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@orlp /ipow.c
Last active Apr 15, 2017

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int64_t ipow(int64_t base, uint8_t exp) {
static const uint8_t highest_bit_set[] = {
0, 1, 2, 2, 3, 3, 3, 3,
4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5,
5, 5, 5, 5, 5, 5, 5, 5,
6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 255, // anything past 63 is a guaranteed overflow with base > 1
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
};
uint64_t result = 1;
switch (highest_bit_set[exp]) {
case 255: // we use 255 as an overflow marker and return 0 on overflow/underflow
if (base == 1) {
return 1;
}
if (base == -1) {
return 1 - 2 * (exp & 1);
}
return 0;
case 6:
if (exp & 1) result *= base;
exp >>= 1;
base *= base;
case 5:
if (exp & 1) result *= base;
exp >>= 1;
base *= base;
case 4:
if (exp & 1) result *= base;
exp >>= 1;
base *= base;
case 3:
if (exp & 1) result *= base;
exp >>= 1;
base *= base;
case 2:
if (exp & 1) result *= base;
exp >>= 1;
base *= base;
case 1:
if (exp & 1) result *= base;
default:
return result;
}
}

woodb commented Nov 17, 2013

That's some really pretty code, nice work.

holtdoa commented Jan 5, 2016

Pretty slick.

Jabro commented Aug 11, 2016 edited

Pretty cool!
There is one bug though, if you call it e.g. with base = 16 and exponent = 8, the int32_t base will overflow --> as 16^8 = 2^32 --> function will return 0 instead of 2^32

Either pass as int64_t or use a temporary variable with 64 bits

prakharsingh95 commented Sep 5, 2016 edited

You can calculate highest set bit as i <= 64 ? (int)(log10(i)/log10(2)) : 255.

mmdts commented Apr 15, 2017

Prakhasingh95, that would prove pointless. The point of this function is that it is so fast, that would just slow it down.

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