Improved readline function to support <- and ->

- readline function now uses raw terminal input mode (if in
interactive session)
- readline function supports cursor movement to the left and to the
right and inline editing (also unicode support)

README.org

@@ -11,7 +11,7 @@ Simple shell for Unix-like systems written in C, that has a funny name (for germ
 - ~cd~ builtin command (~cd~ without arguments moves you to ~$HOME~)
 - ~exit~ builtin command
 - ~CTRL+C~ stops running command
-- Custom ~readline~ function
+- Custom ~readline~ function with support for cursor moving (<-, ->) and inline editing
 - Run scripts with ~arsh /path/to/script~ or by putting ~#!/usr/bin/env arsh~ at the first line of a script and making it executable
 - Run commands with ~arsh -c 'echo $PATH'~ to launch something quickly with ~arsh~.
 * Build

main.c

@@ -8,6 +8,7 @@
 #include <setjmp.h>
 #include <ctype.h>
 #include <fcntl.h>
+#include <termios.h>
 
 constexpr size_t max_length = 1024;
 constexpr size_t max_env_var_length = 256;
@@ -48,18 +49,10 @@ void* ccalloc(size_t size, size_t n)
 
 static constexpr size_t max_line = 1024;
 static char string_buffer[max_line];
-char* readline(char* print, FILE* stream)
+char* __readline_file(FILE* stream)
 {
-  if(print)
-  {
-    printf("%s", print);
-  }
   if(fgets(string_buffer, max_line, stream) == NULL)
   {
-    if (feof(stdin))
-    {
-        return NULL;   // Ctrl+D
-    }
     if(ferror(stream))
     {
       perror("fgets error");
@@ -78,6 +71,185 @@ char* readline(char* print, FILE* stream)
   return string_buffer;
 }
 
+static struct termios orig_termios;
+
+void disable_raw_mode()
+{
+  tcsetattr(STDIN_FILENO, TCSAFLUSH, &orig_termios);
+}
+
+void enable_raw_mode()
+{
+  if (tcgetattr(STDIN_FILENO, &orig_termios) == -1)
+    exit(1);
+
+  atexit(disable_raw_mode);
+
+  struct termios raw = orig_termios;
+  
+  raw.c_lflag &= ~(ECHO | ICANON);
+  raw.c_iflag &= ~(IXON | ICRNL);
+  raw.c_oflag &= ~(OPOST);
+
+  tcsetattr(STDIN_FILENO, TCSAFLUSH, &raw);
+}
+
+size_t utf8_char_len(unsigned char c) {
+  if ((c & 0x80) == 0) return 1;
+  if ((c & 0xE0) == 0xC0) return 2;
+  if ((c & 0xF0) == 0xE0) return 3;
+  if ((c & 0xF8) == 0xF0) return 4;
+  return 1;
+}
+
+size_t prev_char_start(char* buf, size_t pos) {
+  if (pos == 0) return 0;
+
+  pos--;
+  while (pos > 0 && ((buf[pos] & 0xC0) == 0x80)) {
+    pos--;
+  }
+  return pos;
+}
+
+size_t next_char_start(char* buf, size_t len, size_t pos) {
+  if (pos >= len) return len;
+  return pos + utf8_char_len((unsigned char)buf[pos]);
+}
+
+ssize_t read_utf8_char(int fd, char* out) {
+  unsigned char c;
+  if (read(fd, &c, 1) != 1) return -1;
+
+  size_t len = utf8_char_len(c);
+  out[0] = c;
+
+  for (size_t i = 1; i < len; i++) {
+    if (read(fd, &out[i], 1) != 1)
+      return -1;
+  }
+
+  return len;
+}
+
+size_t utf8_display_width(const char *buf, size_t len)
+{
+  size_t i = 0;
+  size_t width = 0;
+
+  while (i < len) {
+    unsigned char c = buf[i];
+
+    if ((c & 0x80) == 0) {
+      i += 1;
+      width += 1;
+    }
+    else if ((c & 0xE0) == 0xC0) {
+      i += 2;
+      width += 1;
+    }
+    else if ((c & 0xF0) == 0xE0) {
+      i += 3;
+      width += 1;
+    }
+    else {
+      i += 4;
+      width += 1;
+    }
+  }
+
+  return width;
+}
+
+char* __readline_interactive(char* prompt)
+{
+  enable_raw_mode();
+  static char buffer[max_length];
+  size_t cursor = 0;
+  size_t len = 0;
+  ssize_t swrite(int fd, const char* str)
+  {
+    return write(fd, str, strlen(str));
+  }
+  void render_line(char *prompt, char *buf, size_t len, size_t cursor)
+  {
+    swrite(STDOUT_FILENO, "\r");
+    swrite(STDOUT_FILENO, prompt);
+    
+    write(STDOUT_FILENO, buf, len);
+
+    swrite(STDOUT_FILENO, "\x1b[K");
+    size_t prompt_width = utf8_display_width(prompt, strlen(prompt));
+    size_t cell_cursor = utf8_display_width(buf, cursor);
+    char seq[64];
+    snprintf(seq, sizeof(seq), "\r\x1b[%zuC", prompt_width + cell_cursor);
+    swrite(STDOUT_FILENO, seq);
+  }
+
+  render_line(prompt, buffer, len, cursor);
+  while (1) {
+    char utf8[4];
+    ssize_t clen = read_utf8_char(STDIN_FILENO, utf8);
+    if (clen <= 0) continue;
+
+    if (utf8[0] == '\x1b') {
+      char seq[2];
+      if (read(STDIN_FILENO, &seq[0], 1) != 1) continue;
+      if (read(STDIN_FILENO, &seq[1], 1) != 1) continue;
+      
+      if (seq[0] == '[') {
+	switch (seq[1])
+	{
+	  case 'A': break; // Arrow up
+	  case 'B': break; // Arrow down
+	  case 'C': // Arrow right
+	    cursor = next_char_start(buffer, len, cursor);
+	    break;
+	  case 'D': // Arrow left
+	    cursor = prev_char_start(buffer, cursor);
+	    break;
+	}
+      }
+    } else if (utf8[0] == '\r') { // enter
+      buffer[len] = '\0';
+      swrite(STDOUT_FILENO, "\r\n");
+      break;
+    } else if (utf8[0] == 127) { // backspace
+      if (len > 0)
+      {
+        if (cursor > 0) {
+	  size_t prev = prev_char_start(buffer, cursor);
+	  memmove(buffer + prev, buffer + cursor, len - cursor);
+	  len -= (cursor - prev);
+	  cursor = prev;
+	}
+      }
+    }
+    else {
+      if(len < sizeof(buffer))
+      {
+	memmove(buffer + cursor + clen, buffer + cursor, len - cursor);
+	memcpy(buffer + cursor, utf8, clen);
+	cursor += clen;
+	len += clen;
+      }
+    }
+    render_line(prompt, buffer, len, cursor);
+  }
+
+  disable_raw_mode();
+  return buffer;
+}
+
+char* readline(char* prompt, FILE* stream)
+{
+  if (isatty(fileno(stream)))
+  {
+    return __readline_interactive(prompt);
+  }
+  return __readline_file(stream);
+}
+
 /**
  * Struct to represent a command and its arguments.
  */
@@ -737,7 +909,7 @@ char* prettify_pwd(char* pwd)
 {
   if (!pwd)
   {
-    fprintf(stderr, "Internal Error: pwd can't be null");
+    fprintf(stderr, "\nInternal Error: pwd can't be null\n");
     exit(1);
   }
   char* home = get_home();
@@ -817,7 +989,7 @@ char* generate_ps1_prompt()
     size_t len = strlen(data);
     if(j + i - 1 - start + len >= sizeof(prompt_buf))
     {
-      fprintf(stderr, "Out of memory(1) for prompt: %d >= %d", j + i - start + len, sizeof(prompt_buf));
+      fprintf(stderr, "\nOut of memory(1) for prompt: %d >= %d\n", j + i - start + len, sizeof(prompt_buf));
       break;
     }
     size_t start_len = min_size_t(sizeof(prompt_buf) - j, i - start - 1);
@@ -825,7 +997,7 @@ char* generate_ps1_prompt()
     j += start_len;
     if(j + len >= sizeof(prompt_buf))
     {
-      fprintf(stderr, "Out of memory(2) for prompt: %d >= %d", j + len, sizeof(prompt_buf));
+      fprintf(stderr, "\nOut of memory(2) for prompt: %d >= %d\n", j + len, sizeof(prompt_buf));
       break;
     }
     memcpy(prompt_buf + j, data, len);