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day15_part2.c
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day15_part2.c
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#include <stdio.h>
#include <stdlib.h>
/* A just-for-fun rewrite of day 15 part 2 in C. On my machine, the python
* solution takes ~8000ms, while this solution takes ~600ms with default gcc
* options and ~430ms with -O3.
*/
int main() {
int numberList[] = {1,0,15,2,10,13}; // problem input
int *numberHistory;
int lastSaidNumberIndex = 5;
int lastSaidNumber = numberList[5];
numberHistory = (int *) malloc(30000000*sizeof(int));
for (int i = 0; i <= 29999999; i++) {
numberHistory[i] = -1;
}
for (int i = 0; i <= 5; i++) {
numberHistory[numberList[i]] = i;
}
while(1) {
int previousLastSaidNumberIndex = numberHistory[lastSaidNumber];
int newNumber;
if (previousLastSaidNumberIndex == -1) {
newNumber = 0;
} else {
newNumber = lastSaidNumberIndex - previousLastSaidNumberIndex;
}
numberHistory[lastSaidNumber] = lastSaidNumberIndex;
lastSaidNumber = newNumber;
lastSaidNumberIndex++;
if (lastSaidNumberIndex == (30000000-1)) {
printf("Solution: %d",lastSaidNumber);
break;
}
}
return 0;
}