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POAlgorithm for copying N bits at arbitrary position from one int to another
 

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  1. POAlgorithm for copying N bits at arbitrary position from one int to another
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    <p>An interesting problem I've been pondering the past few days is how to copy one integer's bits into another integer at a given position in the destination integer. So, for example, given the destination integer <code>0xdeadbeef</code> and the source integer <code>0xabcd</code>, the idea would be to get a result of <code>0xabcdbeef</code> (given a destination position of 16 bits) or <code>0xdeabcdef</code> (given a destination position of 8 bits). </p> <p>With the arbitrary limitation of avoiding conditionals or loops (allowing myself to use just mathematical/bitwise operations), I developed the following function (C++)</p> <pre><code>int setbits(int destination, int source, int at, int numbits) { int ones = ((1&lt;&lt;(numbits))-1)&lt;&lt;at; return (ones|destination)^((~source&lt;&lt;at)&amp;ones); } </code></pre> <p>where <code>at</code> is the place where the source bits should be copied into the destination number (0-31) and <code>numbits</code> is the number of bits being copied from <code>source</code> (1-32). As far as I can tell, this algorithm works for all values except for <code>at</code> = 0 and <code>numbits</code> = 32 (the case when the entire destination integer is being overwritten by the source integer) due to the fact that 1&lt;&lt;32 results in 1 (since the shift wraps around) as opposed to 0. </p> <p>My questions are:</p> <ol> <li>How is this normally done? Are there any particularly notable algorithms used (by notable, I'm asking if there are any particularly efficient tricks that can be used to do this)?</li> <li>Does my algorithm work as well as I think it does (that is, works for all values except at = 0 and numbits = 32)?</li> <li>Related to 1), is there any way to do this only using mathematical/bitwise operators? The algorithm for all values is trivial using conditions or loops, so I'm not interested in that.</li> </ol> <p>Algorithm design is usually a weak point for me, so I have no idea whether or not my algorithm is 'as good as it gets' when only using mathematical/bitwise operations. Thanks</p>
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    1. COThat's pretty slick! I occasionally need to manipulate oddly-sized strings of bits in an environment where every cycle counts -- I'll be sure to add this to my bag of tricks.
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      1. USJim Lewis
    2. COI should caution that I've only done some superficial testing, so I can't guarantee that this method works 100%
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    3. COMight be safer to use unsigned int to avoid any funny business with sign bits.
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      1. POAlgorithm for copying N bits at arbitrary position from one int to another
      1. USCraig McQueen
 

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