package world.bentobox.level.calculators;

import java.text.ParseException;

/**
 * @author tastybento
 */
public class EquationEvaluator {

    private static class Parser {
	private final String input;
	private int pos = -1;
	private int currentChar;

	public Parser(String input) {
	    this.input = input;
	    moveToNextChar();
	}

	private void moveToNextChar() {
	    currentChar = (++pos < input.length()) ? input.charAt(pos) : -1;
	}

	private boolean tryToEat(int charToEat) {
	    while (currentChar == ' ')
		moveToNextChar();
	    if (currentChar == charToEat) {
		moveToNextChar();
		return true;
	    }
	    return false;
	}

	public double evaluate() throws ParseException {
	    double result = parseExpression();
	    if (pos < input.length()) {
		throw new ParseException("Unexpected character: " + (char) currentChar, pos);
	    }
	    return result;
	}

	private double parseExpression() throws ParseException {
	    double result = parseTerm();
	    while (true) {
		if (tryToEat('+'))
		    result += parseTerm();
		else if (tryToEat('-'))
		    result -= parseTerm();
		else
		    return result;
	    }
	}

	private double parseFactor() throws ParseException {
	    if (tryToEat('+'))
		return parseFactor(); // unary plus
	    if (tryToEat('-'))
		return -parseFactor(); // unary minus

	    double x;
	    int startPos = this.pos;
	    if (tryToEat('(')) { // parentheses
		x = parseExpression();
		tryToEat(')');
	    } else if ((currentChar >= '0' && currentChar <= '9') || currentChar == '.') { // numbers
		while ((currentChar >= '0' && currentChar <= '9') || currentChar == '.')
		    moveToNextChar();
		x = Double.parseDouble(input.substring(startPos, this.pos));
	    } else if (currentChar >= 'a' && currentChar <= 'z') { // functions
		while (currentChar >= 'a' && currentChar <= 'z')
		    moveToNextChar();
		String func = input.substring(startPos, this.pos);
		x = parseFactor();
		switch (func) {
		case "sqrt":
		    x = Math.sqrt(x);
		    break;
		case "sin":
		    x = Math.sin(Math.toRadians(x));
		    break;
		case "cos":
		    x = Math.cos(Math.toRadians(x));
		    break;
		case "tan":
		    x = Math.tan(Math.toRadians(x));
		    break;
		case "log":
		    x = Math.log(x);
		    break;
		default:
		    throw new ParseException("Unknown function: " + func, startPos);
		}
	    } else {
		throw new ParseException("Unexpected: " + (char) currentChar, startPos);
	    }

	    if (tryToEat('^'))
		x = Math.pow(x, parseFactor()); // exponentiation

	    return x;
	}

	private double parseTerm() throws ParseException {
	    double x = parseFactor();
	    for (;;) {
		if (tryToEat('*'))
		    x *= parseFactor(); // multiplication
		else if (tryToEat('/'))
		    x /= parseFactor(); // division
		else
		    return x;
	    }
	}

    }

    public static double eval(final String equation) throws ParseException {
	return new Parser(equation).evaluate();
    }

}