rust 37 lines · 6 steps

A generic memoizer in Rust

A reusable struct that caches the results of any pure function, keyed by its input.

Explained by highlit
Intermediate memoization generics caching closures trait-bounds
1use std::collections::HashMap;
2 
3pub struct Memoizer<K, V, F> {
4 cache: HashMap<K, V>,
5 compute: F,
6}
7 
8impl<K, V, F> Memoizer<K, V, F>
9where
10 K: std::hash::Hash + Eq + Clone,
11 V: Clone,
12 F: Fn(&K) -> V,
13{
14 pub fn new(compute: F) -> Self {
15 Self {
16 cache: HashMap::new(),
17 compute,
18 }
19 }
20 
21 pub fn get(&mut self, key: K) -> V {
22 if let Some(value) = self.cache.get(&key) {
23 return value.clone();
24 }
25 let value = (self.compute)(&key);
26 self.cache.insert(key, value.clone());
27 value
28 }
29 
30 pub fn len(&self) -> usize {
31 self.cache.len()
32 }
33 
34 pub fn invalidate(&mut self, key: &K) -> Option<V> {
35 self.cache.remove(key)
36 }
37}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Storing a closure in a generic field lets one struct memoize any compatible function.
  2. 2Trait bounds like Hash, Eq, and Clone express exactly what the cache needs from its types.
  3. 3Checking the cache before computing turns repeated work into a single hash lookup.

Related explainers

rust 
use serde::Deserialize;
 
#[derive(Debug, Deserialize)]
#[serde(untagged)]

Parsing flexible JSON shapes with serde

deserialization enums json
Intermediate 6 steps
rust 
use std::cmp::{Ordering, Reverse};
use std::collections::BinaryHeap;
use std::fs::File;
use std::io::{self, BufRead, BufReader, BufWriter, Lines, Write};

K-way merge of sorted logs in Rust

binary-heap k-way-merge streaming-io
Intermediate 8 steps
javascript 
function evaluate(expression) {
  const tokens = tokenize(expression);
  let pos = 0;
 

Building a recursive descent calculator

parsing recursion operator-precedence
Intermediate 8 steps
rust 
use axum::{
    extract::{Path, State},
    response::sse::{Event, KeepAlive, Sse},
};

Streaming import progress with SSE in Axum

server-sent-events streams watch-channel
Advanced 7 steps
rust 
use std::f64::consts::PI;
 
#[derive(Debug, Clone, Copy)]
pub struct LatLng {

Building geographic bounding boxes in Rust

geospatial value-types option
Intermediate 7 steps
javascript 
import { useState, useEffect, useCallback, useRef } from 'react';
 
const cache = new Map();
const inflight = new Map();

Building a stale-while-revalidate hook in React

caching request-deduplication custom-hooks
Advanced 10 steps

Share this explainer

Here's the card — post it anywhere.

A generic memoizer in Rust — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code