/*
 * Copyright (C) 2011 The Guava Authors
 *
 * 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 com.google.common.math;

import static com.google.common.base.Preconditions.checkArgument;
import static java.lang.Double.MAX_EXPONENT;
import static java.lang.Double.MIN_EXPONENT;
import static java.lang.Double.POSITIVE_INFINITY;
import static java.lang.Double.doubleToRawLongBits;
import static java.lang.Double.isNaN;
import static java.lang.Double.longBitsToDouble;
import static java.lang.Math.getExponent;

import java.math.BigInteger;

/**
 * Utilities for {@code double} primitives.
 *
 * @author Louis Wasserman
 */
final class DoubleUtils {
	private DoubleUtils() {
	}

	static double nextDown(double d) {
		return -Math.nextUp(-d);
	}

	// The mask for the significand, according to the {@link
	// Double#doubleToRawLongBits(double)} spec.
	static final long SIGNIFICAND_MASK = 0x000fffffffffffffL;

	// The mask for the exponent, according to the {@link
	// Double#doubleToRawLongBits(double)} spec.
	static final long EXPONENT_MASK = 0x7ff0000000000000L;

	// The mask for the sign, according to the {@link
	// Double#doubleToRawLongBits(double)} spec.
	static final long SIGN_MASK = 0x8000000000000000L;

	static final int SIGNIFICAND_BITS = 52;

	static final int EXPONENT_BIAS = 1023;

	/**
	 * The implicit 1 bit that is omitted in significands of normal doubles.
	 */
	static final long IMPLICIT_BIT = SIGNIFICAND_MASK + 1;

	static long getSignificand(double d) {
		checkArgument(isFinite(d), "not a normal value");
		int exponent = getExponent(d);
		long bits = doubleToRawLongBits(d);
		bits &= SIGNIFICAND_MASK;
		return (exponent == MIN_EXPONENT - 1) ? bits << 1 : bits | IMPLICIT_BIT;
	}

	static boolean isFinite(double d) {
		return getExponent(d) <= MAX_EXPONENT;
	}

	static boolean isNormal(double d) {
		return getExponent(d) >= MIN_EXPONENT;
	}

	/*
	 * Returns x scaled by a power of 2 such that it is in the range [1, 2). Assumes
	 * x is positive, normal, and finite.
	 */
	static double scaleNormalize(double x) {
		long significand = doubleToRawLongBits(x) & SIGNIFICAND_MASK;
		return longBitsToDouble(significand | ONE_BITS);
	}

	static double bigToDouble(BigInteger x) {
		// This is an extremely fast implementation of BigInteger.doubleValue(). JDK
		// patch pending.
		BigInteger absX = x.abs();
		int exponent = absX.bitLength() - 1;
		// exponent == floor(log2(abs(x)))
		if (exponent < Long.SIZE - 1) {
			return x.longValue();
		} else if (exponent > MAX_EXPONENT) {
			return x.signum() * POSITIVE_INFINITY;
		}

		/*
		 * We need the top SIGNIFICAND_BITS + 1 bits, including the "implicit" one bit.
		 * To make rounding easier, we pick out the top SIGNIFICAND_BITS + 2 bits, so we
		 * have one to help us round up or down. twiceSignifFloor will contain the top
		 * SIGNIFICAND_BITS + 2 bits, and signifFloor the top SIGNIFICAND_BITS + 1.
		 *
		 * It helps to consider the real number signif = absX * 2^(SIGNIFICAND_BITS -
		 * exponent).
		 */
		int shift = exponent - SIGNIFICAND_BITS - 1;
		long twiceSignifFloor = absX.shiftRight(shift).longValue();
		long signifFloor = twiceSignifFloor >> 1;
		signifFloor &= SIGNIFICAND_MASK; // remove the implied bit

		/*
		 * We round up if either the fractional part of signif is strictly greater than
		 * 0.5 (which is true if the 0.5 bit is set and any lower bit is set), or if the
		 * fractional part of signif is >= 0.5 and signifFloor is odd (which is true if
		 * both the 0.5 bit and the 1 bit are set).
		 */
		boolean increment = (twiceSignifFloor & 1) != 0 && ((signifFloor & 1) != 0 || absX.getLowestSetBit() < shift);
		long signifRounded = increment ? signifFloor + 1 : signifFloor;
		long bits = (long) ((exponent + EXPONENT_BIAS)) << SIGNIFICAND_BITS;
		bits += signifRounded;
		/*
		 * If signifRounded == 2^53, we'd need to set all of the significand bits to
		 * zero and add 1 to the exponent. This is exactly the behavior we get from just
		 * adding signifRounded to bits directly. If the exponent is
		 * MAX_DOUBLE_EXPONENT, we round up (correctly) to Double.POSITIVE_INFINITY.
		 */
		bits |= x.signum() & SIGN_MASK;
		return longBitsToDouble(bits);
	}

	/**
	 * Returns its argument if it is non-negative, zero if it is negative.
	 */
	static double ensureNonNegative(double value) {
		checkArgument(!isNaN(value));
		if (value > 0.0) {
			return value;
		} else {
			return 0.0;
		}
	}

	private static final long ONE_BITS = doubleToRawLongBits(1.0);
}
