%include ../include/gigatron.i

rand_colour         EQU     0x06
        
x1                  EQU     0x30
y1                  EQU     x1+2
x2                  EQU     x1+4
y2                  EQU     x1+6
xy1    				EQU     x1+8
xy2    				EQU     x1+10
dxy1   				EQU     x1+12
dxy2   				EQU     x1+14
dx1                 EQU     x1+16
dy1                 EQU     x1+18
dx2                 EQU     x1+20
dy2                 EQU     x1+22
sx     				EQU     x1+24
sy     				EQU     x1+26
h      				EQU     x1+28
num    				EQU     x1+30
count  				EQU     x1+32
tmp    				EQU     x1+34

_singleStepWatch_   EQU     count       ; the single step debugger watches this variable location to decide when to step,
                                        ; choose a variable that is updated often

lineStart	EQU		0x0200
lineExt		EQU		0x0300


lineStart	LDWI    0x0000      
            STW     x1
            STW     y1
            STW     x2 
            STW     y2

line_random LDWI    SYS_Random_34
            STW     giga_sysFn

			SYS     34                  ; x1 = (rnd AND &h7F) + (rnd AND &h1F)
            ANDI    0x7F        
            STW     x1
            SYS     34
            ANDI    0x1F
            ADDW    x1
            STW     x1
            
            SYS     34                  ; y1 = (rnd AND &h3F) + (rnd AND &h3F)
            ANDI    0x3F        
            STW     y1
            SYS     34
            ANDI    0x3F
            ADDW    y1
            ORI     0x08
            STW     y1
            
            SYS     34                  ; x2 = (rnd AND &h7F) + (rnd AND &h1F)
            ANDI    0x7F        
            STW     x2
            SYS     34
            ANDI    0x1F
            ADDW    x2
            STW     x2

            SYS     34                  ; y2 = (rnd AND &h3F) + (rnd AND &h3F)
            ANDI    0x3F        
            STW     y2
            SYS     34
            ANDI    0x3F
            ADDW    y2
            ORI     0x08
            STW     y2
      
            LDI     1
            STW     dx1
            STW     dx2
            STW     dy1
            LDI     0
            STW     dy2                

            LDWI    0x8000
            STW     tmp
            
            LDW     x2         			; sx = x2 - x1
            SUBW    x1
            STW     sx
            ANDW    tmp        
            BEQ     line_dy
            LDWI    -1
            STW     dx1        
            STW     dx2        			; dx1 = dx2 = (sx & 0x8000) ? -1 : 1
            LDI     0                   ; sx = (sx & 0x8000) ? 0 - sx : sx
            SUBW    sx
            STW     sx                
                
line_dy     LDW     y2
            SUBW    y1
            STW     sy
            STW     h          			; h = sy
            ANDW    tmp
            BEQ     line_ext
            
            LDWI    -1
            STW     dy1        			; dy1 = (sy & 0x8000) ? -1 : 1
            LDI     0                   
            SUBW    sy
            STW     sy         			; sy = (sy & 0x8000) ? 0 - sy : sy
            SUBW    sx
            BLE     line_ext           
            LDW     dy1
            STW     dy2        			; if(sx < sy) dy2 = -1
			
line_ext    LDWI    lineExt
            CALL    giga_vAC
			
lineExt		LD      x1	
            ST      xy1	
            LD      y1	
            ST      xy1 + 1    			; xy1 = x1 | (y1<<8)
				
            LD      x2	
            ST      xy2	
            LD      y2	
            ST      xy2 + 1    			; xy2 = x2 | (y2<<8)
                
			LDW     sy
            SUBW    sx
            BLE     line_num
            LDI     0
            STW     dx2        			; if(sx < sy) dx2 = 0
            LDW     sy       
            STW     tmp
            LDW     sx
            STW     sy
            LDW     tmp
            STW     sx         			; swap sx with sy
            LDW     h
            BLE     line_num
            LDI     1
            STW     dy2        			; if(h > 0) dy2 = 1

line_num    LDWI    SYS_LSRW1_48
            STW     giga_sysFn          
            LDW     sx
            SYS     48
            STW     num        			; numerator = sx>>1
            STW     count
            INC     count      			; for(count=sx>>1; counti>=0; --i)
            
			LDWI    SYS_LSLW8_24	
            STW     giga_sysFn      	    
            LDW     dy1	
            SYS     28
            ADDW    dx1	
            STW     dxy1       			; dxy1 = dx1 + (dy1<<8)
	
            LDW     dy2	
            SYS     28
            ADDW    dx2	
            STW     dxy2       			; dxy2 = dx2 + (dy2<<8)
			
line_loop   LD      rand_colour
            POKE    xy1        			; plot start pixel
            POKE    xy2        			; plot end pixel, (meet in middle)      
            
            LDW     num        			; numerator += sy
            ADDW    sy
            STW     num
            SUBW    sx
            BLE     line_flip           ; if(numerator <= sx) goto flip
                
            STW     num        			; numerator -= sx
			LDW     xy1
            ADDW    dxy1
            STW     xy1        			; xy1 += dxy1
				
            LDW     xy2	
            SUBW    dxy1	
            STW     xy2        			; xy2 -= dxy1
            BRA     line_count	
					
line_flip   LDW     xy1        	
            ADDW    dxy2	
            STW     xy1        			; xy1 += dxy2
				
            LDW     xy2        	
            SUBW    dxy2	
            STW     xy2        			; xy2 -= dxy2
					
line_count  LDW     count	
            SUBI    0x01	
            STW     count	
            BGT     line_loop	
			LDWI    line_random
            CALL    giga_vAC