use crate::{
    builtins::string::is_trimmable_whitespace,
    gc::{empty_trace, Finalize, Trace},
};
use rustc_hash::FxHashSet;
use std::{
    alloc::{alloc, dealloc, Layout},
    borrow::Borrow,
    cell::Cell,
    hash::{Hash, Hasher},
    marker::PhantomData,
    ops::Deref,
    ptr::{copy_nonoverlapping, NonNull},
};

const CONSTANTS_ARRAY: [&str; 127] = [
    // Empty string
    "",
    // Misc
    ",",
    ":",
    // Generic use
    "name",
    "length",
    "arguments",
    "prototype",
    "constructor",
    // typeof
    "null",
    "undefined",
    "number",
    "string",
    "symbol",
    "bigint",
    "object",
    "function",
    // Property descriptor
    "value",
    "get",
    "set",
    "writable",
    "enumerable",
    "configurable",
    // Object object
    "Object",
    "assing",
    "create",
    "toString",
    "valueOf",
    "is",
    "seal",
    "isSealed",
    "freeze",
    "isFrozen",
    "keys",
    "values",
    "entries",
    // Function object
    "Function",
    "apply",
    "bind",
    "call",
    // Array object
    "Array",
    "from",
    "isArray",
    "of",
    "get [Symbol.species]",
    "copyWithin",
    "entries",
    "every",
    "fill",
    "filter",
    "find",
    "findIndex",
    "flat",
    "flatMap",
    "forEach",
    "includes",
    "indexOf",
    "join",
    "map",
    "reduce",
    "reduceRight",
    "reverse",
    "shift",
    "slice",
    "some",
    "sort",
    "unshift",
    "push",
    "pop",
    // String object
    "String",
    "charAt",
    "charCodeAt",
    "concat",
    "endsWith",
    "includes",
    "indexOf",
    "lastIndexOf",
    "match",
    "matchAll",
    "normalize",
    "padEnd",
    "padStart",
    "repeat",
    "replace",
    "replaceAll",
    "search",
    "slice",
    "split",
    "startsWith",
    "substring",
    "toLowerString",
    "toUpperString",
    "trim",
    "trimEnd",
    "trimStart",
    // Number object
    "Number",
    // Boolean object
    "Boolean",
    // RegExp object
    "RegExp",
    "exec",
    "test",
    "flags",
    "index",
    "lastIndex",
    // Symbol object
    "Symbol",
    "for",
    "keyFor",
    "description",
    "[Symbol.toPrimitive]",
    "",
    // Map object
    "Map",
    "clear",
    "delete",
    "get",
    "has",
    "set",
    "size",
    // Set object
    "Set",
    // Reflect object
    "Reflect",
    // Error objects
    "Error",
    "TypeError",
    "RangeError",
    "SyntaxError",
    "ReferenceError",
    "EvalError",
    "URIError",
    "message",
    // Date object
    "Date",
    "toJSON",
];

const MAX_CONSTANT_STRING_LENGTH: usize = {
    let mut max = 0;
    let mut i = 0;
    while i < CONSTANTS_ARRAY.len() {
        let len = CONSTANTS_ARRAY[i].len();
        if len > max {
            max = len;
        }
        i += 1;
    }
    max
};

thread_local! {
    static CONSTANTS: FxHashSet<JsString> = {
        let mut constants = FxHashSet::default();

        for s in CONSTANTS_ARRAY.iter() {
            let s = JsString {
                inner: Inner::new(s),
                _marker: PhantomData,
            };
            constants.insert(s);
        }

        constants
    };
}

/// The inner representation of a [`JsString`].
#[repr(C)]
struct Inner {
    /// The utf8 length, the number of bytes.
    len: usize,

    /// The number of references to the string.
    ///
    /// When this reaches `0` the string is deallocated.
    refcount: Cell<usize>,

    /// An empty array which is used to get the offset of string data.
    data: [u8; 0],
}

impl Inner {
    /// Create a new `Inner` from `&str`.
    #[inline]
    fn new(s: &str) -> NonNull<Self> {
        // We get the layout of the `Inner` type and we extend by the size
        // of the string array.
        let inner_layout = Layout::new::<Inner>();
        let (layout, offset) = inner_layout
            .extend(Layout::array::<u8>(s.len()).unwrap())
            .unwrap();

        let inner = unsafe {
            let inner = alloc(layout) as *mut Inner;

            // Write the first part, the Inner.
            inner.write(Inner {
                len: s.len(),
                refcount: Cell::new(1),
                data: [0; 0],
            });

            // Get offset into the string data.
            let data = (*inner).data.as_mut_ptr();

            debug_assert!(std::ptr::eq(inner.cast::<u8>().add(offset), data));

            // Copy string data into data offset.
            copy_nonoverlapping(s.as_ptr(), data, s.len());

            inner
        };

        // Safety: We already know it's not null, so this is safe.
        unsafe { NonNull::new_unchecked(inner) }
    }

    /// Concatenate array of strings.
    #[inline]
    fn concat_array(strings: &[&str]) -> NonNull<Inner> {
        let mut total_string_size = 0;
        for string in strings {
            total_string_size += string.len();
        }

        // We get the layout of the `Inner` type and we extend by the size
        // of the string array.
        let inner_layout = Layout::new::<Inner>();
        let (layout, offset) = inner_layout
            .extend(Layout::array::<u8>(total_string_size).unwrap())
            .unwrap();

        let inner = unsafe {
            let inner = alloc(layout) as *mut Inner;

            // Write the first part, the Inner.
            inner.write(Inner {
                len: total_string_size,
                refcount: Cell::new(1),
                data: [0; 0],
            });

            // Get offset into the string data.
            let data = (*inner).data.as_mut_ptr();

            debug_assert!(std::ptr::eq(inner.cast::<u8>().add(offset), data));

            // Copy the two string data into data offset.
            let mut offset = 0;
            for string in strings {
                copy_nonoverlapping(string.as_ptr(), data.add(offset), string.len());
                offset += string.len();
            }

            inner
        };

        // Safety: We already know it's not null, so this is safe.
        unsafe { NonNull::new_unchecked(inner) }
    }

    /// Deallocate inner type with string data.
    #[inline]
    unsafe fn dealloc(x: NonNull<Inner>) {
        let len = (*x.as_ptr()).len;

        let inner_layout = Layout::new::<Inner>();
        let (layout, _offset) = inner_layout
            .extend(Layout::array::<u8>(len).unwrap())
            .unwrap();

        dealloc(x.as_ptr() as _, layout);
    }
}

/// This represents a JavaScript primitive string.
///
/// This is similar to `Rc<str>`. But unlike `Rc<str>` which stores the length
/// on the stack and a pointer to the data (this is also known as fat pointers).
/// The `JsString` length and data is stored on the heap. and just an non-null
/// pointer is kept, so its size is the size of a pointer.
pub struct JsString {
    inner: NonNull<Inner>,
    _marker: PhantomData<std::rc::Rc<str>>,
}

impl Default for JsString {
    #[inline]
    fn default() -> Self {
        Self::new("")
    }
}

impl JsString {
    /// Create an empty string, same as calling default.
    #[inline]
    pub fn empty() -> Self {
        JsString::default()
    }

    /// Create a new JavaScript string.
    #[inline]
    pub fn new<S: AsRef<str>>(s: S) -> Self {
        let s = s.as_ref();

        if s.len() <= MAX_CONSTANT_STRING_LENGTH {
            if let Some(constant) = CONSTANTS.with(|c| c.get(s).cloned()) {
                return constant;
            }
        }

        Self {
            inner: Inner::new(s),
            _marker: PhantomData,
        }
    }

    /// Concatenate two string.
    pub fn concat<T, U>(x: T, y: U) -> JsString
    where
        T: AsRef<str>,
        U: AsRef<str>,
    {
        let x = x.as_ref();
        let y = y.as_ref();

        let this = Self {
            inner: Inner::concat_array(&[x, y]),
            _marker: PhantomData,
        };

        if this.len() <= MAX_CONSTANT_STRING_LENGTH {
            if let Some(constant) = CONSTANTS.with(|c| c.get(&this).cloned()) {
                return constant;
            }
        }

        this
    }

    /// Concatenate array of string.
    pub fn concat_array(strings: &[&str]) -> JsString {
        let this = Self {
            inner: Inner::concat_array(strings),
            _marker: PhantomData,
        };

        if this.len() <= MAX_CONSTANT_STRING_LENGTH {
            if let Some(constant) = CONSTANTS.with(|c| c.get(&this).cloned()) {
                return constant;
            }
        }

        this
    }

    /// Return the inner representation.
    #[inline]
    fn inner(&self) -> &Inner {
        unsafe { self.inner.as_ref() }
    }

    /// Return the JavaScript string as a rust `&str`.
    #[inline]
    pub fn as_str(&self) -> &str {
        let inner = self.inner();

        unsafe {
            let slice = std::slice::from_raw_parts(inner.data.as_ptr(), inner.len);
            std::str::from_utf8_unchecked(slice)
        }
    }

    /// Gets the number of `JsString`s which point to this allocation.
    #[inline]
    pub fn refcount(this: &Self) -> usize {
        this.inner().refcount.get()
    }

    /// Returns `true` if the two `JsString`s point to the same allocation (in a vein similar to [`ptr::eq`]).
    ///
    /// [`ptr::eq`]: std::ptr::eq
    #[inline]
    pub fn ptr_eq(x: &Self, y: &Self) -> bool {
        x.inner == y.inner
    }

    /// `6.1.4.1 StringIndexOf ( string, searchValue, fromIndex )`
    ///
    /// Note: Instead of returning an isize with `-1` as the "not found" value,
    /// We make use of the type system and return Option<usize> with None as the "not found" value.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-stringindexof
    pub(crate) fn index_of(&self, search_value: &Self, from_index: usize) -> Option<usize> {
        // 1. Assert: Type(string) is String.
        // 2. Assert: Type(searchValue) is String.
        // 3. Assert: fromIndex is a non-negative integer.

        // 4. Let len be the length of string.
        let len = self.encode_utf16().count();

        // 5. If searchValue is the empty String and fromIndex ≤ len, return fromIndex.
        if search_value.is_empty() && from_index <= len {
            return Some(from_index);
        }

        // 6. Let searchLen be the length of searchValue.
        let search_len = search_value.encode_utf16().count();

        // 7. For each integer i starting with fromIndex such that i ≤ len - searchLen, in ascending order, do
        for i in from_index..=len {
            if i as isize > (len as isize - search_len as isize) {
                break;
            }

            // a. Let candidate be the substring of string from i to i + searchLen.
            let candidate = String::from_utf16_lossy(
                &self
                    .encode_utf16()
                    .skip(i)
                    .take(search_len)
                    .collect::<Vec<u16>>(),
            );

            // b. If candidate is the same sequence of code units as searchValue, return i.
            if candidate == search_value.as_str() {
                return Some(i);
            }
        }

        // 8. Return -1.
        None
    }

    pub(crate) fn string_to_number(&self) -> f64 {
        let string = self.trim_matches(is_trimmable_whitespace);

        // TODO: write our own lexer to match syntax StrDecimalLiteral
        match string {
            "" => 0.0,
            "Infinity" | "+Infinity" => f64::INFINITY,
            "-Infinity" => f64::NEG_INFINITY,
            _ if matches!(
                string
                    .chars()
                    .take(4)
                    .collect::<String>()
                    .to_ascii_lowercase()
                    .as_str(),
                "inf" | "+inf" | "-inf" | "nan" | "+nan" | "-nan"
            ) =>
            {
                // Prevent fast_float from parsing "inf", "+inf" as Infinity and "-inf" as -Infinity
                f64::NAN
            }
            _ => fast_float::parse(string).unwrap_or(f64::NAN),
        }
    }
}

impl Finalize for JsString {}

// Safety: [`JsString`] does not contain any objects which recquire trace,
// so this is safe.
unsafe impl Trace for JsString {
    empty_trace!();
}

impl Clone for JsString {
    #[inline]
    fn clone(&self) -> Self {
        let inner = self.inner();
        inner.refcount.set(inner.refcount.get() + 1);

        JsString {
            inner: self.inner,
            _marker: PhantomData,
        }
    }
}

impl Drop for JsString {
    #[inline]
    fn drop(&mut self) {
        let inner = self.inner();
        if inner.refcount.get() == 1 {
            // Safety: If refcount is 1 and we call drop, that means this is the last
            // JsString which points to this memory allocation, so deallocating it is safe.
            unsafe {
                Inner::dealloc(self.inner);
            }
        } else {
            inner.refcount.set(inner.refcount.get() - 1);
        }
    }
}

impl std::fmt::Debug for JsString {
    #[inline]
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.as_str().fmt(f)
    }
}

impl std::fmt::Display for JsString {
    #[inline]
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.as_str().fmt(f)
    }
}

impl From<&str> for JsString {
    #[inline]
    fn from(s: &str) -> Self {
        Self::new(s)
    }
}

impl From<Box<str>> for JsString {
    #[inline]
    fn from(s: Box<str>) -> Self {
        Self::new(s)
    }
}

impl From<String> for JsString {
    #[inline]
    fn from(s: String) -> Self {
        Self::new(s)
    }
}

impl AsRef<str> for JsString {
    #[inline]
    fn as_ref(&self) -> &str {
        self.as_str()
    }
}

impl Borrow<str> for JsString {
    #[inline]
    fn borrow(&self) -> &str {
        self.as_str()
    }
}

impl Deref for JsString {
    type Target = str;

    #[inline]
    fn deref(&self) -> &Self::Target {
        self.as_str()
    }
}

impl PartialEq<JsString> for JsString {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        // If they point at the same memory allocation, then they are equal.
        if Self::ptr_eq(self, other) {
            return true;
        }

        self.as_str() == other.as_str()
    }
}

impl Eq for JsString {}

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

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

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

impl PartialEq<str> for JsString {
    #[inline]
    fn eq(&self, other: &str) -> bool {
        self.as_str() == other
    }
}

impl PartialEq<JsString> for str {
    #[inline]
    fn eq(&self, other: &JsString) -> bool {
        self == other.as_str()
    }
}

impl PartialEq<&str> for JsString {
    #[inline]
    fn eq(&self, other: &&str) -> bool {
        self.as_str() == *other
    }
}

impl PartialEq<JsString> for &str {
    #[inline]
    fn eq(&self, other: &JsString) -> bool {
        *self == other.as_str()
    }
}

#[cfg(test)]
mod tests {
    use super::JsString;
    use std::mem::size_of;

    #[test]
    fn empty() {
        let _ = JsString::new("");
    }

    #[test]
    fn pointer_size() {
        assert_eq!(size_of::<JsString>(), size_of::<*const u8>());
        assert_eq!(size_of::<Option<JsString>>(), size_of::<*const u8>());
    }

    #[test]
    fn refcount() {
        let x = JsString::new("Hello wrold");
        assert_eq!(JsString::refcount(&x), 1);

        {
            let y = x.clone();
            assert_eq!(JsString::refcount(&x), 2);
            assert_eq!(JsString::refcount(&y), 2);

            {
                let z = y.clone();
                assert_eq!(JsString::refcount(&x), 3);
                assert_eq!(JsString::refcount(&y), 3);
                assert_eq!(JsString::refcount(&z), 3);
            }

            assert_eq!(JsString::refcount(&x), 2);
            assert_eq!(JsString::refcount(&y), 2);
        }

        assert_eq!(JsString::refcount(&x), 1);
    }

    #[test]
    fn ptr_eq() {
        let x = JsString::new("Hello");
        let y = x.clone();

        assert!(JsString::ptr_eq(&x, &y));

        let z = JsString::new("Hello");
        assert!(!JsString::ptr_eq(&x, &z));
        assert!(!JsString::ptr_eq(&y, &z));
    }

    #[test]
    fn as_str() {
        let s = "Hello";
        let x = JsString::new(s);

        assert_eq!(x.as_str(), s);
    }

    #[test]
    fn hash() {
        use std::collections::hash_map::DefaultHasher;
        use std::hash::{Hash, Hasher};

        let s = "Hello, world!";
        let x = JsString::new(s);

        assert_eq!(x.as_str(), s);

        let mut hasher = DefaultHasher::new();
        s.hash(&mut hasher);
        let s_hash = hasher.finish();
        let mut hasher = DefaultHasher::new();
        x.hash(&mut hasher);
        let x_hash = hasher.finish();

        assert_eq!(s_hash, x_hash);
    }

    #[test]
    fn concat() {
        let x = JsString::new("hello");
        let y = ", ";
        let z = JsString::new("world");
        let w = String::from("!");

        let xy = JsString::concat(x, y);
        assert_eq!(xy, "hello, ");
        assert_eq!(JsString::refcount(&xy), 1);

        let xyz = JsString::concat(xy, z);
        assert_eq!(xyz, "hello, world");
        assert_eq!(JsString::refcount(&xyz), 1);

        let xyzw = JsString::concat(xyz, w);
        assert_eq!(xyzw, "hello, world!");
        assert_eq!(JsString::refcount(&xyzw), 1);
    }
}