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Task/Left-factorials/C/left-factorials.c

@@ -0,0 +1,50 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <gmp.h>
+
+void mpz_left_fac_ui(mpz_t rop, unsigned long op)
+{
+    mpz_t t1;
+    mpz_init_set_ui(t1, 1);
+    mpz_set_ui(rop, 0);
+
+    size_t i;
+    for (i = 1; i <= op; ++i) {
+        mpz_add(rop, rop, t1);
+        mpz_mul_ui(t1, t1, i);
+    }
+
+    mpz_clear(t1);
+}
+
+size_t mpz_digitcount(mpz_t op)
+{
+    /* mpz_sizeinbase can not be trusted to give accurate base 10 length */
+    char *t    = mpz_get_str(NULL, 10, op);
+    size_t ret = strlen(t);
+    free(t);
+    return ret;
+}
+
+int main(void)
+{
+    mpz_t t;
+    mpz_init(t);
+    size_t i;
+
+    for (i = 0; i <= 110; ++i) {
+        if (i <= 10 || i % 10 == 0) {
+            mpz_left_fac_ui(t, i);
+            gmp_printf("!%u = %Zd\n", i, t);
+        }
+    }
+
+    for (i = 1000; i <= 10000; i += 1000) {
+        mpz_left_fac_ui(t, i);
+        printf("!%u has %u digits\n", i, mpz_digitcount(t));
+    }
+
+    mpz_clear(t);
+    return 0;
+}