Advent of Code 2024 Day 3 in C (custom parser)

day03.c

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

#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <unistd.h>

enum state_type {
  Skip, SawM, SawU, SAW_L, SawD, SawO, SawN, SawT,
  SawComma, SawQuote,
  SawDigitA, SawDigitB,
  SawParenMul, SawParenDo,  SawParenDont
};

int main(int argc, char* argv[]) {
  if (argc == 1) {
    exit(1);
  }
  int fd = open(argv[1], O_RDONLY);
  if (fd == -1) {
    exit(1);
  }
  struct stat s;
  if (fstat(fd, &s) == -1) {
    exit(1);
  }

  char const* addr = (char const*)mmap(nullptr, (size_t)s.st_size, PROT_READ, MAP_PRIVATE | MAP_FILE, fd, 0);
  if (addr == nullptr) {
    exit(1);
  }
  char const* end = addr + s.st_size;

  int a = 0, b = 0, part1 = 0, part2 = 0, valid = 1;
  enum state_type state = Skip;

  for (char const* c = addr; c != end; ++c) {
    switch(*c) {
      case 'm': state = SawM; continue;
      case 'u': if (state == SawM) state = SawU; continue;
      case 'l': if (state == SawU) state = SAW_L; continue;
      case 'd': state = SawD; continue;
      case 'o': if (state == SawD) state = SawO; continue;
      case 'n': if (state == SawO) state = SawN; continue;
      case '\'': if (state == SawN) state = SawQuote; continue;
      case 't': if (state == SawQuote) state = SawT; continue;
      case ',': if (state == SawDigitA) state = SawComma; continue;
      case '(':
        switch (state) {
          case SAW_L: state = SawParenMul; a = 0; b = 0; continue;
          case SawO: state = SawParenDo; continue;
          case SawT: state = SawParenDont; continue;
          default: break;
        }
        break;
      case ')':
        switch (state) {
          case SawDigitB: part1 += a * b; if (valid) part2 += a * b; break;
          case SawParenDo: valid = true; break;
          case SawParenDont: valid = false; break;
          default: break;
        }
        break;
      default:
        if ('0' <= *c && *c <= '9') {
          switch (state) {
            case SawParenMul: state = SawDigitA; [[fallthrough]];
            case SawDigitA: a = a * 10 + (*c - '0'); continue;
            case SawComma: state = SawDigitB; [[fallthrough]];
            case SawDigitB: b = b * 10 + (*c - '0'); continue;
            default: break;
          }
        }
    }
    state = Skip;
  }

  munmap((void*)addr, (size_t)s.st_size);
  close(fd);
  char buffer[256];
  write(0, buffer, (size_t)snprintf(buffer, 255, "%d %d\n", part1, part2));
}