countWhereBranching.swift

import Foundation

func measure<T>(_ block: () -> T) -> T {
    let start = Date()
    let result = block()
    print("elapsed time: \(-start.timeIntervalSinceNow)")
    return result
}

extension Sequence {
    func count(where predicate: (Element) throws -> Bool) rethrows -> Int {
        var count = 0
        for element in self {
            if try predicate(element) {
                count += 1
            }
        }
        return count
    }
    
    func _count(where predicate: (Element) throws -> Bool) rethrows -> Int {
        var count = 0
        for element in self {
            count += try predicate(element) ? 1 : 0
        }
        return count
    }
}

let numbers = 0..<10_000_000_000
let evenCount = measure { numbers.count(where: { $0 % 2 == 0 }) }    // elapsed time: 7.831457018852234
let _evenCount = measure { numbers._count(where: { $0 % 2 == 0 }) }  // elapsed time: 5.888622999191284
precondition(evenCount == _evenCount)