read_fonts/tables/

layout.rs

//! OpenType Layout common table formats

#[cfg(feature = "std")]
mod closure;

mod feature;
mod lookup_flag;
mod script;

use core::cmp::Ordering;

pub use lookup_flag::LookupFlag;
pub use script::{ScriptTags, SelectedScript, UNICODE_TO_NEW_OPENTYPE_SCRIPT_TAGS};

use super::variations::DeltaSetIndex;

#[cfg(feature = "std")]
use crate::collections::IntSet;

#[cfg(feature = "std")]
pub(crate) use closure::{
    ContextFormat1, ContextFormat2, ContextFormat3, LayoutLookupList, LookupClosure,
    LookupClosureCtx, MAX_LOOKUP_VISIT_COUNT, MAX_NESTING_LEVEL,
};

#[cfg(feature = "std")]
pub use closure::Intersect;

#[cfg(test)]
mod spec_tests;

include!("../../generated/generated_layout.rs");

impl<'a, T: FontRead<'a>> Lookup<'a, T> {
    pub fn get_subtable(&self, offset: Offset16) -> Result<T, ReadError> {
        self.resolve_offset(offset)
    }

    #[cfg(feature = "experimental_traverse")]
    fn traverse_lookup_flag(&self) -> traversal::FieldType<'a> {
        self.lookup_flag().to_bits().into()
    }
}

/// A trait that abstracts the behaviour of an extension subtable
///
/// This is necessary because GPOS and GSUB have different concrete types
/// for their extension lookups.
pub trait ExtensionLookup<'a, T: FontRead<'a>>: FontRead<'a> {
    fn extension(&self) -> Result<T, ReadError>;
}

/// an array of subtables, maybe behind extension lookups
///
/// This is used to implement more ergonomic access to lookup subtables for
/// GPOS & GSUB lookup tables.
pub enum Subtables<'a, T: FontRead<'a>, Ext: ExtensionLookup<'a, T>> {
    Subtable(ArrayOfOffsets<'a, T>),
    Extension(ArrayOfOffsets<'a, Ext>),
}

impl<'a, T: FontRead<'a> + 'a, Ext: ExtensionLookup<'a, T> + 'a> Subtables<'a, T, Ext> {
    /// create a new subtables array given offsets to non-extension subtables
    pub(crate) fn new(offsets: &'a [BigEndian<Offset16>], data: FontData<'a>) -> Self {
        Subtables::Subtable(ArrayOfOffsets::new(offsets, data, ()))
    }

    /// create a new subtables array given offsets to extension subtables
    pub(crate) fn new_ext(offsets: &'a [BigEndian<Offset16>], data: FontData<'a>) -> Self {
        Subtables::Extension(ArrayOfOffsets::new(offsets, data, ()))
    }

    /// The number of subtables in this collection
    pub fn len(&self) -> usize {
        match self {
            Subtables::Subtable(inner) => inner.len(),
            Subtables::Extension(inner) => inner.len(),
        }
    }

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

    /// Return the subtable at the given index
    pub fn get(&self, idx: usize) -> Result<T, ReadError> {
        match self {
            Subtables::Subtable(inner) => inner.get(idx),
            Subtables::Extension(inner) => inner.get(idx).and_then(|ext| ext.extension()),
        }
    }

    /// Return an iterator over all the subtables in the collection
    pub fn iter(&self) -> impl Iterator<Item = Result<T, ReadError>> + 'a {
        let (left, right) = match self {
            Subtables::Subtable(inner) => (Some(inner.iter()), None),
            Subtables::Extension(inner) => (
                None,
                Some(inner.iter().map(|ext| ext.and_then(|ext| ext.extension()))),
            ),
        };
        left.into_iter()
            .flatten()
            .chain(right.into_iter().flatten())
    }
}

/// An enum for different possible tables referenced by [Feature::feature_params_offset]
pub enum FeatureParams<'a> {
    StylisticSet(StylisticSetParams<'a>),
    Size(SizeParams<'a>),
    CharacterVariant(CharacterVariantParams<'a>),
}

impl ReadArgs for FeatureParams<'_> {
    type Args = Tag;
}

impl<'a> FontReadWithArgs<'a> for FeatureParams<'a> {
    fn read_with_args(bytes: FontData<'a>, args: &Tag) -> Result<FeatureParams<'a>, ReadError> {
        match *args {
            t if t == Tag::new(b"size") => SizeParams::read(bytes).map(Self::Size),
            // to whoever is debugging this dumb bug I wrote: I'm sorry.
            t if &t.to_raw()[..2] == b"ss" => {
                StylisticSetParams::read(bytes).map(Self::StylisticSet)
            }
            t if &t.to_raw()[..2] == b"cv" => {
                CharacterVariantParams::read(bytes).map(Self::CharacterVariant)
            }
            // NOTE: what even is our error condition here? an offset exists but
            // we don't know the tag?
            _ => Err(ReadError::InvalidFormat(0xdead)),
        }
    }
}

#[cfg(feature = "experimental_traverse")]
impl<'a> SomeTable<'a> for FeatureParams<'a> {
    fn type_name(&self) -> &str {
        match self {
            FeatureParams::StylisticSet(table) => table.type_name(),
            FeatureParams::Size(table) => table.type_name(),
            FeatureParams::CharacterVariant(table) => table.type_name(),
        }
    }

    fn get_field(&self, idx: usize) -> Option<Field<'a>> {
        match self {
            FeatureParams::StylisticSet(table) => table.get_field(idx),
            FeatureParams::Size(table) => table.get_field(idx),
            FeatureParams::CharacterVariant(table) => table.get_field(idx),
        }
    }
}

impl FeatureTableSubstitutionRecord {
    pub fn alternate_feature<'a>(&self, data: FontData<'a>) -> Result<Feature<'a>, ReadError> {
        self.alternate_feature_offset()
            .resolve_with_args(data, &Tag::new(b"NULL"))
    }
}

fn bit_storage(v: u32) -> u32 {
    u32::BITS - v.leading_zeros()
}

impl<'a> CoverageTable<'a> {
    pub fn iter(&self) -> impl Iterator<Item = GlyphId16> + 'a {
        // all one expression so that we have a single return type
        let (iter1, iter2) = match self {
            CoverageTable::Format1(t) => (Some(t.glyph_array().iter().map(|g| g.get())), None),
            CoverageTable::Format2(t) => {
                let iter = t.range_records().iter().flat_map(RangeRecord::iter);
                (None, Some(iter))
            }
        };

        iter1
            .into_iter()
            .flatten()
            .chain(iter2.into_iter().flatten())
    }

    /// If this glyph is in the coverage table, returns its index
    #[inline]
    pub fn get(&self, gid: impl Into<GlyphId>) -> Option<u16> {
        match self {
            CoverageTable::Format1(sub) => sub.get(gid),
            CoverageTable::Format2(sub) => sub.get(gid),
        }
    }

    /// Returns if this table contains at least one glyph in the 'glyphs' set.
    #[cfg(feature = "std")]
    pub fn intersects(&self, glyphs: &IntSet<GlyphId>) -> bool {
        match self {
            CoverageTable::Format1(sub) => sub.intersects(glyphs),
            CoverageTable::Format2(sub) => sub.intersects(glyphs),
        }
    }

    /// Returns the intersection of this table and input 'glyphs' set.
    #[cfg(feature = "std")]
    pub fn intersect_set(&self, glyphs: &IntSet<GlyphId>) -> IntSet<GlyphId> {
        match self {
            CoverageTable::Format1(sub) => sub.intersect_set(glyphs),
            CoverageTable::Format2(sub) => sub.intersect_set(glyphs),
        }
    }

    /// Return the number of glyphs in this table
    pub fn population(&self) -> usize {
        match self {
            CoverageTable::Format1(sub) => sub.population(),
            CoverageTable::Format2(sub) => sub.population(),
        }
    }

    /// Return the cost of looking up a glyph in this table
    pub fn cost(&self) -> u32 {
        match self {
            CoverageTable::Format1(sub) => sub.cost(),
            CoverageTable::Format2(sub) => sub.cost(),
        }
    }
}

impl CoverageFormat1<'_> {
    /// If this glyph is in the coverage table, returns its index
    #[inline]
    pub fn get(&self, gid: impl Into<GlyphId>) -> Option<u16> {
        let gid16: GlyphId16 = gid.into().try_into().ok()?;
        let be_glyph: BigEndian<GlyphId16> = gid16.into();
        self.glyph_array()
            .binary_search(&be_glyph)
            .ok()
            .map(|idx| idx as _)
    }

    /// Returns if this table contains at least one glyph in the 'glyphs' set.
    #[cfg(feature = "std")]
    fn intersects(&self, glyphs: &IntSet<GlyphId>) -> bool {
        let glyph_count = self.glyph_count() as u32;
        if glyph_count > (glyphs.len() as u32) * self.cost() {
            glyphs.iter().any(|g| self.get(g).is_some())
        } else {
            self.glyph_array()
                .iter()
                .any(|g| glyphs.contains(GlyphId::from(g.get())))
        }
    }

    /// Returns the intersection of this table and input 'glyphs' set.
    #[cfg(feature = "std")]
    fn intersect_set(&self, glyphs: &IntSet<GlyphId>) -> IntSet<GlyphId> {
        let glyph_count = self.glyph_count() as u32;
        if glyph_count > (glyphs.len() as u32) * self.cost() {
            glyphs
                .iter()
                .filter_map(|g| self.get(g).map(|_| g))
                .collect()
        } else {
            self.glyph_array()
                .iter()
                .filter(|g| glyphs.contains(GlyphId::from(g.get())))
                .map(|g| GlyphId::from(g.get()))
                .collect()
        }
    }

    /// Return the number of glyphs in this table
    pub fn population(&self) -> usize {
        self.glyph_count() as usize
    }

    /// Return the cost of looking up a glyph in this table
    pub fn cost(&self) -> u32 {
        bit_storage(self.glyph_count() as u32)
    }
}

impl CoverageFormat2<'_> {
    /// If this glyph is in the coverage table, returns its index
    #[inline]
    pub fn get(&self, gid: impl Into<GlyphId>) -> Option<u16> {
        let gid: GlyphId16 = gid.into().try_into().ok()?;
        self.range_records()
            .binary_search_by(|rec| {
                if rec.end_glyph_id() < gid {
                    Ordering::Less
                } else if rec.start_glyph_id() > gid {
                    Ordering::Greater
                } else {
                    Ordering::Equal
                }
            })
            .ok()
            .map(|idx| {
                let rec = &self.range_records()[idx];
                rec.start_coverage_index() + gid.to_u16() - rec.start_glyph_id().to_u16()
            })
    }

    /// Returns if this table contains at least one glyph in the 'glyphs' set.
    #[cfg(feature = "std")]
    fn intersects(&self, glyphs: &IntSet<GlyphId>) -> bool {
        let range_count = self.range_count() as u32;
        if range_count > (glyphs.len() as u32) * self.cost() {
            glyphs.iter().any(|g| self.get(g).is_some())
        } else {
            self.range_records()
                .iter()
                .any(|record| record.intersects(glyphs))
        }
    }

    /// Returns the intersection of this table and input 'glyphs' set.
    #[cfg(feature = "std")]
    fn intersect_set(&self, glyphs: &IntSet<GlyphId>) -> IntSet<GlyphId> {
        let range_count = self.range_count() as u32;
        if range_count > (glyphs.len() as u32) * self.cost() {
            glyphs
                .iter()
                .filter_map(|g| self.get(g).map(|_| g))
                .collect()
        } else {
            let mut out = IntSet::empty();
            let mut last = GlyphId16::from(0);
            for record in self.range_records() {
                // break out of loop for overlapping/broken tables
                let start_glyph = record.start_glyph_id();
                if start_glyph < last {
                    break;
                }
                let end = record.end_glyph_id();
                last = end;

                let start = GlyphId::from(start_glyph);
                if glyphs.contains(start) {
                    out.insert(start);
                }

                for g in glyphs.iter_after(start) {
                    if g.to_u32() > end.to_u32() {
                        break;
                    }
                    out.insert(g);
                }
            }
            out
        }
    }

    /// Return the number of glyphs in this table
    pub fn population(&self) -> usize {
        self.range_records()
            .iter()
            .fold(0, |acc, record| acc + record.population())
    }

    /// Return the cost of looking up a glyph in this table
    pub fn cost(&self) -> u32 {
        bit_storage(self.range_count() as u32)
    }
}

impl RangeRecord {
    pub fn iter(&self) -> impl Iterator<Item = GlyphId16> + '_ {
        (self.start_glyph_id().to_u16()..=self.end_glyph_id().to_u16()).map(GlyphId16::new)
    }

    /// Returns if this table contains at least one glyph in the 'glyphs' set.
    #[cfg(feature = "std")]
    pub fn intersects(&self, glyphs: &IntSet<GlyphId>) -> bool {
        glyphs.intersects_range(
            GlyphId::from(self.start_glyph_id())..=GlyphId::from(self.end_glyph_id()),
        )
    }

    /// Return the number of glyphs in this record
    pub fn population(&self) -> usize {
        let start = self.start_glyph_id().to_u32() as usize;
        let end = self.end_glyph_id().to_u32() as usize;
        if start > end {
            0
        } else {
            end - start + 1
        }
    }
}

impl DeltaFormat {
    pub(crate) fn value_count(self, start_size: u16, end_size: u16) -> usize {
        let range_len = end_size.saturating_add(1).saturating_sub(start_size) as usize;
        let val_per_word = match self {
            DeltaFormat::Local2BitDeltas => 8,
            DeltaFormat::Local4BitDeltas => 4,
            DeltaFormat::Local8BitDeltas => 2,
            _ => return 0,
        };

        let count = range_len / val_per_word;
        let extra = (range_len % val_per_word).min(1);
        count + extra
    }
}

// we as a 'format' in codegen, and the generic error type for an invalid format
// stores the value as an i64, so we need this conversion.
impl From<DeltaFormat> for i64 {
    fn from(value: DeltaFormat) -> Self {
        value as u16 as _
    }
}

impl<'a> ClassDefFormat1<'a> {
    /// Get the class for this glyph id
    #[inline]
    pub fn get(&self, gid: impl Into<GlyphId>) -> u16 {
        let Some(idx) = gid
            .into()
            .to_u32()
            .checked_sub(self.start_glyph_id().to_u32())
        else {
            return 0;
        };
        self.class_value_array()
            .get(idx as usize)
            .map(|x| x.get())
            .unwrap_or(0)
    }

    /// Iterate over each glyph and its class.
    pub fn iter(&self) -> impl Iterator<Item = (GlyphId16, u16)> + 'a {
        let start = self.start_glyph_id();
        self.class_value_array()
            .iter()
            .enumerate()
            .map(move |(i, val)| {
                let gid = start.to_u16().saturating_add(i as u16);
                (GlyphId16::new(gid), val.get())
            })
    }

    /// Return the number of glyphs explicitly assigned to a class in this table
    pub fn population(&self) -> usize {
        self.glyph_count() as usize
    }

    /// Return the cost of looking up a glyph in this table
    pub fn cost(&self) -> u32 {
        1
    }

    /// Returns class values for the intersected glyphs of this table and input 'glyphs' set.
    #[cfg(feature = "std")]
    fn intersect_classes(&self, glyphs: &IntSet<GlyphId>) -> IntSet<u16> {
        let mut out = IntSet::empty();
        if glyphs.is_empty() {
            return out;
        }

        let start_glyph = self.start_glyph_id().to_u32();
        let glyph_count = self.glyph_count();
        let end_glyph = start_glyph + glyph_count as u32 - 1;
        if glyphs.first().unwrap().to_u32() < start_glyph
            || glyphs.last().unwrap().to_u32() > end_glyph
        {
            out.insert(0);
        }

        let class_values = self.class_value_array();
        if glyphs.contains(GlyphId::from(start_glyph)) {
            let Some(start_glyph_class) = class_values.first() else {
                return out;
            };
            out.insert(start_glyph_class.get());
        }

        for g in glyphs.iter_after(GlyphId::from(start_glyph)) {
            let g = g.to_u32();
            if g > end_glyph {
                break;
            }

            let idx = g - start_glyph;
            let Some(class) = class_values.get(idx as usize) else {
                break;
            };
            out.insert(class.get());
        }
        out
    }

    /// Returns intersected glyphs of this table and input 'glyphs' set that are assigned to input class value.
    #[cfg(feature = "std")]
    fn intersected_class_glyphs(&self, glyphs: &IntSet<GlyphId>, class: u16) -> IntSet<GlyphId> {
        let mut out = IntSet::empty();
        if glyphs.is_empty() {
            return out;
        }

        let start_glyph = self.start_glyph_id().to_u32();
        let glyph_count = self.glyph_count();
        let end_glyph = start_glyph + glyph_count as u32 - 1;
        if class == 0 {
            let first = glyphs.first().unwrap();
            if first.to_u32() < start_glyph {
                out.extend(glyphs.range(first..GlyphId::from(start_glyph)));
            }

            let last = glyphs.last().unwrap();
            if last.to_u32() > end_glyph {
                out.extend(glyphs.range(GlyphId::from(end_glyph + 1)..=last));
            }
            return out;
        }

        let class_values = self.class_value_array();
        for g in glyphs.range(GlyphId::from(start_glyph)..=GlyphId::from(end_glyph)) {
            let idx = g.to_u32() - start_glyph;
            let Some(c) = class_values.get(idx as usize) else {
                break;
            };
            if c.get() == class {
                out.insert(g);
            }
        }
        out
    }
}

impl<'a> ClassDefFormat2<'a> {
    /// Get the class for this glyph id
    #[inline]
    pub fn get(&self, gid: impl Into<GlyphId>) -> u16 {
        let gid = gid.into().to_u32();
        let records = self.class_range_records();
        let ix = match records.binary_search_by(|rec| rec.start_glyph_id().to_u32().cmp(&gid)) {
            Ok(ix) => ix,
            Err(ix) => ix.saturating_sub(1),
        };
        if let Some(record) = records.get(ix) {
            if (record.start_glyph_id().to_u32()..=record.end_glyph_id().to_u32()).contains(&gid) {
                return record.class();
            }
        }
        0
    }

    /// Iterate over each glyph and its class.
    pub fn iter(&self) -> impl Iterator<Item = (GlyphId16, u16)> + 'a {
        self.class_range_records().iter().flat_map(|range| {
            let start = range.start_glyph_id().to_u16();
            let end = range.end_glyph_id().to_u16();
            (start..=end).map(|gid| (GlyphId16::new(gid), range.class()))
        })
    }

    /// Return the number of glyphs explicitly assigned to a class in this table
    pub fn population(&self) -> usize {
        self.class_range_records()
            .iter()
            .fold(0, |acc, record| acc + record.population())
    }

    /// Return the cost of looking up a glyph in this table
    pub fn cost(&self) -> u32 {
        bit_storage(self.class_range_count() as u32)
    }

    /// Returns class values for the intersected glyphs of this table and input 'glyphs' set.
    #[cfg(feature = "std")]
    fn intersect_classes(&self, glyphs: &IntSet<GlyphId>) -> IntSet<u16> {
        let mut out = IntSet::empty();
        if glyphs.is_empty() {
            return out;
        }

        if self.class_range_count() == 0 {
            out.insert(0);
            return out;
        }

        let range_records = self.class_range_records();
        let first_record = range_records[0];

        if glyphs.first().unwrap() < first_record.start_glyph_id() {
            out.insert(0);
        } else {
            let mut glyph = GlyphId::from(first_record.end_glyph_id());
            for record in range_records.iter().skip(1) {
                let Some(g) = glyphs.iter_after(glyph).next() else {
                    break;
                };

                if g < record.start_glyph_id() {
                    out.insert(0);
                    break;
                }
                glyph = GlyphId::from(record.end_glyph_id());
            }
            if glyphs.iter_after(glyph).next().is_some() {
                out.insert(0);
            }
        }

        let num_ranges = self.class_range_count();
        if num_ranges as u64 > glyphs.len() * self.cost() as u64 {
            for g in glyphs.iter() {
                let class = self.get(g);
                if class != 0 {
                    out.insert(class);
                }
            }
        } else {
            for record in range_records {
                if glyphs.intersects_range(
                    GlyphId::from(record.start_glyph_id())..=GlyphId::from(record.end_glyph_id()),
                ) {
                    out.insert(record.class());
                }
            }
        }
        out
    }

    /// Returns intersected glyphs of this table and input 'glyphs' set that are assgiend to input class value.
    #[cfg(feature = "std")]
    fn intersected_class_glyphs(&self, glyphs: &IntSet<GlyphId>, class: u16) -> IntSet<GlyphId> {
        let mut out = IntSet::empty();
        if glyphs.is_empty() {
            return out;
        }

        let first = glyphs.first().unwrap().to_u32();
        let last = glyphs.last().unwrap().to_u32();
        if class == 0 {
            let mut start = first;
            for range in self.class_range_records() {
                let range_start = range.start_glyph_id().to_u32();
                if start < range_start {
                    out.extend(glyphs.range(GlyphId::from(start)..GlyphId::from(range_start)));
                }

                let range_end = range.end_glyph_id().to_u32();
                if range_end >= last {
                    break;
                }
                start = range_end + 1;
            }

            if start <= last {
                out.extend(glyphs.range(GlyphId::from(start)..=GlyphId::from(last)));
            }
            return out;
        }

        let num_ranges = self.class_range_count();
        if num_ranges as u64 > glyphs.len() * self.cost() as u64 {
            for g in glyphs.iter() {
                let c = self.get(g);
                if c == class {
                    out.insert(g);
                }
            }
        } else {
            for range in self.class_range_records() {
                let range_start = range.start_glyph_id().to_u32();
                let range_end = range.end_glyph_id().to_u32();
                if range_start > last {
                    break;
                }
                if range.class() != class || range.end_glyph_id().to_u32() < first {
                    continue;
                }
                out.extend(glyphs.range(GlyphId::from(range_start)..=GlyphId::from(range_end)));
            }
        }
        out
    }
}

impl ClassRangeRecord {
    /// Return the number of glyphs explicitly assigned to a class in this table
    pub fn population(&self) -> usize {
        let start = self.start_glyph_id().to_u32() as usize;
        let end = self.end_glyph_id().to_u32() as usize;
        if start > end {
            0
        } else {
            end - start + 1
        }
    }
}

impl ClassDef<'_> {
    /// Get the class for this glyph id
    #[inline]
    pub fn get(&self, gid: impl Into<GlyphId>) -> u16 {
        match self {
            ClassDef::Format1(table) => table.get(gid),
            ClassDef::Format2(table) => table.get(gid),
        }
    }

    /// Iterate over each glyph and its class.
    ///
    /// This will not include class 0 unless it has been explicitly assigned.
    pub fn iter(&self) -> impl Iterator<Item = (GlyphId16, u16)> + '_ {
        let (one, two) = match self {
            ClassDef::Format1(inner) => (Some(inner.iter()), None),
            ClassDef::Format2(inner) => (None, Some(inner.iter())),
        };
        one.into_iter().flatten().chain(two.into_iter().flatten())
    }

    /// Return the number of glyphs explicitly assigned to a class in this table
    pub fn population(&self) -> usize {
        match self {
            ClassDef::Format1(table) => table.population(),
            ClassDef::Format2(table) => table.population(),
        }
    }

    /// Return the cost of looking up a glyph in this table
    pub fn cost(&self) -> u32 {
        match self {
            ClassDef::Format1(sub) => sub.cost(),
            ClassDef::Format2(sub) => sub.cost(),
        }
    }

    /// Returns class values for the intersected glyphs of this table and input 'glyphs' set.
    #[cfg(feature = "std")]
    pub fn intersect_classes(&self, glyphs: &IntSet<GlyphId>) -> IntSet<u16> {
        match self {
            ClassDef::Format1(table) => table.intersect_classes(glyphs),
            ClassDef::Format2(table) => table.intersect_classes(glyphs),
        }
    }

    /// Returns intersected glyphs of this table and input 'glyphs' set that are assgiend to input class value.
    #[cfg(feature = "std")]
    pub fn intersected_class_glyphs(
        &self,
        glyphs: &IntSet<GlyphId>,
        class: u16,
    ) -> IntSet<GlyphId> {
        match self {
            ClassDef::Format1(table) => table.intersected_class_glyphs(glyphs, class),
            ClassDef::Format2(table) => table.intersected_class_glyphs(glyphs, class),
        }
    }
}

impl<'a> Device<'a> {
    /// Iterate over the decoded values for this device
    pub fn iter(&self) -> impl Iterator<Item = i8> + 'a {
        let format = self.delta_format();
        let mut n = (self.end_size() - self.start_size()) as usize + 1;
        let deltas_per_word = match format {
            DeltaFormat::Local2BitDeltas => 8,
            DeltaFormat::Local4BitDeltas => 4,
            DeltaFormat::Local8BitDeltas => 2,
            _ => 0,
        };

        self.delta_value().iter().flat_map(move |val| {
            let iter = iter_packed_values(val.get(), format, n);
            n = n.saturating_sub(deltas_per_word);
            iter
        })
    }
}

fn iter_packed_values(raw: u16, format: DeltaFormat, n: usize) -> impl Iterator<Item = i8> {
    let mut decoded = [None; 8];
    let (mask, sign_mask, bits) = match format {
        DeltaFormat::Local2BitDeltas => (0b11, 0b10, 2usize),
        DeltaFormat::Local4BitDeltas => (0b1111, 0b1000, 4),
        DeltaFormat::Local8BitDeltas => (0b1111_1111, 0b1000_0000, 8),
        _ => (0, 0, 0),
    };

    let max_per_word = 16 / bits;
    #[allow(clippy::needless_range_loop)] // enumerate() feels weird here
    for i in 0..n.min(max_per_word) {
        let mask = mask << ((16 - bits) - i * bits);
        let val = (raw & mask) >> ((16 - bits) - i * bits);
        let sign = val & sign_mask != 0;

        let val = if sign {
            // it is 2023 and I am googling to remember how twos compliment works
            -((((!val) & mask) + 1) as i8)
        } else {
            val as i8
        };
        decoded[i] = Some(val)
    }
    decoded.into_iter().flatten()
}

impl From<VariationIndex<'_>> for DeltaSetIndex {
    fn from(src: VariationIndex) -> DeltaSetIndex {
        DeltaSetIndex {
            outer: src.delta_set_outer_index(),
            inner: src.delta_set_inner_index(),
        }
    }
}

/// Combination of a tag and a child table.
///
/// Used in script and feature lists where a data structure has an array
/// of records with each containing a tag and an offset to a table. This
/// allows us to provide convenience methods that return both values.
#[derive(Clone)]
pub struct TaggedElement<T> {
    pub tag: Tag,
    pub element: T,
}

impl<T> TaggedElement<T> {
    pub fn new(tag: Tag, element: T) -> Self {
        Self { tag, element }
    }
}

impl<T> std::ops::Deref for TaggedElement<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.element
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn coverage_get_format1() {
        // manually generated, corresponding to the glyphs (1, 7, 13, 27, 44);
        const COV1_DATA: FontData = FontData::new(&[0, 1, 0, 5, 0, 1, 0, 7, 0, 13, 0, 27, 0, 44]);

        let coverage = CoverageFormat1::read(COV1_DATA).unwrap();
        assert_eq!(coverage.get(GlyphId::new(1)), Some(0));
        assert_eq!(coverage.get(GlyphId::new(2)), None);
        assert_eq!(coverage.get(GlyphId::new(7)), Some(1));
        assert_eq!(coverage.get(GlyphId::new(27)), Some(3));
        assert_eq!(coverage.get(GlyphId::new(45)), None);
    }

    #[test]
    fn coverage_get_format2() {
        // manually generated, corresponding to glyphs (5..10) and (30..40).
        const COV2_DATA: FontData =
            FontData::new(&[0, 2, 0, 2, 0, 5, 0, 9, 0, 0, 0, 30, 0, 39, 0, 5]);
        let coverage = CoverageFormat2::read(COV2_DATA).unwrap();
        assert_eq!(coverage.get(GlyphId::new(2)), None);
        assert_eq!(coverage.get(GlyphId::new(7)), Some(2));
        assert_eq!(coverage.get(GlyphId::new(9)), Some(4));
        assert_eq!(coverage.get(GlyphId::new(10)), None);
        assert_eq!(coverage.get(GlyphId::new(32)), Some(7));
        assert_eq!(coverage.get(GlyphId::new(39)), Some(14));
        assert_eq!(coverage.get(GlyphId::new(40)), None);
    }

    #[test]
    fn classdef_get_format2() {
        let classdef = ClassDef::read(FontData::new(
            font_test_data::gdef::MARKATTACHCLASSDEF_TABLE,
        ))
        .unwrap();
        assert!(matches!(classdef, ClassDef::Format2(..)));
        let gid_class_pairs = [
            (616, 1),
            (617, 1),
            (618, 1),
            (624, 1),
            (625, 1),
            (626, 1),
            (652, 2),
            (653, 2),
            (654, 2),
            (655, 2),
            (661, 2),
        ];
        for (gid, class) in gid_class_pairs {
            assert_eq!(classdef.get(GlyphId16::new(gid)), class);
        }
        for (gid, class) in classdef.iter() {
            assert_eq!(classdef.get(gid), class);
        }
    }

    #[test]
    fn delta_decode() {
        // these examples come from the spec
        assert_eq!(
            iter_packed_values(0x123f, DeltaFormat::Local4BitDeltas, 4).collect::<Vec<_>>(),
            &[1, 2, 3, -1]
        );

        assert_eq!(
            iter_packed_values(0x5540, DeltaFormat::Local2BitDeltas, 5).collect::<Vec<_>>(),
            &[1, 1, 1, 1, 1]
        );
    }

    #[test]
    fn delta_decode_all() {
        // manually generated with write-fonts
        let bytes: &[u8] = &[0, 7, 0, 13, 0, 3, 1, 244, 30, 245, 101, 8, 42, 0];
        let device = Device::read(bytes.into()).unwrap();
        assert_eq!(
            device.iter().collect::<Vec<_>>(),
            &[1i8, -12, 30, -11, 101, 8, 42]
        );
    }

    #[test]
    fn bit_storage_tests() {
        assert_eq!(bit_storage(0), 0);
        assert_eq!(bit_storage(1), 1);
        assert_eq!(bit_storage(2), 2);
        assert_eq!(bit_storage(4), 3);
        assert_eq!(bit_storage(9), 4);
        assert_eq!(bit_storage(0x123), 9);
        assert_eq!(bit_storage(0x1234), 13);
        assert_eq!(bit_storage(0xffff), 16);
        assert_eq!(bit_storage(0xffff_ffff), 32);
    }

    #[test]
    fn default_coverage() {
        let coverage = CoverageTable::default();
        assert_eq!(coverage.iter().count(), 0)
    }

    #[test]
    fn default_classdef() {
        let classdef = ClassDef::default();
        assert_eq!(classdef.population(), 0);
        assert_eq!(classdef.iter().count(), 0);
    }
}