CS327-Discrete-Structures-II/Desk-Calculator/Evaluator.java

package evaluator;

import java.io.InputStream;
import java.util.LinkedList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Scanner;

/**
 * Simulate a PDA to evaluate a series of postfix expressions provided by a
 * lexer. The constructor argument is the lexer of type Lexer. A single line is
 * evaluated and its value is printed. Expression values can also be assigned to
 * variables for later use. If no variable is explicitly assigned, then the
 * default variable "it" is assigned the value of the most recently evaluated
 * expression.
 *
 * @author YOU NAME HERE
 */
public class Evaluator {

	/**
	 * Run the desk calculator.
	 */
	public static void main(String[] args) {
		Evaluator evaluator = new Evaluator(new Lexer(System.in));
		evaluator.run();
	}

	private Lexer lexer; // providing a stream of tokens
	private LinkedList<Double> stack; // operands
	private HashMap<String, Double> symbols; // symbol table for variables
	private String target; // variable assigned the latest expression value

	public Evaluator(Lexer lexer) {
		this.lexer = lexer;
		stack = new LinkedList<>();
		symbols = new HashMap<>();
		target = "it";
	}

	/**
	 * Evaluate a single line of input, which should be a complete expression
	 * optionally assigned to a variable; if no variable is assigned to, then the
	 * result is assigned to "it". In any case, return the value of the expression,
	 * or "no value" if there was some sort of error.
	 */
	public Double evaluate() {

		stack.clear();
		target = "it";

		int q = 1;

		while (q != 4) {
			int token = lexer.nextToken();

			if (lexer.getText().equals("exit")) {
				System.out.println("Bye");
				System.exit(0);
			}

			switch (q) {
				case 1:
					switch (token) {
						case Lexer.NUMBER -> {
							stack.push(Double.parseDouble(lexer.getText()));
							q = 3;
						}
						case Lexer.VARIABLE -> {
							target = lexer.getText();
							stack.push(symbols.getOrDefault(target, 0.0));
							q = 2;
						}
						default -> error("Invald token encountered in state q1");
					}
					break;
				case 2:
					switch (token) {
						case Lexer.ASSIGN_OP -> {
							stack.pop();
							q = 3;
						}
						case Lexer.MINUS_OP -> {
							target = "it";
							stack.push(-1 * stack.pop());
							q = 3;
						}
						case Lexer.NUMBER -> {
							target = "it";
							stack.push(Double.parseDouble(lexer.getText()));
							q = 3;
						}
						case Lexer.VARIABLE -> {
							target = "it";
							stack.push(symbols.getOrDefault(lexer.getText(), 0.0));
							q = 3;
						}
						case Lexer.EOL -> {
							target = "it";
							symbols.put(target, stack.pop());
							q = 4;
						}
						default -> error("Invald token encountered in state q2");
					}
					break;
				case 3:
					switch (token) {
						case Lexer.MINUS_OP -> stack.push(-1 * stack.pop());
						case Lexer.NUMBER -> stack.push(Double.parseDouble(lexer.getText()));
						case Lexer.VARIABLE -> stack.push(symbols.getOrDefault(lexer.getText(), 0.0));
						case Lexer.ADD_OP -> {
							double a1 = stack.pop();
							double a2 = stack.pop();
							stack.push(a2 + a1);
						}
						case Lexer.SUBTRACT_OP -> {
							double s1 = stack.pop();
							double s2 = stack.pop();
							stack.push(s2 - s1);
						}
						case Lexer.MULTIPLY_OP -> {
							double m1 = stack.pop();
							double m2 = stack.pop();
							stack.push(m2 * m1);
						}
						case Lexer.DIVIDE_OP -> {
							double d1 = stack.pop();
							double d2 = stack.pop();
							stack.push(d2 / d1);
						}
						case Lexer.EOL -> {
							symbols.put(target, stack.pop());
							q = 4;
						}
						default -> error("Invald token encountered in state q3");
					}
					break;
			}
		}

		return symbols.get(target);
	} // evaluate

	/**
	 * Run evaluate on each line of input and print the result forever.
	 */
	public void run() {
		while (true) {
			Double value = evaluate();
			if (value == null)
				System.out.println("no value");
			else
				System.out.println(value);
		}
	}

	/**
	 * Print an error message, display the offending line with the current location
	 * marked, and flush the lexer in preparation for the next line.
	 *
	 * @param msg what to print as an error indication
	 */
	private void error(String msg) {
		System.out.println(msg);
		String line = lexer.getCurrentLine();
		int index = lexer.getCurrentChar();
		System.out.print(line);
		for (int i = 1; i < index; i++)
			System.out.print(' ');
		System.out.println("^");
		lexer.flush();
	}

	////////////////////////////////
	///////// Lexer Class //////////

	/**
	 * Read terminal input and convert it to a token type, and also record the text
	 * of each token. Whitespace is skipped. The input comes from stdin, and each
	 * line is prompted for.
	 */
	public static class Lexer {

		// language token codes
		public static final int ADD_OP = 3;
		public static final int SUBTRACT_OP = 4;
		public static final int MULTIPLY_OP = 5;
		public static final int DIVIDE_OP = 6;
		public static final int MINUS_OP = 7;
		public static final int ASSIGN_OP = 8;
		public static final int EOL = 9;
		public static final int NUMBER = 11;
		public static final int VARIABLE = 12;
		public static final int BAD_TOKEN = 100;

		private Scanner input; // for reading lines from stdin
		private String line; // next input line
		private int index; // current character in this line
		private String text; // text of the current token

		public Lexer(InputStream in) {
			input = new Scanner(in);
			line = "";
			index = 0;
			text = "";
		}

		/**
		 * Fetch the next character from the terminal. If the current line is exhausted,
		 * then prompt the user and wait for input. If end-of-file occurs, then exit the
		 * program.
		 */
		private char nextChar() {
			if (index == line.length()) {
				System.out.print(">> ");
				if (input.hasNextLine()) {
					line = input.nextLine() + "\n";
					index = 0;
				} else {
					System.out.println("\nBye");
					System.exit(0);
				}
			}
			char ch = line.charAt(index);
			index++;
			return ch;
		}

		/**
		 * Put the last character back on the input line.
		 */
		private void unread() {
			index -= 1;
		}

		/**
		 * Return the next token from the terminal.
		 */
		public int nextToken() {
			StringBuffer sb = new StringBuffer();

			char t;
			int q = 0;

			while (true) {
				t = nextChar();

				switch (q) {
					case 0 -> {
						if (Character.isWhitespace(t) && t != '\n') {
							continue;
						}

						sb.append(t);

						if (Character.isDigit(t)) {
							q = 1;
							continue;
						}
						if (Character.isLetter(t)) {
							q = 2;
							continue;
						}

						text = sb.toString();

						return switch (t) {
							case '+' -> ADD_OP;
							case '-' -> SUBTRACT_OP;
							case '*' -> MULTIPLY_OP;
							case '/' -> DIVIDE_OP;
							case '~' -> MINUS_OP;
							case '=' -> ASSIGN_OP;
							case '\n' -> EOL;
							default -> BAD_TOKEN;
						};
					}

					case 1 -> {
						if (Character.isDigit(t)) {
							sb.append(t);
							continue;
						} else if (t == '.') {
							sb.append(t);
							q = 10;
							continue;
						} else {
							unread();
							text = sb.toString();
							return NUMBER;
						}
					}

					case 2 -> {
						if (Character.isLetterOrDigit(t)) {
							sb.append(t);
							continue;
						} else {
							unread();
							text = sb.toString();
							return VARIABLE;
						}
					}

					case 10 -> {
						if (Character.isDigit(t)) {
							sb.append(t);
							continue;
						} else {
							unread();
							text = sb.toString();
							return NUMBER;
						}
					}
				}
			}
		} // nextToken

		/**
		 * Return the current line for error messages.
		 */
		public String getCurrentLine() {
			return line;
		}

		/**
		 * Return the current character index for error messages.
		 */
		public int getCurrentChar() {
			return index;
		}

		/**
		 * /** Return the text of the current token.
		 */
		public String getText() {
			return text;
		}

		/**
		 * Clear the current line after an error
		 */
		public void flush() {
			index = line.length();
		}

	} // Lexer

} // Evaluator