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#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
#define BUF_SIZE 2000
char buf[BUF_SIZE];
int input[200];
int length;
void read_input() {
int fd = open("input.txt", O_RDONLY);
ssize_t bytes_read = read(fd, buf, BUF_SIZE-1);
if (bytes_read <= 0) {
printf("Error reading input\n");
exit(1);
}
buf[bytes_read] = '\0';
}
void parse_input() {
int offset = 0;
int done = 0;
int buf_offset = 0;
int i = 0;
char current[10];
while (!done) {
int current_offset = 0;
while (buf[buf_offset] != '\n' && buf[buf_offset] != '\0') {
current[current_offset++] = buf[buf_offset++];
}
if (buf[buf_offset] == '\0') {
done = 1;
} else {
buf_offset++;
current[current_offset] = '\0';
input[i++] = atoi(current);
length = i;
}
}
}
int int_less(const void *a, const void *b) {
return *(int *)a - *(int *)b;
}
void sort_input() {
qsort(input, length, sizeof(int), int_less);
}
int search_2020() {
int small_pointer = 0;
int large_pointer = length - 1;
int sum = input[small_pointer] + input[large_pointer];
while (sum != 2020 && small_pointer != large_pointer) {
if (sum > 2020) {
large_pointer--;
} else {
small_pointer++;
}
sum = input[small_pointer] + input[large_pointer];
}
return input[small_pointer] * input[large_pointer];
}
void print_input() {
for (int i = 0; i < length; i++) {
printf("%d\n", input[i]);
}
}
int main() {
read_input();
parse_input();
sort_input();
printf("%d\n", search_2020());
}
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