ttf_parser/tables/cmap/
format4.rs

1use core::convert::TryFrom;
2
3use crate::parser::{LazyArray16, Stream};
4use crate::GlyphId;
5
6/// A [format 4](https://docs.microsoft.com/en-us/typography/opentype/spec/cmap#format-4-segment-mapping-to-delta-values)
7/// subtable.
8#[derive(Clone, Copy)]
9pub struct Subtable4<'a> {
10    start_codes: LazyArray16<'a, u16>,
11    end_codes: LazyArray16<'a, u16>,
12    id_deltas: LazyArray16<'a, i16>,
13    id_range_offsets: LazyArray16<'a, u16>,
14    id_range_offset_pos: usize,
15    // The whole subtable data.
16    data: &'a [u8],
17}
18
19impl<'a> Subtable4<'a> {
20    /// Parses a subtable from raw data.
21    pub fn parse(data: &'a [u8]) -> Option<Self> {
22        let mut s = Stream::new(data);
23        s.advance(6); // format + length + language
24        let seg_count_x2 = s.read::<u16>()?;
25        if seg_count_x2 < 2 {
26            return None;
27        }
28
29        let seg_count = seg_count_x2 / 2;
30        s.advance(6); // searchRange + entrySelector + rangeShift
31
32        let end_codes = s.read_array16::<u16>(seg_count)?;
33        s.skip::<u16>(); // reservedPad
34        let start_codes = s.read_array16::<u16>(seg_count)?;
35        let id_deltas = s.read_array16::<i16>(seg_count)?;
36        let id_range_offset_pos = s.offset();
37        let id_range_offsets = s.read_array16::<u16>(seg_count)?;
38
39        Some(Self {
40            start_codes,
41            end_codes,
42            id_deltas,
43            id_range_offsets,
44            id_range_offset_pos,
45            data,
46        })
47    }
48
49    /// Returns a glyph index for a code point.
50    ///
51    /// Returns `None` when `code_point` is larger than `u16`.
52    pub fn glyph_index(&self, code_point: u32) -> Option<GlyphId> {
53        // This subtable supports code points only in a u16 range.
54        let code_point = u16::try_from(code_point).ok()?;
55
56        // A custom binary search.
57        let mut start = 0;
58        let mut end = self.start_codes.len();
59        while end > start {
60            let index = (start + end) / 2;
61            let end_value = self.end_codes.get(index)?;
62            if end_value >= code_point {
63                let start_value = self.start_codes.get(index)?;
64                if start_value > code_point {
65                    end = index;
66                } else {
67                    let id_range_offset = self.id_range_offsets.get(index)?;
68                    let id_delta = self.id_deltas.get(index)?;
69                    if id_range_offset == 0 {
70                        return Some(GlyphId(code_point.wrapping_add(id_delta as u16)));
71                    } else if id_range_offset == 0xFFFF {
72                        // Some malformed fonts have 0xFFFF as the last offset,
73                        // which is invalid and should be ignored.
74                        return None;
75                    }
76
77                    let delta = (u32::from(code_point) - u32::from(start_value)) * 2;
78                    let delta = u16::try_from(delta).ok()?;
79
80                    let id_range_offset_pos =
81                        (self.id_range_offset_pos + usize::from(index) * 2) as u16;
82                    let pos = id_range_offset_pos.wrapping_add(delta);
83                    let pos = pos.wrapping_add(id_range_offset);
84
85                    let glyph_array_value: u16 = Stream::read_at(self.data, usize::from(pos))?;
86
87                    // 0 indicates missing glyph.
88                    if glyph_array_value == 0 {
89                        return None;
90                    }
91
92                    let glyph_id = (glyph_array_value as i16).wrapping_add(id_delta);
93                    return u16::try_from(glyph_id).ok().map(GlyphId);
94                }
95            } else {
96                start = index + 1;
97            }
98        }
99
100        None
101    }
102
103    /// Calls `f` for each codepoint defined in this table.
104    pub fn codepoints(&self, mut f: impl FnMut(u32)) {
105        for (start, end) in self.start_codes.into_iter().zip(self.end_codes) {
106            // OxFFFF value is special and indicates codes end.
107            if start == end && start == 0xFFFF {
108                break;
109            }
110
111            for code_point in start..=end {
112                f(u32::from(code_point));
113            }
114        }
115    }
116}
117
118impl core::fmt::Debug for Subtable4<'_> {
119    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
120        write!(f, "Subtable4 {{ ... }}")
121    }
122}
ttf_parser/tables/cmap/format4.rs

ttf_parser/tables/cmap/
format4.rs
