/**
 * NodeId 解码工具
 */
const ROOT_BITS = 4,
    TYPE_BITS = 4,
    SUBJ_BITS = 20,
    STAGE_BITS = 4,
    GRADE_BITS = 4,
    NODE_BITS = 28;

const ROOT_MASK = mask(ROOT_BITS),
    TYPE_MASK = mask(TYPE_BITS),
    SUBJ_MASK = mask(SUBJ_BITS),
    STAGE_MASK = mask(STAGE_BITS),
    GRADE_MASK = mask(GRADE_BITS),
    NODE_MASK = mask(NODE_BITS);

function mask(bits: number) {
    return (1n << BigInt(bits)) - 1n;
}

function intToSubject(n: number) {
    if (n === 0) return null;
    return n <= 6 ? `SUB0${n}` : `SUB${n}`;
}

/**
 * 解码入口：long → 普通对象
 * @param {string|number} compositeId  后端传来的 long 字符串或数字
 * @returns {Object}
 */
export function decodeNodeId(compositeId: number | string) {
    const id = BigInt(compositeId);

    const root = Number((id >> BigInt(TYPE_BITS + SUBJ_BITS + STAGE_BITS + GRADE_BITS + NODE_BITS)) & ROOT_MASK);
    const type = Number((id >> BigInt(SUBJ_BITS + STAGE_BITS + GRADE_BITS + NODE_BITS)) & TYPE_MASK);
    const subNum = Number((id >> BigInt(STAGE_BITS + GRADE_BITS + NODE_BITS)) & SUBJ_MASK);
    const stage = Number((id >> BigInt(GRADE_BITS + NODE_BITS)) & STAGE_MASK);
    const grade = Number((id >> BigInt(NODE_BITS)) & GRADE_MASK);
    const nodeId = Number(id & NODE_MASK);

    return {
        rootType: root === 0 ? null : root,
        nodeType: type === 0 ? null : type,
        subject: intToSubject(subNum),
        stage: stage === 0 ? null : stage,
        grade: grade === 0 ? null : grade,
        nodeId: nodeId === 0 ? null : nodeId,
    };
}