'use strict';

const db = require('../infra/db');

const AUTO_COLUMNS = new Set(['id', 'created_at', 'updated_at']);
const columnsCache = new Map();

/**
 * 取表列名（缓存）。DDL 稳定，进程级缓存即可。
 * @param {string} table
 * @returns {Promise<string[]>}
 */
async function getColumns(table) {
  if (columnsCache.has(table)) return columnsCache.get(table);
  const rows = await db.query(
    `SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS
      WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = ?
      ORDER BY ORDINAL_POSITION`,
    [table],
  );
  const cols = rows.map((r) => r.COLUMN_NAME);
  columnsCache.set(table, cols);
  return cols;
}

/**
 * 依据表实际列，推断按月过滤条件（兼容三种月份维度命名）。
 * 无月份维度（字典/全量表）→ 返回空条件。
 * @param {string[]} columns
 * @param {number} year
 * @param {number} month
 * @returns {{ clause: string, params: any[] }}
 */
function monthWhere(columns, year, month) {
  if (columns.includes('exam_year') && columns.includes('exam_month')) {
    return { clause: 'exam_year = ? AND exam_month = ?', params: [year, month] };
  }
  if (columns.includes('year') && columns.includes('month')) {
    return { clause: '`year` = ? AND `month` = ?', params: [year, month] };
  }
  if (columns.includes('missing_month')) {
    return { clause: 'missing_month = ?', params: [`${year}-${String(month).padStart(2, '0')}`] };
  }
  return { clause: '', params: [] };
}

/**
 * 值归一化：对象/数组转 JSON 字符串；Buffer/Date/标量原样；undefined→null。
 * @param {any} v
 */
function coerceValue(v) {
  if (v === undefined || v === null) return null;
  if (Buffer.isBuffer(v) || v instanceof Date) return v;
  if (typeof v === 'object') return JSON.stringify(v);
  return v;
}

/**
 * 生成多行 INSERT（列 = 表列 ∩ 行键，排除自增/时间戳自动列）。
 * @param {string} table
 * @param {string[]} tableColumns
 * @param {Object[]} rows
 * @returns {{ sql: string, params: any[], columns: string[] } | null} 无有效列时返回 null
 */
function buildMultiInsert(table, tableColumns, rows) {
  const rowKeys = new Set();
  for (const r of rows) for (const k of Object.keys(r)) rowKeys.add(k);

  const columns = tableColumns.filter((c) => !AUTO_COLUMNS.has(c) && rowKeys.has(c));
  if (columns.length === 0) return null;

  const placeholderRow = `(${columns.map(() => '?').join(', ')})`;
  const sql =
    `INSERT INTO \`${table}\` (${columns.map((c) => `\`${c}\``).join(', ')}) ` +
    `VALUES ${rows.map(() => placeholderRow).join(', ')}`;
  const params = [];
  for (const r of rows) for (const c of columns) params.push(coerceValue(r[c]));
  return { sql, params, columns };
}

/** 将数组按 size 切块 */
function chunk(arr, size) {
  const out = [];
  for (let i = 0; i < arr.length; i += size) out.push(arr.slice(i, i + size));
  return out;
}

module.exports = { getColumns, monthWhere, coerceValue, buildMultiInsert, chunk };