Enum Ordinalize
====================

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This library enables enums to not only obtain the ordinal values of their variants but also allows for the construction of enums from an ordinal value.

## Feature Flags

The default features are `derive` and `traits`. The `derive` feature re-exports the `Ordinalize` derive macro, and the `traits` feature exposes the `Ordinalize` trait and enables the derive macro to implement it automatically.

When only `derive` is enabled, the macro can still generate inherent constants and functions with the `#[ordinalize(...)]` attributes shown below, but it will not implement the `Ordinalize` trait.

## Usage

Use `#[derive(Ordinalize)]` to have an enum (which must only has unit variants) implement the `Ordinalize` trait.

```rust
use enum_ordinalize::Ordinalize;

#[derive(Debug, PartialEq, Eq, Ordinalize)]
enum MyEnum {
    Zero,
    One,
    Two,
}

assert_eq!(3, MyEnum::VARIANT_COUNT);
assert_eq!([MyEnum::Zero, MyEnum::One, MyEnum::Two], MyEnum::VARIANTS);
assert_eq!([0i8, 1i8, 2i8], MyEnum::VALUES);

assert_eq!(0i8, MyEnum::Zero.ordinal());
assert_eq!(1i8, MyEnum::One.ordinal());
assert_eq!(2i8, MyEnum::Two.ordinal());

assert_eq!(Some(MyEnum::Zero), MyEnum::from_ordinal(0i8));
assert_eq!(Some(MyEnum::One), MyEnum::from_ordinal(1i8));
assert_eq!(Some(MyEnum::Two), MyEnum::from_ordinal(2i8));

assert_eq!(MyEnum::Zero, unsafe { MyEnum::from_ordinal_unsafe(0i8) });
assert_eq!(MyEnum::One, unsafe { MyEnum::from_ordinal_unsafe(1i8) });
assert_eq!(MyEnum::Two, unsafe { MyEnum::from_ordinal_unsafe(2i8) });
```

#### Quickly Implement `Serialize` and `Deserialize`

The `Ordinalize` trait can be used to serialize an enum as its ordinal value and safely reject unknown values during deserialization. Add `serde` to your dependencies:

```toml
serde = { version = "1", default-features = false }
```

Then implement the traits using `ordinal` and `from_ordinal`:

```rust
use enum_ordinalize::Ordinalize;
use serde::{Deserialize, Deserializer, Serialize, Serializer};

#[derive(Debug, PartialEq, Eq, Ordinalize)]
#[repr(i8)]
enum MyEnum {
    Zero,
    One,
    Two,
}

impl Serialize for MyEnum {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serializer.serialize_i8(self.ordinal())
    }
}

impl<'de> Deserialize<'de> for MyEnum {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        let ordinal = i8::deserialize(deserializer)?;

        Self::from_ordinal(ordinal).ok_or_else(|| {
            <D::Error as serde::de::Error>::custom("invalid ordinal for MyEnum")
        })
    }
}
```

#### The (Ordinal) Size of an Enum

The ordinal value is an integer whose size is determined by the enum itself. The size of the enum increases with the magnitude of the variants' values, whether larger (or smaller if negative).

For example,

```rust
use enum_ordinalize::Ordinalize;

#[derive(Debug, PartialEq, Eq, Ordinalize)]
enum MyEnum {
    Zero,
    One,
    Two,
    Thousand = 1000,
}

assert_eq!(4, MyEnum::VARIANT_COUNT);
assert_eq!([MyEnum::Zero, MyEnum::One, MyEnum::Two, MyEnum::Thousand], MyEnum::VARIANTS);
assert_eq!([0i16, 1i16, 2i16, 1000i16], MyEnum::VALUES);

assert_eq!(0i16, MyEnum::Zero.ordinal());
assert_eq!(1i16, MyEnum::One.ordinal());
assert_eq!(2i16, MyEnum::Two.ordinal());

assert_eq!(Some(MyEnum::Zero), MyEnum::from_ordinal(0i16));
assert_eq!(Some(MyEnum::One), MyEnum::from_ordinal(1i16));
assert_eq!(Some(MyEnum::Two), MyEnum::from_ordinal(2i16));

assert_eq!(MyEnum::Zero, unsafe { MyEnum::from_ordinal_unsafe(0i16) });
assert_eq!(MyEnum::One, unsafe { MyEnum::from_ordinal_unsafe(1i16) });
assert_eq!(MyEnum::Two, unsafe { MyEnum::from_ordinal_unsafe(2i16) });
```

In order to accommodate the value `1000`, the size of `MyEnum` increases. Consequently, the ordinal is represented in `i16` instead of `i8`.

You can utilize the `#[repr(type)]` attribute to explicitly control the size. For instance,

```rust
use enum_ordinalize::Ordinalize;

#[derive(Debug, PartialEq, Eq, Ordinalize)]
#[repr(usize)]
enum MyEnum {
    Zero,
    One,
    Two,
    Thousand = 1000,
}

assert_eq!(4, MyEnum::VARIANT_COUNT);
assert_eq!([MyEnum::Zero, MyEnum::One, MyEnum::Two, MyEnum::Thousand], MyEnum::VARIANTS);
assert_eq!([0usize, 1usize, 2usize, 1000usize], MyEnum::VALUES);

assert_eq!(0usize, MyEnum::Zero.ordinal());
assert_eq!(1usize, MyEnum::One.ordinal());
assert_eq!(2usize, MyEnum::Two.ordinal());

assert_eq!(Some(MyEnum::Zero), MyEnum::from_ordinal(0usize));
assert_eq!(Some(MyEnum::One), MyEnum::from_ordinal(1usize));
assert_eq!(Some(MyEnum::Two), MyEnum::from_ordinal(2usize));

assert_eq!(MyEnum::Zero, unsafe { MyEnum::from_ordinal_unsafe(0usize) });
assert_eq!(MyEnum::One, unsafe { MyEnum::from_ordinal_unsafe(1usize) });
assert_eq!(MyEnum::Two, unsafe { MyEnum::from_ordinal_unsafe(2usize) });
```

Path constants, casts, binary expressions, and const fn calls used as discriminants require an explicit integer `#[repr(...)]` because the derive macro cannot evaluate them while choosing the ordinal type.

#### Useful Increment

The integers represented by variants can be extended in successive increments and set explicitly from any value.

```rust
use enum_ordinalize::Ordinalize;

#[derive(Debug, PartialEq, Eq, Ordinalize)]
enum MyEnum {
    Two   = 2,
    Three,
    Four,
    Eight = 8,
    Nine,
    NegativeTen = -10,
    NegativeNine,
}

assert_eq!(7, MyEnum::VARIANT_COUNT);
assert_eq!([MyEnum::Two, MyEnum::Three, MyEnum::Four, MyEnum::Eight, MyEnum::Nine, MyEnum::NegativeTen, MyEnum::NegativeNine], MyEnum::VARIANTS);
assert_eq!([2i8, 3i8, 4i8, 8i8, 9i8, -10i8, -9i8], MyEnum::VALUES);

assert_eq!(4i8, MyEnum::Four.ordinal());
assert_eq!(9i8, MyEnum::Nine.ordinal());
assert_eq!(-9i8, MyEnum::NegativeNine.ordinal());

assert_eq!(Some(MyEnum::Four), MyEnum::from_ordinal(4i8));
assert_eq!(Some(MyEnum::Nine), MyEnum::from_ordinal(9i8));
assert_eq!(Some(MyEnum::NegativeNine), MyEnum::from_ordinal(-9i8));

assert_eq!(MyEnum::Four, unsafe { MyEnum::from_ordinal_unsafe(4i8) });
assert_eq!(MyEnum::Nine, unsafe { MyEnum::from_ordinal_unsafe(9i8) });
assert_eq!(MyEnum::NegativeNine, unsafe { MyEnum::from_ordinal_unsafe(-9i8) });
```

#### Implement Functionality for an enum on Itself

For some reason, if you don't want to implement the `Ordinalize` trait for your enum, you can choose to disable the trait implementation and enable the constants/functions one by one. Functions are `const fn`. Names and visibility can also be defined by you.

```rust
use enum_ordinalize::Ordinalize;

#[derive(Debug, PartialEq, Eq, Ordinalize)]
#[ordinalize(impl_trait = false)]
#[ordinalize(variant_count(pub const VARIANT_COUNT, doc = "The count of variants."))]
#[ordinalize(variants(pub const VARIANTS, doc = "List of this enum's variants."))]
#[ordinalize(values(pub const VALUES, doc = "List of values for all variants of this enum."))]
#[ordinalize(ordinal(pub const fn ordinal, doc = "Retrieve the integer number of this variant."))]
#[ordinalize(from_ordinal(pub const fn from_ordinal, doc = "Obtain a variant based on an integer number."))]
#[ordinalize(from_ordinal_unsafe(
    pub const fn from_ordinal_unsafe,
    doc = "Obtain a variant based on an integer number.",
    doc = "# Safety",
    doc = "You have to ensure that the input integer number can correspond to a variant on your own.",
))]
enum MyEnum {
    A,
    B,
}

assert_eq!(2, MyEnum::VARIANT_COUNT);
assert_eq!([MyEnum::A, MyEnum::B], MyEnum::VARIANTS);
assert_eq!([0i8, 1i8], MyEnum::VALUES);

assert_eq!(0i8, MyEnum::A.ordinal());
assert_eq!(1i8, MyEnum::B.ordinal());

assert_eq!(Some(MyEnum::A), MyEnum::from_ordinal(0i8));
assert_eq!(Some(MyEnum::B), MyEnum::from_ordinal(1i8));

assert_eq!(MyEnum::A, unsafe { MyEnum::from_ordinal_unsafe(0i8) });
assert_eq!(MyEnum::B, unsafe { MyEnum::from_ordinal_unsafe(1i8) });
```

## Crates.io

https://crates.io/crates/enum-ordinalize

## Documentation

https://docs.rs/enum-ordinalize

## License

[MIT](LICENSE)
