Extracting ttf font ligature mappings

Question

I have an existing ttf font where I wish to extract all Ligature mappings into this form:

{
    "calendar_today": "E935",
    "calendar_view_day": "E936",
    ...
}

I'm using fontkit with this script:

const fontkit = require('fontkit');
let font = fontkit.openSync('./MaterialIcons-Regular.ttf');
let lookupList = font.GSUB.lookupList.toArray();
let lookupListIndexes = font.GSUB.featureList[0].feature.lookupListIndexes;

lookupListIndexes.forEach(index => {
    let subTable = lookupList[index].subTables[0];
    let ligatureSets = subTable.ligatureSets.toArray();

    ligatureSets.forEach(ligatureSet => {
        ligatureSet.forEach(ligature => {
            let character = font.stringsForGlyph(ligature.glyph)[0];
            let characterCode = character.charCodeAt(0).toString(16).toUpperCase();

            let ligatureText = ligature
                .components
                .map(x => font.stringsForGlyph(x)[0])
                .join('');

            console.log(`${ligatureText} -> ${characterCode}`);
        });
    });
});

However, I'm not getting the full Ligature names. output:

...
alendar_today -> E935
rop_portrait -> E3C5
ontact_phone -> E0CF
ontrol_point -> E3BA
hevron_right -> E5CC
...

What am I doing wrong? Judging by the analysis with FontForge, the font's Ligature names are not missing any characters.

Answer 1

As noted here, the first character is calculated according to a coverage range records.

first, calculate the leading characters

let leadingCharacters = [];
subTable.coverage.rangeRecords.forEach((coverage) => {
    for (let i = coverage.start; i <= coverage.end; i++) {
        let character = font.stringsForGlyph(i)[0];
        leadingCharacters.push(character);
    }
});

then, access these characters by the index of the subtable

let ligatureSets = subTable.ligatureSets.toArray();
ligatureSets.forEach((ligatureSet, ligatureSetIndex) => {

    let leadingCharacter = leadingCharacters[ligatureSetIndex];

    ligatureSet.forEach(ligature => {
        let character = font.stringsForGlyph(ligature.glyph)[0];
        let characterCode = character.charCodeAt(0).toString(16).toUpperCase();

        let ligatureText = ligature
            .components
            .map(x => font.stringsForGlyph(x)[0])
            .join('');

        ligatureText = leadingCharacter + ligatureText;

        console.log(`${ligatureText} -> ${characterCode}`);
    });
});