cold-string

Compact representation of immutable UTF-8 strings. Optimized for memory usage and struct packing.

Usage

Use it like a String:

use cold_string::ColdString;

let s = ColdString::new("qwerty");
assert_eq!(s.as_str(), "qwerty");

Packs well with other types:

use std::mem;
use cold_string::ColdString;

assert_eq!(mem::size_of::<ColdString>(), 8);
assert_eq!(mem::align_of::<ColdString>(), 1);

assert_eq!(mem::size_of::<(ColdString, u8)>(), 9);
assert_eq!(mem::align_of::<(ColdString, u8)>(), 1);

How It Works

ColdString is an 8 byte array (4 bytes on 32-bit machines):

pub struct ColdString([u8; 8]);

The array acts as either a pointer to heap data for strings longer than 7 bytes or is the inlined data itself.

Inline Mode

self.0[1] to self.0[7] store the bytes of string. In the least significant byte, self.0[0], the least significant bit signifies the inline/heap flag, and is set to "1" for inline mode. The next bits encode the length (always between 0 and 7).

b0 b1 b2 b3 b4 b5 b6 b7
b0 = <7 bit len> | 1

For example, "qwerty" = [13, 'q', 'w', 'e', 'r', 't', 'y', 0], where 13 is "qwerty".len() << 1 | 1.

Heap Mode

The bytes act as a pointer to heap. The data on the heap has alignment 2, causing the least significant bit to always be 0 (since alignment 2 implies addr % 2 == 0), signifying heap mode. On the heap, the data starts with a variable length integer encoding of the length, followed by the bytes.

ptr --> <var int length> <data>

Memory Comparisons

string_memory

Memory Usage Comparison (RSS per String)

Crate	0–4 chars	0–8 chars	0–16 chars	0–32 chars	0–64 chars
`std`	36.9 B	38.4 B	46.8 B	55.3 B	71.4 B
`smol_str`	24.0 B	24.0 B	24.0 B	41.1 B	72.2 B
`compact_str`	24.0 B	24.0 B	24.0 B	35.4 B	61.0 B
`compact_string`	24.1 B	25.8 B	32.6 B	40.5 B	56.5 B
`cold-string`	8.0 B	11.2 B	24.9 B	36.5 B	53.5 B

Note: Columns represent string length (bytes/chars). Values represent average Resident Set Size (RSS) in bytes per string instance. Measurements taken with 10M iterations.

License

Licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

cold-string 0.1.0