ZADD, ZSCORE & ZRANGE — Sorted Sets

Sorted sets combine the uniqueness of sets with an ordering score. Every member has an associated floating-point score.

Commands

• ZADD key score member [score member ...] — add with scores, return count of NEW
• ZSCORE key member — return score as bulk string, or $-1
• ZRANGE key start stop [WITHSCORES] — return members ordered by score (low→high)
• ZRANK key member — return 0-based rank (position by score), or $-1
• ZCARD key — return count

Use Cases

Sorted sets are one of Redis's most powerful features:

• Leaderboards: ZADD leaderboard 1500 alice 2000 bob
• Priority queues: score = priority
• Time-based feeds: score = timestamp
• Rate limiting: score = request timestamp

Implementation

You need a data structure that maintains order by score. Options:

• Simple: List of (score, member) tuples, sorted on insert
• Efficient: Skip list + hash map (what Redis actually uses)

For this exercise, the simple approach is fine — we're not benchmarking performance.