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POMantissa Normalization of C# double
 

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    <p>EDIT: Got it to work now, while normalizing the mantiss it is important to first set the implicit bit, when decoding the implicit bit then does not have to be added. I left the marked answer as correct, as the information there really helped.</p> <p>I'm currently implementing an encoding (Distinguished encoding rules) and have a slight problem encoding double values.</p> <p>So, I can get out the sign, exponent and mantissa from a double in c# by using:</p> <pre><code> // get parts double value = 10.0; long bits = BitConverter.DoubleToInt64Bits(value); // Note that the shift is sign-extended, hence the test against -1 not 1 bool negative = (bits &lt; 0); int exponent = (int)((bits &gt;&gt; 52) &amp; 0x7ffL); long mantissa = bits &amp; 0xfffffffffffffL; </code></pre> <p>(using code from <a href="https://stackoverflow.com/questions/389993/extracting-mantissa-and-exponent-from-double-in-c-sharp">here</a>). These values can be encoded and a simple reversal of the process will get me back the original double.</p> <p>However, the DER encoding rules specify that the mantissa should be normalized:</p> <blockquote> <p>In the Canonical Encoding Rules and the Distinguished Encoding Rules normalization is specified and the mantissa (unless it is 0) needs to be repeatedly shifted until the least significant bit is a 1.</p> </blockquote> <p>(see <a href="http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf" rel="nofollow noreferrer">here in section 8.5.6.5</a>).</p> <p>Doing this by hand using:</p> <pre><code> while ((mantissa &amp; 1) == 0) { mantissa &gt;&gt;= 1; exponent++; } </code></pre> <p>will not work, and gives me strange values. (Even when using the whole function Jon Skeet posted in the aforementioned link).</p> <p>I seem to be missing something here, it would be easiest if I first could normalize the mantiassa of the double and the get the "bits". However, I also can't really see why the normalization by hand won't work correctly.</p> <p>Thanks for any help,</p> <p>Danny</p> <p>EDIT: Actual working problem showing my issue with mantiss normalization:</p> <pre><code> static void Main(string[] args) { Console.WriteLine(CalculateDouble(GetBits(55.5, false))); Console.WriteLine(CalculateDouble(GetBits(55.5, true))); Console.ReadLine(); } private static double CalculateDouble(Tuple&lt;bool, int, long&gt; bits) { double result = 0; bool isNegative = bits.Item1; int exponent = bits.Item2; long significand = bits.Item3; if (exponent == 2047 &amp;&amp; significand != 0) { // special case } else if (exponent == 2047 &amp;&amp; significand == 0) { result = isNegative ? double.NegativeInfinity : double.PositiveInfinity; } else if (exponent == 0) { // special case, subnormal numbers } else { /* old code, wont work double actualSignificand = significand*Math.Pow(2, -52) + 1; */ double actualSignificand = significand*Math.Pow(2, -52); int actualExponent = exponent - 1023; if (isNegative) { result = actualSignificand*Math.Pow(2, actualExponent); } else { result = -actualSignificand*Math.Pow(2, actualExponent);**strong text** } } return result; } private static Tuple&lt;bool, int, long&gt; GetBits(double d, bool normalizeSignificand) { // Translate the double into sign, exponent and mantissa. long bits = BitConverter.DoubleToInt64Bits(d); // Note that the shift is sign-extended, hence the test against -1 not 1 bool negative = (bits &lt; 0); int exponent = (int)((bits &gt;&gt; 52) &amp; 0x7ffL); long significand = bits &amp; 0xfffffffffffffL; if (significand == 0) { return Tuple.Create&lt;bool, int, long&gt;(false, 0, 0); } // fix: add implicit bit before normalization if (exponent != 0) { significand = significand | (1L &lt;&lt; 52); } if (normalizeSignificand) { //* Normalize */ while ((significand &amp; 1) == 0) { /* i.e., Mantissa is even */ significand &gt;&gt;= 1; exponent++; } } return Tuple.Create(negative, exponent, significand); } Output: 55.5 2.25179981368527E+15 </code></pre>
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    1. CODoubleToInt64Bits() doesn't get you the mantissa, it gives you the value with the exponent already applied. That code is just wrong, throw it away.
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    2. COOh, ok, I didn't know that, however, have you got a suggestion how I could get what I want? I think I'll now try a union (well, a struct with explicit field layout)
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    3. COJon Skeet is supporting that answer, post a comment there. A union doesn't work, these bits don't fall on byte boundaries.
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