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Task/Langtons-ant/BBC-BASIC/langtons-ant.basic

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+      REM Implementation of Langton's ant for Rosetta Code
+      fieldsize%=100
+      REM Being pedantic, this will actually result in a field of 101 square,
+      REM since arrays start at 0, and my implementation allows them to use it
+      DIM field&(fieldsize%,fieldsize%)   : REM variables with an & suffix are byte variables
+      x%=fieldsize%/2
+      y%=fieldsize%/2
+      d%=0
+      REPEAT
+        IF field&(x%,y%)=0 THEN field&(x%,y%)=1:d%-=1 ELSE field&(x%,y%)=0:d%+=1
+        GCOL 15*field&(x%,y%)
+        PLOT 69,x%*2,y%*2     :REM for historical reasons there are two "plot points" per pixel
+        d%=(d%+4) MOD 4       :REM ensure direction is always between 0 and 3
+        CASE d% OF
+          WHEN 0:y%+=1
+          WHEN 1:x%+=1
+          WHEN 2:y%-=1
+          WHEN 3:x%-=1
+        ENDCASE
+      UNTIL x%>fieldsize% OR x%<0 OR y%>fieldsize% OR y%<0
+      END