So I got to the point I wanted to speed up the parsing of Fit Files

Fast Swift Parsing of Fit Files

The standard fit sdk in c++ was out because of speed, I started to work out how the c version was working to see how to link that into swift.

There are two main files of interest in the c examples of the standard Fit SDK:

  • fit_convert.c where the actual logic of the parsing is implemented
  • fit_example.h where you can find the definitions of the different fields

I found the implementation of the parsing logic quite fascinating. It runs via a loop that maintain a state and is very robust to re-entry and processing arbitrary (small) buffer size. Clearly, this was implemented with keeping the memory tight in mind. You need to call the function FitConvert_Read and feed it more data in a buffer while the return value is FIT_CONVERT_CONTINUE or process a message when the return value is FIT_CONVERT_MESSAGE_AVAILABLE.

C pointers and swift

The first challenge was to figure out the swift syntax to call and interact with the c function. It turns out swift has a very neat way of interacting with pointers.

First you need to pass in a FIT_UINT8 * with the address to read the Fit Data from. In swift, from the Data object you can then access the equivalent UnsafePointer<UInt8> and pass it to the c function using withUnsafeByes:

       while convert_return == FIT_CONVERT_CONTINUE {
            data.withUnsafeBytes({ (ptr: UnsafePointer<UInt8>) in
                repeat {
                    convert_return = FitConvert_Read(&state, ptr, FIT_UINT32(data.count))

The next challenge is when a message is received, you need to typecase the pointer to a pointer the appropriate structure to extract the information. For example, for a struct FIT_RECORD_MESG*, in c you would convert the pointer to (FIT_RECORD_MESG*)ptr. In swift you can achieve the same using withMemoryRebound(to:, capacity:) as follows:

func conv( uptr : UnsafePointer<UInt8>){
	uptr.withMemoryRebound(to: FIT_RECORD_MESG.self, capacity: 1) {
		callfunc( ptr: $0 )

You can see the final swift code calling and processing the convert function here

Processing the types definitions in Fit Files

The next challenge was that all the conceptual definition of the types in the fit files are either typedef of numbers or c structures. And there is almost 8000 lines of such definition in the c header files. I wanted to be able to convert the files from these structures and integer code into a more processing friend key value representation of strings to double.

For this I decided to convert the almost 8000 lines of c typedefs into a corresponding swift file of definition. Given that the SDK and fields are regularly updated it was out of the question to process even semi-manually with regex… So I wrote a python script that would convert the c code to swift…

There are three main type of code I had to convert, enum typedef which I would convert to function mapping the integer code to a string. A good example would be

#define FIT_GENDER_INVALID                                                       FIT_ENUM_INVALID
#define FIT_GENDER_FEMALE                                                        ((FIT_GENDER)0)
#define FIT_GENDER_MALE                                                          ((FIT_GENDER)1)
#define FIT_GENDER_COUNT                                                         2

that the python code would convert to:

func rzfit_gender_string(input : FIT_ENUM) -> String? 
  switch  input {
    case FIT_GENDER_FEMALE: return "female";
    case FIT_GENDER_MALE: return "male";
    default: return nil

The python code is quite straight forward and generate the string by removing the type name and going lower case…

typedef struct
   FIT_MESSAGE_INDEX message_index; //
   FIT_UINT16 high_value; // 1000 * m/s + 0,

That would have to be converted to a function to convert the values high_value and a function to convert the enums FIT_MESSAGE_INDEX:

func rzfit_speed_zone_mesg_value_dict( ptr : UnsafePointer<FIT_SPEED_ZONE_MESG>) -> [String:Double] {
  var rv : [String:Double] = [:]
  let x : FIT_SPEED_ZONE_MESG = ptr.pointee
  if x.high_value != FIT_UINT16_INVALID  {
    let val : Double = (Double(x.high_value))/Double(1000)
    rv[ "high_value" ] = val
  return rv
func rzfit_speed_zone_mesg_enum_dict( ptr : UnsafePointer<FIT_SPEED_ZONE_MESG>) -> [String:String] {
  var rv : [String:String] = [:]
  let x : FIT_SPEED_ZONE_MESG = ptr.pointee
  if( x.message_index != FIT_MESSAGE_INDEX_INVALID ) {
    rv[ "message_index" ] = rzfit_message_index_string(input: x.message_index)
  return rv

Putting it all together

Equipped with all these 9000 lines of auto generated functions to convert the c struct into swift dictionaries, all what remained was to put it all together. A message read from a fit file contains a message number as FIT_MESG_NUM c type, which just required now a big auto generated switch putting the result of the correct function into a swift class container as below:

func rzfit_build_mesg(num : FIT_MESG_NUM, uptr : UnsafePointer<UInt8>) -> RZFitMessage?{
    var rv : RZFitMessage? = nil
    switch num {
    uptr.withMemoryRebound(to: FIT_FILE_ID_MESG.self, capacity: 1) {
      rv = RZFitMessage( mesg_num:    FIT_MESG_NUM_FILE_ID,
                         mesg_values: rzfit_file_id_mesg_value_dict(ptr: $0),
                         mesg_enums:  rzfit_file_id_mesg_enum_dict(ptr: $0))
    uptr.withMemoryRebound(to: FIT_CAPABILITIES_MESG.self, capacity: 1) {
      rv = RZFitMessage( mesg_num:    FIT_MESG_NUM_CAPABILITIES,
                         mesg_values: rzfit_capabilities_mesg_value_dict(ptr: $0),
                         mesg_enums:  rzfit_capabilities_mesg_enum_dict(ptr: $0))

The project contains a small command line utility and a sample file, and it parses in 0.044 second 6718 messages:

Parsing /Users/brice/Development/public/fit-sdk-swift/samples/
6718 messages in 0.04484999179840088 seconds
Program ended with exit code: 0

As a comparison the c++ parsing code from the original sdk would parse the same file in 3.88 seconds

FIT Decode Example Application
Decoded FIT file /Users/brice/Development/public/fit-sdk-swift/samples/ in 3.884835.
Program ended with exit code: 0