Parallel Map, Filter Implementation

parallel.swift

import Foundation

public extension Array {
    
    func pmap<T>(transformer: @escaping (Element) -> T) -> [T] {
        
        var result: [Int: [T]] = [:]
        guard !self.isEmpty else {
            return []
        }
        
        let coreCount = ProcessInfo.processInfo.activeProcessorCount
        let sampleSize = Int(ceil(Double(count) / Double(coreCount)))
        
        let group = DispatchGroup()
        
        for index in 0..<sampleSize {
            
            let startIndex = index * coreCount
            let endIndex = Swift.min((startIndex + (coreCount - 1)), count - 1)
            result[startIndex] = []
            
            group.enter()
            DispatchQueue.global().async {
                for index in startIndex...endIndex {
                    result[startIndex]?.append(transformer(self[index]))
                }
                group.leave()
            }
        }
        
        group.wait()
        return result.sorted(by: { $0.0 < $1.0 }).flatMap { $0.1 }
    }
    
    func pfilter(filter: @escaping (Element) -> Bool) -> [Element] {
        
        var result: [Int: [Element]] = [:]
        guard !self.isEmpty else {
            return []
        }
        
        let coreCount = ProcessInfo.processInfo.activeProcessorCount
        let sampleSize = Int(ceil(Double(count) / Double(coreCount)))
        
        let group = DispatchGroup()
        
        for index in 0..<sampleSize {
            
            let startIndex = index * coreCount
            let endIndex = Swift.min((startIndex + (coreCount - 1)), count - 1)
            result[startIndex] = []
            
            group.enter()
            DispatchQueue.global().async {
                for index in startIndex...endIndex where filter(self[index]) {
                    result[startIndex]?.append(self[index])
                }
                group.leave()
            }
        }
        
        group.wait()
        return result.sorted(by: { $0.0 < $1.0 }).flatMap { $0.1 }
    }

    
}

You have an unsynchronized accesses to result, which critically breaks this code.