font-ligatures/src/processors/6-2.ts

Find ligature replacements for any system font

import { ChainingContextualSubstitutionTable, Lookup } from '../tables';
import { LookupTree } from '../types';
import mergeTrees from '../merge';

import { listGlyphsByIndex } from './coverage';
import getGlyphClass, { listClassGlyphs } from './classDef';
import { processInputPosition, processLookaheadPosition, processBacktrackPosition, getInputTree, EntryMeta } from './helper';

/**
 * Build lookup tree for GSUB lookup table 6, format 2.
 * https://docs.microsoft.com/en-us/typography/opentype/spec/gsub#62-chaining-context-substitution-format-2-class-based-glyph-contexts
 *
 * @param table JSON representation of the table
 * @param lookups List of lookup tables
 * @param tableIndex Index of this table in the overall lookup
 */
export default function buildTree(table: ChainingContextualSubstitutionTable.Format2, lookups: Lookup[], tableIndex: number): LookupTree {
    const results: LookupTree[] = [];

    const firstGlyphs = listGlyphsByIndex(table.coverage);

    for (const { glyphId } of firstGlyphs) {
        const firstInputClass = getGlyphClass(table.inputClassDef, glyphId);
        for (const [glyphId, inputClass] of firstInputClass.entries()) {
            // istanbul ignore next - invalid font
            if (inputClass === null) {
                continue;
            }

            const classSet = table.chainClassSet[inputClass];

            // If the class set is null there's nothing to do with this table.
            if (!classSet) {
                continue;
            }

            for (const [subIndex, subTable] of classSet.entries()) {
                const result: LookupTree = {
                    individual: {},
                    range: []
                };

                let currentEntries: EntryMeta[] = getInputTree(
                    result,
                    subTable.lookupRecords,
                    lookups,
                    0,
                    glyphId
                ).map(({ entry, substitution }) => ({ entry, substitutions: [substitution] }));

                for (const [index, classNum] of subTable.input.entries()) {
                    currentEntries = processInputPosition(
                        listClassGlyphs(table.inputClassDef, classNum),
                        index + 1,
                        currentEntries,
                        subTable.lookupRecords,
                        lookups
                    );
                }

                for (const classNum of subTable.lookahead) {
                    currentEntries = processLookaheadPosition(
                        listClassGlyphs(table.lookaheadClassDef, classNum),
                        currentEntries
                    );
                }

                for (const classNum of subTable.backtrack) {
                    currentEntries = processBacktrackPosition(
                        listClassGlyphs(table.backtrackClassDef, classNum),
                        currentEntries
                    );
                }

                // When we get to the end, all of the entries we've accumulated
                // should have a lookup defined
                for (const { entry, substitutions } of currentEntries) {
                    entry.lookup = {
                        substitutions,
                        index: tableIndex,
                        subIndex,
                        length: subTable.input.length + 1,
                        contextRange: [
                            -1 * subTable.backtrack.length,
                            1 + subTable.input.length + subTable.lookahead.length
                        ]
                    };
                }

                results.push(result);
            }
        }
    }

    return mergeTrees(results);
}