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use crate::{Any, InstFnNameHash, IntoComponent, Item, Protocol};
use serde::{Deserialize, Serialize};
use std::any;
use std::fmt;
use std::hash;
use std::hash::{BuildHasher as _, BuildHasherDefault, Hash as _, Hasher as _};
use std::mem;
use twox_hash::XxHash64;

const SEP: usize = 0x7f;
const TYPE: usize = 1;
const INSTANCE_FUNCTION_HASH: u64 = 0x5ea77ffbcdf5f302;
const FIELD_FUNCTION_HASH: u64 = 0xab53b6a7a53c757e;
const OBJECT_KEYS: usize = 4;

/// The hash of a primitive thing.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
#[repr(transparent)]
pub struct Hash(u64);

impl Hash {
    /// Construct a new raw hash.
    pub(crate) const fn new(hash: u64) -> Self {
        Self(hash)
    }

    /// Construct a simple hash from something that is hashable.
    pub(crate) fn of<T: hash::Hash>(thing: T) -> Self {
        let mut hasher = Self::new_hasher();
        thing.hash(&mut hasher);
        Self(hasher.finish())
    }

    /// Get the hash of a type.
    pub fn type_hash<I>(path: I) -> Self
    where
        I: IntoTypeHash,
    {
        path.into_type_hash()
    }

    /// Construct a hash from the given type id.
    pub fn from_any<T>() -> Self
    where
        T: Any,
    {
        Self::from_type_id(any::TypeId::of::<T>())
    }

    /// Construct a hash from a type id.
    pub fn from_type_id(type_id: any::TypeId) -> Self {
        // Safety: a type id is exactly a 64-bit unsigned integer.
        // And has an identical bit pattern to `Hash`.
        unsafe { mem::transmute(type_id) }
    }

    /// Construct a hash to an instance function, where the instance is a
    /// pre-determined type.
    #[inline]
    pub fn instance_function<N>(type_hash: Hash, name: N) -> Self
    where
        N: InstFnNameHash,
    {
        let name = name.inst_fn_name_hash();
        Self(INSTANCE_FUNCTION_HASH ^ (type_hash.0 ^ name.0))
    }

    /// Construct a hash corresponding to a field function.
    #[inline]
    pub fn field_fn<N>(protocol: Protocol, type_hash: Hash, name: N) -> Self
    where
        N: InstFnNameHash,
    {
        let name = name.inst_fn_name_hash();
        Self(FIELD_FUNCTION_HASH ^ ((type_hash.0 ^ protocol.hash.0) ^ name.0))
    }

    /// Get the hash corresponding to a static byte array.
    pub fn static_bytes(bytes: &[u8]) -> Hash {
        Self::of(bytes)
    }

    /// Get the hash corresponding to a instance function name.
    pub fn instance_fn_name(name: &str) -> Hash {
        Self::of(name)
    }

    /// Hash the given iterator of object keys.
    pub fn object_keys<I>(keys: I) -> Self
    where
        I: IntoIterator,
        I::Item: AsRef<str>,
    {
        let mut hasher = Self::new_hasher();
        OBJECT_KEYS.hash(&mut hasher);

        for key in keys {
            SEP.hash(&mut hasher);
            key.as_ref().hash(&mut hasher);
        }

        Self(hasher.finish())
    }

    /// Construct a new hasher.
    fn new_hasher() -> impl hash::Hasher {
        BuildHasherDefault::<XxHash64>::default().build_hasher()
    }

    /// Construct a hash for an use.
    fn path_hash<I>(kind: usize, path: I) -> Self
    where
        I: IntoIterator,
        I::Item: IntoComponent,
    {
        let mut hasher = Self::new_hasher();
        kind.hash(&mut hasher);

        for c in path {
            c.hash_component(&mut hasher);
        }

        Self(hasher.finish())
    }
}

impl fmt::Display for Hash {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(fmt, "0x{:x}", self.0)
    }
}

impl fmt::Debug for Hash {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(fmt, "Hash(0x{:x})", self.0)
    }
}

/// Helper conversion into a function hash.
pub trait IntoTypeHash: Copy {
    /// Generate a function hash.
    fn into_type_hash(self) -> Hash;

    /// Optionally convert into an item, if appropriate.
    fn into_item(self) -> Item;
}

impl IntoTypeHash for Hash {
    fn into_type_hash(self) -> Hash {
        self
    }

    fn into_item(self) -> Item {
        Item::new()
    }
}

impl<I> IntoTypeHash for I
where
    I: Copy + IntoIterator,
    I::Item: IntoComponent,
{
    fn into_type_hash(self) -> Hash {
        Hash::path_hash(TYPE, self)
    }

    fn into_item(self) -> Item {
        Item::with_item(self)
    }
}