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Task/Modular-inverse/Oforth/modular-inverse.oforth

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+// euclid ( a b -- u v r )
+//    Return r = gcd(a, b) and (u, v) / r = au + bv
+
+: euclid(a, b)
+| q u u1 v v1 |
+
+   b 0 < ifTrue: [ b neg ->b ]
+   a 0 < ifTrue: [ b a neg b mod - ->a ]
+
+   1 dup ->u ->v1
+   0 dup ->v ->u1
+
+   while(b) [
+      b a b /mod ->q ->b ->a
+      u1 u u1 q * - ->u1 ->u
+      v1 v v1 q * - ->v1 ->v
+      ]
+   u v a ;
+
+: invmod(a, modulus)
+   a modulus euclid 1 == ifFalse: [ drop drop null return ]
+   drop dup 0 < ifTrue: [ modulus + ] ;