// This file is part of ICU4X. For terms of use, please see the file

// called LICENSE at the top level of the ICU4X source tree

// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

use super::components::VarZeroVecComponents;

use super::*;

use crate::ule::*;

use core::marker::PhantomData;

use core::mem;

/// A slice representing the index and data tables of a VarZeroVec,

/// *without* any length fields. The length field is expected to be stored elsewhere.

///

/// Without knowing the length this is of course unsafe to use directly.

#[repr(transparent)]

#[derive(PartialEq, Eq)]

pub(crate) struct VarZeroLengthlessSlice<T: ?Sized, F> {

    marker: PhantomData<(F, T)>,

    /// The original slice this was constructed from

    // Safety invariant: This field must have successfully passed through

    // VarZeroVecComponents::parse_bytes_with_length() with the length

    // associated with this value.

    entire_slice: [u8],

}

impl<T: VarULE + ?Sized, F: VarZeroVecFormat> VarZeroLengthlessSlice<T, F> {

    /// Obtain a [`VarZeroVecComponents`] borrowing from the internal buffer

    ///

    /// Safety: `len` must be the length associated with this value

    #[inline]

    pub(crate) unsafe fn as_components<'a>(&'a self, len: u32) -> VarZeroVecComponents<'a, T, F> {

        unsafe {

            // safety: VarZeroSlice is guaranteed to parse here

            VarZeroVecComponents::from_bytes_unchecked_with_length(len, &self.entire_slice)

        }

    }

    /// Parse a VarZeroLengthlessSlice from a slice of the appropriate format

    ///

    /// Slices of the right format can be obtained via [`VarZeroSlice::as_bytes()`]

    pub fn parse_bytes<'a>(len: u32, slice: &'a [u8]) -> Result<&'a Self, UleError> {

        let _ = VarZeroVecComponents::<T, F>::parse_bytes_with_length(len, slice)

            .map_err(|_| UleError::parse::<Self>())?;

        unsafe {

            // Safety: We just verified that it is of the correct format.

            Ok(Self::from_bytes_unchecked(slice))

        }

    }

    /// Uses a `&[u8]` buffer as a `VarZeroLengthlessSlice<T>` without any verification.

    ///

    /// # Safety

    ///

    /// `bytes` need to be an output from [`VarZeroLengthlessSlice::as_bytes()`], or alternatively

    /// successfully pass through `parse_bytes` (with `len`)

    ///

    /// The length associated with this value will be the length associated with the original slice.

    pub(crate) const unsafe fn from_bytes_unchecked(bytes: &[u8]) -> &Self {

        // self is really just a wrapper around a byte slice

        mem::transmute(bytes)

    }

    /// Uses a `&mut [u8]` buffer as a `VarZeroLengthlessSlice<T>` without any verification.

    ///

    /// # Safety

    ///

    /// `bytes` need to be an output from [`VarZeroLengthlessSlice::as_bytes()`], or alternatively

    /// be valid to be passed to `from_bytes_unchecked_with_length`

    ///

    /// The length associated with this value will be the length associated with the original slice.

    pub(crate) unsafe fn from_bytes_unchecked_mut(bytes: &mut [u8]) -> &mut Self {

        // self is really just a wrapper around a byte slice

        mem::transmute(bytes)

    }

    /// Get one of this slice's elements

    ///

    /// # Safety

    ///

    /// `index` must be in range, and `len` must be the length associated with this

    /// instance of VarZeroLengthlessSlice.

    pub(crate) unsafe fn get_unchecked(&self, len: u32, idx: usize) -> &T {

        self.as_components(len).get_unchecked(idx)

    }

    /// Get a reference to the entire encoded backing buffer of this slice

    ///

    /// The bytes can be passed back to [`Self::parse_bytes()`].

    ///

    /// To take the bytes as a vector, see [`VarZeroVec::into_bytes()`].

    #[inline]

    pub(crate) const fn as_bytes(&self) -> &[u8] {

        &self.entire_slice

    }

    /// Get the bytes behind this as a mutable slice

    ///

    /// # Safety

    ///

    ///  - `len` is the length associated with this VarZeroLengthlessSlice

    ///  - The resultant slice is only mutated in a way such that it remains a valid `T`

    ///

    /// # Panics

    ///

    ///  Panics when idx is not in bounds for this slice

    pub(crate) unsafe fn get_bytes_at_mut(&mut self, len: u32, idx: usize) -> &mut [u8] {

        let components = self.as_components(len);

        let range = components.get_things_range(idx);

        let offset = components.get_indices_size();

        // get_indices_size() returns the start of the things slice, and get_things_range()

        // returns a range in-bounds of the things slice

        #[expect(clippy::indexing_slicing)]

        &mut self.entire_slice[offset..][range]

    }

}