subsetter 0.2.3

Reduces the size and coverage of OpenType fonts.
Documentation 
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use crate::cff::cid_font::CIDMetadata;
use crate::cff::dict::operators::*;
use crate::cff::dict::DictionaryParser;
use crate::cff::number::Number;
use crate::cff::remapper::FontDictRemapper;
use crate::cff::Offsets;
use crate::write::Writer;
use crate::Error::{MalformedFont, SubsetError};
use crate::Result;
use std::array;

// The parsing logic was adapted from ttf-parser.

/// Parse the subroutine offset from a private dict.
pub fn parse_subr_offset(data: &[u8]) -> Option<usize> {
    let mut operands_buffer: [Number; 48] = array::from_fn(|_| Number::zero());
    let mut dict_parser = DictionaryParser::new(data, &mut operands_buffer);

    while let Some(operator) = dict_parser.parse_next() {
        if operator == SUBRS {
            return dict_parser.parse_offset();
        }
    }

    None
}

/// Write the private dicts of a CID font for each font dict.
pub fn rewrite_cid_private_dicts(
    fd_remapper: &FontDictRemapper,
    offsets: &mut Offsets,
    metadata: &CIDMetadata,
    w: &mut Writer,
) -> Result<()> {
    for (new_df, old_df) in fd_remapper.sorted_iter().enumerate() {
        let font_dict = metadata.font_dicts.get(old_df as usize).ok_or(SubsetError)?;
        rewrite_private_dict(offsets, font_dict.private_dict, w, new_df)?;
    }

    Ok(())
}

pub(crate) fn rewrite_private_dict(
    offsets: &mut Offsets,
    private_dict_data: &[u8],
    w: &mut Writer,
    dict_index: usize,
) -> Result<()> {
    let private_dict_offset = w.len();

    let private_dict_data = {
        let mut operands_buffer: [Number; 48] = array::from_fn(|_| Number::zero());
        let mut dict_parser =
            DictionaryParser::new(private_dict_data, &mut operands_buffer);

        let mut sub_w = Writer::new();

        // We just make sure that no subroutine offset gets written, all other operators stay the
        // same.
        while let Some(operator) = dict_parser.parse_next() {
            match operator {
                SUBRS => {
                    // We don't have any subroutines, so don't rewrite this DICT entry.
                }
                _ => {
                    dict_parser.parse_operands().ok_or(MalformedFont)?;
                    let operands = dict_parser.operands();

                    sub_w.write(operands);
                    sub_w.write(operator);
                }
            }
        }

        sub_w.finish()
    };

    let private_dict_len = private_dict_data.len();

    offsets
        .private_dicts_lens
        .get_mut(dict_index)
        .ok_or(SubsetError)?
        .update_value(private_dict_len)?;

    offsets
        .private_dicts_offsets
        .get_mut(dict_index)
        .ok_or(SubsetError)?
        .update_value(private_dict_offset)?;

    w.extend(&private_dict_data);

    Ok(())
}