underflow
        ;; 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:
# This triangle has 10 posibol cases, see table bellow
# ----------------------------------------------------
# | name left_side               right_side          |
# ----------------------------------------------------
# | RR   right-sloaping-steep    right-sloaping-steep|
# | rR   right-sloaping          right-sloaping-steep|
# | lR   left-sloaping           right-sloaping-steep|
# | LR   left-sloaping-steep     right-sloaping-steep|
# | rr   right-sloaping          right-sloaping      |
# | lr   left-sloaping           right-sloaping      |
# | Lr   left-sloaping-steep     right-sloaping      |
# | ll   left-sloaping           left-sloaping       |
# | Ll   left-sloaping-sleep     left-sloaping       |
# | LL   left-sloaping-sleep     left-sloaping-sleep |
# ----------------------------------------------------
RTS
.endproc