math.extras: adding round-to-step.

extra/math/extras/extras-tests.factor

@@ -120,3 +120,9 @@ IN: math.extras.test
         12345.6789012 12345.67890123 12345.678901235
     }
 } [ 12345.67890123456 -6 9 [a,b] [ round-to-decimal ] with map ] unit-test
+
+{ 0 } [ 0 5 round-to-step ] unit-test
+{ 0 } [ 1 5 round-to-step ] unit-test
+{ 5 } [ 3 5 round-to-step ] unit-test
+{ 10 } [ 12 5 round-to-step ] unit-test
+{ 15 } [ 13 5 round-to-step ] unit-test

extra/math/extras/extras.factor

@@ -255,3 +255,6 @@ M: float round-to-even
 
 : round-to-decimal ( x n -- y )
     10^ [ * 0.5 over 0 > [ + ] [ - ] if truncate ] [ / ] bi ;
+
+: round-to-step ( x step -- y )
+    [ [ / round ] [ * ] bi ] unless-zero ;