stator_jse 0.3.5

//! Thread-local string interning pool for JavaScript property names.
//!
//! Interning ensures that identical property-name strings share a single
//! [`Rc<str>`] allocation, which reduces memory usage and enables O(1)
//! pointer-based equality checks via [`interned_eq`] and [`str_eq_fast`].
//!
//! The pool is pre-seeded with [`WELL_KNOWN`] property names so that the
//! most frequently accessed names (e.g. `"length"`, `"prototype"`) are
//! available immediately without a cache miss on first use.

use std::cell::RefCell;
use std::collections::HashSet;
use std::rc::Rc;

/// Well-known property names used frequently in JavaScript.
///
/// These are pre-loaded into every thread-local intern pool so that the
/// first access to any of these names returns an already-interned
/// [`Rc<str>`], avoiding a fresh allocation.
const WELL_KNOWN: &[&str] = &[
    "length",
    "prototype",
    "constructor",
    "__proto__",
    "toString",
    "valueOf",
    "hasOwnProperty",
    "name",
    "message",
    "stack",
    "undefined",
    "null",
    "NaN",
    "Infinity",
    "arguments",
    "caller",
    "callee",
    "apply",
    "call",
    "bind",
    "then",
    "catch",
    "finally",
    "next",
    "done",
    "value",
    "get",
    "set",
    "writable",
    "enumerable",
    "configurable",
];

/// Build the initial pool contents from [`WELL_KNOWN`].
fn seed_pool() -> HashSet<Rc<str>> {
    let mut pool = HashSet::with_capacity(WELL_KNOWN.len());
    for &s in WELL_KNOWN {
        pool.insert(Rc::from(s));
    }
    pool
}

thread_local! {
    static INTERN_POOL: RefCell<HashSet<Rc<str>>> = RefCell::new(seed_pool());
}

/// Intern a string, returning a shared `Rc<str>` reference.
/// If the string is already interned, returns the existing reference.
/// This allows O(1) pointer comparison for string equality.
pub fn intern(s: &str) -> Rc<str> {
    INTERN_POOL.with(|pool| {
        let mut pool = pool.borrow_mut();
        if let Some(existing) = pool.get(s) {
            Rc::clone(existing)
        } else {
            let rc: Rc<str> = s.into();
            pool.insert(Rc::clone(&rc));
            rc
        }
    })
}

/// Clear the thread-local interning pool, then re-seed it with
/// [`WELL_KNOWN`] names so that subsequent lookups still hit the pool.
///
/// Used by test harnesses to reclaim memory between test runs.
pub fn clear_intern_pool() {
    INTERN_POOL.with(|pool| {
        let mut pool = pool.borrow_mut();
        pool.clear();
        *pool = seed_pool();
    });
}

/// Check if two interned strings are the same by pointer comparison.
#[inline]
pub fn interned_eq(a: &Rc<str>, b: &Rc<str>) -> bool {
    Rc::ptr_eq(a, b)
}

/// Fast string equality: pointer check first, then byte comparison.
///
/// When both slices originate from the same interned [`Rc<str>`] the
/// pointer check succeeds in O(1), avoiding the O(n) content scan.
#[inline(always)]
pub fn str_eq_fast(a: &str, b: &str) -> bool {
    std::ptr::eq(a, b) || a == b
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_intern_returns_same_rc() {
        let a = intern("hello");
        let b = intern("hello");
        assert!(Rc::ptr_eq(&a, &b));
    }

    #[test]
    fn test_intern_different_strings() {
        let a = intern("foo");
        let b = intern("bar");
        assert!(!Rc::ptr_eq(&a, &b));
    }

    #[test]
    fn test_interned_eq() {
        let a = intern("test");
        let b = intern("test");
        let c = intern("other");
        assert!(interned_eq(&a, &b));
        assert!(!interned_eq(&a, &c));
    }

    #[test]
    fn test_well_known_pre_seeded() {
        for &name in WELL_KNOWN {
            let a = intern(name);
            let b = intern(name);
            assert!(Rc::ptr_eq(&a, &b), "well-known name {name:?} not interned");
        }
    }

    #[test]
    fn test_clear_reseeds_well_known() {
        let before = intern("length");
        clear_intern_pool();
        let after = intern("length");
        // After clearing and re-seeding, a new Rc is created but the
        // name is still present in the pool.
        assert_eq!(&*before, &*after);
        // The old and new Rc are distinct allocations.
        assert!(!Rc::ptr_eq(&before, &after));
    }

    #[test]
    fn test_str_eq_fast_same_pointer() {
        let s = intern("prototype");
        let a: &str = &s;
        let b: &str = &s;
        assert!(str_eq_fast(a, b));
    }

    #[test]
    fn test_str_eq_fast_different_pointer_same_content() {
        let a = String::from("hello");
        let b = String::from("hello");
        assert!(str_eq_fast(&a, &b));
    }

    #[test]
    fn test_str_eq_fast_different_content() {
        assert!(!str_eq_fast("foo", "bar"));
    }
}