Data commit

Task/Executable-library/Factor/executable-library-1.factor

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+! rosetta/hailstone/hailstone.factor
+USING: arrays io kernel math math.ranges prettyprint sequences vectors ;
+IN: rosetta.hailstone
+
+: hailstone ( n -- seq )
+    [ 1vector ] keep
+    [ dup 1 number= ]
+    [
+        dup even? [ 2 / ] [ 3 * 1 + ] if
+        2dup swap push
+    ] until
+    drop ;
+
+<PRIVATE
+: main ( -- )
+    27 hailstone dup dup
+    "The hailstone sequence from 27:" print
+    "  has length " write length .
+    "  starts with " write 4 head [ unparse ] map ", " join print
+    "  ends with " write 4 tail* [ unparse ] map ", " join print
+
+    ! Maps n => { length n }, and reduces to longest Hailstone sequence.
+    1 100000 [a,b)
+    [ [ hailstone length ] keep 2array ]
+    [ [ [ first ] bi@ > ] most ] map-reduce
+    first2
+    "The hailstone sequence from " write pprint
+    " has length " write pprint "." print ;
+PRIVATE>
+
+MAIN: main