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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2020 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
//
//===----------------------------------------------------------------------===//
extension BinaryFloatingPoint where Self.RawSignificand: FixedWidthInteger {
/// Returns a random value within the specified range, using the given
/// generator as a source for randomness.
///
/// Use this method to generate a floating-point value within a specific
/// range when you are using a custom random number generator. This example
/// creates three new values in the range `10.0 ..< 20.0`.
///
/// for _ in 1...3 {
/// print(Double.random(in: 10.0 ..< 20.0, using: &myGenerator))
/// }
/// // Prints "18.1900709259179"
/// // Prints "14.2286325689993"
/// // Prints "13.1485686260762"
///
/// The `random(in:using:)` static method chooses a random value from a
/// continuous uniform distribution in `range`, and then converts that value
/// to the nearest representable value in this type. Depending on the size
/// and span of `range`, some concrete values may be represented more
/// frequently than others.
///
/// - Note: The algorithm used to create random values may change in a future
/// version of Swift. If you're passing a generator that results in the
/// same sequence of floating-point values each time you run your program,
/// that sequence may change when your program is compiled using a
/// different version of Swift.
///
/// - Parameters:
/// - range: The range in which to create a random value.
/// `range` must be finite and non-empty.
/// - generator: The random number generator to use when creating the
/// new random value.
/// - Returns: A random value within the bounds of `range`.
@inlinable
public static func random<T: RandomNumberGenerator>(
in range: Range<Self>,
using generator: inout T
) -> Self {
_precondition(
!range.isEmpty,
"Can't get random value with an empty range"
)
let delta = range.upperBound - range.lowerBound
// TODO: this still isn't quite right, because the computation of delta
// can overflow (e.g. if .upperBound = .maximumFiniteMagnitude and
// .lowerBound = -.upperBound); this should be re-written with an
// algorithm that handles that case correctly, but this precondition
// is an acceptable short-term fix.
_precondition(
delta.isFinite,
"There is no uniform distribution on an infinite range"
)
let rand: Self.RawSignificand
if Self.RawSignificand.bitWidth == Self.significandBitCount + 1 {
rand = generator.next()
} else {
let significandCount = Self.significandBitCount + 1
let maxSignificand: Self.RawSignificand = 1 << significandCount
// Rather than use .next(upperBound:), which has to work with arbitrary
// upper bounds, and therefore does extra work to avoid bias, we can take
// a shortcut because we know that maxSignificand is a power of two.
rand = generator.next() & (maxSignificand - 1)
}
let unitRandom = Self.init(rand) * (Self.ulpOfOne / 2)
let randFloat = delta * unitRandom + range.lowerBound
if randFloat == range.upperBound {
return Self.random(in: range, using: &generator)
}
return randFloat
}
/// Returns a random value within the specified range.
///
/// Use this method to generate a floating-point value within a specific
/// range. This example creates three new values in the range
/// `10.0 ..< 20.0`.
///
/// for _ in 1...3 {
/// print(Double.random(in: 10.0 ..< 20.0))
/// }
/// // Prints "18.1900709259179"
/// // Prints "14.2286325689993"
/// // Prints "13.1485686260762"
///
/// The `random()` static method chooses a random value from a continuous
/// uniform distribution in `range`, and then converts that value to the
/// nearest representable value in this type. Depending on the size and span
/// of `range`, some concrete values may be represented more frequently than
/// others.
///
/// This method is equivalent to calling `random(in:using:)`, passing in the
/// system's default random generator.
///
/// - Parameter range: The range in which to create a random value.
/// `range` must be finite and non-empty.
/// - Returns: A random value within the bounds of `range`.
@inlinable
public static func random(in range: Range<Self>) -> Self {
var g = SystemRandomNumberGenerator()
return Self.random(in: range, using: &g)
}
/// Returns a random value within the specified range, using the given
/// generator as a source for randomness.
///
/// Use this method to generate a floating-point value within a specific
/// range when you are using a custom random number generator. This example
/// creates three new values in the range `10.0 ... 20.0`.
///
/// for _ in 1...3 {
/// print(Double.random(in: 10.0 ... 20.0, using: &myGenerator))
/// }
/// // Prints "18.1900709259179"
/// // Prints "14.2286325689993"
/// // Prints "13.1485686260762"
///
/// The `random(in:using:)` static method chooses a random value from a
/// continuous uniform distribution in `range`, and then converts that value
/// to the nearest representable value in this type. Depending on the size
/// and span of `range`, some concrete values may be represented more
/// frequently than others.
///
/// - Note: The algorithm used to create random values may change in a future
/// version of Swift. If you're passing a generator that results in the
/// same sequence of floating-point values each time you run your program,
/// that sequence may change when your program is compiled using a
/// different version of Swift.
///
/// - Parameters:
/// - range: The range in which to create a random value. Must be finite.
/// - generator: The random number generator to use when creating the
/// new random value.
/// - Returns: A random value within the bounds of `range`.
@inlinable
public static func random<T: RandomNumberGenerator>(
in range: ClosedRange<Self>,
using generator: inout T
) -> Self {
_precondition(
!range.isEmpty,
"Can't get random value with an empty range"
)
let delta = range.upperBound - range.lowerBound
// TODO: this still isn't quite right, because the computation of delta
// can overflow (e.g. if .upperBound = .maximumFiniteMagnitude and
// .lowerBound = -.upperBound); this should be re-written with an
// algorithm that handles that case correctly, but this precondition
// is an acceptable short-term fix.
_precondition(
delta.isFinite,
"There is no uniform distribution on an infinite range"
)
let rand: Self.RawSignificand
if Self.RawSignificand.bitWidth == Self.significandBitCount + 1 {
rand = generator.next()
let tmp: UInt8 = generator.next() & 1
if rand == Self.RawSignificand.max && tmp == 1 {
return range.upperBound
}
} else {
let significandCount = Self.significandBitCount + 1
let maxSignificand: Self.RawSignificand = 1 << significandCount
rand = generator.next(upperBound: maxSignificand + 1)
if rand == maxSignificand {
return range.upperBound
}
}
let unitRandom = Self.init(rand) * (Self.ulpOfOne / 2)
let randFloat = delta * unitRandom + range.lowerBound
return randFloat
}
/// Returns a random value within the specified range.
///
/// Use this method to generate a floating-point value within a specific
/// range. This example creates three new values in the range
/// `10.0 ... 20.0`.
///
/// for _ in 1...3 {
/// print(Double.random(in: 10.0 ... 20.0))
/// }
/// // Prints "18.1900709259179"
/// // Prints "14.2286325689993"
/// // Prints "13.1485686260762"
///
/// The `random()` static method chooses a random value from a continuous
/// uniform distribution in `range`, and then converts that value to the
/// nearest representable value in this type. Depending on the size and span
/// of `range`, some concrete values may be represented more frequently than
/// others.
///
/// This method is equivalent to calling `random(in:using:)`, passing in the
/// system's default random generator.
///
/// - Parameter range: The range in which to create a random value. Must be finite.
/// - Returns: A random value within the bounds of `range`.
@inlinable
public static func random(in range: ClosedRange<Self>) -> Self {
var g = SystemRandomNumberGenerator()
return Self.random(in: range, using: &g)
}
}
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