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math.factorials: making double-factorial work for negative numbers.

db4
John Benediktsson 2013-04-08 21:44:35 -07:00
parent 50c8033301
commit ad419301af
2 changed files with 14 additions and 6 deletions
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10
extra/math/factorials/factorials-tests.factor
View File

@ -1,4 +1,4 @@
USING: kernel math.functions sequences tools.test ;
USING: kernel math.functions math.ranges sequences tools.test ;
IN: math.factorials
[ 1 ] [ -1 factorial ] unit-test ! not necessarily correct
@ -6,8 +6,12 @@ IN: math.factorials
[ 1 ] [ 1 factorial ] unit-test
[ 3628800 ] [ 10 factorial ] unit-test
{ { 1 1 2 3 8 15 48 105 384 945 } }
[ 10 iota [ double-factorial ] map ] unit-test
{
{
1/0. 1/105 1/0. -1/15 1/0. 1/3 1/0. -1 1/0.
1 1 1 2 3 8 15 48 105 384 945 3840
}
} [ -10 10 [a,b] [ double-factorial ] map ] unit-test
{ 1 } [ 10 10 factorial/ ] unit-test
{ 720 } [ 10 7 factorial/ ] unit-test

10
extra/math/factorials/factorials.factor
View File

@ -12,9 +12,13 @@ MEMO: factorial ( n -- n! )
ALIAS: n! factorial
MEMO: double-factorial ( n -- n!! )
dup 1 > [
dup even? 2 1 ? swap 2 <range> product
] [ drop 1 ] if ;
dup [ even? ] [ 0 < ] bi [
[ drop 1/0. ] [
2 + -1 swap -2 <range> product recip
] if
] [
2 3 ? swap 2 <range> product
] if ;
ALIAS: n!! double-factorial