Problem 74: Digit factorial chains

eu074.c

@@ -0,0 +1,43 @@
+#include "euler.h"
+
+static int digfact[] = { 1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880 };
+
+static int factsum(int n) {
+  int t = 0;
+  while (n > 0) {
+    int d = n % 10;
+    t += digfact[d];
+    n /= 10;
+  }
+  return t;
+}
+
+#define N 1000000
+#define LEN 60
+
+void eu074(char *ans) {
+  int count = 0;
+
+  for (int i = 1; i < N; i++) {
+    int chain[LEN+1];
+    chain[0] = i;
+    int n = 1, v = i;
+    int done = 0;
+    while (n <= LEN) {
+      v = factsum(v);
+
+      for (int j = 0; j < n-1; j++) {
+        if (chain[j] == v) {
+          done = 1;
+          break;
+        }
+      }
+      if (done) break;
+
+      chain[n++] = v;
+    }
+
+    if (n == LEN) count++;
+  }
+  sprintf(ans, "%d", count);
+}

euler.c

@@ -295,6 +295,7 @@ struct puzzle puzzles[] = {
   { "071", &eu071, "428570" },
   { "072", &eu072, "303963552391" },
   { "073", &eu073, "7295372" },
+  { "074", &eu074, "402" },
 };
 
 #define NPUZZLES (sizeof puzzles / sizeof(puzzles[0]))