Discussion on: Advent of Code 2019 Solution Megathread - Day 2: 1202 Program Alarm

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

typedef unsigned int opcode;

struct program {
    size_t size;
    opcode base[];
};

size_t program_size(size_t n) {
    return sizeof(size_t) + sizeof(opcode) * n;
}

struct program *load_program(char *filename) {
    size_t size = 100;
    struct program *program = (struct program *)malloc(program_size(size));
    size_t pos = 0;
    opcode cur = 0;
    FILE *f = fopen(filename, "rt");
    for (;;) {
        int c = fgetc(f);
        if (c == EOF || c == ',') {
            if (pos == size) {
                size *= 2;
                program = (struct program *)realloc(program, program_size(size));
            }
            program->base[pos++] = cur;
            cur = 0;
        } else if ('0' <= c && c <= '9') {
            cur = (cur * 10) + (c - '0');
        }
        if (c == EOF) break;
    }
    fclose(f);
    program->size = pos;
    return program;
}

struct program *copy_program(struct program *program) {
    size_t size = program_size(program->size);
    struct program *copy = (struct program *)malloc(size);
    memcpy(copy, program, size);
    return copy;
}

int read_program(struct program *program, size_t pos, opcode *x) {
    if (pos >= program->size) {
        return 1;
    }
    *x = program->base[pos];
    return 0;
}

int write_program(struct program *program, size_t pos, opcode value) {
    if (pos >= program->size) {
        return 1;
    }
    program->base[pos] = value;
    return 0;
}

void print_program(struct program *program) {
    printf("%u", program->base[0]);
    for (int i = 1; i < program->size; ++i) {
        printf(",%u", program->base[i]);
    }
    printf("\n");
}

int run_program(struct program *program, int debug) {
    size_t pc = 0;
    opcode i;
    while (read_program(program, pc, &i) == 0 && i != 99) {
        opcode x, y, z;
        if (read_program(program, pc+1, &x) != 0) return 1;
        if (read_program(program, x,    &x) != 0) return 1;
        if (read_program(program, pc+2, &y) != 0) return 1;
        if (read_program(program, y,    &y) != 0) return 1;
        if (read_program(program, pc+3, &z) != 0) return 1;
        if (debug) printf("pc=%u i=%u x=%u y=%u z=%u\n", pc, i, x, y, z);
        switch (i) {
            case 1: 
                if (write_program(program, z, x + y) != 0) return 1;
                break;
            case 2:
                if (write_program(program, z, x * y) != 0) return 1;
                break;
            default:
                return 1;
        }
        if (debug) print_program(program);
        pc += 4;
    }
    return 0;
}

void modify_program(struct program *program) {
    program->base[1] = 12;
    program->base[2] = 2;
}

void part1(struct program *original) {
    printf("Part 1\n");
    struct program *program = copy_program(original);
    modify_program(program);
    run_program(program, 0);
    print_program(program);
    free(program);  
}

void part2(struct program *original) {
    opcode noun = 0;
    opcode verb = 0;
    int result = 0;
    while (noun < 100 && verb < 100) {
        struct program *program = copy_program(original);
        write_program(program, 1, noun);
        write_program(program, 2, verb);
        int result = run_program(program, 0) == 0 && program->base[0] == 19690720;
        free(program);

        if (result) {
            break;
        }

        if (++noun > 99) {
            noun = 0;
            ++verb;
        }
    }
    printf("Part 2\nnoun = %u\nverb = %u\nresult = %u\n", noun, verb, 100*noun+verb);
}

static struct program test_program = {
    .size = 12,
    .base = { 1,9,10,3,2,3,11,0,99,30,40,50 }
};

int main(int argc, char **argv) {
    struct program *program = (argc < 2) ? load_program("input.txt") : &test_program;
    part1(program);
    part2(program);

    if (program != &test_program) {
        free(program);
    }
    return 0;
}