function copyWith (obj, key, value) {
    const result = typeof key === 'number' ? obj == null ? [] : [...obj] : obj == null ? {} : {...obj};
    result[key] = value;
    return result;
}

export const cowValueModel = (...keyDescriptions) => {
    const keys = [];

    function fillKeys (keyDescriptions) {
        keyDescriptions.forEach(each => {
            if (typeof each === 'number') keys.push(each);
            else if (Array.isArray(each)) fillKeys(each);
            else keys.push(...each.toString().split('.'));
        });
    }

    fillKeys(keyDescriptions);

    function setField (x, index, val) {
        if (index >= keys.length) return val;
        const key = keys[index],
            value = x == null ? undefined : x[key],
            modified = setField(value, index + 1, val);
        return value === modified ? x : copyWith(x, key, modified);
    }

    const GET_SENTINEL = {};

    return (obj, val = GET_SENTINEL) =>
        val === GET_SENTINEL ?
            keys.reduce((x, key) => x == null ? undefined : x[key], obj) :
            setField(obj, 0, val);
};

export const cowWorkshop = (keys, fn = x => x) => (obj, {result = obj, resultKeys = keys, diff} = {}) => {
    keys.forEach((key, index) => {
        const value = fn(cowValueModel(key)(obj));
        if (typeof value === "undefined") return;
        const modifier = cowValueModel(resultKeys[index]);
        const oldResult = result;
        result = modifier(oldResult, value);
        if (result !== oldResult) {
            diff = modifier(diff, value);
        }
    });
    return {result, diff};
};