use std::{borrow::Borrow, convert::TryInto, fmt, io, ops::Deref};

use crate::{borrowed::oid, Kind, SIZE_OF_SHA1_DIGEST};

/// An owned hash identifying objects, most commonly Sha1
#[derive(PartialEq, Eq, Hash, Ord, PartialOrd, Clone, Copy)]
#[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
pub enum ObjectId {
    /// A SHA 1 hash digest
    Sha1([u8; SIZE_OF_SHA1_DIGEST]),
}

impl std::fmt::Debug for ObjectId {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ObjectId::Sha1(_hash) => f.write_str("Sha1(")?,
        }
        for b in self.as_bytes() {
            write!(f, "{:02x}", b)?;
        }
        f.write_str(")")
    }
}

/// Access and conversion
impl ObjectId {
    /// Returns the kind of hash used in this `Id`
    pub fn kind(&self) -> crate::Kind {
        crate::Kind::Sha1
    }
    /// Return the raw byte slice representing this hash
    pub fn as_slice(&self) -> &[u8] {
        match self {
            Self::Sha1(b) => b.as_ref(),
        }
    }
    /// Return the raw mutable byte slice representing this hash
    pub fn as_mut_slice(&mut self) -> &mut [u8] {
        match self {
            Self::Sha1(b) => b.as_mut(),
        }
    }

    /// Write ourselves to `out` in hexadecimal notation
    pub fn write_hex_to(&self, mut out: impl io::Write) -> io::Result<()> {
        out.write_all(&self.to_sha1_hex())
    }

    /// The hash of an empty tree
    pub const fn empty_tree(hash: Kind) -> ObjectId {
        match hash {
            Kind::Sha1 => {
                ObjectId::Sha1(*b"\x4b\x82\x5d\xc6\x42\xcb\x6e\xb9\xa0\x60\xe5\x4b\xf8\xd6\x92\x88\xfb\xee\x49\x04")
            }
        }
    }

    /// Returns true if this hash consists of all null bytes
    pub fn is_null(&self) -> bool {
        match self {
            ObjectId::Sha1(digest) => &digest[..] == Self::null_sha1().as_bytes(),
        }
    }

    /// Returns an Digest representing a hash with whose memory is zeroed.
    pub const fn null(kind: crate::Kind) -> ObjectId {
        match kind {
            crate::Kind::Sha1 => Self::null_sha1(),
        }
    }
}

/// Sha1 hash specific methods
impl ObjectId {
    /// Returns ourselves as slice of 20 bytes.
    ///
    /// Panics if this instance is not a sha1 hash.
    pub fn sha1(&self) -> &[u8; SIZE_OF_SHA1_DIGEST] {
        match self {
            Self::Sha1(b) => b,
        }
    }

    /// Return ourselves as array of 40 hexadecimal bytes.
    ///
    /// Panics if this instance is not a sha1 hash.
    pub fn to_sha1_hex(self) -> [u8; SIZE_OF_SHA1_DIGEST * 2] {
        match self {
            Self::Sha1(b) => {
                let mut hex_buf = [0u8; 40];
                hex::encode_to_slice(b, &mut hex_buf).expect("we can count");
                hex_buf
            }
        }
    }

    /// Return ourselves as hexadecimal string with a length of 40 bytes.
    ///
    /// Panics if this instance is not a sha1 hash.
    pub fn to_sha1_hex_string(self) -> String {
        let buf = self.to_sha1_hex();
        std::str::from_utf8(&buf).expect("hex is valid UTF-8").to_string()
    }

    /// Instantiate an Digest from 20 bytes of a Sha1 digest.
    pub fn new_sha1(id: [u8; SIZE_OF_SHA1_DIGEST]) -> Self {
        ObjectId::Sha1(id)
    }

    /// Instantiate an Digest from a slice 20 borrowed bytes of a Sha1 digest.
    ///
    /// Panics of the slice doesn't have a length of 20.
    pub fn from_20_bytes(b: &[u8]) -> ObjectId {
        let mut id = [0; SIZE_OF_SHA1_DIGEST];
        id.copy_from_slice(b);
        ObjectId::Sha1(id)
    }

    /// Instantiate an Digest from a borrowed array of 20 bytes of a Sha1 digest.
    pub fn from_borrowed_sha1(b: &[u8; SIZE_OF_SHA1_DIGEST]) -> ObjectId {
        let mut id = [0; SIZE_OF_SHA1_DIGEST];
        id.copy_from_slice(&b[..]);
        ObjectId::Sha1(id)
    }

    /// Returns an Digest representing a Sha1 with whose memory is zeroed.
    pub const fn null_sha1() -> ObjectId {
        ObjectId::Sha1([0u8; 20])
    }
}

impl From<[u8; SIZE_OF_SHA1_DIGEST]> for ObjectId {
    fn from(v: [u8; 20]) -> Self {
        Self::new_sha1(v)
    }
}

impl From<&[u8]> for ObjectId {
    fn from(v: &[u8]) -> Self {
        match v.len() {
            20 => Self::Sha1(v.try_into().expect("prior length validation")),
            other => panic!("BUG: unsupported hash len: {}", other),
        }
    }
}

impl From<&crate::oid> for ObjectId {
    fn from(v: &oid) -> Self {
        match v.kind() {
            crate::Kind::Sha1 => ObjectId::from_20_bytes(v.as_bytes()),
        }
    }
}

impl Deref for ObjectId {
    type Target = oid;

    fn deref(&self) -> &Self::Target {
        self.as_ref()
    }
}

impl AsRef<crate::oid> for ObjectId {
    fn as_ref(&self) -> &oid {
        oid::try_from(self.as_slice()).expect("sibling type always implements all hashes we support")
    }
}

impl Borrow<crate::oid> for ObjectId {
    fn borrow(&self) -> &oid {
        self.as_ref()
    }
}

impl fmt::Display for ObjectId {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        for b in self.as_bytes() {
            write!(f, "{:02x}", b)?;
        }
        Ok(())
    }
}

impl PartialEq<&crate::oid> for ObjectId {
    fn eq(&self, other: &&oid) -> bool {
        self.as_ref() == *other
    }
}