Zack Williams | e940c7a | 2019-08-21 14:25:39 -0700 | [diff] [blame] | 1 | /* |
| 2 | Copyright 2015 The Kubernetes Authors. |
| 3 | |
| 4 | Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | you may not use this file except in compliance with the License. |
| 6 | You may obtain a copy of the License at |
| 7 | |
| 8 | http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | |
| 10 | Unless required by applicable law or agreed to in writing, software |
| 11 | distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | See the License for the specific language governing permissions and |
| 14 | limitations under the License. |
| 15 | */ |
| 16 | |
| 17 | package resource |
| 18 | |
| 19 | import ( |
| 20 | "math" |
| 21 | "math/big" |
| 22 | "sync" |
| 23 | ) |
| 24 | |
| 25 | var ( |
| 26 | // A sync pool to reduce allocation. |
| 27 | intPool sync.Pool |
| 28 | maxInt64 = big.NewInt(math.MaxInt64) |
| 29 | ) |
| 30 | |
| 31 | func init() { |
| 32 | intPool.New = func() interface{} { |
| 33 | return &big.Int{} |
| 34 | } |
| 35 | } |
| 36 | |
| 37 | // scaledValue scales given unscaled value from scale to new Scale and returns |
| 38 | // an int64. It ALWAYS rounds up the result when scale down. The final result might |
| 39 | // overflow. |
| 40 | // |
| 41 | // scale, newScale represents the scale of the unscaled decimal. |
| 42 | // The mathematical value of the decimal is unscaled * 10**(-scale). |
| 43 | func scaledValue(unscaled *big.Int, scale, newScale int) int64 { |
| 44 | dif := scale - newScale |
| 45 | if dif == 0 { |
| 46 | return unscaled.Int64() |
| 47 | } |
| 48 | |
| 49 | // Handle scale up |
| 50 | // This is an easy case, we do not need to care about rounding and overflow. |
| 51 | // If any intermediate operation causes overflow, the result will overflow. |
| 52 | if dif < 0 { |
| 53 | return unscaled.Int64() * int64(math.Pow10(-dif)) |
| 54 | } |
| 55 | |
| 56 | // Handle scale down |
| 57 | // We have to be careful about the intermediate operations. |
| 58 | |
| 59 | // fast path when unscaled < max.Int64 and exp(10,dif) < max.Int64 |
| 60 | const log10MaxInt64 = 19 |
| 61 | if unscaled.Cmp(maxInt64) < 0 && dif < log10MaxInt64 { |
| 62 | divide := int64(math.Pow10(dif)) |
| 63 | result := unscaled.Int64() / divide |
| 64 | mod := unscaled.Int64() % divide |
| 65 | if mod != 0 { |
| 66 | return result + 1 |
| 67 | } |
| 68 | return result |
| 69 | } |
| 70 | |
| 71 | // We should only convert back to int64 when getting the result. |
| 72 | divisor := intPool.Get().(*big.Int) |
| 73 | exp := intPool.Get().(*big.Int) |
| 74 | result := intPool.Get().(*big.Int) |
| 75 | defer func() { |
| 76 | intPool.Put(divisor) |
| 77 | intPool.Put(exp) |
| 78 | intPool.Put(result) |
| 79 | }() |
| 80 | |
| 81 | // divisor = 10^(dif) |
| 82 | // TODO: create loop up table if exp costs too much. |
| 83 | divisor.Exp(bigTen, exp.SetInt64(int64(dif)), nil) |
| 84 | // reuse exp |
| 85 | remainder := exp |
| 86 | |
| 87 | // result = unscaled / divisor |
| 88 | // remainder = unscaled % divisor |
| 89 | result.DivMod(unscaled, divisor, remainder) |
| 90 | if remainder.Sign() != 0 { |
| 91 | return result.Int64() + 1 |
| 92 | } |
| 93 | |
| 94 | return result.Int64() |
| 95 | } |