Dividing one 32-bit number by another is a constant-time operation even if on some machines (e.g. ARM) it is implemented as a loop. This is what matters. Raising a number to the nth power is an O(log n) operation.
To decode? Why does the user care how long it takes to decode, they care how long it takes to run. We can provide upper bounds for exp, sin, and erfc too.
