Suggest an editImprove this articleRefine the answer for “Why is a perfect hash function impossible in practice?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)A **perfect hash function** is one that produces **unique, collision-free hashes** for all possible keys and distributes them evenly across the table. **Key point:** a perfect hash function exists only for a known, fixed set of keys in advance; in all other cases, approximations are used that minimize collisions but cannot eliminate them entirely.Shown above the full answer for quick recall.Answer (EN)ImageA **perfect hash function** is one that produces **unique, collision-free hashes** for all possible keys and distributes them evenly across the table. In practice this is **impossible**, because: 1. **There are infinitely many keys, but a limited number of slots.** - The table has a finite size `N`, while the number of possible inputs (strings, numbers, objects) is practically infinite. - So different keys will inevitably land in the same slot. 2. **A hash function is a trade-off between speed and uniformity.** - To be fast, it must work simply (arithmetic operations, bit shifts). - But simplicity makes a perfectly even distribution impossible. 3. **Data is unpredictable.** - It is impossible to know in advance exactly which keys will be used, so it is impossible to build a universal "perfect" function. 4. **Any range restriction (via** `% N`**) creates overlaps.** - Even if hashes are unique in a 64-bit space, once reduced to the table's range (for example, 0-999), collisions will still appear. ## Conclusion A perfect hash function exists only **for a known, fixed set of keys in advance**. For all other cases, **approximations** are used: functions that minimize collisions but cannot eliminate them completely.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.