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Very simple java library to generate permutations, combinations and other combinatorial sequences.

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/**
 * Combinatorics Library
 * Copyright 2012 Dmytro Paukov [email protected]
 */
package org.paukov.tool;

import java.util.LinkedList;

/**
 * @author Dmytro Paukov
 * @version 2.0
 */
public class IntExpressionParser {

  static public boolean isDelim(char c) {
    return c == ' ';
  }

  static public boolean isOperator(char c) {
    return c == '+' || c == '-' || c == '*' || c == '/' || c == '%';
  }

  static public int priority(char op) {
    switch (op) {
      case '+':
      case '-':
        return 1;
      case '*':
      case '/':
      case '%':
        return 2;
      default:
        return -1;
    }
  }

  static public void processOperator(LinkedList st, char op) {
    int r = st.removeLast();
    int l = st.removeLast();
    switch (op) {
      case '+':
        st.add(l + r);
        break;
      case '-':
        st.add(l - r);
        break;
      case '*':
        st.add(l * r);
        break;
      case '/':
        st.add(l / r);
        break;
      case '%':
        st.add(l % r);
        break;
    }
  }

  public static int eval(String s) {
    LinkedList st = new LinkedList();
    LinkedList op = new LinkedList();
    for (int i = 0; i < s.length(); i++) {
      char c = s.charAt(i);
      if (isDelim(c)) {
        continue;
      }
      if (c == '(') {
        op.add('(');
      } else if (c == ')') {
        while (op.getLast() != '(') {
          processOperator(st, op.removeLast());
        }
        op.removeLast();
      } else if (isOperator(c)) {
        while (!op.isEmpty() && priority(op.getLast()) >= priority(c)) {
          processOperator(st, op.removeLast());
        }
        op.add(c);
      } else {
        String operand = "";
        while (i < s.length() && Character.isDigit(s.charAt(i))) {
          operand += s.charAt(i++);
        }
        --i;
        st.add(Integer.parseInt(operand));
      }
    }
    while (!op.isEmpty()) {
      processOperator(st, op.removeLast());
    }
    return st.get(0);
  }
}