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authorAleksey Kladov <[email protected]>2021-01-20 11:47:42 +0000
committerAleksey Kladov <[email protected]>2021-01-20 11:49:29 +0000
commit74f8201586435a7a2e7f8fd49c7eb0750a089180 (patch)
treeed1e997cb8dc1537d4d2a64b3a73d4b2f78b9d3e /docs
parent724059569b4c775ee4723640e0eaabe0da7cdeaf (diff)
Avoid intermediate collections
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@@ -421,12 +421,44 @@ fn frobnicate(s: &str) {
421**Rationale:** reveals the costs. 421**Rationale:** reveals the costs.
422It is also more efficient when the caller already owns the allocation. 422It is also more efficient when the caller already owns the allocation.
423 423
424## Collection types 424## Collection Types
425 425
426Prefer `rustc_hash::FxHashMap` and `rustc_hash::FxHashSet` instead of the ones in `std::collections`. 426Prefer `rustc_hash::FxHashMap` and `rustc_hash::FxHashSet` instead of the ones in `std::collections`.
427 427
428**Rationale:** they use a hasher that's significantly faster and using them consistently will reduce code size by some small amount. 428**Rationale:** they use a hasher that's significantly faster and using them consistently will reduce code size by some small amount.
429 429
430## Avoid Intermediate Collections
431
432When writing a recursive function to compute a sets of things, use an accumulator parameter instead of returning a fresh collection.
433Accumulator goes first in the list of arguments.
434
435```rust
436// GOOD
437pub fn reachable_nodes(node: Node) -> FxHashSet<Node> {
438 let mut res = FxHashSet::default();
439 go(&mut res, node);
440 res
441}
442fn go(acc: &mut FxHashSet<Node>, node: Node) {
443 acc.insert(node);
444 for n in node.neighbors() {
445 go(acc, n);
446 }
447}
448
449// BAD
450pub fn reachable_nodes(node: Node) -> FxHashSet<Node> {
451 let mut res = FxHashSet::default();
452 res.insert(node);
453 for n in node.neighbors() {
454 res.extend(reachable_nodes(n));
455 }
456 res
457}
458```
459
460**Rational:** re-use allocations, accumulator style is more concise for complex cases.
461
430# Style 462# Style
431 463
432## Order of Imports 464## Order of Imports