Swift 4 - Double 到 big-endian 的高效转换

Question

我需要将 Double 格式转换为大端格式，以便使用石油行业二进制文件标准将其写入文件，该标准最初是在 1970 年代为 IBM 半英寸 9 磁道磁带定义的！

我需要非常高效的 Swift 4 代码，因为这个转换在两个嵌套循环中进行，将执行超过 100,000 次。

Answer 1

您可以创建一个包含大端表示的 UInt64 Double 和

let value = 1.0
var n = value.bitPattern.bigEndian

为了将其写入文件，您可能需要将其转换到 Data:

let data = Data(buffer: UnsafeBufferPointer(start: &n, count: 1))
print(data as NSData) // <3ff00000 00000000>

如果将许多连续的浮点值写入文件那么创建一个 [UInt64] 数组会更有效使用大端表示并将其转换为 Data，例如

let values = [1.0, 2.0, 3.0, 4.0]
let array = values.map { [=12=].bitPattern.bigEndian }
let data = array.withUnsafeBufferPointer { Data(buffer: [=12=]) }

（以上所有内容均使用 Swift 3 和 4 进行编译。）

Answer 2

我成功实现了 Martin 的数组建议。我决定我应该使用一些 "interesting" 测试值，一件事导致另一件事！这是我的测试游乐场。希望大家感兴趣：

//: Playground - noun: a place where people can play

import UIKit

func convert(doubleArray: [Double]) {
    let littleEndianArray = doubleArray.map { [=10=].bitPattern}
    var data = littleEndianArray.withUnsafeBufferPointer { Data(buffer: [=10=]) }
    print("Little-endian : ", data as NSData)

    // Convert and display the big-endian bytes
    let bigEndianArray = doubleArray.map { [=10=].bitPattern.bigEndian }
    data = bigEndianArray.withUnsafeBufferPointer { Data(buffer: [=10=]) }
    print("Big-endian    : ", data as NSData)
}
// Values below are from:
//      https://en.wikipedia.org/wiki/Double-precision_floating-point_format

let nan = Double.nan
let plusInfinity                  =  +1.0 / 0.0
let maxDouble                     =  +1.7976931348623157E308
let smallestNumberGreaterThanOne  =  +1.0000000000000002
let plusOne                       =  +1.0
let maxSubnormalPositiveDouble    =  +2.2250738585072009E-308
let minSubnormalPositiveDouble    =  +4.9E-324
let plusZero                      =  +0.0

let minusZero                     =  -0.0
let maxSubnormalNegativeDouble    =  -4.9E-324
let minSubnormalNegativeDouble    =  -2.2250738585072009E-308
let minusOne                      =  -1.0
let largestNumberLessThanOne      =  -1.0000000000000002
let minDouble                     =  -1.7976931348623157E308
let minusInfinity                 =  -1.0 / 0.0


let smallestNumber = "+1.0000000000000002"
let largestNumber  = "-1.0000000000000002"



print("\n\nPrint little-endian and big-endian Doubles")
print("\n\nDisplay: NaN and +0.0 to +1.0")
print("                                                      Min. Subnormal    Max. Subnormal")
print("                  Not a Number      Plus Zero         Positive Double   Positive Double   Plus One")
print(String(format: "Decimal       :   NaN               %+8.6e     %+8.6e    %+8.6e    %+8.6e", plusZero, minSubnormalPositiveDouble, maxSubnormalPositiveDouble, plusOne))
var doubleArray = [nan, plusZero, minSubnormalPositiveDouble, maxSubnormalPositiveDouble, plusOne]
convert(doubleArray: doubleArray)

print("\n\nDisplay: +1.0 to +Infinity")
print("                                    Smallest Number                  ")
print("                  Plus One          Greater Than 1.0  Max. Double       +Infinity")
print(String(format: "Decimal       :   %+8.6e   \(smallestNumber) %+8.6e%+8.6e", plusOne, maxDouble, plusInfinity))
doubleArray = [plusOne, smallestNumberGreaterThanOne, maxDouble, plusInfinity]
convert(doubleArray: doubleArray)

print("\n\nDisplay: NaN and -0.0 to -1.0")
print("                                                      Min. Subnormal    Max. Subnormal")
print("                  Not a Number      Minus Zero        Negative Double   Negative Double   Minus One")
print(String(format: "Decimal       :   NaN               %+8.6e     %+8.6e    %+8.6e    %+8.6e", minusZero, maxSubnormalNegativeDouble, minSubnormalNegativeDouble, minusOne))
doubleArray = [nan, minusZero, maxSubnormalNegativeDouble, minSubnormalNegativeDouble, minusOne]
convert(doubleArray: doubleArray)

print("\n\nDisplay: -1.0 to -Infinity")
print("                                    Smallest Number                  ")
print("                  Minus One         Less Than -1.0    Min. Double       -Infinity")
print(String(format: "Decimal       :   %+8.6e   \(largestNumber) %+8.6e%+8.6e", minusOne, minDouble, minusInfinity))
doubleArray = [minusOne, largestNumberLessThanOne, minDouble, minusInfinity]
convert(doubleArray: doubleArray)

Swift 4 - Double 到 big-endian 的高效转换

Swift 4 - efficient conversion of Double to big-endian

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double

data-conversion

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swift