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#charvec: Benchmark character sequence random access Bart Massey 2022

In a Reddit comment user /u/pingveno proposed that accessing random characters of a UTF-8 string in Rust by iteration might be faster than accessing random elements of a Vec<char> even for small char sequences.

This weak microbenchmark tests this hypothesis. The benchmark is run separately over sequences of 7-bit ASCII characters and sequences of arbitrary Unicode code points. For each sequence type, a random sequence of length m is generated, then indexed 100,000,000 times at random positions in three ways: not at all ("baseline" condition), by indexing into a UTF-8 String ("string" condition), and by indexing into an array of Unicode char.

Care is taken to make the printed output a function of the input so that nothing is optimized out. Results are for my Debian Bullseye Linux box with a Ryzen 9 3900X processor.

Just a single representative run is shown. Changing PRNG seed and repeated runs gave identical results. Even changing PRNG gave comparable results.

Time for 100,000,000 random accesses of m random ASCII characters.

Time for 100,000,000 random accesses of m random Unicode code points.

As expected Vec<char> dominates String at longer sequence lengths. Slightly more surprisingly, it is not faster at any length.

• cargo build --release
• sh bench.sh >bench.txt (this will take a while)
• sh graphs.sh

This work is available under the "MIT License". Please see the file LICENSE.txt in this distribution for license terms.