/
ipow.go
97 lines (91 loc) · 2.11 KB
/
ipow.go
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package goja
// ported from https://gist.github.com/orlp/3551590
var highest_bit_set = [256]byte{
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,
}
func ipow(base, exp int64) (result int64) {
result = 1
switch highest_bit_set[byte(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 != 0 {
result *= base
}
exp >>= 1
base *= base
fallthrough
case 5:
if exp&1 != 0 {
result *= base
}
exp >>= 1
base *= base
fallthrough
case 4:
if exp&1 != 0 {
result *= base
}
exp >>= 1
base *= base
fallthrough
case 3:
if exp&1 != 0 {
result *= base
}
exp >>= 1
base *= base
fallthrough
case 2:
if exp&1 != 0 {
result *= base
}
exp >>= 1
base *= base
fallthrough
case 1:
if exp&1 != 0 {
result *= base
}
fallthrough
default:
return result
}
}