2018/Day16/Day16B.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <inttypes.h>
#include <assert.h>

typedef struct {
    uint8_t opcode;
    uint32_t a, b, c;
} Instruction;

typedef struct {
    uint32_t before[4];
    Instruction instruction;
    uint32_t after[4];
} Sample;

enum {
    ADDR, ADDI, MULR, MULI, BANR, BANI, BORR, BORI,
    SETR, SETI, GTIR, GTRI, GTRR, EQIR, EQRI, EQRR,
};

#ifdef DEBUG
const char *names[] = {
    "addr", "addi", "mulr", "muli", "banr", "bani", "borr", "bori",
    "setr", "seti", "gtir", "gtri", "gtrr", "eqir", "eqri", "eqrr",
};
#endif

int opcodes[16];

int sample_read(Sample *s, FILE *file) {
    int read = 0;

    read = fscanf(file, "Before: [%u, %u, %u, %u]\n",
                  &s->before[0], &s->before[1], &s->before[2], &s->before[3]);
    if (read != 4) return 0;

    Instruction *i = &s->instruction;
    read = fscanf(file, "%" SCNu8 " %u %u %u\n", &i->opcode, &i->a, &i->b, &i->c);
    if (read != 4) return 0;

    read = fscanf(file, "After: [%u, %u, %u, %u]\n\n",
                  &s->after[0], &s->after[1], &s->after[2], &s->after[3]);
    if (read != 4) return 0;

    return 1;
}

uint16_t sample_possible_opcodes(const Sample s) {
    uint16_t possible_opcodes = 0;

    Instruction i = s.instruction;

    // Addition
    if (s.after[i.c] == s.before[i.a] + s.before[i.b])    possible_opcodes |= 1 << ADDR;
    if (s.after[i.c] == s.before[i.a] + i.b)              possible_opcodes |= 1 << ADDI;

    // Multiplication
    if (s.after[i.c] == s.before[i.a] * s.before[i.b])    possible_opcodes |= 1 << MULR;
    if (s.after[i.c] == s.before[i.a] * i.b)              possible_opcodes |= 1 << MULI;

    // Bitwise AND
    if (s.after[i.c] == (s.before[i.a] & s.before[i.b]))  possible_opcodes |= 1 << BANR;
    if (s.after[i.c] == (s.before[i.a] & i.b))            possible_opcodes |= 1 << BANI;

    // Bitwise OR
    if (s.after[i.c] == (s.before[i.a] | s.before[i.b]))  possible_opcodes |= 1 << BORR;
    if (s.after[i.c] == (s.before[i.a] | i.b))            possible_opcodes |= 1 << BORI;

    // Assignment
    if (s.after[i.c] == s.before[i.a])                    possible_opcodes |= 1 << SETR;
    if (s.after[i.c] == i.a)                              possible_opcodes |= 1 << SETI;

    // Greater-than testing
    if (s.after[i.c] == (i.a > s.before[i.b]))            possible_opcodes |= 1 << GTIR;
    if (s.after[i.c] == (s.before[i.a] > i.b))            possible_opcodes |= 1 << GTRI;
    if (s.after[i.c] == (s.before[i.a] > s.before[i.b]))  possible_opcodes |= 1 << GTRR;

    // Equality testing
    if (s.after[i.c] == (i.a == s.before[i.b]))           possible_opcodes |= 1 << EQIR;
    if (s.after[i.c] == (s.before[i.a] == i.b))           possible_opcodes |= 1 << EQRI;
    if (s.after[i.c] == (s.before[i.a] == s.before[i.b])) possible_opcodes |= 1 << EQRR;

    return possible_opcodes;
}

int bits_set(uint16_t n) {
    int bit = -1;
    for (int i = 0; i < 16; i++) {
        if ((n >> i) & 1) {
            if (bit != -1) return -1;
            bit = i;
        }
    }
    return bit;
}

void find_opcodes(FILE *file) {
    int unknown = 16;
    uint16_t table[16] = {
        0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
        0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
    };

    Sample sample;
    while (sample_read(&sample, file)) {
        uint16_t possible_opcodes = sample_possible_opcodes(sample);
        table[sample.instruction.opcode] &= possible_opcodes;
    }

    while (unknown > 0) {
        for (int opcode = 0; opcode < 16; opcode++) {
            int bit = bits_set(table[opcode]);
            if (bit == -1) continue;

            // Set every row in this column to 0
            for (int j = 0; j < 16; j++)
                table[j] &= ~(1 << bit);

            opcodes[opcode] = bit;
            unknown -= 1;
        }
    }

#ifdef DEBUG
    for (int i = 0; i < 16; i++)
        printf("%d: %s\n", i, names[opcodes[i]]);
#endif
}

int instruction_read(Instruction *i, FILE *file) {
    return fscanf(file, "%" SCNu8 " %u %u %u\n",
                  &i->opcode, &i->a, &i->b, &i->c) == 4;
}

void instruction_execute(const Instruction *i, uint32_t *r) {
    switch (opcodes[i->opcode]) {
    // Addition
    case ADDR: r[i->c] = r[i->a] + r[i->b];    break;
    case ADDI: r[i->c] = r[i->a] + i->b;       break;

    // Multiplication
    case MULR: r[i->c] = r[i->a] * r[i->b];    break;
    case MULI: r[i->c] = r[i->a] * i->b;       break;

    // Bitwise AND
    case BANR: r[i->c] = (r[i->a] & r[i->b]);  break;
    case BANI: r[i->c] = (r[i->a] & i->b);     break;

    // Bitwise OR
    case BORR: r[i->c] = (r[i->a] | r[i->b]);  break;
    case BORI: r[i->c] = (r[i->a] | i->b);     break;

    // Assignment
    case SETR: r[i->c] = r[i->a];              break;
    case SETI: r[i->c] = i->a;                 break;

    // Greater-than testing
    case GTIR: r[i->c] = (i->a > r[i->b]);     break;
    case GTRI: r[i->c] = (r[i->a] > i->b);     break;
    case GTRR: r[i->c] = (r[i->a] > r[i->b]);  break;

    // Equality testing
    case EQIR: r[i->c] = (i->a == r[i->b]);    break;
    case EQRI: r[i->c] = (r[i->a] == i->b);    break;
    case EQRR: r[i->c] = (r[i->a] == r[i->b]); break;

    default: assert(0);
    }
}

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("Usage: %s [input]\n", argv[0]);
        return EXIT_FAILURE;
    }

    FILE *file = fopen(argv[1], "r");
    find_opcodes(file);

    uint32_t registers[4] = {0};
    Instruction instruction = {0};
    while (instruction_read(&instruction, file)) {
#ifdef DEBUG
        const char *name = names[opcodes[instruction.opcode]];
        printf("opcode: %2d (%s), a: %d, b: %d, c: %d\n", instruction.opcode,
               name, instruction.a, instruction.b, instruction.c);
#endif

        instruction_execute(&instruction, registers);

#ifdef DEBUG
        printf("registers: %d %d %d %d\n", registers[0], registers[1],
               registers[2], registers[3]);
#endif
    }

    fclose(file);

    printf("%d\n", registers[0]);
    return EXIT_SUCCESS;
}