#sort #strings #flag #american


American Flag sort implementation for faster sorting of Strings

5 unstable releases

Uses old Rust 2015

0.3.1 Mar 16, 2019
0.3.0 Aug 12, 2018
0.2.0 Oct 22, 2017
0.1.1 Oct 2, 2017
0.1.0 Oct 1, 2017

#485 in Rust patterns

35 downloads per month
Used in crate-race

MIT license

362 lines

American Flag sort for Rust

Linux build status Crates.io Version badge

The afsort crate implements a sorting algorithm based on American Flag sort. The implementation is currently limited to sort byte slices, e.g. Strings. The main motivation is to sort strings of text, so most of the benchmarks are based on English text strings. When sorting English words, this implementation seems to be about 40% faster than sort_unstable from the Rust standard library.

For small input, this method falls back to the standard library.


Add the dependency to your Cargo.toml:

afsort = "0.2"

In your crate root:

extern crate afsort;

Warning: Version 0.1.0 is flawed(slow), use 0.1.1 or later.

Note on upgrading 0.1.x -> 0.2.x: The method afsort::sort_unstable(&mut [AsRef<u8>]) has been removed. Use the af_sort_unstable from the AFSortable trait instead.


You can now use afsort to e.g. sort arrays of strings or string slices.

use afsort::AFSortable;
let mut strings = vec!("red", "green", "blue");
assert_eq!(strings, vec!["blue", "green", "red"]);

It also works on u8, u16, u32 and u64:

use afsort::AFSortable;
let mut strings = vec!(1u32, 2u32, 7u32);
assert_eq!(strings, vec![1u32, 2u32, 7u32]);

You can also sort by an extractor function, e.g.:

use afsort;
let mut tuples = vec![("b", 2), ("a", 1)];
afsort::sort_unstable_by(&mut tuples, |t| &t.0);
assert_eq!(tuples, vec![("a", 1), ("b", 2)]);

The af_sort_unstable() method is implemented for all slices of values that implement the afsort::DigitAt and the Ord traits. The DigitAt trait is implemented for &str , String, [u8], u8, u16, u32 and u64. All of these also implement Ord. You can also implement this trait for any other type.


Essentially, I noticed that sorting of strings took a long time when using the fst crate, since it requires the input to be ordered. Since sorting strings is a general problem, this is now a crate.


As mentioned, this implementation seems to be about 40% faster than the sort in the standard library, when sorting strings of random English words. It is slower for strings that are already sorted. The implementation is fairly naive, so I would not be surprised if it could be improved further.

For numbers, it currently seems to be slower than the standard library. I suspect this is due to more swaps happening in afsort than in the standard library. I want to fix this.

This will be heavily affected by the distribution of values in the input though. As always with performance: your milage may vary. Profile your usage.

You can run the benchmark tests using cargo bench (currently requires nightly rust), like this:

% cargo bench
    Finished release [optimized] target(s) in 0.0 secs
     Running target/release/deps/afsort-2f0d4e495216be99
running 10 tests
test tests::correct_radix_for_u16 ... ignored
test tests::correct_radix_for_u32 ... ignored
test tests::correct_radix_for_u64 ... ignored
test tests::correct_radix_for_u8 ... ignored
test tests::sorts_strings_same_as_unstable ... ignored
test tests::sorts_tuples_same_as_unstable ... ignored
test tests::sorts_u16_same_as_unstable ... ignored
test tests::sorts_u32_same_as_unstable ... ignored
test tests::sorts_u64_same_as_unstable ... ignored
test tests::sorts_u8_same_as_unstable ... ignored
test result: ok. 0 passed; 0 failed; 10 ignored; 0 measured; 0 filtered out
     Running target/release/deps/bench-42a0c77149fb906a
running 16 tests
test sort_en_strings_lower_10_000_af   ... bench:   1,881,300 ns/iter (+/- 618,858)
test sort_en_strings_lower_10_000_std  ... bench:   2,594,388 ns/iter (+/- 767,774)
test sort_en_strings_rand_100_000_af   ... bench:  23,101,465 ns/iter (+/- 12,052,025)
test sort_en_strings_rand_100_000_std  ... bench:  31,536,516 ns/iter (+/- 12,910,887)
test sort_en_strings_rand_10_000_af    ... bench:   1,588,372 ns/iter (+/- 568,509)
test sort_en_strings_rand_10_000_std   ... bench:   2,193,132 ns/iter (+/- 648,297)
test sort_en_strings_sorted_10_000_af  ... bench:     806,419 ns/iter (+/- 128,186)
test sort_en_strings_sorted_10_000_std ... bench:     589,161 ns/iter (+/- 340,707)
test sort_u16_1_000_000_af             ... bench:  19,442,855 ns/iter (+/- 1,642,992)
test sort_u16_1_000_000_std            ... bench:  21,401,736 ns/iter (+/- 3,607,120)
test sort_u32_1_000_000_af             ... bench:  31,682,863 ns/iter (+/- 5,254,810)
test sort_u32_1_000_000_std            ... bench:  30,809,651 ns/iter (+/- 1,623,271)
test sort_u64_1_000_000_af             ... bench:  39,730,940 ns/iter (+/- 6,139,556)
test sort_u64_1_000_000_std            ... bench:  32,477,660 ns/iter (+/- 1,733,969)
test sort_u8_1_000_af                  ... bench:      11,330 ns/iter (+/- 702)
test sort_u8_1_000_std                 ... bench:       8,764 ns/iter (+/- 163)
test result: ok. 0 passed; 0 failed; 0 ignored; 16 measured; 0 filtered out


The American Flag algorithm is unstable, in the same way that sort_unstable in the standard library. That is, equal elements might be re-ordered.

This crate can only sort strings based on their utf-8 byte values. For many problems, this is fine. However, if you want to sort strings for display to a user, Locale might matter. This crate does not try to address this issue.


Testing is done using the quickcheck crate. We run about 50k different variations of input strings & numbers. We treat the standard library's sort_unstable as the gold standard. This means that it is very likely that this library is as bug-free (at least in a functional sense) as the standard library.


This repository is licensed under the MIT license, with the exception of the english-american.txt dictionary file, which comes from the Linux words package, and thus is under GPL. The dictionary is only used for testing purposes.

No runtime deps