Add divisor? math function

basis/math/functions/functions-docs.factor

@@ -13,7 +13,8 @@ ARTICLE: "integer-functions" "Integer functions"
 "Tests:"
 { $subsection power-of-2? }
 { $subsection even? }
-{ $subsection odd? } ;
+{ $subsection odd? }
+{ $subsection divisor? } ;
 
 ARTICLE: "arithmetic-functions" "Arithmetic functions"
 "Computing additive and multiplicative inverses:"
@@ -269,6 +270,11 @@ HELP: gcd
 { $description "Computes the positive greatest common divisor " { $snippet "d" } " of " { $snippet "x" } " and " { $snippet "y" } ", and another value " { $snippet "a" } " satisfying:" { $code "a*y = d mod x" } }
 { $notes "If " { $snippet "d" } " is 1, then " { $snippet "a" } " is the inverse of " { $snippet "y" } " modulo " { $snippet "x" } "." } ;
 
+HELP: divisor?
+{ $values { "x" integer } { "y" integer } { "?" "a boolean" } }
+{ $description "Tests if " { $snippet "y" } " is a divisor of " { $snippet "x" } ". This is the same thing as saying " { $snippet "x" } " is divisible by " { $snippet "y" } "." }
+{ $notes "Returns t for both negative and positive divisors, as well as trivial and non-trivial divisors." } ;
+
 HELP: mod-inv
 { $values { "x" integer } { "n" integer } { "y" integer } }
 { $description "Outputs an integer " { $snippet "y" } " such that " { $snippet "xy = 1 (mod n)" } "." }

basis/math/functions/functions-tests.factor

@@ -102,6 +102,12 @@ IN: math.functions.tests
 [ t ] [ 123 124 verify-gcd ] unit-test
 [ t ] [ 50 120 verify-gcd ] unit-test
 
+[ t ] [ 0 42 divisor? ] unit-test
+[ t ] [ 42 7 divisor? ] unit-test
+[ t ] [ 42 -7 divisor? ] unit-test
+[ t ] [ 42 42 divisor? ] unit-test
+[ f ] [ 42 16 divisor? ] unit-test
+
 [ 3 ] [ 5 7 mod-inv ] unit-test
 [ 78572682077 ] [ 234829342 342389423843 mod-inv ] unit-test
 

basis/math/functions/functions.factor

@@ -111,6 +111,9 @@ PRIVATE>
 : lcm ( a b -- c )
     [ * ] 2keep gcd nip /i ; foldable
 
+: divisor? ( x y -- ? )
+    mod 0 = ;
+
 : mod-inv ( x n -- y )
     [ nip ] [ gcd 1 = ] 2bi
     [ dup 0 < [ + ] [ nip ] if ]