p5.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define ARG0 (modes[0] ? p[i+1] : p[p[i+1]])
#define ARG1 (modes[1] ? p[i+2] : p[p[i+2]])

int run_program(int *program, int length, int input) {
    int *p = (int*)calloc(length, sizeof(int));
    memcpy(p, program, sizeof(int)*length);
    int i = 0, output = 0;
    while (p[i] != 99) {
        int modes[4] = { 100,1000,10000,10000};
        int instruction = p[i] % 100;
        for (size_t j = 0; j < 4; j++) modes[j] = (p[i] / modes[j]) % 10;        
        switch (instruction) {
        case 1: p[p[i+3]] = ARG0 + ARG1; i += 4; break;
        case 2: p[p[i+3]] = ARG0 * ARG1; i += 4; break;
        case 7: p[p[i+3]] = (ARG0 < ARG1) ? 1 : 0; i += 4; break;
        case 8: p[p[i+3]] = (ARG0 == ARG1) ? 1 : 0; i += 4; break;
        case 3: p[p[i+1]] = input; i += 2; break;
        case 4: output = ARG0; i += 2; break;
        case 5: i = (ARG0 != 0) ? ARG1 : (i+3); break;
        case 6: i = (ARG0 == 0) ? ARG1 : (i+3); break;
        default: fprintf(stderr, "Invalid opcode %d.\n", p[i]); exit(1);
        }
    }
    free(p);
    return output;
}

int main(int argc, char **argv) {
    int a[1024], i = 0;
    FILE *fp = fopen(argv[1], "r");
    while(fscanf(fp, "%d,", a + i++) > 0);
    fclose(fp);
    i--;

    printf("Pt 1: %d\n", run_program(a, i, 1));
    printf("Pt 2: %d\n", run_program(a, i, 5));

    return 0;
}