/*******************************************************************************
 *  Copyright (c) 2010 Association for Decentralized Information Management in
 *  Industry THTH ry.
 *  All rights reserved. This program and the accompanying materials
 *  are made available under the terms of the Eclipse Public License v1.0
 *  which accompanies this distribution, and is available at
 *  http://www.eclipse.org/legal/epl-v10.html
 *
 *  Contributors:
 *      VTT Technical Research Centre of Finland - initial API and implementation
 *******************************************************************************/
package org.simantics.databoard.units.internal.parser;

import org.simantics.databoard.units.internal.UnitParseException;

public abstract class UnitParser {

	String string;
	int pos;
	
	int nextChar;
	int sign;
	
	public UnitParser() {
	}

	void next() {
		++pos;
		if(pos >= string.length())
			nextChar = -1;
		else
			nextChar = string.charAt(pos);
	}
	
	public void unit(String string, int sign) throws UnitParseException {
		this.string = string;
		this.pos = -1;
		this.sign = sign;
		next();
		
		expression();
		expect(-1);
	}
	
	/* Grammar
	 * 
	 * unit_expression: unit_numerator [ "/" unit_denominator ]
	 * unit_numerator: "1" | unit_factors | "(" unit_expression ")"
	 * unit_denominator: unit_factor | "(" unit_expression ")"
	 * unit_factors: unit_factor [ "." unit_factors ]
	 * unit_factor: unit_operand [ unit_exponent ]
	 * unit_exponent: [ "+" | "-" ] integer
	 * unit_operand: unit_symbol | unit_prefix unit_symbol 
	 */
	
	void expression() throws UnitParseException {
		numerator();
		if(nextChar == '/') {
			next();
			denominator();
		}		
	}
	
	void numerator() throws UnitParseException {
		if(nextChar == '1')
			next();
		else if(nextChar == '(') {
			next();
			expression();
			skip(')');
		}
		else
			factors();
	}

	void denominator() throws UnitParseException {
		sign = -sign;
		if(nextChar == '(') {
			next();
			expression();
			skip(')');
		}
		else
			factor();
		sign = -sign;
	}
	
	void factors() throws UnitParseException {
		while(true) {
			factor();
			if(nextChar == '.')
				next();
			else
			    break;
		}
	}
	
	void factor() throws UnitParseException {
		// Base
		int factorBegin = pos;
		while((nextChar >= 'a' && nextChar <= 'z') || (nextChar >= 'A' && nextChar <= 'Z') || (nextChar=='µ') || nextChar=='(' || nextChar==')' || nextChar=='%') 
			next();
		int exponentBegin = pos;
		if(factorBegin == exponentBegin)
			throw new UnitParseException(string, 
					"Expected base unit name, but got " + charName(nextChar) + " at position " + pos + ".");	
		String baseUnit = string.substring(factorBegin, exponentBegin);
		
		// Exponent
		if(nextChar == '-' || nextChar == '+' || (nextChar >= '0' && nextChar <= '9')) {
			next();
			while(nextChar >= '0' && nextChar <= '9')
				next();
		}
		int exponentEnd = pos;
		if(exponentEnd > exponentBegin) {
			String exponentString = string.substring(exponentBegin, exponentEnd);
			try {
				visit(baseUnit, sign * Integer.parseInt(exponentString));
			} catch(NumberFormatException e) {
				throw new UnitParseException(string, 
						"Invalid exponent \"" + exponentString + "\" at position " + pos + ".");	
			}
		}
		else
			visit(baseUnit, sign);
	}
			
	private void skip(char c) throws UnitParseException {
		expect(c);			
		next();
	}
	
	private static String charName(int c) {
		if(c >= 0)
			return "'" + Character.toString((char)c) + "'";
		else
			return "end of unit";
	}
	
	private void expect(int c) throws UnitParseException {
		if(nextChar != c)
			throw new UnitParseException(string, 
					"Expected " + charName(c) + ", but got " + charName(nextChar) + " at position " + pos + ".");	
	}
	
	public abstract void visit(String baseUnit, int exponent);
	
}