bit manipulation - Alt answer to classic bitwise 2^n test: valid? -


Quick question: I used to test single-n for format 2 ^ n (n> = 0) Press the line expression.

See:

An associate came up with this unorthodox answer: 

  ((n-1) xor n == 2 * n - 1 ) & Amp; Amp; (n! = 0)   
 I used a test to test it, and it seems right. I almost tested almost n = 10000  
 Does anyone see any problem with this solution? I can not find this reference online.  
 Thank you!  
 - David   
 
 
 In fact you  all  input (For example, using an SMT solver) should be tested.  
 There are 31 cases in which this formula is correct even though  n  does not have the power of two, in cases where the top bit is set, apart from some other bit. For example, 0xC0000000 or 0x80000001.  
  n ^ (n - 1)  A known method is "Remove the right set bit and tidy it to the right". Among the cases I have described,  n * 2 - 1  is one of the same mistakes, the upper bit is moved and the smear on the right side of -1 (because bit now One is on the left side where it was to be).

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