Data commit

Task/Modular-inverse/CLU/modular-inverse.clu

@@ -0,0 +1,36 @@
+mul_inv = proc (a, b: int) returns (int) signals (no_inverse)
+    if b<0 then b := -b end
+    if a<0 then a := b - (-a // b) end
+    t: int := 0
+    nt: int := 1
+    r: int := b
+    nr: int := a // b
+    while nr ~= 0 do
+        q: int := r / nr
+        t, nt := nt, t - q*nt
+        r, nr := nr, r - q*nr
+    end
+    if r>1 then signal no_inverse end
+    if t<0 then t := t+b end
+    return(t)
+end mul_inv
+
+start_up = proc ()
+    pair = struct[a, b: int]
+    tests: sequence[pair] := sequence[pair]$
+       [pair${a: 42, b: 2017},
+        pair${a: 40, b: 1},
+        pair${a: 52, b: -217},
+        pair${a: -486, b: 217},
+        pair${a: 40, b: 2018}]
+
+    po: stream := stream$primary_output()
+    for test: pair in sequence[pair]$elements(tests) do
+        stream$puts(po, int$unparse(test.a) || ", "
+                     || int$unparse(test.b) || " -> ")
+        stream$putl(po, int$unparse(mul_inv(test.a, test.b)))
+        except when no_inverse:
+            stream$putl(po, "no modular inverse")
+        end
+    end
+end start_up