URL Shortener

medium · hashing / encoding, idempotency, expiry

Brief-reported interview attribution (unverified): a standard LLD question at Amazon, Microsoft, Google (the class-level version, not the distributed system)

How to approach an LLD round

Design the core of a URL shortener as a single class. Short codes are base62 encodings of a counter starting at 1 (1, 2, …, 9, A, …, Z, a, …, z, 10, …). Shortening the same URL twice returns the same code. A custom alias may be requested; a taken alias returns None. A link may have a TTL in seconds; after it expires, redirect returns None. Time comes from a clock callable passed to the constructor (default: time.time), so tests can use a fake clock.

class URLShortener:
    def __init__(self, clock=None):
        # clock: callable returning the current time in seconds
    def shorten(self, long_url, custom_alias=None, ttl_seconds=None):
        # -> short code (str), or None if the custom alias is taken
    def redirect(self, code):
        # -> the long URL, or None for an unknown or expired code
    def clicks(self, code):
        # -> number of successful redirects for the code (0 for an unknown code)

Rules. Base62 alphabet is 0-9A-Za-z in that order. The counter starts at 1, so the first generated code is "1". An expired code may be reused (shortening the same URL again after expiry creates a fresh code; a custom alias equal to an expired code is free again). redirect counts a click only on success. Expiry is checked as now >= created_at + ttl_seconds.

Tests use explicit inputs and injected clocks/callbacks. No network, real threads or application-server execution is involved.

Public demo

# Public demo: run after implementing the class API above.
now = [1000.0]
u = URLShortener(clock=lambda: now[0])
print(u.shorten('https://example.com/a'))
print(u.redirect('1'))

Starter code

class URLShortener:

    def __init__(self, clock=None):
        """clock: callable returning the current time in seconds"""
        pass

    def shorten(self, long_url, custom_alias=None, ttl_seconds=None):
        """-> short code (str), or None if the custom alias is taken"""
        pass

    def redirect(self, code):
        """-> the long URL, or None for an unknown or expired code"""
        pass

    def clicks(self, code):
        """-> number of successful redirects for the code (0 for an unknown code)"""
        pass

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