Keyboard.jack

// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/12/Keyboard.jack

/**
 * A library for handling user input from the keyboard.
 */
class Keyboard {

	static int KEYBOARD_ADDRESS;
	static int LINE_BUFFER_LENGTH;

    /** Initializes the keyboard. */
    function void init() {
		let KEYBOARD_ADDRESS = 24576;
		let LINE_BUFFER_LENGTH = 64;
		return;
    } 

    /**
     * Returns the character of the currently pressed key on the keyboard;
     * if no key is currently pressed, returns 0.
     *
     * Recognizes all ASCII characters, as well as the following keys:
     * new line = 128 = String.newline()
     * backspace = 129 = String.backspace()
     * left arrow = 130
     * up arrow = 131
     * right arrow = 132
     * down arrow = 133
     * home = 134
     * End = 135
     * page up = 136
     * page down = 137
     * insert = 138
     * delete = 139
     * ESC = 140
     * F1 - F12 = 141 - 152
     */
    function char keyPressed() {
		return Memory.peek(KEYBOARD_ADDRESS);
    }

    /**								
     * Waits until a key is pressed on the keyboard and released,
     * then echoes the key to the screen, and returns the character 
     * of the pressed key.
     */
    function char readChar() {
		var char key;
		while (key = 0) {
			let key = Keyboard.keyPressed();
		}
		while (~(Keyboard.keyPressed() = 0)) {}
		return key;
    }

    /**								
     * Displays the message on the screen, reads from the keyboard the entered
     * text until a newline character is detected, echoes the text to the screen,
     * and returns its value. Also handles user backspaces.
     */
	function String readLine(String message) {
		return Keyboard._readLine(message, false);
	}
	   

    /**								
     * Displays the message on the screen, reads from the keyboard the entered
     * text until a newline character is detected, echoes the text to the screen,
     * and returns its integer value (until the first non-digit character in the
     * entered text is detected). Also handles user backspaces. 
     */
    function int readInt(String message) {
		var String str;
		let str = Keyboard._readLine(message, true);
		while(str.length() = 0){
			let str = Keyboard._readLine(message, true);
		}
		return str.intValue();
    }
	
	function String _readLine(String message, boolean onlyDigits) {
		var char key;
		var String ret;
		let ret = String.new(LINE_BUFFER_LENGTH);
		do Output.printString(message);
		let key = Keyboard.readChar();
		while ((~(key = 128)) & (~(ret.length() > LINE_BUFFER_LENGTH))) { // intro
			if(key = 129) { // backspace
				do Output.backSpace();
				do Output.printChar(32);
				do Output.backSpace();
				do ret.eraseLastChar();
			} else {
				if((~onlyDigits) | ((key > 47) & (key < 58))){
					do ret.appendChar(key);
					do Output.printChar(key);
				} else {
					if((ret.length() = 0) & (key = 45)){
						do ret.appendChar(key);
						do Output.printChar(key);
					}
				}
			}
			let key = Keyboard.readChar();
		}
		do Output.println();
		return ret;
    }   
}