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Optimisation of the parallelCompactMap implementation #20

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Optimisation of the parallelCompactMap implementation #20

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72 changes: 3 additions & 69 deletions Sources/Carpaccio/Collection+Extensions.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,7 @@ extension Swift.Collection where Index == Int {
return []
}

var result: [(Int, T?)] = []
var result: [T?] = Array(repeating: nil, count: self.count)

let group = DispatchGroup()
let lock = DispatchQueue(label: "pcompactmap")
Expand All @@ -33,7 +33,7 @@ extension Swift.Collection where Index == Int {
do {
let t = try transform(self[i])
lock.async(group: group) {
result += [(i, t)]
result[i] = t
}
}
catch {
Expand All @@ -47,72 +47,6 @@ extension Swift.Collection where Index == Int {
throw error
}

return result.sorted { $0.0 < $1.0 }.compactMap { $0.1 }
return result.compactMap { $0 }
}
}

// Commented out, for now, due to unreliable operation: some elements might go missing from the result.
/*
// Inspired by http://moreindirection.blogspot.co.uk/2015/07/gcd-and-parallel-collections-in-swift.html
extension Swift.Sequence {
public func parallelMap<T>(maxParallelism:Int? = nil, _
transform: @escaping ((Iterator.Element) throws -> T)) throws -> [T]
{
return try self.parallelCompactMap(maxParallelism: maxParallelism, transform)
}

public func parallelCompactMap<T>(maxParallelism:Int? = nil, _ transform: @escaping ((Iterator.Element) throws -> T?)) throws -> [T]
{
if let maxParallelism = maxParallelism, maxParallelism == 1 {
return try self.compactMap(transform)
}

var result: [(Int64, T)] = []
let group = DispatchGroup()
let lock = DispatchQueue(label: "pcompactmap")

let parallelism:Int = {
if let maxParallelism = maxParallelism {
precondition(maxParallelism > 0)
return maxParallelism
}
return ProcessInfo.processInfo.activeProcessorCount
}()

let semaphore = DispatchSemaphore(value: parallelism)
var iterator = self.makeIterator()
var index:Int64 = 0
var caughtError: Swift.Error? = nil

repeat {
guard let item = iterator.next() else {
break
}
semaphore.wait()
DispatchQueue.global().async { [index] in
do {
if let mappedElement = try transform(item) {
lock.async {
result += [(index, mappedElement)]
}
}
}
catch {
caughtError = error
}
semaphore.signal()
}
index += 1
} while true

group.wait()

if let error = caughtError {
throw error
}

return result.sorted { $0.0 < $1.0 }
.compactMap { $0.1 }
}
}
*/