// Copyright © 2016 Abcum Ltd
//
// 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 math

import "math"

func Variance(vals []float64, sample bool) float64 {

	if len(vals) == 0 {
		return math.NaN()
	}

	var out float64

	m := Mean(vals)

	for _, v := range vals {
		out += (float64(v) - m) * (float64(v) - m)
	}

	return out / float64((len(vals) - (1 * Bool(sample))))

}

func SampleVariance(vals []float64) float64 {

	if len(vals) == 0 {
		return math.NaN()
	}

	return Variance(vals, true)

}

func PopulationVariance(vals []float64) float64 {

	if len(vals) == 0 {
		return math.NaN()
	}

	return Variance(vals, false)

}