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package sun.security.util.math;

import java.math.BigInteger;

/**

 * The base interface for integers modulo a prime value. Objects of this

 * type may be either mutable or immutable, and subinterfaces can be used

 * to specify that an object is mutable or immutable. This type should never

 * be used to declare local/member variables, but it may be used for

 * formal parameters of a method. None of the methods in this interface

 * modify the value of arguments or this.

 *

 * The behavior of this interface depends on the particular implementation.

 * For example, some implementations only support a limited number of add

 * operations before each multiply operation. See the documentation of the

 * implementation for details.

 *

 * @see ImmutableIntegerModuloP

 * @see MutableIntegerModuloP

 */

public interface IntegerModuloP {

    /**

     * Get the field associated with this element.

     *

     * @return the field

     */

    IntegerFieldModuloP getField();

    /**

     * Get the canonical value of this element as a BigInteger. This value

     * will always be in the range [0, p), where p is the prime that defines

     * the field. This method performs reduction and other computation to

     * produce the result.

     *

     * @return the value as a BigInteger

     */

    BigInteger asBigInteger();

    /**

     * Return this value as a fixed (immutable) element. This method will

     * copy the underlying representation if the object is mutable.

     *

     * @return a fixed element with the same value

     */

    ImmutableIntegerModuloP fixed();

    /**

     * Return this value as a mutable element. This method will always copy

     * the underlying representation.

     *

     * @return a mutable element with the same value

     */

    MutableIntegerModuloP mutable();

    /**

     * Add this field element with the supplied element and return the result.

     *

     * @param b the sumand

     * @return this + b

     */

    ImmutableIntegerModuloP add(IntegerModuloP b);

    /**

     * Compute the additive inverse of the field element

     * @return the addditiveInverse (0 - this)

     */

    ImmutableIntegerModuloP additiveInverse();

    /**

     * Multiply this field element with the supplied element and return the

     * result.

     *

     * @param b the multiplicand

     * @return this * b

     */

    ImmutableIntegerModuloP multiply(IntegerModuloP b);

    /**

     * Perform an addition modulo a power of two and return the little-endian

     * encoding of the result. The value is (this' + b') % 2^(8 * len),

     * where this' and b' are the canonical integer values equivalent to

     * this and b.

     *

     * @param b the sumand

     * @param len the length of the desired array

     * @return a byte array of length len containing the result

     */

    default byte[] addModPowerTwo(IntegerModuloP b, int len) {

        byte[] result = new byte[len];

        addModPowerTwo(b, result);

        return result;

    }

    /**

     * Perform an addition modulo a power of two and store the little-endian

     * encoding of the result in the supplied array. The value is

     * (this' + b') % 2^(8 * result.length), where this' and b' are the

     * canonical integer values equivalent to this and b.

     *

     * @param b the sumand

     * @param result an array which stores the result upon return

     */

    void addModPowerTwo(IntegerModuloP b, byte[] result);

    /**

     * Returns the little-endian encoding of this' % 2^(8 * len), where this'

     * is the canonical integer value equivalent to this.

     *

     * @param len the length of the desired array

     * @return a byte array of length len containing the result

     */

    default byte[] asByteArray(int len) {

        byte[] result = new byte[len];

        asByteArray(result);

        return result;

    }

    /**

     * Places the little-endian encoding of this' % 2^(8 * result.length)

     * into the supplied array, where this' is the canonical integer value

     * equivalent to this.

     *

     * @param result an array which stores the result upon return

     */

    void asByteArray(byte[] result);

    /**

     * Compute the multiplicative inverse of this field element.

     *

     * @return the multiplicative inverse (1 / this)

     */

    default ImmutableIntegerModuloP multiplicativeInverse() {

        return pow(getField().getSize().subtract(BigInteger.valueOf(2)));

    }

    /**

     * Subtract the supplied element from this one and return the result.

     * @param b the subtrahend

     *

     * @return the difference (this - b)

     */

    default ImmutableIntegerModuloP subtract(IntegerModuloP b) {

        return add(b.additiveInverse());

    }

    /**

     * Calculate the square of this element and return the result. This method

     * should be used instead of a.multiply(a) because implementations may

     * include optimizations that only apply to squaring.

     *

     * @return the product (this * this)

     */

    default ImmutableIntegerModuloP square() {

        return multiply(this);

    }

    /**

     * Calculate the power this^b and return the result.

     *

     * @param b the exponent

     * @return the value of this^b

     */

    default ImmutableIntegerModuloP pow(BigInteger b) {

        //Default implementation is square and multiply

        MutableIntegerModuloP y = getField().get1().mutable();

        MutableIntegerModuloP x = mutable();

        int bitLength = b.bitLength();

        for (int bit = 0; bit < bitLength; bit++) {

            if (b.testBit(bit)) {

                // odd

                y.setProduct(x);

            }

            x.setSquare();

        }

        return y.fixed();

    }

}