forked from dop251/goja
-
Notifications
You must be signed in to change notification settings - Fork 0
/
ipow.go
97 lines (91 loc) · 2.11 KB
/
ipow.go
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
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
}
}