Fraction.java

/*
 * Copyright 2002-2005 The Apache Software Foundation.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.lang.math;

import java.io.Serializable;
import java.math.BigInteger;

/**
 * <p><code>Fraction</code> is a <code>Number</code> implementation that
 * stores fractions accurately.</p>
 *
 * <p>This class is immutable, and interoperable with most methods that accept
 * a <code>Number</code>.</p>
 *
 * @author Travis Reeder
 * @author Stephen Colebourne
 * @author Tim O'Brien
 * @author Pete Gieser
 * @author C. Scott Ananian
 * @since 2.0
 * @version $Id$
 */
public final class Fraction extends Number implements Serializable, Comparable {

    /** Serialization lock, Lang version 2.0 */
    private static final long serialVersionUID = 65382027393090L;

    /**
     * <code>Fraction</code> representation of 0.
     */
    public static final Fraction ZERO = new Fraction(0, 1);
    /**
     * <code>Fraction</code> representation of 1.
     */
    public static final Fraction ONE = new Fraction(1, 1);
    /**
     * <code>Fraction</code> representation of 1/2.
     */
    public static final Fraction ONE_HALF = new Fraction(1, 2);
    /**
     * <code>Fraction</code> representation of 1/3.
     */
    public static final Fraction ONE_THIRD = new Fraction(1, 3);
    /**
     * <code>Fraction</code> representation of 2/3.
     */
    public static final Fraction TWO_THIRDS = new Fraction(2, 3);
    /**
     * <code>Fraction</code> representation of 1/4.
     */
    public static final Fraction ONE_QUARTER = new Fraction(1, 4);
    /**
     * <code>Fraction</code> representation of 2/4.
     */
    public static final Fraction TWO_QUARTERS = new Fraction(2, 4);
    /**
     * <code>Fraction</code> representation of 3/4.
     */
    public static final Fraction THREE_QUARTERS = new Fraction(3, 4);
    /**
     * <code>Fraction</code> representation of 1/5.
     */
    public static final Fraction ONE_FIFTH = new Fraction(1, 5);
    /**
     * <code>Fraction</code> representation of 2/5.
     */
    public static final Fraction TWO_FIFTHS = new Fraction(2, 5);
    /**
     * <code>Fraction</code> representation of 3/5.
     */
    public static final Fraction THREE_FIFTHS = new Fraction(3, 5);
    /**
     * <code>Fraction</code> representation of 4/5.
     */
    public static final Fraction FOUR_FIFTHS = new Fraction(4, 5);


    /**
     * The numerator number part of the fraction (the three in three sevenths).
     */
    private final int numerator;
    /**
     * The denominator number part of the fraction (the seven in three sevenths).
     */
    private final int denominator;

    /**
     * Cached output hashCode (class is immutable).
     */
    private transient int hashCode = 0;
    /**
     * Cached output toString (class is immutable).
     */
    private transient String toString = null;
    /**
     * Cached output toProperString (class is immutable).
     */
    private transient String toProperString = null;

    /**
     * <p>Constructs a <code>Fraction</code> instance with the 2 parts
     * of a fraction Y/Z.</p>
     *
     * @param numerator  the numerator, for example the three in 'three sevenths'
     * @param denominator  the denominator, for example the seven in 'three sevenths'
     */
    private Fraction(int numerator, int denominator) {
        super();
        this.numerator = numerator;
        this.denominator = denominator;
    }

    /**
     * <p>Creates a <code>Fraction</code> instance with the 2 parts
     * of a fraction Y/Z.</p>
     *
     * <p>Any negative signs are resolved to be on the numerator.</p>
     *
     * @param numerator  the numerator, for example the three in 'three sevenths'
     * @param denominator  the denominator, for example the seven in 'three sevenths'
     * @return a new fraction instance
     * @throws ArithmeticException if the denomiator is <code>zero</code>
     */
    public static Fraction getFraction(int numerator, int denominator) {
        if (denominator == 0) {
            throw new ArithmeticException("The denominator must not be zero");
        }
        if (denominator < 0) {
            if (numerator==Integer.MIN_VALUE ||
                    denominator==Integer.MIN_VALUE) {
                throw new ArithmeticException("overflow: can't negate");
            }
            numerator = -numerator;
            denominator = -denominator;
        }
        return new Fraction(numerator, denominator);
    }

    /**
     * <p>Creates a <code>Fraction</code> instance with the 3 parts
     * of a fraction X Y/Z.</p>
     *
     * <p>The negative sign must be passed in on the whole number part.</p>
     *
     * @param whole  the whole number, for example the one in 'one and three sevenths'
     * @param numerator  the numerator, for example the three in 'one and three sevenths'
     * @param denominator  the denominator, for example the seven in 'one and three sevenths'
     * @return a new fraction instance
     * @throws ArithmeticException if the denomiator is <code>zero</code>
     * @throws ArithmeticException if the denominator is negative
     * @throws ArithmeticException if the numerator is negative
     * @throws ArithmeticException if the resulting numerator exceeds 
     *  <code>Integer.MAX_VALUE</code>
     */
    public static Fraction getFraction(int whole, int numerator, int denominator) {
        if (denominator == 0) {
            throw new ArithmeticException("The denominator must not be zero");
        }
        if (denominator < 0) {
            throw new ArithmeticException("The denominator must not be negative");
        }
        if (numerator < 0) {
            throw new ArithmeticException("The numerator must not be negative");
        }
        long numeratorValue;
        if (whole < 0) {
            numeratorValue = whole * (long)denominator - numerator;
        } else {
            numeratorValue = whole * (long)denominator + numerator;
        }
        if (numeratorValue < Integer.MIN_VALUE ||
                numeratorValue > Integer.MAX_VALUE)  {
            throw new ArithmeticException("Numerator too large to represent as an Integer.");
        }
        return new Fraction((int) numeratorValue, denominator);
    }

    /**
     * <p>Creates a <code>Fraction</code> instance with the 2 parts
     * of a fraction Y/Z.</p>
     *
     * <p>Any negative signs are resolved to be on the numerator.</p>
     *
     * @param numerator  the numerator, for example the three in 'three sevenths'
     * @param denominator  the denominator, for example the seven in 'three sevenths'
     * @return a new fraction instance, with the numerator and denominator reduced
     * @throws ArithmeticException if the denominator is <code>zero</code>
     */
    public static Fraction getReducedFraction(int numerator, int denominator) {
        if (denominator == 0) {
            throw new ArithmeticException("The denominator must not be zero");
        }
        if (numerator==0) {
            return ZERO; // normalize zero.
        }
        // allow 2^k/-2^31 as a valid fraction (where k>0)
        if (denominator==Integer.MIN_VALUE && (numerator&1)==0) {
            numerator/=2; denominator/=2;
        }
        if (denominator < 0) {
            if (numerator==Integer.MIN_VALUE ||
                    denominator==Integer.MIN_VALUE) {
                throw new ArithmeticException("overflow: can't negate");
            }
            numerator = -numerator;
            denominator = -denominator;
        }
        // simplify fraction.
        int gcd = greatestCommonDivisor(numerator, denominator);
        numerator /= gcd;
        denominator /= gcd;
        return new Fraction(numerator, denominator);
    }

    /**
     * <p>Creates a <code>Fraction</code> instance from a <code>double</code> value.</p>
     *
     * <p>This method uses the <a href="http://archives.math.utk.edu/articles/atuyl/confrac/">
     *  continued fraction algorithm</a>, computing a maximum of
     *  25 convergents and bounding the denominator by 10,000.</p>
     *
     * @param value  the double value to convert
     * @return a new fraction instance that is close to the value
     * @throws ArithmeticException if <code>|value| > Integer.MAX_VALUE</code> 
     *  or <code>value = NaN</code>
     * @throws ArithmeticException if the calculated denominator is <code>zero</code>
     * @throws ArithmeticException if the the algorithm does not converge
     */
    public static Fraction getFraction(double value) {
        int sign = (value < 0 ? -1 : 1);
        value = Math.abs(value);
        if (value  > Integer.MAX_VALUE || Double.isNaN(value)) {
            throw new ArithmeticException
                ("The value must not be greater than Integer.MAX_VALUE or NaN");
        }
        int wholeNumber = (int) value;
        value -= wholeNumber;
        
        int numer0 = 0;  // the pre-previous
        int denom0 = 1;  // the pre-previous
        int numer1 = 1;  // the previous
        int denom1 = 0;  // the previous
        int numer2 = 0;  // the current, setup in calculation
        int denom2 = 0;  // the current, setup in calculation
        int a1 = (int) value;
        int a2 = 0;
        double x1 = 1;
        double x2 = 0;
        double y1 = value - a1;
        double y2 = 0;
        double delta1, delta2 = Double.MAX_VALUE;
        double fraction;
        int i = 1;
//        System.out.println("---");
        do {
            delta1 = delta2;
            a2 = (int) (x1 / y1);
            x2 = y1;
            y2 = x1 - a2 * y1;
            numer2 = a1 * numer1 + numer0;
            denom2 = a1 * denom1 + denom0;
            fraction = (double) numer2 / (double) denom2;
            delta2 = Math.abs(value - fraction);
//            System.out.println(numer2 + " " + denom2 + " " + fraction + " " + delta2 + " " + y1);
            a1 = a2;
            x1 = x2;
            y1 = y2;
            numer0 = numer1;
            denom0 = denom1;
            numer1 = numer2;
            denom1 = denom2;
            i++;
//            System.out.println(">>" + delta1 +" "+ delta2+" "+(delta1 > delta2)+" "+i+" "+denom2);
        } while ((delta1 > delta2) && (denom2 <= 10000) && (denom2 > 0) && (i < 25));
        if (i == 25) {
            throw new ArithmeticException("Unable to convert double to fraction");
        }
        return getReducedFraction((numer0 + wholeNumber * denom0) * sign, denom0);
    }

    /**
     * <p>Creates a Fraction from a <code>String</code>.</p>
     *
     * <p>The formats accepted are:</p>
     *
     * <ol>
     *  <li><code>double</code> String containing a dot</li>
     *  <li>'X Y/Z'</li>
     *  <li>'Y/Z'</li>
     *  <li>'X' (a simple whole number)</li>
     * </ol>
     * and a .</p>
     *
     * @param str  the string to parse, must not be <code>null</code>
     * @return the new <code>Fraction</code> instance
     * @throws IllegalArgumentException if the string is <code>null</code>
     * @throws NumberFormatException if the number format is invalid
     */
    public static Fraction getFraction(String str) {
        if (str == null) {
            throw new IllegalArgumentException("The string must not be null");
        }
        // parse double format
        int pos = str.indexOf('.');
        if (pos >= 0) {
            return getFraction(Double.parseDouble(str));
        }

        // parse X Y/Z format
        pos = str.indexOf(' ');
        if (pos > 0) {
            int whole = Integer.parseInt(str.substring(0, pos));
            str = str.substring(pos + 1);
            pos = str.indexOf('/');
            if (pos < 0) {
                throw new NumberFormatException("The fraction could not be parsed as the format X Y/Z");
            } else {
                int numer = Integer.parseInt(str.substring(0, pos));
                int denom = Integer.parseInt(str.substring(pos + 1));
                return getFraction(whole, numer, denom);
            }
        }

        // parse Y/Z format
        pos = str.indexOf('/');
        if (pos < 0) {
            // simple whole number
            return getFraction(Integer.parseInt(str), 1);
        } else {
            int numer = Integer.parseInt(str.substring(0, pos));
            int denom = Integer.parseInt(str.substring(pos + 1));
            return getFraction(numer, denom);
        }
    }

    // Accessors
    //-------------------------------------------------------------------

    /**
     * <p>Gets the numerator part of the fraction.</p>
     *
     * <p>This method may return a value greater than the denominator, an
     * improper fraction, such as the seven in 7/4.</p>
     *
     * @return the numerator fraction part
     */
    public int getNumerator() {
        return numerator;
    }

    /**
     * <p>Gets the denominator part of the fraction.</p>
     *
     * @return the denominator fraction part
     */
    public int getDenominator() {
        return denominator;
    }

    /**
     * <p>Gets the proper numerator, always positive.</p>
     *
     * <p>An improper fraction 7/4 can be resolved into a proper one, 1 3/4.
     * This method returns the 3 from the proper fraction.</p>
     *
     * <p>If the fraction is negative such as -7/4, it can be resolved into
     * -1 3/4, so this method returns the positive proper numerator, 3.</p>
     *
     * @return the numerator fraction part of a proper fraction, always positive
     */
    public int getProperNumerator() {
        return Math.abs(numerator % denominator);
    }

    /**
     * <p>Gets the proper whole part of the fraction.</p>
     *
     * <p>An improper fraction 7/4 can be resolved into a proper one, 1 3/4.
     * This method returns the 1 from the proper fraction.</p>
     *
     * <p>If the fraction is negative such as -7/4, it can be resolved into
     * -1 3/4, so this method returns the positive whole part -1.</p>
     *
     * @return the whole fraction part of a proper fraction, that includes the sign
     */
    public int getProperWhole() {
        return numerator / denominator;
    }

    // Number methods
    //-------------------------------------------------------------------

    /**
     * <p>Gets the fraction as an <code>int</code>. This returns the whole number
     * part of the fraction.</p>
     *
     * @return the whole number fraction part
     */
    public int intValue() {
        return numerator / denominator;
    }

    /**
     * <p>Gets the fraction as a <code>long</code>. This returns the whole number
     * part of the fraction.</p>
     *
     * @return the whole number fraction part
     */
    public long longValue() {
        return (long) numerator / denominator;
    }

    /**
     * <p>Gets the fraction as a <code>float</code>. This calculates the fraction
     * as the numerator divided by denominator.</p>
     *
     * @return the fraction as a <code>float</code>
     */
    public float floatValue() {
        return ((float) numerator) / ((float) denominator);
    }

    /**
     * <p>Gets the fraction as a <code>double</code>. This calculates the fraction
     * as the numerator divided by denominator.</p>
     *
     * @return the fraction as a <code>double</code>
     */
    public double doubleValue() {
        return ((double) numerator) / ((double) denominator);
    }

    // Calculations
    //-------------------------------------------------------------------

    /**
     * <p>Reduce the fraction to the smallest values for the numerator and
     * denominator, returning the result..</p>
     *
     * @return a new reduce fraction instance, or this if no simplification possible
     */
    public Fraction reduce() {
        int gcd = greatestCommonDivisor(Math.abs(numerator), denominator);
        return Fraction.getFraction(numerator / gcd, denominator / gcd);
    }

    /**
     * <p>Gets a fraction that is the inverse (1/fraction) of this one.</p>
     * 
     * <p>The returned fraction is not reduced.</p>
     *
     * @return a new fraction instance with the numerator and denominator
     *         inverted.
     * @throws ArithmeticException if the fraction represents zero.
     */
    public Fraction invert() {
        if (numerator == 0) {
            throw new ArithmeticException("Unable to invert zero.");
        }
        if (numerator==Integer.MIN_VALUE) {
            throw new ArithmeticException("overflow: can't negate numerator");
        }
        if (numerator<0) {
            return new Fraction(-denominator, -numerator);
        } else {
            return new Fraction(denominator, numerator);
        }
    }

    /**
     * <p>Gets a fraction that is the negative (-fraction) of this one.</p>
     *
     * <p>The returned fraction is not reduced.</p>
     *
     * @return a new fraction instance with the opposite signed numerator
     */
    public Fraction negate() {
        // the positive range is one smaller than the negative range of an int.
        if (numerator==Integer.MIN_VALUE) {
            throw new ArithmeticException("overflow: too large to negate");
        }
        return new Fraction(-numerator, denominator);
    }

    /**
     * <p>Gets a fraction that is the positive equivalent of this one.</p>
     * <p>More precisely: <code>(fraction >= 0 ? this : -fraction)</code></p>
     *
     * <p>The returned fraction is not reduced.</p>
     *
     * @return <code>this</code> if it is positive, or a new positive fraction
     *  instance with the opposite signed numerator
     */
    public Fraction abs() {
        if (numerator >= 0) {
            return this;
        }
        return negate();
    }

    /**
     * <p>Gets a fraction that is raised to the passed in power.</p>
     *
     * <p>The returned fraction is in reduced form.</p>
     *
     * @param power  the power to raise the fraction to
     * @return <code>this</code> if the power is one, <code>ONE</code> if the power
     * is zero (even if the fraction equals ZERO) or a new fraction instance 
     * raised to the appropriate power
     * @throws ArithmeticException if the resulting numerator or denominator exceeds
     *  <code>Integer.MAX_VALUE</code>
     */
    public Fraction pow(int power) {
        if (power == 1) {
            return this;
        } else if (power == 0) {
            return ONE;
        } else if (power < 0) {
            if (power==Integer.MIN_VALUE) { // MIN_VALUE can't be negated.
                return this.invert().pow(2).pow(-(power/2));
            }
            return this.invert().pow(-power);
        } else {
            Fraction f = this.multiplyBy(this);
            if ((power % 2) == 0) { // if even...
                return f.pow(power/2);
            } else { // if odd...
                return f.pow(power/2).multiplyBy(this);
            }
        }
    }

    /**
     * <p>Gets the greatest common divisor of the absolute value of
     * two numbers, using the "binary gcd" method which avoids
     * division and modulo operations.  See Knuth 4.5.2 algorithm B.
     * This algorithm is due to Josef Stein (1961).</p>
     *
     * @param u  a non-zero number
     * @param v  a non-zero number
     * @return the greatest common divisor, never zero
     */
    private static int greatestCommonDivisor(int u, int v) {
        // keep u and v negative, as negative integers range down to
        // -2^31, while positive numbers can only be as large as 2^31-1
        // (i.e. we can't necessarily negate a negative number without
        // overflow)
        /* assert u!=0 && v!=0; */
        if (u>0) { u=-u; } // make u negative
        if (v>0) { v=-v; } // make v negative
        // B1. [Find power of 2]
        int k=0;
        while ((u&1)==0 && (v&1)==0 && k<31) { // while u and v are both even...
            u/=2; v/=2; k++; // cast out twos.
        }
        if (k==31) {
            throw new ArithmeticException("overflow: gcd is 2^31");
        }
        // B2. Initialize: u and v have been divided by 2^k and at least
        //     one is odd.
        int t = ((u&1)==1) ? v : -(u/2)/*B3*/;
        // t negative: u was odd, v may be even (t replaces v)
        // t positive: u was even, v is odd (t replaces u)
        do {
            /* assert u<0 && v<0; */
            // B4/B3: cast out twos from t.
            while ((t&1)==0) { // while t is even..
                t/=2; // cast out twos
            }
            // B5 [reset max(u,v)]
            if (t>0) {
                u = -t;
            } else {
                v = t;
            }
            // B6/B3. at this point both u and v should be odd.
            t = (v - u)/2;
            // |u| larger: t positive (replace u)
            // |v| larger: t negative (replace v)
        } while (t!=0);
        return -u*(1<<k); // gcd is u*2^k
    }

    // Arithmetic
    //-------------------------------------------------------------------

    /** 
     * Multiply two integers, checking for overflow.
     * 
     * @param x a factor
     * @param y a factor
     * @return the product <code>x*y</code>
     * @throws ArithmeticException if the result can not be represented as
     *                             an int
     */
    private static int mulAndCheck(int x, int y) {
        long m = ((long)x)*((long)y);
        if (m < Integer.MIN_VALUE ||
            m > Integer.MAX_VALUE) {
            throw new ArithmeticException("overflow: mul");
        }
        return (int)m;
    }
    
    /**
     *  Multiply two non-negative integers, checking for overflow.
     * 
     * @param x a non-negative factor
     * @param y a non-negative factor
     * @return the product <code>x*y</code>
     * @throws ArithmeticException if the result can not be represented as
     * an int
     */
    private static int mulPosAndCheck(int x, int y) {
        /* assert x>=0 && y>=0; */
        long m = ((long)x)*((long)y);
        if (m > Integer.MAX_VALUE) {
            throw new ArithmeticException("overflow: mulPos");
        }
        return (int)m;
    }
    
    /** 
     * Add two integers, checking for overflow.
     * 
     * @param x an addend
     * @param y an addend
     * @return the sum <code>x+y</code>
     * @throws ArithmeticException if the result can not be represented as
     * an int
     */
    private static int addAndCheck(int x, int y) {
        long s = (long)x+(long)y;
        if (s < Integer.MIN_VALUE ||
            s > Integer.MAX_VALUE) {
            throw new ArithmeticException("overflow: add");
        }
        return (int)s;
    }
    
    /** 
     * Subtract two integers, checking for overflow.
     * 
     * @param x the minuend
     * @param y the subtrahend
     * @return the difference <code>x-y</code>
     * @throws ArithmeticException if the result can not be represented as
     * an int
     */
    private static int subAndCheck(int x, int y) {
        long s = (long)x-(long)y;
        if (s < Integer.MIN_VALUE ||
            s > Integer.MAX_VALUE) {
            throw new ArithmeticException("overflow: add");
        }
        return (int)s;
    }
    
    /**
     * <p>Adds the value of this fraction to another, returning the result in reduced form.
     * The algorithm follows Knuth, 4.5.1.</p>
     *
     * @param fraction  the fraction to add, must not be <code>null</code>
     * @return a <code>Fraction</code> instance with the resulting values
     * @throws IllegalArgumentException if the fraction is <code>null</code>
     * @throws ArithmeticException if the resulting numerator or denominator exceeds
     *  <code>Integer.MAX_VALUE</code>
     */
    public Fraction add(Fraction fraction) {
        return addSub(fraction, true /* add */);
    }

    /**
     * <p>Subtracts the value of another fraction from the value of this one, 
     * returning the result in reduced form.</p>
     *
     * @param fraction  the fraction to subtract, must not be <code>null</code>
     * @return a <code>Fraction</code> instance with the resulting values
     * @throws IllegalArgumentException if the fraction is <code>null</code>
     * @throws ArithmeticException if the resulting numerator or denominator
     *   cannot be represented in an <code>int</code>.
     */
    public Fraction subtract(Fraction fraction) {
        return addSub(fraction, false /* subtract */);
    }

    /** 
     * Implement add and subtract using algorithm described in Knuth 4.5.1.
     * 
     * @param fraction the fraction to subtract, must not be <code>null</code>
     * @param isAdd true to add, false to subtract
     * @return a <code>Fraction</code> instance with the resulting values
     * @throws IllegalArgumentException if the fraction is <code>null</code>
     * @throws ArithmeticException if the resulting numerator or denominator
     *   cannot be represented in an <code>int</code>.
     */
    private Fraction addSub(Fraction fraction, boolean isAdd) {
        if (fraction == null) {
            throw new IllegalArgumentException("The fraction must not be null");
        }
        // zero is identity for addition.
        if (numerator == 0) {
            return isAdd ? fraction : fraction.negate();
        }
        if (fraction.numerator == 0) {
            return this;
        }     
        // if denominators are randomly distributed, d1 will be 1 about 61%
        // of the time.
        int d1 = greatestCommonDivisor(denominator, fraction.denominator);
        if (d1==1) {
            // result is ( (u*v' +/- u'v) / u'v')
            int uvp = mulAndCheck(numerator, fraction.denominator);
            int upv = mulAndCheck(fraction.numerator, denominator);
            return new Fraction
                (isAdd ? addAndCheck(uvp, upv) : subAndCheck(uvp, upv),
                 mulPosAndCheck(denominator, fraction.denominator));
        }
        // the quantity 't' requires 65 bits of precision; see knuth 4.5.1
        // exercise 7.  we're going to use a BigInteger.
        // t = u(v'/d1) +/- v(u'/d1)
        BigInteger uvp = BigInteger.valueOf(numerator)
            .multiply(BigInteger.valueOf(fraction.denominator/d1));
        BigInteger upv = BigInteger.valueOf(fraction.numerator)
            .multiply(BigInteger.valueOf(denominator/d1));
        BigInteger t = isAdd ? uvp.add(upv) : uvp.subtract(upv);
        // but d2 doesn't need extra precision because
        // d2 = gcd(t,d1) = gcd(t mod d1, d1)
        int tmodd1 = t.mod(BigInteger.valueOf(d1)).intValue();
        int d2 = (tmodd1==0)?d1:greatestCommonDivisor(tmodd1, d1);

        // result is (t/d2) / (u'/d1)(v'/d2)
        BigInteger w = t.divide(BigInteger.valueOf(d2));
        if (w.bitLength() > 31) {
            throw new ArithmeticException
                ("overflow: numerator too large after multiply");
        }
        return new Fraction
            (w.intValue(),
             mulPosAndCheck(denominator/d1, fraction.denominator/d2));
    }

    /**
     * <p>Multiplies the value of this fraction by another, returning the 
     * result in reduced form.</p>
     *
     * @param fraction  the fraction to multiply by, must not be <code>null</code>
     * @return a <code>Fraction</code> instance with the resulting values
     * @throws IllegalArgumentException if the fraction is <code>null</code>
     * @throws ArithmeticException if the resulting numerator or denominator exceeds
     *  <code>Integer.MAX_VALUE</code>
     */
    public Fraction multiplyBy(Fraction fraction) {
        if (fraction == null) {
            throw new IllegalArgumentException("The fraction must not be null");
        }
        if (numerator == 0 || fraction.numerator == 0) {
            return ZERO;
        }
        // knuth 4.5.1
        // make sure we don't overflow unless the result *must* overflow.
        int d1 = greatestCommonDivisor(numerator, fraction.denominator);
        int d2 = greatestCommonDivisor(fraction.numerator, denominator);
        return getReducedFraction
            (mulAndCheck(numerator/d1, fraction.numerator/d2),
             mulPosAndCheck(denominator/d2, fraction.denominator/d1));
    }

    /**
     * <p>Divide the value of this fraction by another.</p>
     *
     * @param fraction  the fraction to divide by, must not be <code>null</code>
     * @return a <code>Fraction</code> instance with the resulting values
     * @throws IllegalArgumentException if the fraction is <code>null</code>
     * @throws ArithmeticException if the fraction to divide by is zero
     * @throws ArithmeticException if the resulting numerator or denominator exceeds
     *  <code>Integer.MAX_VALUE</code>
     */
    public Fraction divideBy(Fraction fraction) {
        if (fraction == null) {
            throw new IllegalArgumentException("The fraction must not be null");
        }
        if (fraction.numerator == 0) {
            throw new ArithmeticException("The fraction to divide by must not be zero");
        }
        return multiplyBy(fraction.invert());
    }

    // Basics
    //-------------------------------------------------------------------

    /**
     * <p>Compares this fraction to another object to test if they are equal.</p>.
     *
     * <p>To be equal, both values must be equal. Thus 2/4 is not equal to 1/2.</p>
     *
     * @param obj the reference object with which to compare
     * @return <code>true</code> if this object is equal
     */
    public boolean equals(Object obj) {
        if (obj == this) {
            return true;
        }
        if (obj instanceof Fraction == false) {
            return false;
        }
        Fraction other = (Fraction) obj;
        return (getNumerator() == other.getNumerator() &&
                getDenominator() == other.getDenominator());
    }

    /**
     * <p>Gets a hashCode for the fraction.</p>
     *
     * @return a hash code value for this object
     */
    public int hashCode() {
        if (hashCode == 0) {
            // hashcode update should be atomic.
            hashCode = 37 * (37 * 17 + getNumerator()) + getDenominator();
        }
        return hashCode;
    }

    /**
     * <p>Compares this object to another based on size.</p>
     *
     * @param object  the object to compare to
     * @return -1 if this is less, 0 if equal, +1 if greater
     * @throws ClassCastException if the object is not a <code>Fraction</code>
     * @throws NullPointerException if the object is <code>null</code>
     */
    public int compareTo(Object object) {
        Fraction other = (Fraction) object;
        if (this==other) {
            return 0;
        }
        if (numerator == other.numerator && denominator == other.denominator) {
            return 0;
        }

        // otherwise see which is less
        long first = (long) numerator * (long) other.denominator;
        long second = (long) other.numerator * (long) denominator;
        if (first == second) {
            return 0;
        } else if (first < second) {
            return -1;
        } else {
            return 1;
        }
    }

    /**
     * <p>Gets the fraction as a <code>String</code>.</p>
     *
     * <p>The format used is '<i>numerator</i>/<i>denominator</i>' always.
     *
     * @return a <code>String</code> form of the fraction
     */
    public String toString() {
        if (toString == null) {
            toString = new StringBuffer(32)
                .append(getNumerator())
                .append('/')
                .append(getDenominator()).toString();
        }
        return toString;
    }

    /**
     * <p>Gets the fraction as a proper <code>String</code> in the format X Y/Z.</p>
     *
     * <p>The format used in '<i>wholeNumber</i> <i>numerator</i>/<i>denominator</i>'.
     * If the whole number is zero it will be ommitted. If the numerator is zero,
     * only the whole number is returned.</p>
     *
     * @return a <code>String</code> form of the fraction
     */
    public String toProperString() {
        if (toProperString == null) {
            if (numerator == 0) {
                toProperString = "0";
            } else if (numerator == denominator) {
                toProperString = "1";
            } else if ((numerator>0?-numerator:numerator) < -denominator) {
                // note that we do the magnitude comparison test above with
                // NEGATIVE (not positive) numbers, since negative numbers
                // have a larger range.  otherwise numerator==Integer.MIN_VALUE
                // is handled incorrectly.
                int properNumerator = getProperNumerator();
                if (properNumerator == 0) {
                    toProperString = Integer.toString(getProperWhole());
                } else {
                    toProperString = new StringBuffer(32)
                        .append(getProperWhole()).append(' ')
                        .append(properNumerator).append('/')
                        .append(getDenominator()).toString();
                }
            } else {
                toProperString = new StringBuffer(32)
                    .append(getNumerator()).append('/')
                    .append(getDenominator()).toString();
            }
        }
        return toProperString;
    }
}