// SPDX-License-Identifier: Apache-2.0
//
// Copyright The Prometheus 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 multiplier applies the optional multiplier of a user-defined metric
// of the wmi, performancecounter and registry collectors. It converts values
// to base units, for example milliseconds to seconds with 0.001.
package multiplier

import (
	"fmt"
	"math"
)

// maxExactInteger is the largest integer up to which every integer has an
// exact float64 representation.
const maxExactInteger = 1 << 53

// Multiplier scales values. The zero value leaves them unchanged.
type Multiplier struct {
	factor  float64
	divisor float64
}

// New validates the multiplier of the configuration; nil means none. It must
// be a positive finite number, so that counters stay monotonic.
func New(multiplier *float64) (Multiplier, error) {
	if multiplier == nil {
		return Multiplier{}, nil
	}

	m := *multiplier

	if math.IsNaN(m) || math.IsInf(m, 0) || m <= 0 {
		return Multiplier{}, fmt.Errorf("multiplier must be a positive finite number, got %v", m)
	}

	if m == 1 {
		return Multiplier{}, nil
	}

	// x * 0.001 is not always the correctly rounded x / 1000, because 0.001
	// has no exact binary representation: 9 * 0.001 is 0.009000000000000001.
	// Divide by the reciprocal instead when it is an integer.
	if m < 1 {
		if d := math.Round(1 / m); d <= maxExactInteger && 1/d == m {
			return Multiplier{divisor: d}, nil
		}
	}

	return Multiplier{factor: m}, nil
}

// Apply returns v scaled by the multiplier.
func (m Multiplier) Apply(v float64) float64 {
	switch {
	case m.divisor != 0:
		return v / m.divisor
	case m.factor != 0:
		return v * m.factor
	default:
		return v
	}
}