facet_reflect/peek/

list_like.rs

use facet_core::{IterVTable, PtrConst, PtrMut, Shape, ShapeLayout};

use super::Peek;
use core::{fmt::Debug, marker::PhantomData, ptr::NonNull};

/// Fields for types which act like lists
#[derive(Clone, Copy)]
pub enum ListLikeDef {
    /// Ordered list of homogenous values, variable size
    ///
    /// e.g. `Vec<T>`
    List(facet_core::ListDef),

    /// Fixed-size array of homogenous values
    ///
    /// e.g. `[T; 32]`
    Array(facet_core::ArrayDef),

    /// Slice — a contiguous sequence of elements
    ///
    /// e.g. `[T]`
    Slice(facet_core::SliceDef),
}

impl ListLikeDef {
    /// Returns the shape of the items in the list
    #[inline]
    pub fn t(&self) -> &'static Shape {
        match self {
            ListLikeDef::List(v) => v.t(),
            ListLikeDef::Array(v) => v.t(),
            ListLikeDef::Slice(v) => v.t(),
        }
    }
}

/// Iterator over a `PeekListLike`
pub struct PeekListLikeIter<'mem, 'facet> {
    state: PeekListLikeIterState<'mem>,
    index: usize,
    len: usize,
    def: ListLikeDef,
    _list: PhantomData<Peek<'mem, 'facet>>,
}

impl<'mem, 'facet> Iterator for PeekListLikeIter<'mem, 'facet> {
    type Item = Peek<'mem, 'facet>;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        let item_ptr = match &self.state.kind {
            PeekListLikeIterStateKind::Ptr { data, stride } => {
                if self.index >= self.len {
                    return None;
                }

                unsafe { data.field(stride * self.index) }
            }
            PeekListLikeIterStateKind::Iter { iter, vtable } => unsafe { (vtable.next)(*iter)? },
        };

        // Update the index. This is used pointer iteration and for
        // calculating the iterator's size
        self.index += 1;

        Some(unsafe { Peek::unchecked_new(item_ptr, self.def.t()) })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.len.saturating_sub(self.index);
        (remaining, Some(remaining))
    }
}

impl<'mem, 'facet> ExactSizeIterator for PeekListLikeIter<'mem, 'facet> {}

impl<'mem, 'facet> IntoIterator for &'mem PeekListLike<'mem, 'facet> {
    type Item = Peek<'mem, 'facet>;
    type IntoIter = PeekListLikeIter<'mem, 'facet>;

    #[inline]
    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

struct PeekListLikeIterState<'mem> {
    kind: PeekListLikeIterStateKind,
    _phantom: PhantomData<&'mem ()>,
}

enum PeekListLikeIterStateKind {
    Ptr {
        data: PtrConst,
        stride: usize,
    },
    Iter {
        iter: PtrMut,
        vtable: &'static IterVTable<PtrConst>,
    },
}

impl Drop for PeekListLikeIterState<'_> {
    #[inline]
    fn drop(&mut self) {
        match &self.kind {
            PeekListLikeIterStateKind::Iter { iter, vtable } => unsafe { (vtable.dealloc)(*iter) },
            PeekListLikeIterStateKind::Ptr { .. } => {
                // Nothing to do
            }
        }
    }
}

/// Lets you read from a list, array or slice
#[derive(Clone, Copy)]
pub struct PeekListLike<'mem, 'facet> {
    pub(crate) value: Peek<'mem, 'facet>,
    pub(crate) def: ListLikeDef,
    len: usize,
}

impl<'mem, 'facet> Debug for PeekListLike<'mem, 'facet> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_struct("PeekListLike").finish_non_exhaustive()
    }
}

impl<'mem, 'facet> PeekListLike<'mem, 'facet> {
    /// Creates a new peek list
    ///
    /// # Safety
    ///
    /// The caller must ensure that `def` contains valid vtable function pointers that:
    /// - Correctly implement the list-like operations for the actual type
    /// - Do not cause undefined behavior when called
    /// - Return pointers within valid memory bounds
    /// - Match the element type specified in `def.t()`
    ///
    /// Violating these requirements can lead to memory safety issues.
    #[inline]
    pub unsafe fn new(value: Peek<'mem, 'facet>, def: ListLikeDef) -> Self {
        let len = match def {
            ListLikeDef::List(v) => unsafe { (v.vtable.len)(value.data()) },
            ListLikeDef::Slice(_) => {
                let slice_as_units = unsafe { value.data.get::<[()]>() };
                slice_as_units.len()
            }
            ListLikeDef::Array(v) => v.n,
        };
        Self { value, def, len }
    }

    /// Get the length of the list
    #[inline]
    pub fn len(&self) -> usize {
        self.len
    }

    /// Returns true if the list is empty
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Get an item from the list at the specified index
    ///
    /// Return `None` if the index is out of bounds
    pub fn get(&self, index: usize) -> Option<Peek<'mem, 'facet>> {
        // Special handling for bare slices with wide pointers
        if let ListLikeDef::Slice(_) = &self.def {
            if index >= self.len() {
                return None;
            }

            // Get the element type layout
            let elem_layout = match self.def.t().layout {
                ShapeLayout::Sized(layout) => layout,
                ShapeLayout::Unsized => return None,
            };

            // Get the data pointer directly from the wide pointer
            let data_ptr = self.value.data().raw_ptr();

            // Calculate the element pointer
            let elem_ptr = unsafe { data_ptr.add(index * elem_layout.size()) };

            // Create a Peek for the element
            return Some(unsafe {
                Peek::unchecked_new(
                    PtrConst::new(NonNull::new_unchecked(elem_ptr as *mut u8).as_ptr()),
                    self.def.t(),
                )
            });
        }

        let as_ptr = match self.def {
            ListLikeDef::List(def) => {
                // Call get from the list's vtable directly if available
                let item =
                    unsafe { (def.vtable.get)(self.value.data(), index, self.value.shape())? };
                return Some(unsafe { Peek::unchecked_new(item, self.def.t()) });
            }
            ListLikeDef::Array(def) => def.vtable.as_ptr,
            ListLikeDef::Slice(def) => def.vtable.as_ptr,
        };

        if index >= self.len() {
            return None;
        }

        // Get the base pointer of the array
        let base_ptr = unsafe { as_ptr(self.value.data()) };

        // Get the layout of the element type
        let elem_layout = match self.def.t().layout {
            ShapeLayout::Sized(layout) => layout,
            ShapeLayout::Unsized => return None, // Cannot handle unsized elements
        };

        // Calculate the offset based on element size
        let offset = index * elem_layout.size();

        // Apply the offset to get the item's pointer
        let item_ptr = unsafe { base_ptr.field(offset) };

        Some(unsafe { Peek::unchecked_new(item_ptr, self.def.t()) })
    }

    /// Returns an iterator over the list
    pub fn iter(self) -> PeekListLikeIter<'mem, 'facet> {
        let (as_ptr_fn, iter_vtable) = match self.def {
            ListLikeDef::List(def) => (def.vtable.as_ptr, def.iter_vtable()),
            ListLikeDef::Array(def) => (Some(def.vtable.as_ptr), None),
            ListLikeDef::Slice(def) => (Some(def.vtable.as_ptr), None),
        };

        let state = match (as_ptr_fn, iter_vtable) {
            (Some(as_ptr_fn), _) => {
                // Special handling for bare slices with wide pointers
                let data = if let ListLikeDef::Slice(_) = &self.def {
                    // Get the data pointer directly from the wide pointer
                    PtrConst::new(unsafe {
                        NonNull::new_unchecked(self.value.data().raw_ptr() as *mut u8).as_ptr()
                    })
                } else {
                    unsafe { as_ptr_fn(self.value.data()) }
                };

                let layout = self
                    .def
                    .t()
                    .layout
                    .sized_layout()
                    .expect("can only iterate over sized list elements");
                let stride = layout.size();

                PeekListLikeIterState {
                    kind: PeekListLikeIterStateKind::Ptr { data, stride },
                    _phantom: PhantomData,
                }
            }
            (None, Some(vtable)) => {
                let iter = unsafe { (vtable.init_with_value.unwrap())(self.value.data()) };
                PeekListLikeIterState {
                    kind: PeekListLikeIterStateKind::Iter { iter, vtable },
                    _phantom: PhantomData,
                }
            }
            (None, None) => unreachable!(),
        };

        PeekListLikeIter {
            state,
            index: 0,
            len: self.len(),
            def: self.def(),
            _list: PhantomData,
        }
    }

    /// Def getter
    #[inline]
    pub fn def(&self) -> ListLikeDef {
        self.def
    }
}