Swift 4 - efficient conversion of Double to big-endian - swift

Question

I need to convert a Double to big-endian in order to write it to a file, using an oil-industry binary file standard, that was originally defined for IBM half-inch 9 track tapes in the 1970s!
I need really efficient Swift 4 code, because this conversion is inside two nested-loops and will be executed upwards of 100,000 times.

Answer 1

You can create an UInt64 containing the big-endian representation
of the Double with
let value = 1.0
var n = value.bitPattern.bigEndian
In order to write that to a file you might need to convert it
to Data:
let data = Data(buffer: UnsafeBufferPointer(start: &n, count: 1))
print(data as NSData) // <3ff00000 00000000>
If many contiguous floating point values are written to the file
then it would be more effective to create an [UInt64] array
with the big-endian representations and convert that to Data,
for example
let values = [1.0, 2.0, 3.0, 4.0]
let array = values.map { $0.bitPattern.bigEndian }
let data = array.withUnsafeBufferPointer { Data(buffer: $0) }
(All the above compiles with Swift 3 and 4.)

Answer 2

I successfully implemented Martin's array suggestion. I decided I should use some "interesting" test values and one thing led to another! Here's my test playground. I hope it is of interest:
//: Playground - noun: a place where people can play
import UIKit
func convert(doubleArray: [Double]) {
let littleEndianArray = doubleArray.map { $0.bitPattern}
var data = littleEndianArray.withUnsafeBufferPointer { Data(buffer: $0) }
print("Little-endian : ", data as NSData)
// Convert and display the big-endian bytes
let bigEndianArray = doubleArray.map { $0.bitPattern.bigEndian }
data = bigEndianArray.withUnsafeBufferPointer { Data(buffer: $0) }
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)