scale_int.go

/*
Copyright 2015 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package resource

import (
	"math"
	"math/big"
	"sync"
)

var (
	// A sync pool to reduce allocation.
	intPool  sync.Pool
	maxInt64 = big.NewInt(math.MaxInt64)
)

func init() {
	intPool.New = func() interface{} {
		return &big.Int{}
	}
}

// scaledValue scales given unscaled value from scale to new Scale and returns
// an int64. It ALWAYS rounds up the result when scale down. The final result might
// overflow.
//
// scale, newScale represents the scale of the unscaled decimal.
// The mathematical value of the decimal is unscaled * 10**(-scale).
func scaledValue(unscaled *big.Int, scale, newScale int) int64 {
	dif := scale - newScale
	if dif == 0 {
		return unscaled.Int64()
	}

	// Handle scale up
	// This is an easy case, we do not need to care about rounding and overflow.
	// If any intermediate operation causes overflow, the result will overflow.
	if dif < 0 {
		return unscaled.Int64() * int64(math.Pow10(-dif))
	}

	// Handle scale down
	// We have to be careful about the intermediate operations.

	// fast path when unscaled < max.Int64 and exp(10,dif) < max.Int64
	const log10MaxInt64 = 19
	if unscaled.Cmp(maxInt64) < 0 && dif < log10MaxInt64 {
		divide := int64(math.Pow10(dif))
		result := unscaled.Int64() / divide
		mod := unscaled.Int64() % divide
		if mod != 0 {
			return result + 1
		}
		return result
	}

	// We should only convert back to int64 when getting the result.
	divisor := intPool.Get().(*big.Int)
	exp := intPool.Get().(*big.Int)
	result := intPool.Get().(*big.Int)
	defer func() {
		intPool.Put(divisor)
		intPool.Put(exp)
		intPool.Put(result)
	}()

	// divisor = 10^(dif)
	// TODO: create loop up table if exp costs too much.
	divisor.Exp(bigTen, exp.SetInt64(int64(dif)), nil)
	// reuse exp
	remainder := exp

	// result = unscaled / divisor
	// remainder = unscaled % divisor
	result.DivMod(unscaled, divisor, remainder)
	if remainder.Sign() != 0 {
		return result.Int64() + 1
	}

	return result.Int64()
}