function parseObjLine(line, obj) {
    line = line.trim();
    if (line[0] === '#' || line.length < 1) {
        return;
    }
    const elements = line.split(/\s+/);
    obj[elements[0]].push(elements.slice(1));
}

function getFaces(obj) {
    return obj.f.map(f => {
        const face = {
            'vertices': [],
            'normals': [],
            'textures': [],
        };
        if (f.length === 4) {  // add duplicate vertices to make triangles
            f.splice(3, 0, f[0]);
            f.splice(4, 0, f[2]);
        }
        f.forEach(v => {
            const [vidx, vtidx, vnidx] = v.split('/');
            face.vertices.push((obj.v[vidx - 1] || []).map(Number));
            face.normals.push((obj.vn[vnidx - 1] || []).map(Number));
            //face.textures.push((obj.vt[vtidx] || []).map(Number));
            face.textures.push([0, 0]);
        });
        return face;
    });
}

export async function loadObjModel(url) {
    const stlDataStream = (await fetch(url)).body;
    const faces = [];
    const obj = new Proxy({}, {
        get: (target, name) =>{
            if (!(name in target)) {
                target[name] = [];
            }
            return target[name];
        },
    });

    return new Promise((resolve, reject) => {
        let partialLine = "";
        const reader = stlDataStream.getReader();
        const decoder = new TextDecoder();

        function pump() {
            reader.read().then(({ done, value }) => {
                if (done) {
                    parseObjLine(partialLine, obj);
                    return resolve(getFaces(obj));
                }

                const textInput = decoder.decode(value);
                const lines = textInput.split('\n');

                if (lines.length > 1) {
                    parseObjLine(partialLine + lines[0], obj);
                    partialLine = "";
                    lines.slice(0, -1).forEach(line => {
                        parseObjLine(line, obj);
                    });
                }
                partialLine = partialLine + lines.at(-1);
                pump();
            });
        }
        pump();
    });
}