adventofcode2024/day17/c/day17.c

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

#define CHARS_MAX 128
#define MAX_INSTRUCTIONS 128

#define BIT_JUMPED 0

enum {
    A,
    B,
    C,
    REGISTERS_LEN
};

enum {
    ADV,
    BXL,
    BST,
    JNZ,
    BXC,
    OUT,
    BDV,
    CDV
};

void execute(uint64_t registers[REGISTERS_LEN], uint8_t *instructions, uint32_t *pc, uint8_t *status, char *output_buffer, int *buf_len);
uint64_t combo(uint64_t registers[REGISTERS_LEN], uint8_t operand);
uint64_t revert(uint8_t *instructions_inverted, uint8_t *instructions, uint32_t instruction_count, uint64_t current_A, uint32_t i, int print_register);

int main() {
    char *p, *buf, c;

    buf = calloc(CHARS_MAX, sizeof(char));
    p = buf;

    uint64_t registers[3];
    int reading_register = 0, operand = 0;
    uint8_t *instructions = calloc(MAX_INSTRUCTIONS, sizeof(instructions[0]));
    uint32_t instruction_count = 0;

    while ((c = getchar()) != EOF) {
        if (reading_register < 3) {
            *p++ = c;
            if (c != '\n') {
                continue;
            }

            p = buf;
            while(*p++ != ':');
            p++;
            sscanf(p, "%li", &registers[reading_register++]);

            memset(buf, 0, CHARS_MAX);
            p = buf;
        } else {
            if (c < 48 || c > 57) continue;
            uint8_t num = c - 48;
            instructions[instruction_count++] = num;
        }
    }

    uint32_t pc = 0;
    uint8_t status = 0;
    int buf_len = 0;
    while (pc < instruction_count) {
        execute(registers, instructions, &pc, &status, buf, &buf_len);
        if (!(status & (1 << BIT_JUMPED))) {
            pc += 2;
        }
    }

    buf[buf_len - 1] = '\0';
    printf("%s\n", buf);

    // Find print register
    int print_register = A;
    for (uint32_t i = 0; i < instruction_count; i+=2) {
        if (instructions[i] == OUT) {
            print_register = instructions[i+1] - 4;
            break;
        }
    }

    // Invert instruction list
    uint8_t *instructions_inverted = calloc(instruction_count, sizeof(instructions[0]));
    for (uint32_t i = 0; i < instruction_count; i++) {
        instructions_inverted[i] = instructions[instruction_count - i - 1];
    }
    
    printf("%lu\n", revert(instructions_inverted, instructions, instruction_count, 0, 0, print_register));


    free(buf);
    free(instructions);
    free(instructions_inverted);
}

uint64_t revert(uint8_t *instructions_inverted, uint8_t *instructions, uint32_t instruction_count, uint64_t current_A, uint32_t i, int print_register) {
    if (i == instruction_count) {
        return current_A >> 3;
    }

    // Try each 3-bit LSB of A
    for (uint32_t j = 0; j < 8; j++) {
        uint64_t next_A = current_A + j;
        uint64_t registers_copy[3] = {next_A, 0, 0};
        uint32_t pc = 0;
        uint8_t status = 0;
        for (pc = 0; pc < instruction_count - 4; pc+=2) {
            execute(registers_copy, instructions, &pc, &status, NULL, NULL);
        }
        // Check print register
        uint64_t expected_output = instructions_inverted[i];
        if (registers_copy[print_register] % 8 == expected_output) {
            uint64_t a = revert(instructions_inverted, instructions, instruction_count, next_A << 3, i + 1, print_register);
            if (a) {
                return a;
            }
        }
    }

    return 0;
}

void execute(uint64_t registers[REGISTERS_LEN], uint8_t *instructions, uint32_t *pc, uint8_t *status, char *output_buffer, int *buf_len) {
    uint8_t opcode = instructions[*pc];
    uint8_t operand = instructions[(*pc) + 1];
    uint8_t current_status = *status;
    uint8_t next_status = 0;

    switch (opcode) {
        case ADV:
            registers[A] = registers[A] / (1 << combo(registers, operand));
            break;
        case BXL:
            registers[B] = registers[B] ^ operand;
            break;
        case BST:
            registers[B] = combo(registers, operand) % 8;
            break;
        case JNZ:
            if (registers[A] != 0) {
                *pc = operand;
                next_status |= 1 << BIT_JUMPED;
            }
            break;
        case BXC:
            registers[B] = registers[B] ^ registers[C];
            break;
        case OUT:
            output_buffer[(*buf_len)++] = ((uint8_t)combo(registers, operand) & 7) + 48;
            output_buffer[(*buf_len)++] = ',';
            break;
        case BDV:
            registers[B] = registers[A] / combo(registers, operand);
            break;
        case CDV:
            registers[C] = registers[A] / (1 << combo(registers, operand));
            break;
    }

    *status = next_status;
}

uint64_t combo(uint64_t registers[REGISTERS_LEN], uint8_t operand) {
    if (operand < 4) {
        return operand;
    }

    if (operand == 7) {
        printf("INVALID PROGRAM\n");
        return 0;
    }

    return registers[operand - 4];
}