/*!
# UTF-8 Width

To determine the width of a UTF-8 character by providing its first byte.

References: https://tools.ietf.org/html/rfc3629

## Examples

```rust
assert_eq!(1, utf8_width::get_width(b'1'));
assert_eq!(3, utf8_width::get_width("中".as_bytes()[0]));
```

## Benchmark

```bash
cargo bench
```
*/

#![no_std]

pub const MIN_0_1: u8 = 0x80;
pub const MAX_0_1: u8 = 0xC1;
pub const MIN_0_2: u8 = 0xF5;
pub const MAX_0_2: u8 = 0xFF;
pub const MIN_1: u8 = 0x00;
pub const MAX_1: u8 = 0x7F;
pub const MIN_2: u8 = 0xC2;
pub const MAX_2: u8 = 0xDF;
pub const MIN_3: u8 = 0xE0;
pub const MAX_3: u8 = 0xEF;
pub const MIN_4: u8 = 0xF0;
pub const MAX_4: u8 = 0xF4;

#[inline]
pub const fn is_width_1(byte: u8) -> bool {
    byte <= MAX_1 // no need to check `MIN_1 <= byte`
}

#[inline]
pub const fn is_width_2(byte: u8) -> bool {
    byte >= MIN_2 && byte <= MAX_2
}

#[inline]
pub const fn is_width_3(byte: u8) -> bool {
    byte >= MIN_3 && byte <= MAX_3
}

#[inline]
pub const fn is_width_4(byte: u8) -> bool {
    byte >= MIN_4 && byte <= MAX_4
}

#[inline]
pub const fn is_width_0(byte: u8) -> bool {
    byte >= MIN_0_1 && byte <= MAX_0_1 || MIN_0_2 <= byte // no need to check `byte <= MAX_0_2`
}

/// Given a first byte, determine how many bytes are in this UTF-8 character. If the UTF-8 character is invalid, return `0`; otherwise, return `1` to `4`.
#[inline]
pub const fn get_width(byte: u8) -> usize {
    if is_width_1(byte) {
        1
    } else if is_width_2(byte) {
        2
    } else if byte <= MAX_3 {
        // no need to check `MIN_3 <= byte`
        3
    } else if byte <= MAX_4 {
        // no need to check `MIN_4 <= byte`
        4
    } else {
        0
    }
}

/// *Assuming the input first byte is from a valid UTF-8 character*, determine how many bytes are in this UTF-8 character. It returns `1` to `4`.
///
/// # Safety
///
/// You must ensure that the input byte is a valid UTF-8 first byte on your own.
#[inline]
pub const unsafe fn get_width_assume_valid(byte: u8) -> usize {
    if byte <= MAX_1 {
        1
    } else if byte <= MAX_2 {
        2
    } else if byte <= MAX_3 {
        3
    } else {
        4
    }
}