advent-of-code/2018/11/day11_2.c

87 lines
2.3 KiB
C

#include <stdio.h>
#include <stdlib.h>
#define SERIAL 7347
#define DIMENSIONS 300
int initialFieldValue(int x, int y) {
int id = x + 10;
int power = id * y + SERIAL;
power *= id;
char itoa [10];
int len = sprintf(itoa, "%d", power);
if (len < 3) {
return 0;
}
// manually set a nullbyte after the hundreds digit
itoa[len-2] = 0;
int parsed = (int) strtol(&itoa[len-3], NULL, 10);
return parsed - 5;
}
void setFields(int fields[DIMENSIONS][DIMENSIONS]) {
for (int i=0; i<DIMENSIONS; i++) {
for (int j=0; j<DIMENSIONS; j++) {
fields[i][j] = initialFieldValue(i+1, j+1);
}
}
}
// https://en.wikipedia.org/wiki/Summed-area_table
void setSummedArea(int sums[DIMENSIONS][DIMENSIONS], int input[DIMENSIONS][DIMENSIONS]) {
sums[0][0] = input[0][0];
// Topmost row
for (int x=1; x<DIMENSIONS; x++) {
int y = 0;
sums[x][y] = input[x][y] + sums[x-1][y];
}
// Leftmost column
for (int y=1; y<DIMENSIONS; y++) {
int x = 0;
sums[x][y] = input[x][y] + sums[x][y-1];
}
for (int x=1; x<DIMENSIONS; x++) {
for (int y=1; y<DIMENSIONS; y++) {
sums[x][y] = input[x][y] + sums[x][y-1] + sums[x-1][y] - sums[x-1][y-1];
}
}
}
int max(int a, int b) {
return a > b ? a : b;
}
int calculateNeighborhood(int fields[DIMENSIONS][DIMENSIONS], int x, int y, int size) {
return fields[x-1][y-1] + fields[x+size][y+size] - fields[x-1][y+size] - fields[x+size][y-1];
}
int main() {
int fields[DIMENSIONS][DIMENSIONS];
setFields(fields);
int summedAreaFields[DIMENSIONS][DIMENSIONS];
setSummedArea(summedAreaFields, fields);
// Part 1
int maxPower = -999999999;
int maxX, maxY;
int maxSize = 0;
for (int x=1; x<DIMENSIONS-3; x++) {
for (int y=1; y<DIMENSIONS-3; y++) {
for (int size=3; size<DIMENSIONS-max(x, y); size++) {
int moore = calculateNeighborhood(summedAreaFields, x, y, size);
if (moore > maxPower) {
maxPower = moore;
maxX = x + 1;
maxY = y + 1;
maxSize = size + 1;
}
}
}
}
printf("x: %d, y: %d, size: %d, power: %d\n", maxX, maxY, maxSize, maxPower);
return 0;
}