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@orlp /ipow.c
Created Aug 31, 2012

int64_t ipow(int32_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
woodb commented Nov 17, 2013

That's some really pretty code, nice work.

@holtdoa
holtdoa commented Jan 5, 2016

Pretty slick.

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