use {
    snafu::prelude::*,
    std::str::FromStr,
};

/// Id of a device, as can be found in MetadataExt.dev().
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct DeviceId {
    pub major: u32,
    pub minor: u32,
}

#[derive(Debug, Snafu)]
#[snafu(display("Could not parse {string} as a device id"))]
pub struct ParseDeviceIdError {
    string: String,
}

impl FromStr for DeviceId {
    type Err = ParseDeviceIdError;
    /// this code is based on `man 5 proc` and my stochastic interpretation
    fn from_str(string: &str) -> Result<Self, Self::Err> {
        (|| {
            let mut parts = string.split(':').fuse();
            match (parts.next(), parts.next(), parts.next()) {
                (Some(major), Some(minor), None) => {
                    let major = major.parse().ok()?;
                    let minor = minor.parse().ok()?;
                    Some(Self { major, minor })
                }
                (Some(int), None, None) => {
                    let int: u64 = int.parse().ok()?;
                    Some( int.into() )
                }
                _ => None,
            }
        })().with_context(|| ParseDeviceIdSnafu { string })
    }
}

impl From<u64> for DeviceId {
    fn from(num: u64) -> Self {
        Self {
            major: (num >> 8) as u32,
            minor: (num & 0xFF) as u32,
        }
    }
}

impl DeviceId {
    pub fn new(major: u32, minor: u32) -> Self {
        Self { major, minor }
    }
}

#[test]
fn test_from_str() {
    assert_eq!(DeviceId::new(8, 16), DeviceId::from_str("8:16").unwrap());
}

#[test]
fn test_from_u64() {
    assert_eq!(DeviceId::new(8, 16), DeviceId::from(2064u64));
}