Data commit

Task/Modular-inverse/C/modular-inverse-1.c

@@ -0,0 +1,20 @@
+#include <stdio.h>
+
+int mul_inv(int a, int b)
+{
+	int b0 = b, t, q;
+	int x0 = 0, x1 = 1;
+	if (b == 1) return 1;
+	while (a > 1) {
+		q = a / b;
+		t = b, b = a % b, a = t;
+		t = x0, x0 = x1 - q * x0, x1 = t;
+	}
+	if (x1 < 0) x1 += b0;
+	return x1;
+}
+
+int main(void) {
+	printf("%d\n", mul_inv(42, 2017));
+	return 0;
+}

Task/Modular-inverse/C/modular-inverse-2.c

@@ -0,0 +1,25 @@
+#include <stdio.h>
+
+int mul_inv(int a, int b)
+{
+        int t, nt, r, nr, q, tmp;
+        if (b < 0) b = -b;
+        if (a < 0) a = b - (-a % b);
+        t = 0;  nt = 1;  r = b;  nr = a % b;
+        while (nr != 0) {
+          q = r/nr;
+          tmp = nt;  nt = t - q*nt;  t = tmp;
+          tmp = nr;  nr = r - q*nr;  r = tmp;
+        }
+        if (r > 1) return -1;  /* No inverse */
+        if (t < 0) t += b;
+        return t;
+}
+int main(void) {
+        printf("%d\n", mul_inv(42, 2017));
+        printf("%d\n", mul_inv(40, 1));
+        printf("%d\n", mul_inv(52, -217));  /* Pari semantics for negative modulus */
+        printf("%d\n", mul_inv(-486, 217));
+        printf("%d\n", mul_inv(40, 2018));
+        return 0;
+}