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POUnderstanding Left and Right Bitwise shift on a 128-bit number
 

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    <p>Artichoke101 <a href="https://stackoverflow.com/questions/5996384/bitwise-shift-operation-on-a-128-bit-number">asked this</a>:</p> <blockquote> <p>Lets say that I have an array of 4 32-bit integers which I use to store the 128-bit number</p> <p>How can I perform left and right shift on this 128-bit number?"</p> </blockquote> <p>My question is related to the answer Remus Rusanu gave:</p> <pre><code>void shiftl128 ( unsigned int&amp; a, unsigned int&amp; b, unsigned int&amp; c, unsigned int&amp; d, size_t k) { assert (k &lt;= 128); if (k &gt; 32) { a=b; b=c; c=d; d=0; shiftl128(a,b,c,d,k-32); } else { a = (a &lt;&lt; k) | (b &gt;&gt; (32-k)); b = (b &lt;&lt; k) | (c &gt;&gt; (32-k)); c = (c &lt;&lt; k) | (d &gt;&gt; (32-k)); d = (d &lt;&lt; k); } } void shiftr128 ( unsigned int&amp; a, unsigned int&amp; b, unsigned int&amp; c, unsigned int&amp; d, size_t k) { assert (k &lt;= 128); if (k &gt; 32) { d=c; c=b; b=a; a=0; shiftr128(a,b,c,d,k-32); } else { d = (c &lt;&lt; (32-k)) | (d &gt;&gt; k); \ c = (b &lt;&lt; (32-k)) | (c &gt;&gt; k); \ b = (a &lt;&lt; (32-k)) | (b &gt;&gt; k); \ a = (a &gt;&gt; k); } } </code></pre> <p>Lets just focus on one shift, the left shift say. Specifically, </p> <pre><code>a = (a &lt;&lt; k) | (b &gt;&gt; (32-k)); b = (b &lt;&lt; k) | (c &gt;&gt; (32-k)); c = (c &lt;&lt; k) | (d &gt;&gt; (32-k)); d = (d &lt;&lt; k); </code></pre> <p>How is this left shifting the 128-bit number? I understand what bit shifting is, &lt;&lt; shifts bits left, (8-bit number) like 00011000 left shifted 2 is 01100000. Same goes for the right shift, but to the right. Then the single "pipe" | is OR meaning any 1 in either 32-bit number will be in the result.</p> <p>How is <code>a = (a &lt;&lt; k) | (b &gt;&gt; (32-k))</code> shifting the first part (32) of the 128-bit number correctly?</p>
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    1. COInstead of hardcoding the `32` I would advise using `sizeof(unsigned int)`. It should be `32`, but at least you'll make sure of it.
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    2. CO`sizeof(unsigned int)` would be 4. :) So, you need `sizeof(unsigned int)*CHAR_BIT`.
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