Code Explainers

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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
java 
package com.acme.billing.config;
 
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.context.properties.ConfigurationProperties;

Feature-flagged beans with Spring @ConditionalOnProperty

feature-flags conditional-beans strategy-pattern
Intermediate 5 steps
typescript 
import { Injectable, effect, signal, computed } from '@angular/core';
 
interface Preferences {
  theme: 'light' | 'dark';

A signal-based preferences store in Angular

signals state-management persistence
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
go 
func (w *Watcher) resetDebounce(d time.Duration) {
	if !w.timer.Stop() {
		select {
		case <-w.timer.C:

Debouncing a stream of events in Go

debounce timers channels
Advanced 7 steps
ruby 
class WeeklySignupsReport
  DEFAULT_WEEKS = 12
 
  def initialize(weeks: DEFAULT_WEEKS, source: User.all)

Building a weekly signups report in Rails

service object aggregation group by
Intermediate 7 steps
python 
import random
from typing import Iterator, List
 
 

How reservoir sampling picks k items

reservoir-sampling streaming randomness
Intermediate 5 steps
php 
<?php
 
namespace App\Services;
 

Building a cached daily leaderboard in Laravel

caching aggregation eager-loading
Intermediate 9 steps
rust 
use chrono::{Duration, NaiveDate};
 
#[derive(Debug)]
pub struct DateRange {

Parsing and iterating date ranges in Rust

error-handling iterators parsing
Intermediate 7 steps
java 
public static Map<String, String> parseCookieHeader(String header) {
    Map<String, String> cookies = new LinkedHashMap<>();
    if (header == null || header.isBlank()) {
        return cookies;

Parsing an HTTP Cookie header in Java

string-parsing http url-decoding
Intermediate 6 steps
typescript 
import { useEffect, useState } from "react";
 
interface Section {
  id: string;

Building a scroll-spy hook in React

custom-hooks intersectionobserver dom-observation
Intermediate 8 steps
go 
package logging
 
import (
	"context"

Deduplicating log attributes in Go's slog

decorator-pattern structured-logging immutability
Intermediate 8 steps
ruby 
class ApplicationController < ActionController::Base
  EXPERIMENTS = {
    checkout_button_color: %w[control blue green],
    onboarding_flow: %w[control streamlined]

How A/B test cohorts are assigned in Rails

a-b-testing cookies hashing
Intermediate 8 steps
python 
import secrets
 
from django.contrib.auth import authenticate, login
from django.core.cache import cache

Two-factor login with OTP in Django

two-factor-auth one-time-passwords caching
Intermediate 9 steps
php 
<?php
 
final class RotatingFileLogger
{

How a rotating file logger works in PHP

logging file-rotation io
Intermediate 9 steps
rust 
use std::collections::VecDeque;
use std::sync::{Arc, Condvar, Mutex};
use std::time::{Duration, Instant};
 

Building a counting semaphore in Rust

concurrency synchronization condition-variable
Advanced 9 steps
java 
public class TimedSocketReader {
 
    private static final int READ_TIMEOUT_MS = 5_000;
    private static final int CONNECT_TIMEOUT_MS = 3_000;

Reading a socket with connect and read timeouts

sockets timeouts io
Intermediate 8 steps
typescript 
import { Injectable, Scope, Inject, NotFoundException } from '@nestjs/common';
import { REQUEST } from '@nestjs/core';
import { Request } from 'express';
import { DataSource } from 'typeorm';

Per-tenant database connections in NestJS

multi-tenancy connection-pooling dependency-injection
Advanced 8 steps