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package java.security.spec;

import java.math.BigInteger;

import java.util.Arrays;

/**

 * This immutable class defines an elliptic curve (EC)

 * characteristic 2 finite field.

 *

 * @see ECField

 *

 * @author Valerie Peng

 *

 * @since 1.5

 */

public class ECFieldF2m implements ECField {

    private int m;

    private int[] ks;

    private BigInteger rp;

    /**

     * Creates an elliptic curve characteristic 2 finite

     * is not positive.

     */

    public ECFieldF2m(int m) {

        if (m <= 0) {

            throw new IllegalArgumentException("m is not positive");

        }

        this.m = m;

        this.ks = null;

        this.rp = null;

    }

    /**

     * Creates an elliptic curve characteristic 2 finite

     * polynomial basis.

     * The reduction polynomial for this field is based

     * the i-th coefficient of the reduction polynomial.<p>

     * Note: A valid reduction polynomial is either a

     * @param rp the BigInteger whose i-th bit corresponds to

     * the i-th coefficient of the reduction polynomial.

     * a valid reduction polynomial.

     */

    public ECFieldF2m(int m, BigInteger rp) {

        // check m and rp

        this.m = m;

        this.rp = rp;

        if (m <= 0) {

            throw new IllegalArgumentException("m is not positive");

        }

        int bitCount = this.rp.bitCount();

        if (!this.rp.testBit(0) || !this.rp.testBit(m) ||

            ((bitCount != 3) && (bitCount != 5))) {

            throw new IllegalArgumentException

                ("rp does not represent a valid reduction polynomial");

        }

        // convert rp into ks

        BigInteger temp = this.rp.clearBit(0).clearBit(m);

        this.ks = new int[bitCount-2];

        for (int i = this.ks.length-1; i >= 0; i--) {

            int index = temp.getLowestSetBit();

            this.ks[i] = index;

            temp = temp.clearBit(index);

        }

    }

    /**

     * Creates an elliptic curve characteristic 2 finite

     * polynomial basis. The reduction polynomial for this

     * contains the order of the middle term(s) of the

     * reduction polynomial.

     * Note: A valid reduction polynomial is either a

     * have length 1 or 3.

     * @param ks the order of the middle term(s) of the

     * reduction polynomial. Contents of this array are copied

     * to protect against subsequent modification.

     * and in descending order.

     */

    public ECFieldF2m(int m, int[] ks) {

        // check m and ks

        this.m = m;

        this.ks = ks.clone();

        if (m <= 0) {

            throw new IllegalArgumentException("m is not positive");

        }

        if ((this.ks.length != 1) && (this.ks.length != 3)) {

            throw new IllegalArgumentException

                ("length of ks is neither 1 nor 3");

        }

        for (int i = 0; i < this.ks.length; i++) {

            if ((this.ks[i] < 1) || (this.ks[i] > m-1)) {

                throw new IllegalArgumentException

                    ("ks["+ i + "] is out of range");

            }

            if ((i != 0) && (this.ks[i] >= this.ks[i-1])) {

                throw new IllegalArgumentException

                    ("values in ks are not in descending order");

            }

        }

        // convert ks into rp

        this.rp = BigInteger.ONE;

        this.rp = rp.setBit(m);

        for (int j = 0; j < this.ks.length; j++) {

            rp = rp.setBit(this.ks[j]);

        }

    }

    /**

     * for this characteristic 2 finite field.

     * @return the field size in bits.

     */

    public int getFieldSize() {

        return m;

    }

    /**

     * 2 finite field.

     * number of elements.

     */

    public int getM() {

        return m;

    }

    /**

     * Returns a BigInteger whose i-th bit corresponds to the

     * i-th coefficient of the reduction polynomial for polynomial

     * basis or null for normal basis.

     * @return a BigInteger whose i-th bit corresponds to the

     * i-th coefficient of the reduction polynomial for polynomial

     * basis or null for normal basis.

     */

    public BigInteger getReductionPolynomial() {

        return rp;

    }

    /**

     * Returns an integer array which contains the order of the

     * middle term(s) of the reduction polynomial for polynomial

     * basis or null for normal basis.

     * @return an integer array which contains the order of the

     * middle term(s) of the reduction polynomial for polynomial

     * basis or null for normal basis. A new array is returned

     * each time this method is called.

     */

    public int[] getMidTermsOfReductionPolynomial() {

        if (ks == null) {

            return null;

        } else {

            return ks.clone();

        }

    }

    /**

     * Compares this finite field for equality with the

     * specified object.

     * @param obj the object to be compared.

     * polynomial match, false otherwise.

     */

    public boolean equals(Object obj) {

        if (this == obj) return true;

        if (obj instanceof ECFieldF2m) {

            // no need to compare rp here since ks and rp

            // should be equivalent

            return ((m == ((ECFieldF2m)obj).m) &&

                    (Arrays.equals(ks, ((ECFieldF2m) obj).ks)));

        }

        return false;

    }

    /**

     * Returns a hash code value for this characteristic 2

     * finite field.

     * @return a hash code value.

     */

    public int hashCode() {

        int value = m << 5;

        value += (rp==null? 0:rp.hashCode());

        // no need to involve ks here since ks and rp

        // should be equivalent.

        return value;

    }

}