Drawing a line
------------------------------------------------------------------
I can't even count how many hours I've spent writing the following
blob in tal. It draws a line using Bresenham's [1] algorithm. It
doesn't handle all the cases (e.g. x1 needs to be > x2), but I'm
fairly happy with it.

```

@line ( x1* y1* x2* y2* c -- )
,&color STR ROT2 LTH2k DUP ,&dec STR #01 EOR #06 JCN SWP2
SUB2k NIP2 OVR2 ADD2 SWP2 STH2 STH2 SWP2 OVR2 OVR2 STH2 STH2
SUB2 ROT2r STH2r ROT2r STH2r OVR2 OVR2 STH2 STH2 SUB2 GTH2
,&high JCN #01 ,&mode STR SWP2r STH2r ROT2r STH2r STH2r STH2r
,&paint JMP &high SWP2r STH2r ROT2r STH2r SWP2 STH2r STH2r
SWP2 ,&paint JMP BRK &mode $1 &dec $1 &color $1 &paint ROT2
SUB2k STH2 NIP2 ROT2 ROT2 SWP2 SUB2k DUP2 2** STH2rk ADD2 SWP2
STH2 STH2 SWP2 &loop GTH2k ,&end JCN ,&mode LDR ,&inv JCN SWP2
DUP2 .Screen/x DEO2 ROT2 DUP2 .Screen/y DEO2 ,&cont JMP &inv
SWP2 DUP2 .Screen/y DEO2 ROT2 DUP2 .Screen/x DEO2 &cont
,&color LDR .Screen/pixel DEO SWP2 INC2 STH2r DUP2 STH2r DUP2
#0003 MUL2 ROT2 SWP2 LTH2 ,&no-inc-y JCN DUP2 ROT2 STH2rk 2**
ADD2 SWP2 2** SUB2 STH2 STH2 SWP2r SWP2 ,&dec LDR ,&s JCN INC2
,&c JMP &s #0001 SUB2 &c SWP2 ROT2 ,&loop JMP &no-inc-y STH2
SWP2r STH2r SWP2 OVR2 2** ADD2 STH2 STH2 ROT2 ROT2r ROT2r
,&loop JMP &end POP2r POP2r POP2r
RTN
```

=> https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm [1]
Bresenham line algorithm

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