# Rust-PHF

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[Documentation](https://docs.rs/phf)

Rust-PHF is a library to generate efficient lookup tables at compile time using
[perfect hash functions](http://en.wikipedia.org/wiki/Perfect_hash_function).

It currently uses the
[CHD algorithm](http://cmph.sourceforge.net/papers/esa09.pdf) by default and
also ships an experimental `ptrhash` feature for an alternative MPHF layout.

MSRV (minimum supported rust version) is Rust 1.85.

## Usage

PHF data structures can be constructed via either the procedural
macros in the `phf_macros` crate or code generation supported by the
`phf_codegen` crate.

To compile the `phf` crate with a dependency on
libcore instead of libstd, enabling use in environments where libstd
will not work, set `default-features = false` for the dependency:

```toml
[dependencies]
# to use `phf` in `no_std` environments
phf = { version = "0.14.0", default-features = false }
```

### phf_macros

```rust
use phf::phf_map;

#[derive(Clone)]
pub enum Keyword {
    Loop,
    Continue,
    Break,
    Fn,
    Extern,
}

static KEYWORDS: phf::Map<&'static str, Keyword> = phf_map! {
    "loop" => Keyword::Loop,
    "continue" => Keyword::Continue,
    "break" => Keyword::Break,
    "fn" => Keyword::Fn,
    "extern" => Keyword::Extern,
};

// You can also use OR (`|`) patterns to map multiple keys to the same value:
static OPERATORS: phf::Map<&'static str, &'static str> = phf_map! {
    "+" | "add" | "plus" => "addition",
    "-" | "sub" | "minus" => "subtraction",
    "*" | "mul" | "times" => "multiplication",
};

pub fn parse_keyword(keyword: &str) -> Option<Keyword> {
    KEYWORDS.get(keyword).cloned()
}

pub fn parse_operator(operator: &str) -> Option<&'static str> {
    OPERATORS.get(operator).copied()
}
```

```toml
[dependencies]
phf = { version = "0.14.0", features = ["macros"] }
```

To try the experimental alternative, enable `ptrhash` on both the generator and
runtime crates you use:

```toml
[dependencies]
phf = { version = "0.14.0", features = ["macros", "ptrhash"] }
```

#### Note

Currently, the macro syntax has some limitations and may not
work as you want. See [#196] for example.
Integer literals in the first key's type shape must use explicit suffixes, such
as `0u32`, `[0u8, 1]`, or `(0u32, 1u32)`; later keys infer unsuffixed integer
literals from the same position in that first key.
Tuple keys are supported up to 12 elements.
All keys must use the same supported key expression type as the first key.

[#196]: https://github.com/rust-phf/rust-phf/issues/196

### phf_codegen

To use `phf_codegen` on build.rs, you have to add dependencies under `[build-dependencies]`:

```toml
[build-dependencies]
phf = { version = "0.14.0", default-features = false }
phf_codegen = "0.14.0"
```

When using the experimental `ptrhash` layout, enable the `ptrhash` feature on
both `phf_codegen` and the runtime `phf` dependency so the generated constants
match the runtime struct layout.

Then put code on build.rs:

```rust
use std::env;
use std::fs::File;
use std::io::{BufWriter, Write};
use std::path::Path;

fn main() {
    let path = Path::new(&env::var("OUT_DIR").unwrap()).join("codegen.rs");
    let mut file = BufWriter::new(File::create(&path).unwrap());

    write!(
        &mut file,
        "static KEYWORDS: phf::Map<&'static str, Keyword> = {}",
        phf_codegen::Map::new()
            .entry("loop", "Keyword::Loop")
            .entry("continue", "Keyword::Continue")
            .entry("break", "Keyword::Break")
            .entry("fn", "Keyword::Fn")
            .entry("extern", "Keyword::Extern")
            .build()
    )
    .unwrap();
    write!(&mut file, ";\n").unwrap();

    // Example with OR patterns (note: phf_codegen doesn't support OR patterns directly)
    write!(
        &mut file,
        "static OPERATORS: phf::Map<&'static str, &'static str> = {}",
        phf_codegen::Map::new()
            .entry("+", "\"addition\"")
            .entry("add", "\"addition\"")
            .entry("plus", "\"addition\"")
            .entry("-", "\"subtraction\"")
            .entry("sub", "\"subtraction\"")
            .entry("minus", "\"subtraction\"")
            .build()
    )
    .unwrap();
    write!(&mut file, ";\n").unwrap();
}
```

and lib.rs:

```rust
#[derive(Clone)]
enum Keyword {
    Loop,
    Continue,
    Break,
    Fn,
    Extern,
}

include!(concat!(env!("OUT_DIR"), "/codegen.rs"));

pub fn parse_keyword(keyword: &str) -> Option<Keyword> {
    KEYWORDS.get(keyword).cloned()
}

pub fn parse_operator(operator: &str) -> Option<&'static str> {
    OPERATORS.get(operator).copied()
}
```
