use std::{convert::TryInto, fmt};

use crate::{ObjectId, SIZE_OF_SHA1_DIGEST};

/// A borrowed reference to a hash identifying objects.
///
/// # Future Proofing
///
/// In case we wish to support multiple hashes with the same length we cannot discriminate
/// using the slice length anymore. To make that work, we will use the high bits of the
/// internal `bytes` slice length (a fat pointer, pointing to data and its length in bytes)
/// to encode additinoal information. Before accessing or returning the bytes, a new adjusted
/// slice will be constructed, while the high bits will be used to help resolving the
/// hash `[`kind()`][oid::kind()]`.
/// We expect to have quite a few bits available for such 'conflict resolution' as most hashes aren't longer
/// than 64 bytes.
#[derive(PartialEq, Eq, Hash, Ord, PartialOrd)]
#[repr(transparent)]
#[allow(non_camel_case_types)]
#[cfg_attr(feature = "serde1", derive(serde::Serialize))]
pub struct oid {
    bytes: [u8],
}

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

use quick_error::quick_error;

quick_error! {
    #[derive(Debug)]
    pub enum Error {
        InvalidByteSliceLength(length: usize) {
            display("Cannot instantiate git hash from a digest of length {}", length)
        }
    }
}

/// Conversion
impl oid {
    /// Try to create a shared object id from a slice of bytes representing a hash `digest`
    pub fn try_from(digest: &[u8]) -> Result<&Self, Error> {
        match digest.len() {
            20 => Ok(
                #[allow(unsafe_code)]
                unsafe {
                    &*(digest as *const [u8] as *const oid)
                },
            ),
            len => Err(Error::InvalidByteSliceLength(len)),
        }
    }

    /// Only from code that statically assures correct sizes using array conversions
    fn from(value: &[u8]) -> &Self {
        #[allow(unsafe_code)]
        unsafe {
            &*(value as *const [u8] as *const oid)
        }
    }
}

/// Access
impl oid {
    /// The kind of hash used for this Digest
    pub fn kind(&self) -> crate::Kind {
        match self.bytes.len() {
            20 => crate::Kind::Sha1,
            _ => unreachable!("creating this instance is checked and fails on unknown lengths"),
        }
    }

    /// The first byte of the hash, commonly used to partition a set of `Id`s
    pub fn first_byte(&self) -> u8 {
        self.bytes[0]
    }

    #[inline]
    /// Interpret this object id as raw byte slice.
    pub fn as_bytes(&self) -> &[u8] {
        &self.bytes
    }
}

/// Sha1 specific methods
impl oid {
    /// Returns an array with a hexadecimal encoded version of the Sha1 hash this `Id` represents.
    ///
    /// **Panics** if this is not a Sha1 hash, as identifiable by [`ObjectId::kind()`].
    pub fn to_sha1_hex(&self) -> [u8; SIZE_OF_SHA1_DIGEST * 2] {
        let mut buf = [0u8; SIZE_OF_SHA1_DIGEST * 2];
        hex::encode_to_slice(&self.bytes, &mut buf).expect("to count correctly");
        buf
    }

    /// Returns the bytes making up the Sha1.
    ///
    /// **Panics** if this is not a Sha1 hash, as identifiable by [`ObjectId::kind()`].
    pub fn sha1(&self) -> &[u8; SIZE_OF_SHA1_DIGEST] {
        self.bytes.try_into().expect("correctly sized slice")
    }

    /// Returns a Sha1 digest with all bytes being initialized to zero.
    pub fn null_sha1() -> &'static Self {
        oid::from([0u8; SIZE_OF_SHA1_DIGEST].as_ref())
    }
}

impl AsRef<oid> for &oid {
    fn as_ref(&self) -> &oid {
        self
    }
}

impl ToOwned for oid {
    type Owned = crate::ObjectId;

    fn to_owned(&self) -> Self::Owned {
        match self.kind() {
            crate::Kind::Sha1 => crate::ObjectId::Sha1(self.bytes.try_into().expect("no bug in hash detection")),
        }
    }
}

impl<'a> From<&'a [u8; SIZE_OF_SHA1_DIGEST]> for &'a oid {
    fn from(v: &'a [u8; SIZE_OF_SHA1_DIGEST]) -> Self {
        oid::from(v.as_ref())
    }
}

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

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

/// Manually created from a version that uses a slice, and we forcefully try to convert it into a borrowed array of the desired size
/// Could be improved by fitting this into serde
/// Unfortunately the serde::Deserialize derive wouldn't work for borrowed arrays.
#[cfg(feature = "serde1")]
impl<'de: 'a, 'a> serde::Deserialize<'de> for &'a oid {
    fn deserialize<D>(deserializer: D) -> Result<Self, <D as serde::Deserializer<'de>>::Error>
    where
        D: serde::Deserializer<'de>,
    {
        struct __Visitor<'de: 'a, 'a> {
            marker: std::marker::PhantomData<&'a oid>,
            lifetime: std::marker::PhantomData<&'de ()>,
        }
        impl<'de: 'a, 'a> serde::de::Visitor<'de> for __Visitor<'de, 'a> {
            type Value = &'a oid;
            fn expecting(&self, __formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                std::fmt::Formatter::write_str(__formatter, "tuple struct Digest")
            }
            #[inline]
            fn visit_newtype_struct<__E>(self, __e: __E) -> std::result::Result<Self::Value, __E::Error>
            where
                __E: serde::Deserializer<'de>,
            {
                let __field0: &'a [u8] = match <&'a [u8] as serde::Deserialize>::deserialize(__e) {
                    Ok(__val) => __val,
                    Err(__err) => {
                        return Err(__err);
                    }
                };
                Ok(oid::try_from(__field0).expect("exactly 20 bytes"))
            }
            #[inline]
            fn visit_seq<__A>(self, mut __seq: __A) -> std::result::Result<Self::Value, __A::Error>
            where
                __A: serde::de::SeqAccess<'de>,
            {
                let __field0 = match match serde::de::SeqAccess::next_element::<&'a [u8]>(&mut __seq) {
                    Ok(__val) => __val,
                    Err(__err) => {
                        return Err(__err);
                    }
                } {
                    Some(__value) => __value,
                    None => {
                        return Err(serde::de::Error::invalid_length(
                            0usize,
                            &"tuple struct Digest with 1 element",
                        ));
                    }
                };
                Ok(oid::try_from(__field0).expect("exactly 20 bytes"))
            }
        }
        serde::Deserializer::deserialize_newtype_struct(
            deserializer,
            "Digest",
            __Visitor {
                marker: std::marker::PhantomData::<&'a oid>,
                lifetime: std::marker::PhantomData,
            },
        )
    }
}