{"id":27164,"date":"2019-11-02T15:22:48","date_gmt":"2019-11-02T19:22:48","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=27164"},"modified":"2019-11-02T21:24:09","modified_gmt":"2019-11-03T01:24:09","slug":"efficiently-mutating-nested-swift-data-structures","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2019\/11\/02\/efficiently-mutating-nested-swift-data-structures\/","title":{"rendered":"Efficiently Mutating Nested Swift Data Structures"},"content":{"rendered":"

Radek Pietruszewski<\/a>:<\/p>\n

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That feel when you’re profiling a React Native app, and you’re sure JavaScript is the source of your problems… but no, it’s ridiculously misoptimized Swift doing… well… nothing much for two minutes.<\/p>\n

[…]<\/p>\n

Code does something that seems<\/em> completely reasonable, but ends up creating a new copy of a large Set<\/code> 10,000s times. Not entirely sure if Swift is supposed to be smart enough to figure out that it can safely mutate it, or it’s programmer’s responsibility.<\/p>\n

The intention is to insert to a Set<\/code> that’s in a Dictionary<\/code> that’s a mutable variable on a class<\/p>\n<\/blockquote>\n\n

Joe Groff<\/a>:<\/p>\n

\n

Older versions could not modify values in dictionaries without copying them, but 5.0 and later should be able to.<\/p>\n<\/blockquote>\n\n

Karoy Lorentey<\/a>:<\/p>\n

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Yep, Array<\/code> & Dictionary<\/code> both support in-place mutations. For Dictionary<\/code>, the recommended way is to use the defaulting subscript. E.g., this won’t make a CoW copy:<\/p>\n

sets[key, default: Set()].insert(foo)<\/pre>\n
But this will:<\/p>\n
var tmp = sets[key]\ntmp.insert(foo)\nsets[key] = tmp<\/pre>\n
The Dictionary.Values<\/code> collection also supports this, and it may come handy if you need to distinguish between existing and new keys:<\/p>\n
if let i = sets.index(forKey: key) {\n  sets.values[i].insert(foo)\n} else {\n  fatalError(\"Unknown key \\(key)\")\n}<\/pre>\n<\/blockquote>\n\n
I’ve written about this before, but I think it’s worth re-emphasizing how this works. It’s counterintuitive and unusual in programming for introducing a temporary variable, as people often do when debugging, to radically change the behavior of the code. Copy-on-write is a leaky abstraction.<\/p>\n\n
Previously:<\/p>\n