Convert an array of HKQuantitySample into splits.

WorkoutSplit.swift

import Foundation
import HealthKit

struct WorkoutSplit: Hashable {
  let label: String
  let distance: HKQuantity
  let duration: TimeInterval
}

extension WorkoutSplit {
  
  static func array(
    fromHKQuantitySamples samples: [HKQuantitySample], 
    fromStartDate workoutStartDate: Date, 
    withDistanceUnit distanceUnit: HKUnit
  ) -> [WorkoutSplit] {
    var splits = [WorkoutSplit]()
    
    // The size of each split, e.g. 1 km / 1 mile
    let splitSize: Double = 1.0
    
    // A place to keep a reference to the previous iteration of the loop, so that
    // any time gaps between measurements can be accounted for.
    var previousSample: HKQuantitySample? = nil
    
    var currentDistanceTotal: Double = 0
    var currentDurationTotal: TimeInterval = 0
    var splitCount = 1
    
    for sample in samples {
                  
      let distance = sample.quantity.doubleValue(for: distanceUnit)
      let duration = sample.endDate - sample.startDate
      
      // Add the current sample to the running total.
      currentDistanceTotal += distance
      currentDurationTotal += duration
      
      // Check for gaps between sample times, and add them where necessary.
      if let previousSample = previousSample {
        let durationGapBetweenSamples = sample.startDate - previousSample.endDate
        if durationGapBetweenSamples > 0 {
          currentDurationTotal += durationGapBetweenSamples
        }
      } 
      // Otherwise this is the first sample, so take into account the gap between the workout's start
      // date and the start of this sample
      else {
        let gapBetweenWorkoutStartingAndFirstSample = sample.startDate - workoutStartDate
        currentDurationTotal += gapBetweenWorkoutStartingAndFirstSample
      }
      
      // If the current split is over 1km/1mi, we need to reset and start a new split.
      if currentDistanceTotal > splitSize {
        
        // How much the current distance 'overflows' the split by.
        let distanceOverflow = currentDistanceTotal - splitSize
        
        // We need to adjust the durations for this and next split to account for the overflow.
        let durationProportionBelongingToThisSplit = (distance - distanceOverflow) / distance
        let durationProportionBelongingToNextSplit = 1 - durationProportionBelongingToThisSplit
        let durationOverflow = duration * durationProportionBelongingToNextSplit
        
        currentDurationTotal -= durationOverflow
        
        // Record the split.
        splits.append(WorkoutSplit(
          label: "\(distanceUnit.unitString) \(splitCount)",
          distance: HKQuantity(unit: distanceUnit, doubleValue: splitSize),
          duration: currentDurationTotal
        ))
        
        // Reset the counters, beginning them with the overflows.
        currentDistanceTotal = distanceOverflow
        currentDurationTotal = durationOverflow
        splitCount += 1
        
      } // if
      
      previousSample = sample
      
    } // for
    
    return splits
  }
}