#include <dirent.h>
#include <libgen.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

char *__progname;

void usage() {
  printf("Usage: %s [COMMAND]\n", __progname);
  exit(1);
}

int main(int argc, char **argv) {
  char *path, *seg, *__path;
  const char sep[2] = ":";

  __progname = basename(argv[0]);
  if (argc < 2) {
    fprintf(stderr, "%s: Error: missing argument\n", __progname);
    usage();
  } else if (argc > 2) {
    fprintf(stderr, "%s: Error: missing argument\n", __progname);
    usage();
  }
  path = getenv("PATH");
  if (path == NULL) {
    path = "/usr/bin:/bin";
  }
  // Save a copy for later use, since strtok will modify the original
  __path = strndup(path, strlen(path) + 1);
  seg = strtok(path, sep);
  while (seg != NULL) {
    struct dirent *dp;
    DIR *dir = opendir(seg);
    if (dir) {
      while ((dp = readdir(dir)) != NULL) {
        if (strcmp(argv[1], dp->d_name) == 0) {
          char *p = malloc(strlen(argv[1] + strlen(seg) + 2));
          (void)strncpy(p, seg, strlen(seg) + 1);
          (void)strncat(p, "/", 2);
          (void)strncat(p, argv[1], strlen(argv[1]) + 1);
          struct stat path_stat;
          stat(p, &path_stat);
          if (access(p, X_OK) == 0 && S_ISREG(path_stat.st_mode)) {
            printf("%s\n", p);
            exit(0);
          }
        }
      }
    }
    seg = strtok(NULL, sep);
  }
  printf("%s: no %s in (%s)\n", __progname, argv[1], __path);
}