#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>

static char** split_command_into_argv(const char* cmd)
{
    size_t argc = 1;
    char* str = strdup(cmd);
    char** arr = calloc(sizeof(char*), argc);

    char* segment = strtok(str, " \n");
    arr[argc - 1] = segment;
    if (segment == NULL) return arr;

    argc++;
    arr = realloc(arr, sizeof(char*) * argc);

    for (;;)
    {
        char* segment = strtok(NULL, " \n");
        arr[argc - 1] = segment;

        if (segment == NULL) break;

        argc++;
        arr = realloc(arr, sizeof(char*) * argc);
    }

    return arr;
}

static char* join_path(const char* str1, const char* str2)
{
    char* buf = malloc(strlen(str1) + strlen(str2) + 1);
    strlcpy(buf, str1, strlen(str1) + 1);
    strncat(buf, str2, strlen(str2));
    return buf;
}

static int execute_in_path(const char* dir, char** argv)
{
    char* path = join_path(dir, argv[0]);
    int err = errno;

    int status = execv(path, argv);
    free(path);

    if (errno == ENOENT)
    {
        errno = err;
        return 0;
    }

    return status;
}

static int sh_execvp(char** argv)
{
    if (strchr(argv[0], '/'))
    {
        // Relative paths do not need path resolution.
        return execv(argv[0], argv);
    }

    if (execute_in_path("/bin/", argv) != 0) return -1;
    if (execute_in_path("/sbin/", argv) != 0) return -1;
    if (execute_in_path("/usr/bin/", argv) != 0) return -1;
    if (execute_in_path("/usr/local/bin/", argv) != 0) return -1;

    errno = ENOENT;
    return -1;
}

int main()
{
    while (1)
    {
        fputs("sh$ ", stdout);

        char command[4096];
        char* rc = fgets(command, sizeof(command), stdin);
        if (!rc) return 0;

        if (strspn(command, " \n") == strlen(command)) continue;

        pid_t child = fork();
        if (child < 0)
        {
            perror("fork");
            return 1;
        }

        if (child == 0)
        {
            char** argv = split_command_into_argv(command);
            if (argv[0] == NULL) return 0;
            sh_execvp(argv);
            perror(argv[0]);
            return 1;
        }

        if (waitpid(child, NULL, 0) < 0)
        {
            perror("waitpid");
            return 1;
        }
    }
}