Data commit

Task/Modular-inverse/BCPL/modular-inverse.bcpl

@@ -0,0 +1,31 @@
+get "libhdr"
+
+let mulinv(a, b) =
+    b<0 -> mulinv(a, -b),
+    a<0 -> mulinv(b - (-a rem b), b),
+    valof
+    $(  let t, nt, r, nr = 0, 1, b, a rem b
+        until nr = 0
+        $(  let tmp, q = ?, r / nr
+            tmp := nt ; nt := t - q*nt ; t := tmp
+            tmp := nr ; nr := r - q*nr ; r := tmp
+        $)
+        resultis r>1 -> -1,
+                 t<0 -> t + b,
+                 t
+    $)
+
+let show(a, b) be
+$(  let mi = mulinv(a, b)
+    test mi>=0
+        do writef("%N, %N -> %N*N", a, b, mi)
+        or writef("%N, %N -> no inverse*N", a, b)
+$)
+
+let start() be
+$(  show(42, 2017)
+    show(40, 1)
+    show(52, -217)
+    show(-486, 217)
+    show(40, 2018)
+$)