advent2025

commit 3a18add1a8675056d1ce9cf706876a5e17f3c2ba
parent 60bfcfb33293320a139d24e57b6c4867784b6e09
Author: bsandro <email@bsandro.tech>
Date:   Tue,  2 Dec 2025 21:22:56 +0200

day02 p2 alternative - no strings comparison, pure numerical solution

Diffstat:
M
day02.c
 | 
23
+++++++++++++++--------

1 file changed, 15 insertions(+), 8 deletions(-)
diff --git a/day02.c b/day02.c
@@ -5,6 +5,9 @@
 #include <stdbool.h>
 #include "cputime.h"
 
+static const uint64_t pow10[] = {1, 10, 100, 1000, 10000, 100000, 1000000, 10000000,
+                    100000000, 1000000000, 10000000000, 100000000000, 1000000000000};
+
 typedef struct Range {
     uint64_t first;
     uint64_t last;
@@ -41,16 +44,20 @@ bool is_silly1(uint64_t num) {
 }
 
 bool is_silly2(uint64_t num) {
-    int d = digits(num);
-    char s[16];
-    itoa(num, s);
-    for (int i=1;i<=d/2;++i) { // checking pattern 0-i
+    unsigned int d = digits(num);
+    for (unsigned int i=1;i<=d/2;++i) { // checking pattern 0-i
         if (d%i!=0) continue;
-        int rep = 0;
-        for (int j=0;j<d;j+=i) {
-            rep |= strncmp(s, s+j, i);
+        uint64_t stride = pow10[i];
+        uint64_t block = num%stride; //first block (right)
+        bool funny = true;
+        for (unsigned int j=0;j<d/i;++j) {
+            uint64_t jj = pow10[i*j];
+            if ((num/jj)%stride!=block) {
+                funny = false;
+                break;
+            }
         }
-        if (rep==0) return true;
+        if (funny) return true;
     }
     return false;
 }