LeetCode JavaScript 30 Days Challenge – Day12 – 2637. Promise Time Limit

    AllenLi
    LeetCode JavaScript 30 Days Challenge

    Given an asynchronous function fn and a time t in milliseconds, return a new time limited version of the input function.
    A time limited function is a function that is identical to the original unless it takes longer than t milliseconds to fullfill.
    In that case, it will reject with “Time Limit Exceeded”.
    Note that it should reject with a string, not an Error.

    給予一個非同步的函式 fn 和一個以豪秒為單位的時間 t，回傳一個新的限時函式。
這個限時函式和原始函式完全相同，除非它超過 t 豪秒才能完成，
在這個情況下，它將拒絕並顯示“Time Limit Exceeded”。
請注意，它應該回傳文字訊息而非一個錯誤。

    Example 1:

    Input: 
fn = async (n) => { 
  await new Promise(res => setTimeout(res, 100)); 
  return n * n; 
}
inputs = [5]
t = 50
Output: {"rejected":"Time Limit Exceeded","time":50}

    Example 2:

    Input: 
fn = async (n) => { 
  await new Promise(res => setTimeout(res, 100)); 
  return n * n; 
}
inputs = [5]
t = 150
Output: {"resolved":25,"time":100}

    Example 3:

    Input: 
fn = async (a, b) => { 
  await new Promise(res => setTimeout(res, 120)); 
  return a + b; 
}
inputs = [5,10]
t = 150
Output: {"resolved":15,"time":120}

    Example 4:

    Input: 
fn = async () => { 
  throw "Error";
}
inputs = []
t = 1000
Output: {"rejected":"Error","time":0}

    solution:
    以 Example 1 為例，呼叫 limited() 函式後，
    進入到 timeLimit() 中建立一個含有 setTimeout() 的 new Promise()，
    如果時間經過 t 豪秒後會執行 reject(“Time Limit Exceeded”)。
    而 fn 函式放入 …args，並等待 100 豪秒後回傳 n * n，
    此時因為超過 50 豪秒的時間限制，而 .catch(console.log) 抓取到 reject 的結果，
    故回傳(“Time Limit Exceeded”)訊息。
    ex.如果 fn(…args) 提前完成，則需取消 id 的限時函式。

    Code 1: BigO(1)

    var timeLimit = function(fn, t) {
    return async function(...args) {
        return new Promise((resolve, reject) => {
            const id = setTimeout(() => reject("Time Limit Exceeded"), t);
            fn(...args)
                .then((res) => resolve(res))
                .catch((err) => reject(err))
                .finally(() => clearTimeout(id))
        })
    }
};

    FlowChart:
    Example 1

    const limited = timeLimit(async (n) => {
  await new Promise((res) => setTimeout(res, 100));
  return n * n;
}, 50);
limited(5).catch(console.log); // "Time Limit Exceeded" at t=50ms

    Example 2

    const limited = timeLimit(async (n) => {
  await new Promise((res) => setTimeout(res, 100));
  return n * n;
}, 150);
limited(5).then(console.log).catch(console.log); // return 25 at t=100ms

    Example 3

    const limited = timeLimit(async (a, b) => {
  await new Promise((res) => setTimeout(res, 120));
  return a + b;
}, 150);
limited(5, 10).then(console.log).catch(console.log); // return 15 at t=120ms

    Example 4

    const limited = timeLimit(async () => {
  throw "Error";
}, 1000);
limited().then(console.log).catch(console.log); // "Error" at t=0ms

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