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

See Benchmark results

Trade-offs

InlineString16 is optimized for small fixed-size strings.

• Advantages:
  • Compact storage
  • Predictable memory layout
  • Fast copying
  • 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.