Add 'dice7' implementation for RosettaCode.org.

http://rosettacode.org/wiki/Seven-sided_dice_from_five-sided_dice#Factor
http://rosettacode.org/wiki/Verify_distribution_uniformity/Naive#Factor

dice7.factor

@@ -0,0 +1,62 @@
+! Copyright (C) 2015 Alexander Ilin, John Benediktsson.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel random sequences assocs locals sorting prettyprint
+  math math.functions math.statistics math.vectors math.ranges ;
+IN: rosetta-code.dice7
+
+! http://rosettacode.org/wiki/Seven-sided_dice_from_five-sided_dice
+! http://rosettacode.org/wiki/Simple_Random_Distribution_Checker
+
+! Output a random integer 1..5.
+: dice5 ( -- x )
+   5 [1,b] random
+;
+
+! Output a random integer 1..7 using dice5 as randomness source.
+: dice7 ( -- x )
+   0 [ dup 21 < ] [ drop dice5 5 * dice5 + 6 - ] do until
+   7 rem 1 +
+;
+
+! Roll the die by calling the quotation the given number of times and return
+! an array with roll results.
+! Sample call: 1000 [ dice7 ] roll
+: roll ( times quot: ( -- x ) -- array )
+   [ call( --  x ) ] curry replicate
+;
+
+! Input array contains outcomes of a number of die throws. Each die result is
+! an integer in the range 1..X. Calculate and return the number of each
+! of the results in the array so that in the first position of the result
+! there is the number of ones in the input array, in the second position
+! of the result there is the number of twos in the input array, etc.
+: count-dice-outcomes ( X array -- array )
+    histogram
+    swap [1,b] [ over [ 0 or ] change-at ] each
+    sort-keys values
+;
+
+! Verify distribution uniformity/Naive. Delta is the acceptable deviation
+! from the ideal number of items in each bucket, expressed as a fraction of
+! the total count. Sides is the number of die sides. Die-func is a word that
+! produces a random number on stack in the range [1..sides], times is the
+! number of times to call it.
+! Sample call: 0.02 7 [ dice7 ] 100000 verify
+:: verify ( delta sides die-func: ( -- random ) times -- )
+   sides
+   times die-func roll
+   count-dice-outcomes
+   dup .
+   times sides / :> ideal-count
+   ideal-count v-n vabs
+   times v/n
+   delta [ < ] curry all?
+   [ "Random enough" . ] [ "Not random enough" . ] if
+;
+
+
+! Call verify with 1, 10, 100, ... 1000000 rolls of 7-sided die.
+: verify-all ( -- )
+   { 1 10 100 1000 10000 100000 1000000 }
+   [| times | 0.02 7 [ dice7 ] times verify ] each
+;