font-ligatures/src/walk.ts

Find ligature replacements for any system font

import { FlattenedLookupTree, LookupResult } from './types';

export default function walkTree(tree: FlattenedLookupTree, sequence: number[], startIndex: number, index: number): LookupResult | undefined {
    const glyphId = sequence[index];
    let subtree = tree[glyphId];
    if (!subtree) {
        return undefined;
    }

    let lookup = subtree.lookup;
    if (subtree.reverse) {
        const reverseLookup = walkReverse(subtree.reverse, sequence, startIndex);

        if (
            (!lookup && reverseLookup) ||
            (
                reverseLookup && lookup && (
                    lookup.index > reverseLookup.index ||
                    (lookup.index === reverseLookup.index && lookup.subIndex > reverseLookup.subIndex)
                )
            )
        ) {
            lookup = reverseLookup;
        }
    }

    if (++index >= sequence.length || !subtree.forward) {
        return lookup;
    }

    const forwardLookup = walkTree(subtree.forward, sequence, startIndex, index);

    if (
        (!lookup && forwardLookup) ||
        (
            forwardLookup && lookup && (
                lookup.index > forwardLookup.index ||
                (lookup.index === forwardLookup.index && lookup.subIndex > forwardLookup.subIndex)
            )
        )
    ) {
        lookup = forwardLookup;
    }

    return lookup;
}

function walkReverse(tree: FlattenedLookupTree, sequence: number[], index: number): LookupResult | undefined {
    let subtree = tree[sequence[--index]];
    let lookup: LookupResult | undefined = subtree && subtree.lookup;
    while (subtree) {
        if (
            (!lookup && subtree.lookup) ||
            (subtree.lookup && lookup && lookup.index > subtree.lookup.index)
        ) {
            lookup = subtree.lookup;
        }

        if (--index < 0 || !subtree.reverse) {
            break;
        }

        subtree = subtree.reverse[sequence[index]];
    }

    return lookup;
}