Memoization ni nini na unaitekeleza vipi?

Question

IT Interview Practice · Accepted Answer

**Memoization** huhifadhi matokeo ya function kwa kutumia hoja zake kama ufunguo (key), ili miito ya kurudia yenye ingizo lile lile irudishe matokeo yaliyohifadhiwa papo hapo badala ya kuyakokotoa upya. Hubadilishana **kumbukumbu kwa kasi (memory for speed)** na hufanya kazi tu kwa functions **safi (pure)** (ingizo lile lile → matokeo yale yale, bila madhara ya kando).

## Kifunika cha memoize cha jumla

```js
function memoize(fn) {
  const cache = new Map();
  return (...args) => {
    const key = JSON.stringify(args); // build a cache key from the arguments
    if (cache.has(key)) {
      return cache.get(key);          // cache hit — skip the work
    }
    const result = fn(...args);
    cache.set(key, result);
    return result;
  };
}

const slowSquare = n => { /* expensive */ return n * n; };
const fastSquare = memoize(slowSquare);
fastSquare(4); // computes → 16
fastSquare(4); // cached → 16 instantly
```

## Ushindi wa kawaida: Fibonacci ya recursive

```js
const fib = memoize(n => (n < 2 ? n : fib(n - 1) + fib(n - 2)));
// turns exponential O(2^n) into roughly O(n) by caching subproblems
```

## Mizania (trade-offs) na tahadhari

- **Mkakati wa cache key**: `JSON.stringify` hushindwa kwa functions, na mpangilio wa funguo una umuhimu kwa objects. Kwa hoja za aina ya object, `WeakMap` (yenye ufunguo wa rejea) huepuka uvujaji wa kumbukumbu (memory leaks).
- **Ukuaji usio na mipaka**: cache haipungui kamwe — tumia cache ya **LRU** kwa michakato inayoendelea kwa muda mrefu.
- Tumia memoize tu pale ambapo ukokotoaji upya ni gharama kubwa kweli na ingizo hujirudia.

## Kwa nini ni muhimu

Memoization ipo kila mahali: `useMemo`/`React.memo` za React, selectors za Reselect, dynamic programming.

Dhana (hifadhi matokeo safi kwa ingizo) ni ile ile; ustadi ni kuchagua ufunguo na kuweka mipaka kwa cache.

Kifunika cha memoize cha jumla

Ushindi wa kawaida: Fibonacci ya recursive

Mizania (trade-offs) na tahadhari

Kwa nini ni muhimu

Maswali yanayohusiana