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

static void read_line (char line[], size_t);
static bool backspace (char **pos, char line[]);

int
main (void)
{
  printf ("Shell starting...\n");
  for (;;) 
    {
      char command[80];

      /* Read command. */
      printf ("--");
      read_line (command, sizeof command);
      
      /* Execute command. */
      if (!strcmp (command, "exit"))
        break;
      else if (!memcmp (command, "cd ", 3)) 
        {
          if (!chdir (command + 3))
            printf ("\"%s\": chdir failed\n", command + 3);
        }
      else if (command[0] == '\0') 
        {
          /* Empty command. */
        }
      else
        {
          pid_t pid = exec (command);
          if (pid != PID_ERROR)
            printf ("\"%s\": exit code %d\n", command, wait (pid));
          else
            printf ("exec failed\n");
        }
    }

  printf ("Shell exiting.");
  return EXIT_SUCCESS;
}

/* Reads a line of input from the user into LINE, which has room
   for SIZE bytes.  Handles backspace and Ctrl+U in the ways
   expected by Unix users.  On return, LINE will always be
   null-terminated and will not end in a new-line character. */
static void
read_line (char line[], size_t size) 
{
  char *pos = line;
  for (;;)
    {
      char c;
      read (STDIN_FILENO, &c, 1);

      switch (c) 
        {
        case '\r':
          *pos = '\0';
          putchar ('\n');
          return;

        case '\b':
          backspace (&pos, line);
          break;

        case ('U' - 'A') + 1:       /* Ctrl+U. */
          while (backspace (&pos, line))
            continue;
          break;

        default:
          /* Add character to line. */
          if (pos < line + size - 1) 
            {
              putchar (c);
              *pos++ = c;
            }
          break;
        }
    }
}

/* If *POS is past the beginning of LINE, backs up one character
   position.  Returns true if successful, false if nothing was
   done. */
static bool
backspace (char **pos, char line[]) 
{
  if (*pos > line)
    {
      /* Back up cursor, overwrite character, back up
         again. */
      printf ("\b \b");
      (*pos)--;
      return true;
    }
  else
    return false;
}