[Feat] Implement ReservoirLongsSketch for sampling package

[Fengzdadi]

Description

I'd like to contribute an implementation of ReservoirLongsSketch to the Go library. This would address the ❌ status for "ReservoirLongsSketch" in the README.

Proposed Implementation

Based on the Java reference implementation, I've created:

File	Description
sampling/reservoir_longs_sketch.go	Core reservoir sampling for int64 values
sampling/reservoir_longs_union.go	Union for merging multiple sketches
sampling/reservoir_longs_sketch_test.go	Unit tests (11 tests)
examples/reservoir_example_test.go	Usage examples

Algorithm

The classic Reservoir Sampling algorithm (Vitter's Algorithm R):

1. Initial Phase (n < k): Store all items
2. Steady State (n ≥ k): Replace random item with probability k/n

API

// Create sketch with capacity k
sketch, _ := sampling.NewReservoirLongsSketch(10)

// Add items
sketch.Update(42)

// Get uniform random sample
samples := sketch.GetSamples()

// Serialization
bytes, _ := sketch.ToByteArray()
restored, _ := sampling.NewReservoirLongsSketchFromSlice(bytes)

Feedback Requested

I have a working implementation ready. Before submitting a PR, I'd appreciate feedback on:

1. Serialization Format: I followed the general pattern from PreambleUtil.java. Should I verify cross-language compatibility with specific test cases?
2. Scope: Should I include ReservoirItemsSketch<T> (generic version) in the same PR, or keep it as a separate contribution?
3. Any design concerns with the current approach?

Testing

All tests pass locally:

go test -v ./sampling/ ./examples/
# 13 tests pass

I'm happy to adjust the implementation based on your feedback!

[jmalkin]

There's no need for the longs sketch if there's a generic version. It was just done in java to ensure everything was working correctly prior to making a generic sampling sketch.

For c++ there's only a templatized version since you can use that with primitives. And we had a known-working version to compare against by then.

[Fengzdadi]

There's no need for the longs sketch if there's a generic version. It was just done in java to ensure everything was working correctly prior to making a generic sampling sketch.

For c++ there's only a templatized version since you can use that with primitives. And we had a known-working version to compare against by then.

Got it, thanks for the explanation! I'll refactor to a generic ReservoirItemsSketch[T] instead. Will update the PR when ready.

[Fengzdadi]

Serialization is deferred to a follow-up PR as generic type serialization needs some design consideration.

[Fengzdadi]

I'm closing this issue because it's already been completed!