subsetter 0.2.3

Reduces the size and coverage of OpenType fonts.
Documentation 
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use crate::cff::dict::private_dict::parse_subr_offset;
use crate::cff::dict::top_dict::TopDictData;
use crate::cff::index::{parse_index, Index};
use crate::read::Reader;
use crate::write::Writer;
use crate::GlyphRemapper;

/// Metadata required for handling SID-keyed fonts.
#[derive(Clone, Copy, Default, Debug)]
pub struct SIDMetadata<'a> {
    pub local_subrs: Index<'a>,
    pub private_dict_data: &'a [u8],
}

// The parsing logic was taken from ttf-parser.
pub fn parse_sid_metadata<'a>(data: &'a [u8], top_dict: &TopDictData) -> SIDMetadata<'a> {
    top_dict
        .private
        .clone()
        .and_then(|private_dict_range| {
            let mut metadata = SIDMetadata::default();
            let private_dict_data = data.get(private_dict_range.clone())?;

            metadata.local_subrs =
                if let Some(subrs_offset) = parse_subr_offset(private_dict_data) {
                    let start = private_dict_range.start.checked_add(subrs_offset)?;
                    let subrs_data = data.get(start..)?;
                    let mut r = Reader::new(subrs_data);
                    parse_index::<u16>(&mut r)?
                } else {
                    Index::default()
                };

            metadata.private_dict_data = private_dict_data;
            Some(metadata)
        })
        .unwrap_or_default()
}

/// Write the FD INDEX for SID-keyed fonts.
/// They all get mapped to the font DICT 0.
pub fn generate_fd_index(
    gid_remapper: &GlyphRemapper,
    w: &mut Writer,
) -> crate::Result<()> {
    // Format
    w.write::<u8>(3);
    // nRanges
    w.write::<u16>(1);
    // first
    w.write::<u16>(0);
    // fd index
    w.write::<u8>(0);
    // sentinel
    w.write::<u16>(gid_remapper.num_gids());
    Ok(())
}