Function regFunc

packages/office-viewer/src/excel/formula/functions/functions.ts:10–15

(name: FunctionName, func: Func)

export const functions: Map<FunctionName, Func> = new Map();

export function regFunc(name: FunctionName, func: Func) {
  if (functions.has(name)) {
    throw new Error(`function ${name} has been registered`);
  }
  functions.set(name, func);
}

// 这些函数每次计算结果都不一样，不能缓存
const volatileFunctions = new Set(['RAND', 'RANDBETWEEN', 'NOW', 'TODAY']);