//! This module implements the global `Symbol` object.
//!
//! The data type symbol is a primitive data type.
//! The `Symbol()` function returns a value of type symbol, has static properties that expose
//! several members of built-in objects, has static methods that expose the global symbol registry,
//! and resembles a built-in object class, but is incomplete as a constructor because it does not
//! support the syntax "`new Symbol()`".
//!
//! Every symbol value returned from `Symbol()` is unique.
//!
//! More information:
//! - [MDN documentation][mdn]
//! - [ECMAScript reference][spec]
//!
//! [spec]: https://tc39.es/ecma262/#sec-symbol-value
//! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol

use crate::{
    gc::{empty_trace, Finalize, Trace},
    JsString,
};
use std::{
    cell::Cell,
    fmt::{self, Display},
    hash::{Hash, Hasher},
    rc::Rc,
};

/// A structure that contains the JavaScript well known symbols.
///
/// # Examples
/// ```
///# use boa::symbol::WellKnownSymbols;
///
/// let iterator = WellKnownSymbols::iterator();
/// assert_eq!(iterator.description().as_deref(), Some("Symbol.iterator"));
/// ```
/// This is equivalent to `let iterator = Symbol.iterator` in JavaScript.
#[derive(Debug, Clone)]
pub struct WellKnownSymbols {
    async_iterator: JsSymbol,
    has_instance: JsSymbol,
    is_concat_spreadable: JsSymbol,
    iterator: JsSymbol,
    match_: JsSymbol,
    match_all: JsSymbol,
    replace: JsSymbol,
    search: JsSymbol,
    species: JsSymbol,
    split: JsSymbol,
    to_primitive: JsSymbol,
    to_string_tag: JsSymbol,
    unscopables: JsSymbol,
}

/// Reserved number of symbols.
///
/// This is where the well known symbol live
/// and internal engine symbols.
const RESERVED_SYMBOL_HASHES: u64 = 128;

thread_local! {
    /// Cached well known symbols
    static WELL_KNOW_SYMBOLS: WellKnownSymbols = WellKnownSymbols::new();

    /// Symbol hash.
    ///
    /// For now this is an incremented u64 number.
    static SYMBOL_HASH_COUNT: Cell<u64> = Cell::new(RESERVED_SYMBOL_HASHES);
}

impl WellKnownSymbols {
    /// Create the well known symbols.
    fn new() -> Self {
        let mut count = 0;

        let async_iterator = JsSymbol::with_hash(count, Some("Symbol.asyncIterator".into()));
        count += 1;
        let has_instance = JsSymbol::with_hash(count, Some("Symbol.hasInstance".into()));
        count += 1;
        let is_concat_spreadable =
            JsSymbol::with_hash(count, Some("Symbol.isConcatSpreadable".into()));
        count += 1;
        let iterator = JsSymbol::with_hash(count, Some("Symbol.iterator".into()));
        count += 1;
        let match_ = JsSymbol::with_hash(count, Some("Symbol.match".into()));
        count += 1;
        let match_all = JsSymbol::with_hash(count, Some("Symbol.matchAll".into()));
        count += 1;
        let replace = JsSymbol::with_hash(count, Some("Symbol.replace".into()));
        count += 1;
        let search = JsSymbol::with_hash(count, Some("Symbol.search".into()));
        count += 1;
        let species = JsSymbol::with_hash(count, Some("Symbol.species".into()));
        count += 1;
        let split = JsSymbol::with_hash(count, Some("Symbol.split".into()));
        count += 1;
        let to_primitive = JsSymbol::with_hash(count, Some("Symbol.toPrimitive".into()));
        count += 1;
        let to_string_tag = JsSymbol::with_hash(count, Some("Symbol.toStringTag".into()));
        count += 1;
        let unscopables = JsSymbol::with_hash(count, Some("Symbol.unscopables".into()));

        Self {
            async_iterator,
            has_instance,
            is_concat_spreadable,
            iterator,
            match_,
            match_all,
            replace,
            search,
            species,
            split,
            to_primitive,
            to_string_tag,
            unscopables,
        }
    }

    /// The `Symbol.asyncIterator` well known symbol.
    ///
    /// A method that returns the default AsyncIterator for an object.
    /// Called by the semantics of the `for-await-of` statement.
    #[inline]
    pub fn async_iterator() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.async_iterator.clone())
    }

    /// The `Symbol.hasInstance` well known symbol.
    ///
    /// A method that determines if a `constructor` object
    /// recognizes an object as one of the `constructor`'s instances.
    /// Called by the semantics of the instanceof operator.
    #[inline]
    pub fn has_instance() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.has_instance.clone())
    }

    /// The `Symbol.isConcatSpreadable` well known symbol.
    ///
    /// A Boolean valued property that if `true` indicates that
    /// an object should be flattened to its array elements
    /// by `Array.prototype.concat`.
    #[inline]
    pub fn is_concat_spreadable() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.is_concat_spreadable.clone())
    }

    /// The `Symbol.iterator` well known symbol.
    ///
    /// A method that returns the default Iterator for an object.
    /// Called by the semantics of the `for-of` statement.
    #[inline]
    pub fn iterator() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.iterator.clone())
    }

    /// The `Symbol.match` well known symbol.
    ///
    /// A regular expression method that matches the regular expression
    /// against a string. Called by the `String.prototype.match` method.
    #[inline]
    pub fn match_() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.match_.clone())
    }

    /// The `Symbol.matchAll` well known symbol.
    ///
    /// A regular expression method that returns an iterator, that yields
    /// matches of the regular expression against a string.
    /// Called by the `String.prototype.matchAll` method.
    #[inline]
    pub fn match_all() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.match_all.clone())
    }

    /// The `Symbol.replace` well known symbol.
    ///
    /// A regular expression method that replaces matched substrings
    /// of a string. Called by the `String.prototype.replace` method.
    #[inline]
    pub fn replace() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.replace.clone())
    }

    /// The `Symbol.search` well known symbol.
    ///
    /// A regular expression method that returns the index within a
    /// string that matches the regular expression.
    /// Called by the `String.prototype.search` method.
    #[inline]
    pub fn search() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.search.clone())
    }

    /// The `Symbol.species` well known symbol.
    ///
    /// A function valued property that is the `constructor` function
    /// that is used to create derived objects.
    #[inline]
    pub fn species() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.species.clone())
    }

    /// The `Symbol.split` well known symbol.
    ///
    /// A regular expression method that splits a string at the indices
    /// that match the regular expression.
    /// Called by the `String.prototype.split` method.
    #[inline]
    pub fn split() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.split.clone())
    }

    /// The `Symbol.toPrimitive` well known symbol.
    ///
    /// A method that converts an object to a corresponding primitive value.
    /// Called by the `ToPrimitive` (`Value::to_primitve`) abstract operation.
    #[inline]
    pub fn to_primitive() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.to_primitive.clone())
    }

    /// The `Symbol.toStringTag` well known symbol.
    ///
    /// A String valued property that is used in the creation of the default
    /// string description of an object.
    /// Accessed by the built-in method `Object.prototype.toString`.
    #[inline]
    pub fn to_string_tag() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.to_string_tag.clone())
    }

    /// The `Symbol.unscopables` well known symbol.
    ///
    /// An object valued property whose own and inherited property names are property
    /// names that are excluded from the `with` environment bindings of the associated object.
    #[inline]
    pub fn unscopables() -> JsSymbol {
        WELL_KNOW_SYMBOLS.with(|symbols| symbols.unscopables.clone())
    }
}

/// The inner representation of a JavaScript symbol.
#[derive(Debug, Clone)]
struct Inner {
    hash: u64,
    description: Option<JsString>,
}

/// This represents a JavaScript symbol primitive.
#[derive(Debug, Clone)]
pub struct JsSymbol {
    inner: Rc<Inner>,
}

impl JsSymbol {
    /// Create a new symbol.
    #[inline]
    pub fn new(description: Option<JsString>) -> Self {
        let hash = SYMBOL_HASH_COUNT.with(|count| {
            let hash = count.get();
            count.set(hash + 1);
            hash
        });

        Self {
            inner: Rc::new(Inner { hash, description }),
        }
    }

    /// Create a new symbol with a specified hash and description.
    #[inline]
    fn with_hash(hash: u64, description: Option<JsString>) -> Self {
        Self {
            inner: Rc::new(Inner { hash, description }),
        }
    }

    /// Returns the `Symbol`s description.
    #[inline]
    pub fn description(&self) -> Option<JsString> {
        self.inner.description.clone()
    }

    /// Returns the `Symbol`s hash.
    ///
    /// The hash is guaranteed to be unique.
    #[inline]
    pub fn hash(&self) -> u64 {
        self.inner.hash
    }
}

impl Finalize for JsSymbol {}

// Safety: `JsSymbol` does not contain any object that require trace,
// so this is safe.
unsafe impl Trace for JsSymbol {
    empty_trace!();
}

impl Display for JsSymbol {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match &self.inner.description {
            Some(desc) => write!(f, "Symbol({})", desc),
            None => write!(f, "Symbol()"),
        }
    }
}

impl Eq for JsSymbol {}

impl PartialEq for JsSymbol {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        self.inner.hash == other.inner.hash
    }
}

impl PartialOrd for JsSymbol {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        self.inner.hash.partial_cmp(&other.inner.hash)
    }
}

impl Ord for JsSymbol {
    #[inline]
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.inner.hash.cmp(&other.inner.hash)
    }
}

impl Hash for JsSymbol {
    #[inline]
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.inner.hash.hash(state);
    }
}