read_fonts/

array.rs

//! Custom array types

#![deny(clippy::arithmetic_side_effects)]

use bytemuck::AnyBitPattern;
use font_types::FixedSize;

use crate::read::{ComputeSize, FontReadWithArgs, ReadArgs, VarSize};
use crate::{FontData, FontRead, ReadError};

/// An array whose items size is not known at compile time.
///
/// This requires the inner type to implement [`FontReadWithArgs`] as well as
/// [`ComputeSize`].
///
/// At runtime, `Args` are provided which will be used to compute the size
/// of each item; this size is then used to compute the positions of the items
/// within the underlying data, from which they will be read lazily.
#[derive(Clone)]
pub struct ComputedArray<'a, T: ReadArgs> {
    // the length of each item
    item_len: usize,
    len: usize,
    data: FontData<'a>,
    args: T::Args,
}

impl<'a, T: ComputeSize> ComputedArray<'a, T> {
    pub fn new(data: FontData<'a>, args: T::Args) -> Result<Self, ReadError> {
        let item_len = T::compute_size(&args)?;
        let len = data.len().checked_div(item_len).unwrap_or(0);
        Ok(ComputedArray {
            item_len,
            len,
            data,
            args,
        })
    }

    /// The number of items in the array
    pub fn len(&self) -> usize {
        self.len
    }

    pub fn is_empty(&self) -> bool {
        self.len == 0
    }
}

impl<T: ReadArgs> ReadArgs for ComputedArray<'_, T> {
    type Args = T::Args;
}

impl<'a, T> FontReadWithArgs<'a> for ComputedArray<'a, T>
where
    T: ComputeSize + FontReadWithArgs<'a>,
    T::Args: Copy,
{
    fn read_with_args(data: FontData<'a>, args: &Self::Args) -> Result<Self, ReadError> {
        Self::new(data, *args)
    }
}

impl<'a, T> ComputedArray<'a, T>
where
    T: FontReadWithArgs<'a>,
    T::Args: Copy + 'static,
{
    pub fn iter(&self) -> impl Iterator<Item = Result<T, ReadError>> + 'a {
        let mut i = 0;
        let data = self.data;
        let args = self.args;
        let item_len = self.item_len;
        let len = self.len;

        std::iter::from_fn(move || {
            if i == len {
                return None;
            }
            let item_start = item_len.checked_mul(i)?;
            i = i.checked_add(1)?;
            let data = data.split_off(item_start)?;
            Some(T::read_with_args(data, &args))
        })
    }

    #[inline]
    pub fn get(&self, idx: usize) -> Result<T, ReadError> {
        let item_start = idx
            .checked_mul(self.item_len)
            .ok_or(ReadError::OutOfBounds)?;
        self.data
            .split_off(item_start)
            .ok_or(ReadError::OutOfBounds)
            .and_then(|data| T::read_with_args(data, &self.args))
    }
}

impl<T: ReadArgs> std::fmt::Debug for ComputedArray<'_, T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        f.debug_struct("DynSizedArray")
            .field("bytes", &self.data)
            .finish()
    }
}

/// An array of items of non-uniform length.
///
/// Random access into this array cannot be especially efficient, since it requires
/// a linear scan.
pub struct VarLenArray<'a, T> {
    data: FontData<'a>,
    phantom: std::marker::PhantomData<*const T>,
}

impl<'a, T: FontRead<'a> + VarSize> VarLenArray<'a, T> {
    /// Return the item at the provided index.
    ///
    /// # Performance
    ///
    /// Determining the position of an item in this collection requires looking
    /// at all the preceding items; that is, it is `O(n)` instead of `O(1)` as
    /// it would be for a `Vec`.
    ///
    /// As a consequence, calling this method in a loop could potentially be
    /// very slow. If this is something you need to do, it will probably be
    /// much faster to first collect all the items into a `Vec` beforehand,
    /// and then fetch them from there.
    pub fn get(&self, idx: usize) -> Option<Result<T, ReadError>> {
        let mut pos = 0usize;
        for _ in 0..idx {
            pos = pos.checked_add(T::read_len_at(self.data, pos)?)?;
        }
        self.data.split_off(pos).map(T::read)
    }

    /// Return an iterator over this array's items.
    pub fn iter(&self) -> impl Iterator<Item = Result<T, ReadError>> + 'a {
        let mut data = self.data;
        std::iter::from_fn(move || {
            if data.is_empty() {
                return None;
            }

            let item_len = T::read_len_at(data, 0)?;
            // If the length is 0 then then it's not useful to continue
            // iteration. The subsequent read will probably fail but if
            // the user is skipping malformed elements (which is common)
            // this this iterator will continue forever.
            if item_len == 0 {
                return None;
            }
            let item_data = data.slice(..item_len)?;
            let next = T::read(item_data);
            data = data.split_off(item_len)?;
            Some(next)
        })
    }
}

impl<'a, T> FontRead<'a> for VarLenArray<'a, T> {
    fn read(data: FontData<'a>) -> Result<Self, ReadError> {
        Ok(VarLenArray {
            data,
            phantom: core::marker::PhantomData,
        })
    }
}

impl<T: AnyBitPattern> ReadArgs for &[T] {
    type Args = u16;
}

impl<'a, T: AnyBitPattern + FixedSize> FontReadWithArgs<'a> for &'a [T] {
    fn read_with_args(data: FontData<'a>, args: &u16) -> Result<Self, ReadError> {
        let len = (*args as usize)
            .checked_mul(T::RAW_BYTE_LEN)
            .ok_or(ReadError::OutOfBounds)?;
        data.read_array(0..len)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::codegen_test::records::VarLenItem;
    use font_test_data::bebuffer::BeBuffer;

    impl VarSize for VarLenItem<'_> {
        type Size = u32;

        fn read_len_at(data: FontData, pos: usize) -> Option<usize> {
            data.read_at::<u32>(pos).ok().map(|len| len as usize)
        }
    }

    /// HB/HarfRuzz test "shlana_9_006" has a morx table containing a chain
    /// with a length of 0. This caused the VarLenArray iterator to loop
    /// indefinitely.
    #[test]
    fn var_len_iter_with_zero_length_item() {
        // Create a buffer containing three elements where the last
        // has zero length
        let mut buf = BeBuffer::new();
        buf = buf.push(8u32).extend([0u8; 4]);
        buf = buf.push(18u32).extend([0u8; 14]);
        buf = buf.push(0u32);
        let arr: VarLenArray<VarLenItem> = VarLenArray::read(FontData::new(buf.data())).unwrap();
        // Ensure we don't iterate forever and only read two elements (the
        // take() exists so that the test fails rather than hanging if the
        // code regresses in the future)
        assert_eq!(arr.iter().take(10).count(), 2);
    }
}