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//===--- Statistics.swift -------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
/// For a given p-value, returns the critical chi-square value for
/// a distribution with 1 degree of freedom.
func _chiSquaredUniform1DFCritical(_ pValue: Double) -> Double {
if abs(pValue - 0.05) < 0.00001 { return 0.00393214 }
if abs(pValue - 0.02) < 0.00001 { return 0.000628450 }
if abs(pValue - 0.01) < 0.00001 { return 0.000157088 }
if abs(pValue - 0.007) < 0.00001 { return 0.000076971 }
if abs(pValue - 0.005) < 0.00001 { return 0.0000392704 }
if abs(pValue - 0.003) < 0.00001 { return 0.0000141372 }
if abs(pValue - 0.002) < 0.00001 { return 6.2832e-6 }
if abs(pValue - 0.001) < 0.00001 { return 1.5708e-6 }
fatalError("unknown value")
}
/// Perform chi-squared test for a discrete uniform distribution with
/// 2 outcomes.
public func chiSquaredUniform2(
_ trials: Int, _ observedACount: Int, _ pValue: Double
) -> Bool {
func square(_ x: Double) -> Double {
return x * x
}
let expectedA = 0.5
let expectedB = 0.5
let observedA = Double(observedACount) / Double(trials)
let observedB = 1.0 - observedA
let chiSq =
square(observedA - expectedA) / expectedA +
square(observedB - expectedB) / expectedB
if chiSq > _chiSquaredUniform1DFCritical(pValue) {
print("chi-squared test failed: \(trials) \(observedACount) \(chiSq)")
return false
}
return true
}
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