aoc_omni/c/2020/8/main.c

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

#include "../lib/util.h"

#define NUM_INSTRUCTIONS 654

typedef enum InstructionType {
  NOP,
  ACC,
  JMP
} InstructionType;

typedef struct Instruction {
  InstructionType type;
  int argument;
} Instruction;

typedef struct Console {
  size_t pc;
  int acc;
  Instruction program[NUM_INSTRUCTIONS];
  bool visited_instructions[NUM_INSTRUCTIONS];
} Console;

void parse_instruction(char *text, Instruction *i) {
  switch (*text) {
  case 'n':
    i->type = NOP;
    break;
  case 'a':
    i->type = ACC;
    break;
  case 'j':
    i->type = JMP;
    break;
  default:
    fprintf(stderr, "FATAL: Illegal instruction %s\n", text);
    exit(1);
  }

  i->argument = atoi(text + 4);
}

void console_step(Console *console) {
  Instruction i = console->program[console->pc];
  switch(i.type) {
  case NOP:
    console->pc++;
    break;
  case ACC:
    console->pc++;
    console->acc += i.argument;
    break;
  case JMP:
    console->pc += i.argument;
    break;
  }
}

void console_print(Console *console) {
  printf("PC: %d, acc: %d\n", console->pc, console->acc);
}

void console_reset(Console *console) {
  console->acc = 0;
  console->pc = 0;
}

void console_run_until_loop(Console *console) {
  for (int i = 0; i < NUM_INSTRUCTIONS; i++) {
    console->visited_instructions[i] = false;
  }

  while (!console->visited_instructions[console->pc]) {
    console->visited_instructions[console->pc] = true;
    console_step(console);
  }
}

// true = overflow
bool console_run_until_loop_or_overflow(Console *console) {
  for (int i = 0; i < NUM_INSTRUCTIONS; i++) {
    console->visited_instructions[i] = false;
  }

  while (!console->visited_instructions[console->pc]) {
    console->visited_instructions[console->pc] = true;
    console_step(console);
    if (console->pc >= NUM_INSTRUCTIONS) return true;
  }

  return false;
}

bool console_is_current_instruction_corrupted(Console *console) {
  Instruction i = console->program[console->pc];
  printf("%d\n", i.type);
  fflush(stdout);
  switch(i.type) {
  case NOP:
    return console->pc + i.argument > NUM_INSTRUCTIONS - 7;
  case ACC:
    return false;
  case JMP:
    return console->pc == NUM_INSTRUCTIONS || console->pc == 648;
  }
}

void console_run_until_corruption(Console *console) {
  int corruption_index = 0;
  for (int i = 0; i < NUM_INSTRUCTIONS; i++) {
    Instruction *inst = &(console->program[i]);
    if (inst->type == ACC) continue;
    if (inst->type == NOP) {
      inst->type = JMP;
      if (console_run_until_loop_or_overflow(console)) return;
      inst->type = NOP;
      console_reset(console);
    } else {
      inst->type = NOP;
      if (console_run_until_loop_or_overflow(console)) return;
      inst->type = JMP;
      console_reset(console);
    }
  }
}

int part_1() {
  Console *console = malloc(sizeof(Console));
  console_reset(console);
  char *data_buffer = load_input();

  char *instruction_text = strtok(data_buffer, "\n");

  do {
    Instruction *inst = &(console->program[console->pc++]);
    parse_instruction(instruction_text, inst);
  } while (instruction_text = strtok(NULL, "\n"));

  console_reset(console);
  console_run_until_loop(console);

  int answer = console->acc;
  free(console);
  free(data_buffer);
  return answer;
}

int part_2() {
  Console *console = malloc(sizeof(Console));
  console_reset(console);
  char *data_buffer = load_input();

  char *instruction_text = strtok(data_buffer, "\n");

  console_reset(console);
  do {
    Instruction *inst = &(console->program[console->pc++]);
    parse_instruction(instruction_text, inst);
  } while (instruction_text = strtok(NULL, "\n"));

  console_reset(console);
  console_run_until_corruption(console);

  int answer = console->acc;
  free(console);
  free(data_buffer);
  return answer;
}

int main() {
  printf("Part 1: %d\n", part_1());
  printf("Part 2: %d\n", part_2());
}