main/src/test/java/org/apache/poi/ss/util/TestExpandedDouble.java

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   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
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       http://www.apache.org/licenses/LICENSE-2.0

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   distributed under the License is distributed on an "AS IS" BASIS,
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   See the License for the specific language governing permissions and
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package org.apache.poi.ss.util;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertNotEquals;
import static org.junit.jupiter.api.Assertions.assertTrue;

import java.math.BigInteger;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.ValueSource;

/**
 * Tests for {@link ExpandedDouble}
 */
final class TestExpandedDouble {
	private static final BigInteger BIG_POW_10 = BigInteger.valueOf(1000000000);

	@Test
	void testNegative() {
		ExpandedDouble hd = new ExpandedDouble(0xC010000000000000L);
		assertNotEquals(-2046, hd.getBinaryExponent(), "identified bug - sign bit not masked out of exponent");
		assertEquals(2, hd.getBinaryExponent());
		BigInteger frac = hd.getSignificand();
		assertEquals(64, frac.bitLength());
		assertEquals(1, frac.bitCount());
	}

	@Test
	void testSubnormal() {
		ExpandedDouble hd = new ExpandedDouble(0x0000000000000001L);
		assertNotEquals(-1023, hd.getBinaryExponent(), "identified bug - subnormal numbers not decoded properly");
		assertEquals(-1086, hd.getBinaryExponent());
		BigInteger frac = hd.getSignificand();
		assertEquals(64, frac.bitLength());
		assertEquals(1, frac.bitCount());
	}

	/**
	 * Tests specific values for conversion from {@link ExpandedDouble} to {@link NormalisedDecimal} and back
	 */
	@ParameterizedTest
	@ValueSource(longs = {
		0x4010000000000004L,
		0x7010000000000004L,
		0x1010000000000004L,
		0x0010000000000001L, // near lowest normal number
		0x0010000000000000L, // lowest normal number
		0x000FFFFFFFFFFFFFL, // highest subnormal number
		0x0008000000000000L, // subnormal number

		0xC010000000000004L,
		0xE230100010001004L,
		0x403CE0FFFFFFFFF2L,
		0x0000000000000001L, // smallest non-zero number (subnormal)
		0x6230100010000FFEL,
		0x6230100010000FFFL,
		0x6230100010001000L,
		0x403CE0FFFFFFFFF0L, // has single digit round trip error
		0x2B2BFFFF10001079L,
	})
	void confirmRoundTrip(long rawBitsA) {
		double a = Double.longBitsToDouble(rawBitsA);
		if (a == 0.0) {
			// Can't represent 0.0 or -0.0 with NormalisedDecimal
			return;
		}
		ExpandedDouble ed1 = new ExpandedDouble(rawBitsA);
		NormalisedDecimal nd2 = ed1.normaliseBaseTen();
		checkNormaliseBaseTenResult(ed1, nd2);

		ExpandedDouble ed3 = nd2.normaliseBaseTwo();
		assertEquals(ed3.getBinaryExponent(), ed1.getBinaryExponent(), "bin exp mismatch");

		BigInteger diff = ed3.getSignificand().subtract(ed1.getSignificand()).abs();
		if (diff.signum() == 0) {
			return;
		}
		// original quantity only has 53 bits of precision
		// these quantities may have errors in the 64th bit, which hopefully don't make any difference

		// errors in the 64th bit happen from time to time
		// this is well below the 53 bits of precision required
		assertTrue(diff.bitLength() < 2);
	}


	private static void checkNormaliseBaseTenResult(ExpandedDouble orig, NormalisedDecimal result) {
		String sigDigs = result.getSignificantDecimalDigits();
		BigInteger frac = orig.getSignificand();
		while (frac.bitLength() + orig.getBinaryExponent() < 200) {
			frac = frac.multiply(BIG_POW_10);
		}
		int binaryExp = orig.getBinaryExponent() - orig.getSignificand().bitLength();

		String origDigs = frac.shiftLeft(binaryExp+1).toString(10);
		assertTrue(origDigs.startsWith(sigDigs));

		double dO = Double.parseDouble("0." + origDigs.substring(sigDigs.length()));
		double d1 = Double.parseDouble(result.getFractionalPart().toPlainString());
		BigInteger subDigsO = BigInteger.valueOf((int) (dO * 32768 + 0.5));
		BigInteger subDigsB = BigInteger.valueOf((int) (d1 * 32768 + 0.5));

		if (subDigsO.equals(subDigsB)) {
			return;
		}
		BigInteger diff = subDigsB.subtract(subDigsO).abs();
		// 100/32768 ~= 0.003
		assertTrue(diff.intValue() <= 100, "minor mistake");
	}
}