export function calcSplices()

in packages/web-components/fast-element/src/observation/array-change-records.ts [222:319]
80 lines of code
20 McCabe index (conditional complexity)

export function calcSplices(
    current: any[],
    currentStart: number,
    currentEnd: number,
    old: any[],
    oldStart: number,
    oldEnd: number
): ReadonlyArray<never> | Splice[] {
    let prefixCount = 0;
    let suffixCount = 0;

    const minLength = Math.min(currentEnd - currentStart, oldEnd - oldStart);
    if (currentStart === 0 && oldStart === 0) {
        prefixCount = sharedPrefix(current, old, minLength);
    }

    if (currentEnd === current.length && oldEnd === old.length) {
        suffixCount = sharedSuffix(current, old, minLength - prefixCount);
    }

    currentStart += prefixCount;
    oldStart += prefixCount;
    currentEnd -= suffixCount;
    oldEnd -= suffixCount;

    if (currentEnd - currentStart === 0 && oldEnd - oldStart === 0) {
        return emptyArray;
    }

    if (currentStart === currentEnd) {
        const splice = newSplice(currentStart, [], 0);

        while (oldStart < oldEnd) {
            splice.removed.push(old[oldStart++]);
        }

        return [splice];
    } else if (oldStart === oldEnd) {
        return [newSplice(currentStart, [], currentEnd - currentStart)];
    }

    const ops = spliceOperationsFromEditDistances(
        calcEditDistances(current, currentStart, currentEnd, old, oldStart, oldEnd)
    );

    const splices: Splice[] = [];
    let splice: Splice | undefined = void 0;
    let index = currentStart;
    let oldIndex = oldStart;

    for (let i = 0; i < ops.length; ++i) {
        switch (ops[i]) {
            case EDIT_LEAVE:
                if (splice !== void 0) {
                    splices.push(splice);
                    splice = void 0;
                }

                index++;
                oldIndex++;
                break;
            case EDIT_UPDATE:
                if (splice === void 0) {
                    splice = newSplice(index, [], 0);
                }

                splice.addedCount++;
                index++;

                splice.removed.push(old[oldIndex]);
                oldIndex++;
                break;
            case EDIT_ADD:
                if (splice === void 0) {
                    splice = newSplice(index, [], 0);
                }

                splice.addedCount++;
                index++;
                break;
            case EDIT_DELETE:
                if (splice === void 0) {
                    splice = newSplice(index, [], 0);
                }

                splice.removed.push(old[oldIndex]);
                oldIndex++;
                break;
            // no default
        }
    }

    if (splice !== void 0) {
        splices.push(splice);
    }

    return splices;
}