InlineString

0.1.2

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A proof-of-concept fixed-capacity UTF-8 inline string type for Swift, storing up to 16 bytes and supporting bitwise copy semantics.
memory-layout performance string swift trivial-type utf8 value-type zero-allocation
0xdea110c8/InlineString

What's New

0.1.2

2026-07-30T03:36:46Z

InlineString v. 0.1.2

  • add last byte limitation notes #3

InlineString

`InlineString' is a proof of concept exploring a fixed-capacity UTF-8 string representation optimized for small strings.

The goal of this project is to investigate whether storing short strings directly inside a value type can provide performance and memory-layout benefits compared to Swift's general-purpose String.

InlineString16

Stores up to 16 UTF-8 bytes directly inside the value.

Usage

Creating from string literal:

let city: InlineString16 = "Berlin"

Creating from String:

let string: String = "Tokyo"
let location = InlineString16(string)

Creating from String with capacity validation:

let string: String = "Hanoi"
guard let place = InlineString16(validating: string) else {
    // String exceeds capacity
}

Features

  • Bitwise-copyable value type
  • No heap allocation for stored content
  • Fixed capacity: 16 UTF-8 bytes
  • Inline UTF-8 byte storage using two UInt64 values
  • Fast copying and equality checks
  • Protocol conformances:
    • BitwiseCopyable
    • Sendable
    • Equatable
    • Hashable
    • Codable
    • ExpressibleByStringLiteral
    • CustomStringConvertible
    • CustomDebugStringConvertible

Storage

InlineString16 stores up to 16 UTF-8 bytes:

┌───────────────┬───────────────┐
│    UInt64     │    UInt64     │
│  bytes 0...7  │ bytes 8...15  │
└───────────────┴───────────────┘

The string length is stored as metadata inside the representation.

Important

When the storage is not fully utilized, the last byte is reserved for storing the string length. As a result, the final UTF-8 byte of the string cannot be in the range 0x00...0x0F, since those values are reserved for the length encoding.

Performance

Benchmarks were performed against Swift String. Each benchmark was executed repeatedly to obtain stable measurements.

Benchmark String InlineString16 Result
Copy 3.12 ms 0.67 ms ~4.7x faster
Array iteration 3.11 ms 0.69 ms ~4.5x faster
Equality (different values) 1.80 ms 0.70 ms ~2.6x faster
Array search 6.36 ms 4.03 ms ~1.6x faster
Accessing count 0.68 ms 0.67 ms Comparable performance
Hashing 15.16 ms 15.23 ms Comparable performance
Decoding 379 ms 373 ms Comparable performance
Encoding 350 ms 363 ms Comparable performance
Initialization 3.10 ms 4.72 ms ~1.5x slower
Initialization (16-byte strings) 3.12 ms 7.12 ms ~2.3x slower
Conversion to String 3.11 ms 9.02 ms ~2.9x slower

Trade-offs

InlineString16 is optimized for small fixed-size strings.

  • Advantages:
    • Compact storage
    • Predictable memory layout
    • Fast copying
    • Fast comparison
    • Fast iteration
  • Limitations:
    • Maximum size is 16 UTF-8 bytes
    • Converting back to String requires creating a String value
    • Not intended as a replacement for general-purpose String
    • Last UTF-8 byte cannot be in the range 0x00...0x0F, as these values are reserved for encoding the string length

Intended use cases

Good use cases for InlineString16:

  • Identifiers
  • Keys
  • Tags
  • Event names
  • Small fixed-size values

Non-goals

InlineString16 is not intended to:

  • replace Swift String;
  • support arbitrary-length strings;
  • provide the full String API;
  • optimize Unicode processing.

Installation

Add the following dependency to your Package.swift:

.package(url: "https://github.com/0xdea110c8/InlineString.git", from: "0.1.0")

Then add InlineString to your target dependencies.

License

This project is licensed under the MIT License. See LICENSE for details.

Description

  • Swift Tools 6.0.0
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Dependencies

Last updated: Sun Aug 02 2026 00:16:38 GMT-0900 (Hawaii-Aleutian Daylight Time)