Design a URL Shortener

// 1. Create Short URL
POST /api/shorten
Request Body:
{
  "originalUrl": "https://www.example.com/very/long/url",
  "customAlias": "mylink" // optional
}
Response:
{
  "shortUrl": "https://short.url/abc123",
  "originalUrl": "https://www.example.com/very/long/url",
  "createdAt": "2024-01-15T10:30:00Z"
}
// 2. Redirect Short URL
GET /{shortCode}
Response: HTTP 301/302 Redirect to original URL
// 3. Get URL Stats
GET /api/stats/{shortCode}
Response:
{
  "shortCode": "abc123",
  "originalUrl": "https://www.example.com/very/long/url",
  "clicks": 1523,
  "createdAt": "2024-01-15T10:30:00Z",
  "expiresAt": "2025-01-15T10:30:00Z"
}
// 4. Delete URL
DELETE /api/url/{shortCode}
Response: 204 No Content

-- URLs Table
CREATE TABLE urls (
    id BIGINT PRIMARY KEY AUTO_INCREMENT,
    short_code VARCHAR(10) UNIQUE NOT NULL,
    original_url TEXT NOT NULL,
    user_id BIGINT,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    expires_at TIMESTAMP,
    is_custom BOOLEAN DEFAULT FALSE,
    INDEX idx_short_code (short_code),
    INDEX idx_user_id (user_id)
);
-- Analytics Table
CREATE TABLE url_analytics (
    id BIGINT PRIMARY KEY AUTO_INCREMENT,
    short_code VARCHAR(10) NOT NULL,
    clicked_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    ip_address VARCHAR(45),
    user_agent TEXT,
    referrer TEXT,
    INDEX idx_short_code (short_code),
    INDEX idx_clicked_at (clicked_at)
);
-- Users Table (Optional)
CREATE TABLE users (
    id BIGINT PRIMARY KEY AUTO_INCREMENT,
    email VARCHAR(255) UNIQUE,
    api_key VARCHAR(64) UNIQUE,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

public class Base62Encoder {
    // Base62 character set: a-z, A-Z, 0-9 (62 characters)
    private static final String BASE62_CHARS =
        "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    /**
     * Encode a number to Base62 string
     * @param num - The number to encode (database auto-increment ID)
     * @return Base62 encoded string
     */
    public static String encodeBase62(long num) {
        if (num == 0) {
            return String.valueOf(BASE62_CHARS.charAt(0));
        }
        StringBuilder encoded = new StringBuilder();
        while (num > 0) {
            int remainder = (int)(num % 62);
            encoded.insert(0, BASE62_CHARS.charAt(remainder));
            num = num / 62;
        }
        return encoded.toString();
    }
    /**
     * Decode a Base62 string back to number
     * @param str - The Base62 string to decode
     * @return Decoded number
     */
    public static long decodeBase62(String str) {
        long decoded = 0;
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            int value = BASE62_CHARS.indexOf(c);
            decoded = decoded * 62 + value;
        }
        return decoded;
    }
    public static void main(String[] args) {
        // Example usage:
        long id = 125348L; // Auto-increment database ID
        String shortCode = encodeBase62(id);
        System.out.println("Short code: " + shortCode); // "w7e"
        // Decode back
        long decodedId = decodeBase62(shortCode);
        System.out.println("Decoded ID: " + decodedId); // 125348
        // More examples
        System.out.println("ID 1: " + encodeBase62(1L));           // "b"
        System.out.println("ID 62: " + encodeBase62(62L));         // "ba"
        System.out.println("ID 1000000: " + encodeBase62(1000000L)); // "4c92"
        // Capacity calculations:
        // 7 characters = 62^7 = 3.5 trillion unique URLs!
        // 6 characters = 62^6 = 56 billion unique URLs
        // 5 characters = 62^5 = 916 million unique URLs
    }
}