From 87291b54e54099033a9720e706e811aafb2aba9e Mon Sep 17 00:00:00 2001
From: Mia Herkt <mia@0x0.st>
Date: Mon, 12 Dec 2022 00:08:30 +0100
Subject: [PATCH] 11: C: Get LCM by prime factorization

---
 11/solution.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/11/solution.c b/11/solution.c
index a37272d..6c838da 100644
--- a/11/solution.c
+++ b/11/solution.c
@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <math.h>
 
 typedef struct Monkey {
     unsigned long business;
@@ -20,15 +21,37 @@ int compar(const void *a, const void *b) {
     return 0;
 }
 
+unsigned long calc_lcm(Monkey *monkeys, int count) {
+    unsigned long lcm = 1;
+
+    for (int m = 0; m < count; m++) {
+        unsigned long v = monkeys[m].test;
+
+        while (!(v % 2)) {
+            lcm *= 2;
+            v /= 2;
+        }
+
+        for (unsigned long i = 3; i <= sqrt(v); i += 3) {
+            while (!(v % i)) {
+                lcm *= i;
+                v /= i;
+            }
+        }
+
+        if (v > 2)
+            lcm *= v;
+    }
+
+    return lcm;
+}
+
 unsigned long monkey_business(Monkey *monkeys, int count, int steps, int part2) {
     Monkey *copy = malloc(count * sizeof(Monkey));
     memcpy(copy, monkeys, count * sizeof(Monkey));
     monkeys = copy;
 
-    unsigned long lcm = 1;
-    if (part2)
-        for (int m = 0; m < count; m++)
-            lcm *= monkeys[m].test; // these are all primes
+    unsigned long lcm = part2 ? calc_lcm(monkeys, count) : 0;
 
     for (int s = 0; s < steps; s++) {
         for (int m = 0; m < count; m++) {
@@ -43,8 +66,7 @@ unsigned long monkey_business(Monkey *monkeys, int count, int steps, int part2)
                     case '*': new *= val; break;
                 }
 
-                if (part2) new %= lcm;
-                else new /= 3;
+                new = part2 ? new % lcm : new / 3;
 
                 int target = monkeys[m].true;
                 if (new % monkeys[m].test)