Zack Zhu thornpig

libdispatch efficiency tips

The libdispatch is one of the most misused API due to the way it was presented to us when it was introduced and for many years after that, and due to the confusing documentation and API. This page is a compilation of important things to know if you're going to use this library. Many references are available at the end of this document pointing to comments from Apple's very own libdispatch maintainer (Pierre Habouzit).

My take-aways are:

You should create very few, long-lived, well-defined queues. These queues should be seen as execution contexts in your program (gui, background work, ...) that benefit from executing in parallel. An important thing to note is that if these queues are all active at once, you will get as many threads running. In most apps, you probably do not need to create more than 3 or 4 queues.
Go serial first, and as you find performance bottle necks, measure why, and if concurrency helps, apply with care, always validating under system pressure. Reuse

What is the Strict Aliasing Rule and Why do we care?

(OR Type Punning, Undefined Behavior and Alignment, Oh My!)

What is strict aliasing? First we will describe what is aliasing and then we can learn what being strict about it means.

In C and C++ aliasing has to do with what expression types we are allowed to access stored values through. In both C and C++ the standard specifies which expression types are allowed to alias which types. The compiler and optimizer are allowed to assume we follow the aliasing rules strictly, hence the term strict aliasing rule. If we attempt to access a value using a type not allowed it is classified as undefined behavior(UB). Once we have undefined behavior all bets are off, the results of our program are no longer reliable.

Unfortunately with strict aliasing violations, we will often obtain the results we expect, leaving the possibility the a future version of a compiler with a new optimization will break code we th

Transforming Code into Beautiful, Idiomatic Python

Notes from Raymond Hettinger's talk at pycon US 2013 video, slides.

The code examples and direct quotes are all from Raymond's talk. I've reproduced them here for my own edification and the hopes that others will find them as handy as I have!

Looping over a range of numbers

for i in [0, 1, 2, 3, 4, 5]:

	import UIKit

	@UIApplicationMain
	class AppDelegate: UIResponder, UIApplicationDelegate {

	var window: UIWindow?

	func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

	let window = UIWindow(frame: UIScreen.main.bounds)

	import Foundation
	final class Sample: NSObject {
	@objc dynamic var name: String = ""
	}

	class MyObj: NSObject {
	@objc dynamic var test: String = ""
	}

	extension NSObjectProtocol where Self: NSObject {

	import UIKit

	class OnboardingManager {
	private let userDefaults: UserDefaults

	init(userDefaults: UserDefaults = .standard) {
	self.userDefaults = userDefaults
	}

	func presentOnboardingControllerIfNeeded(in viewController: UIViewController) {

	// -------------------------------------
	// Swift Integer Quick Guide
	// -------------------------------------

	// Created by Keith Harrison http://useyourloaf.com
	// Copyright (c) 2017 Keith Harrison. All rights reserved.
	//
	// Redistribution and use in source and binary forms, with or without
	// modification, are permitted provided that the following conditions are met:
	//

	import Foundation

	struct Country {
	let name: String
	let capital: String
	var visited: Bool
	}

	extension Country: Equatable {
	static func == (lhs: Country, rhs: Country) -> Bool {

	/// An unescaped string from a potentially unsafe
	/// source (such as user input)
	struct UnsafeString {
	var value: String
	}

	/// A string that either comes from a safe source
	/// (e.g. a string literal in the source code)
	/// or has been escaped.
	struct SanitizedHTML {

	extension Collection {
	func eachConsecutive(_ size: Int) -> Array<SubSequence> {
	let droppedIndices = indices.dropFirst(size - 1)
	return zip(indices, droppedIndices)
	.map { return self[$0...$1] }
	}
	}