Given 2^n, find n using logarithm

Question

Given a integer(2^n) which is power of 2, I want to find out n, the index value using logarithm. The formula to find index is : log(number) / log(2). Following is the code snipp

Answer 1

When using double math, the log result or quotient may not be exactly the mathematical result but 1 (or 2) next representable double away.

Calculating log() only returns an exact mathematical result for log(0), all other mathematical results are irrational. All double are rational.

This may result in an answer like 29.999999..., which saved as an int is 29.

Recommend using integer math instead

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int mylog2(unsigned long x) {
  int y = 0;
  #if (ULONG_MAX>>16 > 1lu<<16)
    if (x >= 1lu<<32) { x >>= 32; y += 32;
  #endif
  if (x >= 1lu<<16) { x >>= 16; y += 16; }
  if (x >= 1lu<<8) { x >>= 8; y += 8; }
  if (x >= 1lu<<4) { x >>= 4; y += 4; }
  if (x >= 1lu<<2) { x >>= 2; y += 2; }
  if (x >= 1lu<<1) { x >>= 1; y += 1; }
  return y;
}