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POConstrain a 16 bit signed value between 0 and 4095 using Bit Manipulation only (without branching)
 

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  1. POConstrain a 16 bit signed value between 0 and 4095 using Bit Manipulation only (without branching)
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    <p>I want to constrain the value of a <code>signed short</code> variable between 0 and 4095, after which I take the most significant 8 bits as my final value for use elsewhere. Right now I'm doing it in a basic manner as below:</p> <pre><code>short color = /* some external source */; /* * I get the color value as a 16 bit signed integer from an * external source I cannot trust. 16 bits are being used here * for higher precision. */ if ( color &lt; 0 ) { color = 0; } else if ( color &gt; 4095 ) { color = 4095; } unsigned char color8bit = 0xFF &amp; (color &gt;&gt; 4); /* * color8bit is my final value which I would actually use * in my application. */ </code></pre> <p>Is there any way this can be done using bit manipulation only, i.e. without using any conditionals? It might help quite a bit in speeding things up as this operation is happening thousands of time in the code.</p> <p>The following won't help as it doesn't take care of edge cases such as negative values and overflows:</p> <pre><code>unsigned char color8bit = 0xFF &amp; (( 0x0FFF &amp; color ) &gt;&gt; 4 ); </code></pre> <p><strong>Edit:</strong> <a href="https://stackoverflow.com/a/12308926/1044750">Adam Rosenfield's answer</a> is the one which takes the correct approach but its incorrectly implemented. <a href="https://stackoverflow.com/a/12309664/1044750">ouah's answer</a> gives correct results but takes a different approach that what I originally intended to find out.</p> <p>This is what I ended up using:</p> <pre><code>const static short min = 0; const static short max = 4095; color = min ^ (( min ^ color ) &amp; -( min &lt; color )); color = max ^ (( color ^ max ) &amp; -( color &lt; max )); unsigned char color8bit = 0xFF &amp; (( 0x0FFF &amp; color ) &gt;&gt; 4 ); </code></pre>
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    1. COwhat if you cast to unsigned as the first step in that "the following won't help" bit?
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    2. COWhy not an unsigned short? Then for -ve values it would unsigned overflow (which is not UB), then you could just bitmask for 4095 (which is `(1 << 12) - 1`).
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    3. COcan the value really be negative?
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      1. USDavid Heffernan
 

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