Swift 5 Create 3D Array of Doubles And Pass To C Function - arrays

Question

I need to call a legacy C function (from swift) that expects a 3D array of Doubles as an argument. I am fairly new to Swift and have begun converting a large ObjC and C code base written for iOS and Mac over to Swift. The C code does a lot of complex astronomical math and for which Swift is just too cumbersome. I will not convert those, but I need to use them from Swift
The C function is declared like this and the .H file is visible to swift:
void readSWEDayData(double dData[DATA_ROWS_PER_DAY][NUM_PLANET_ELEMENTS][NUM_ELEMENTS_PER_PLANET]);
The Constants used in the declaration are defined to be:
DATA_ROWS_PER_DAY = 1
NUM_PLANET_ELEMENTS = 35
NUM_ELEMENTS_PER_PLANET = 4
I am struggling with declaring the array of doubles in a way that Swift will allow to be passed to the C function. I've tried several approaches.
First Approach:
I declare the array and call it like so:
var data = Array(repeating: Double(EPHEMERIS_NA), count:Int(DATA_ROWS_PER_DAY * NUM_PLANET_ELEMENTS * NUM_ELEMENTS_PER_PLANET))
readSWEDayData(&data)
I get this error: Cannot convert value of type 'UnsafeMutablePointer' to expected argument type 'UnsafeMutablePointer<((Double, Double, Double, Double),...
Second Approach:
If I declare the array this way:
var data = [(Double, Double, Double, Double)](repeating: (EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA), count: Int(NUM_PLANET_ELEMENTS))
readSWEDayData(&data)
I get this error: Cannot convert value of type 'UnsafeMutablePointer<(Double, Double, Double, Double)>' to expected argument type 'UnsafeMutablePointer<((Double, Double, Double, Double),
So, how the heck does one declare a 3D Array in Swift of a specific size so that it can be passed to a C Function?

Answer 1

The function needs an UnsafeMutablePointer to a 35-tuple of things, where each of those things are 4-tuples of Doubles. Yes, C arrays translate to tuples in Swift, because Swift doesn't have fixed size arrays. You could do:
var giantTuple = (
(EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA),
(EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA),
(EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA),
// 32 more times...
)
readSWEDayData(&giantTuple)
But I don't think you'd like that. You can create an array, and use some pointer magic to convert that to a tuple, as discussed in this Swift Forums post. In fact, that post is highly relevant to your situation.
To save some typing, we can write some type aliases first:
typealias Tuple35<T> = (T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T,T)
typealias Double4x35 = Tuple35<(Double, Double, Double, Double)>
Then we can do:
var giantTuple = Array(repeating: (EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA), count: NUM_PLANET_ELEMENTS).withUnsafeBytes { p in
p.bindMemory(to: Double4x35.self)[0]
}
readSWEDayData(&giantTuple)
This works because tuples and arrays have essentially the same "layout" in memory.
Note that I "cheated" a little bit here, since DATA_ROWS_PER_DAY is 1, you can just create one such giantTuple, and get a pointer to it. However, if it is greater than 1, you'd have to do something like:
var giantTuples = Array(repeating:
Array(repeating: (EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA, EPHEMERIS_NA), count: NUM_PLANET_ELEMENTS).withUnsafeBytes { p in
p.bindMemory(to: Double4x35.self)[0]
},
count: DATA_ROWS_PER_DAY)
readSWEDayData(&giantTuples)
To convert from the giant tuple back to an array, you can do something like this:
// converting the first giantTuples in "giantTuples" as an example
let arrayOf4Tuples = asCollection(giantTuples[0], Array.init)
let finalArray = arrayOf4Tuples.map { asCollection($0, Array.init) }
// these are adapted from the Swift forum thread
// you'll need two of these, because you have 2 types of tuples
// yes, working with C arrays is hard :(
func asCollection<T, E>(_ tuple: Tuple35<E>, _ perform: (UnsafeBufferPointer<E>)->T) -> T {
return withUnsafeBytes(of: tuple) { ptr in
let buffer = ptr.bindMemory(to: (E.self))
return perform(buffer)
}
}
func asCollection<T, E>(_ tuple: (E, E, E, E), _ perform: (UnsafeBufferPointer<E>)->T) -> T {
return withUnsafeBytes(of: tuple) { ptr in
let buffer = ptr.bindMemory(to: (E.self))
return perform(buffer)
}
}

Answer 2

Because Swift 5 lacks support for interoperability with C language multi-dimensional Arrays of fixed size except via tuples of explicitly declared structure (See Sweeper's answer above) and which is something I wish to avoid to keep my code flexible for future changes to the C Library being used, I opted to write a wrapper for the C function and make it appear to Swift as a 1 dimensional array.
This was necessary because the Constants used in the C Code change when readSWEDayData increases the array sizes to support additional elements and tuple declarations like this:
let Double4x35 = Tuple35<(Double, Double, Double, Double)>
will DEFINITELY break in a way that will be hard to find:
So my C wrapper function looks like so:
void readSWEDayDataForSwift(double *dData) {
readSWEDayData((double (*)[NUM_PLANET_ELEMENTS][NUM_ELEMENTS_PER_PLANET])dData);
}
Making it easy to call it from Swift like so:
var data = Array(repeating: Double(EPHEMERIS_NA), count:Int(DATA_ROWS_PER_DAY * NUM_PLANET_ELEMENTS * NUM_ELEMENTS_PER_PLANET))
I was surprised that this far into Swift's evolution there is no better way to do this!

Answer 3

My two cents for others..Hoping will help.
I got a similar problem, but hope can save time for other.
I had to pass down:
path (from String to char *)
title (from String to char *)
columns ([String] to array of char *)
a counter
to sum up I had to call "C" function:
bool OpenXLSXManager_saveIn(const char * cFullPath,
const char * sheetName,
char *const columnTitles[],
double *const values[],
int columnCount);
I started from excellent:
// https://oleb.net/blog/2016/10/swift-array-of-c-strings/
expanded a bit:
public func withArrayOfCStringsAndValues<R>(
_ args: [String],
_ values: [[Double]],
_ body: ([UnsafeMutablePointer<CChar>?] , [UnsafeMutablePointer<Double>?] ) -> R ) -> R {
var cStrings = args.map { strdup($0) }
cStrings.append(nil)
let cValuesArrr = values.map { (numbers: [Double]) -> UnsafeMutablePointer<Double> in
let pointer = UnsafeMutablePointer<Double>.allocate(capacity: numbers.count)
for (index, value) in numbers.enumerated() {
pointer.advanced(by: index).pointee = value
}
return pointer
}
defer {
cStrings.forEach { free($0) }
for pointer in cValuesArrr{
pointer.deallocate()
}
}
return body(cStrings, cValuesArrr)
}
so I can call:
func passDown(
filePath: String,
sheetName:
String,
colNames: [String],
values: [[Double]]
) -> Bool
{
let columnCount = Int32(colNames.count)
return withArrayOfCStringsAndValues(colNames, values) {
columnTitles, values in
let retval = OpenXLSXManager_saveIn(filePath, sheetName, columnTitles, values, columnCount)
return retval
}
}
(SORRY for formatting, S.O. formatter has BIG issues ..)