use std::{cmp::min, hash::Hasher};

/// A trait for keys that can be used for hashing.
pub trait KeyBase {
    /// Returns the length of the key in bytes.
    fn len(&self) -> usize;
    fn is_empty(&self) -> bool {
        self.len() == 0
    }
    /// Hashes the key. It should not include the structure of the key, only the data. E.g. `([1,
    /// 2], [3, 4])` should hash the same as `[1, 2, 3, 4]`.
    fn hash<H: Hasher>(&self, state: &mut H);
}

impl KeyBase for &'_ [u8] {
    fn len(&self) -> usize {
        <[u8]>::len(self)
    }

    fn is_empty(&self) -> bool {
        <[u8]>::is_empty(self)
    }

    fn hash<H: Hasher>(&self, state: &mut H) {
        for item in *self {
            state.write_u8(*item);
        }
    }
}

impl<const N: usize> KeyBase for [u8; N] {
    fn len(&self) -> usize {
        N
    }

    fn is_empty(&self) -> bool {
        N > 0
    }

    fn hash<H: Hasher>(&self, state: &mut H) {
        for item in self {
            state.write_u8(*item);
        }
    }
}

impl KeyBase for Vec<u8> {
    fn len(&self) -> usize {
        self.len()
    }

    fn is_empty(&self) -> bool {
        self.is_empty()
    }

    fn hash<H: Hasher>(&self, state: &mut H) {
        for item in self {
            state.write_u8(*item);
        }
    }
}

impl KeyBase for u8 {
    fn len(&self) -> usize {
        1
    }

    fn is_empty(&self) -> bool {
        false
    }

    fn hash<H: Hasher>(&self, state: &mut H) {
        state.write_u8(*self);
    }
}

impl<A: KeyBase, B: KeyBase> KeyBase for (A, B) {
    fn len(&self) -> usize {
        let (a, b) = self;
        a.len() + b.len()
    }

    fn is_empty(&self) -> bool {
        let (a, b) = self;
        a.is_empty() && b.is_empty()
    }

    fn hash<H: Hasher>(&self, state: &mut H) {
        let (a, b) = self;
        KeyBase::hash(a, state);
        KeyBase::hash(b, state);
    }
}

impl<T: KeyBase> KeyBase for &'_ T {
    fn len(&self) -> usize {
        (*self).len()
    }

    fn is_empty(&self) -> bool {
        (*self).is_empty()
    }

    fn hash<H: Hasher>(&self, state: &mut H) {
        (*self).hash(state)
    }
}

/// A trait for keys that can be used to query the database. They need to allow hashing and
/// comparison with a byte slice (total order).
pub trait QueryKey: KeyBase {
    fn cmp(&self, key: &[u8]) -> std::cmp::Ordering;
}

impl QueryKey for &'_ [u8] {
    fn cmp(&self, key: &[u8]) -> std::cmp::Ordering {
        Ord::cmp(self, &key)
    }
}

impl<const N: usize> QueryKey for [u8; N] {
    fn cmp(&self, key: &[u8]) -> std::cmp::Ordering {
        Ord::cmp(&self[..], key)
    }
}

impl QueryKey for Vec<u8> {
    fn cmp(&self, key: &[u8]) -> std::cmp::Ordering {
        Ord::cmp(&**self, key)
    }
}

impl QueryKey for u8 {
    fn cmp(&self, key: &[u8]) -> std::cmp::Ordering {
        Ord::cmp(&[*self][..], key)
    }
}

impl<A: QueryKey, B: QueryKey> QueryKey for (A, B) {
    fn cmp(&self, mut key: &[u8]) -> std::cmp::Ordering {
        let (a, b) = self;
        let len = a.len();
        let key_len = key.len();
        let key_part = &key[..min(key_len, len)];
        match a.cmp(key_part) {
            std::cmp::Ordering::Equal => {
                key = &key[len..];
                b.cmp(key)
            }
            ord => ord,
        }
    }
}

impl<T: QueryKey> QueryKey for &'_ T {
    fn cmp(&self, key: &[u8]) -> std::cmp::Ordering {
        (*self).cmp(key)
    }
}

/// A trait for keys that can be stored in the database. They need to allow hashing and comparison.
pub trait StoreKey: KeyBase + Ord {
    fn write_to(&self, buf: &mut Vec<u8>);
}

impl<const N: usize> StoreKey for [u8; N] {
    fn write_to(&self, buf: &mut Vec<u8>) {
        buf.extend_from_slice(&self[..]);
    }
}

impl StoreKey for Vec<u8> {
    fn write_to(&self, buf: &mut Vec<u8>) {
        buf.extend_from_slice(self);
    }
}

impl StoreKey for &'_ [u8] {
    fn write_to(&self, buf: &mut Vec<u8>) {
        buf.extend_from_slice(self);
    }
}

impl StoreKey for u8 {
    fn write_to(&self, buf: &mut Vec<u8>) {
        buf.push(*self);
    }
}

impl<A: StoreKey, B: StoreKey> StoreKey for (A, B) {
    fn write_to(&self, buf: &mut Vec<u8>) {
        self.0.write_to(buf);
        self.1.write_to(buf);
    }
}

impl<T: StoreKey> StoreKey for &'_ T {
    fn write_to(&self, buf: &mut Vec<u8>) {
        (*self).write_to(buf);
    }
}

/// Hashes a key with a fast, deterministic hash function.
pub fn hash_key(key: &impl KeyBase) -> u64 {
    let mut hasher = twox_hash::XxHash64::with_seed(0);
    key.hash(&mut hasher);
    hasher.finish()
}

#[cfg(test)]
mod tests {
    use std::cmp::Ordering;

    use crate::{QueryKey, key::hash_key};

    #[test]
    fn tuple() {
        let key = (&[1, 2], &[3, 4]);
        assert_eq!(QueryKey::cmp(&key, &[1, 2, 3, 4]), Ordering::Equal);
        assert_eq!(QueryKey::cmp(&key, &[1, 2, 3, 3]), Ordering::Greater);
        assert_eq!(QueryKey::cmp(&key, &[1, 2, 3, 5]), Ordering::Less);
        assert_eq!(QueryKey::cmp(&key, &[0, 2, 3, 4]), Ordering::Greater);
        assert_eq!(QueryKey::cmp(&key, &[2, 2, 3, 4]), Ordering::Less);
        assert_eq!(QueryKey::cmp(&key, &[1, 2, 3, 4, 5]), Ordering::Less);
        assert_eq!(QueryKey::cmp(&key, &[1, 2, 3]), Ordering::Greater);
        assert_eq!(QueryKey::cmp(&key, &[1, 2]), Ordering::Greater);
        assert_eq!(QueryKey::cmp(&key, &[1]), Ordering::Greater);
        assert_eq!(QueryKey::cmp(&key, &[]), Ordering::Greater);
    }

    #[test]
    fn hash() {
        let h1 = hash_key(&[1, 2, 3, 4]);
        let h2 = hash_key(&(&[1, 2], &[3, 4]));
        let h3 = hash_key(&(vec![1, 2, 3], 4u8));
        assert_eq!(h2, h1);
        assert_eq!(h3, h1);
    }
}