c++ - How get smallest n, that 2 ^ n >= x for given integer x in O(1)? -

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how given unsigned integer x find smallest n, 2 ^ n ≥ x in o(1)? in other words want find index of higher set bit in binary format of x (plus 1 if x not power of 2) in o(1) (not depended on size of integer , size of byte).

if have no memory constraints, can use lookup table (one entry each possible value of x) achieve o(1) time.

if want practical solution, processors have kind of "find highest bit set" opcode. on x86, instance, it's bsr. compilers have mechanism write raw assembler.


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