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HyperLogLog Serialization Format (HLL.NET)

This document describes the binary format used for serializing HyperLogLog<T> instances.

Format Structure

The serialized data is a byte array with the following layout:

Offset Size Description
0 1 byte Format version (currently 2)
1 1 byte Type name length (L)
2 1 byte Precision value (p, between 4–16)
3 L UTF-8 encoded full type name (typeof(T).FullName)
3+L N bytes HLL registers (one byte per register; length = 2^p)

Example

For a HyperLogLog<string> with precision 14:

  • Format version = 2
  • Type name = System.String, which has length = 13
  • Registers count = 2^14 = 16384
  • Total bytes = 1 + 1 + 1 + 13 + 16384 = 16400 bytes

Versioning

  • Current format version: 2
  • Future versions must increment the first byte and adjust parsing logic accordingly.

Notes

  • If the deserialized type T does not match the stored type name, deserialization will fail.
  • Type names are used to avoid logic errors when deserializing to the wrong generic type.