use core::fmt;
use core::mem;
use core::ops::{Deref, DerefMut};
use core::ptr::NonNull;
use core::slice;

/// String whose contents can't be mutated, just like how Java strings work.
///
/// Operations like mutation are, in all but a select few cases, O(n) time.
/// No amortization here buddy.
///
/// Maintains invariants:
/// 1. Internal pointer is always big endian if valid
/// 2. `data` is only a valid pointer if its big-endian representation is aligned
///    to 2 bytes.
#[repr(C)]
pub struct RawJavaString {
    len: usize,
    data: NonNull<u8>,
}

impl RawJavaString {
    /// Returns the maxiumum length of an interned string on the target architecture.
    #[inline(always)]
    pub const fn max_intern_len() -> usize {
        mem::size_of::<usize>() * 2 - 1
    }

    /// Returns whether or not this string is interned.
    #[inline(always)]
    pub fn is_interned(&self) -> bool {
        ((self.read_ptr() as usize) % 2) == 1 // Check if the pointer value is even
    }

    #[inline(always)]
    pub fn read_ptr(&self) -> *mut u8 {
        usize::from_be(self.data.as_ptr() as usize) as *mut u8
    }

    #[inline(always)]
    pub fn write_ptr(&mut self, ptr: *mut u8) {
        self.data = NonNull::new(usize::to_be(ptr as usize) as *mut u8)
            .expect("Wrote null to JavaString pointer.");
    }

    #[inline(always)]
    pub unsafe fn write_ptr_unchecked(&mut self, ptr: *mut u8) {
        self.data = NonNull::new_unchecked(usize::to_be(ptr as usize) as *mut u8);
    }

    /// Returns the length of this string.
    #[inline(always)]
    pub fn len(&self) -> usize {
        if self.is_interned() {
            (self.read_ptr() as usize as u8 >> 1) as usize
        } else {
            self.len
        }
    }

    /// Returns the current memory layout of this object. If None, then we're looking
    /// at an interned string.
    #[inline(always)]
    fn get_memory_layout(&self) -> Option<alloc::alloc::Layout> {
        if self.len() > Self::max_intern_len() {
            Some(unsafe { alloc::alloc::Layout::from_size_align_unchecked(self.len(), 2) })
        } else {
            None
        }
    }

    pub fn get_bytes(&self) -> &[u8] {
        #[cfg(test)]
        println!("Calling get_bytes");
        let (ptr, len) = if self.is_interned() {
            let len = ((self.read_ptr() as usize as u8) >> 1) as usize;
            let ptr = (&self.len) as *const usize as *const u8 as *mut u8;
            (ptr, len)
        } else {
            (self.read_ptr(), self.len)
        };

        unsafe { slice::from_raw_parts(ptr, len) }
    }

    #[inline]
    pub fn get_bytes_mut(&mut self) -> &mut [u8] {
        unsafe { &mut *(self.get_bytes() as *const [u8] as *mut [u8]) }
    }

    /// Creates a new, empty, RawJavaString.
    pub const fn new() -> Self {
        Self {
            len: 0,
            data: unsafe { NonNull::new_unchecked(usize::to_be(1) as *mut u8) },
        }
    }

    /// Builds a new string from raw bytes.
    ///
    /// Complexity is O(n) in the length of `bytes`.
    pub fn from_bytes(bytes: impl Deref<Target = [u8]>) -> Self {
        let bytes_list: &[_] = &[bytes];
        Self::from_bytes_array_inline(bytes_list)
    }

    /// Builds a new string from raw bytes.
    ///
    /// Complexity is O(n) in the sum of the lengths of the elements of `bytes`.
    pub fn from_bytes_array(bytes_list: impl Deref<Target = [impl Deref<Target = [u8]>]>) -> Self {
        Self::from_bytes_array_inline(bytes_list)
    }

    /// Builds a new string from raw bytes.
    ///
    /// Complexity is O(n) in the sum of the lengths of the elements of `bytes`.
    #[inline(always)]
    fn from_bytes_array_inline(
        bytes_list: impl Deref<Target = [impl Deref<Target = [u8]>]>,
    ) -> Self {
        let mut new = Self::new();
        let len = bytes_list
            .iter()
            .map(|bytes| bytes.len())
            .fold(0, |sum, len| sum + len);

        let (mut write_location, data_pointer_value) = if len <= Self::max_intern_len() {
            let pointer_value = (len << 1) + 1;
            (
                (&mut new.len) as *mut usize as *mut u8,
                (pointer_value as usize as *mut u8),
            )
        } else {
            use alloc::alloc::*;
            // TODO use safe version and put this version behind flag
            let ptr = unsafe { alloc(Layout::from_size_align_unchecked(len, 2)) };
            new.len = len;
            (ptr, ptr)
        };

        unsafe {
            new.write_ptr_unchecked(data_pointer_value);
        }

        for bytes in bytes_list.iter() {
            unsafe {
                core::ptr::copy_nonoverlapping(bytes.as_ptr(), write_location, len);
                write_location = write_location.add(len);
            }
        }

        new
    }

    /// Overwrites what was previously in this buffer with the contents of bytes.
    ///
    /// Complexity is O(n) in the length of `bytes`.
    #[inline(always)]
    pub fn set_bytes(&mut self, bytes: impl Deref<Target = [u8]>) {
        *self = Self::from_bytes(bytes);
    }
}

impl Drop for RawJavaString {
    fn drop(&mut self) {
        #[cfg(test)]
        println!("Dropping");
        if !self.is_interned() {
            #[cfg(test)]
            println!("Dropping non-interned string");
            use alloc::alloc::{dealloc, Layout};
            unsafe {
                dealloc(
                    self.read_ptr(),
                    Layout::from_size_align_unchecked(self.len(), 2),
                );
            }
        }
    }
}

impl Clone for RawJavaString {
    #[inline(always)]
    fn clone(&self) -> Self {
        Self::from_bytes(self.get_bytes())
    }
}

impl fmt::Debug for RawJavaString {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        write!(formatter, "{:?}", &self.get_bytes())
    }
}

impl Deref for RawJavaString {
    type Target = [u8];
    #[inline(always)]
    fn deref(&self) -> &[u8] {
        self.get_bytes()
    }
}

impl DerefMut for RawJavaString {
    #[inline(always)]
    fn deref_mut(&mut self) -> &mut [u8] {
        self.get_bytes_mut()
    }
}

#[cfg(test)]
mod tests {

    use super::*;

    #[test]
    fn new_does_not_use_heap() {
        let string = RawJavaString::new();
        assert!(string.is_interned(), "Empty RawJavaString isn't interned!");
    }

    #[test]
    fn option_size() {
        assert!(
            mem::size_of::<Option<RawJavaString>>() == 2 * mem::size_of::<usize>(),
            "Size of Option<JavaString> is incorrect!"
        );
    }

    #[test]
    fn size() {
        assert!(
            mem::size_of::<RawJavaString>() == 2 * mem::size_of::<usize>(),
            "Size of JavaString is incorrect!"
        );
    }

    #[test]
    fn from_bytes() {
        let bytes: &[u8] = &[12, 3, 2, 1];
        let string = RawJavaString::from_bytes(bytes);
        assert!(string.is_interned(), "String should be interned but isn't.");

        assert!(
            bytes == string.get_bytes(),
            "String should have value `{:?}`, but instead has value `{:?}`",
            bytes,
            string
        );
    }

    #[test]
    fn from_bytes_large_with_nulls() {
        let bytes: &[u8] = &[0; 127];

        let string = RawJavaString::from_bytes(bytes);
        assert!(
            !string.is_interned(),
            "String shouldn't be interned but is."
        );

        assert!(
            bytes == string.get_bytes(),
            "String should have value `{:?}`, but instead has value `{:?}`",
            bytes,
            string
        );
    }

    #[test]
    fn large_interned() {
        let bytes: &[u8] = &[0; RawJavaString::max_intern_len()];
        let bytes_2: &[u8] = &[1; RawJavaString::max_intern_len()];

        let mut string = RawJavaString::from_bytes(bytes);
        assert!(string.is_interned(), "String should be interned but isn't.");

        assert!(
            bytes == string.get_bytes(),
            "String should have value `{:?}`, but instead has value `{:?}`",
            bytes,
            string
        );

        string.set_bytes(bytes_2);
        assert!(string.is_interned(), "String should be interned but isn't.");

        assert!(
            bytes_2 == string.get_bytes(),
            "String should have value `{:?}`, but instead has value `{:?}`",
            bytes,
            string
        );
    }

    #[test]
    fn from_bytes_large() {
        let bytes: &[u8] = &[1; 255];

        let string = RawJavaString::from_bytes(bytes);
        assert!(
            !string.is_interned(),
            "String shouldn't be interned but is."
        );

        assert!(
            bytes == string.get_bytes(),
            "String should have value `{:?}`, but instead has value `{:?}`",
            bytes,
            string
        );
    }
}