Fengzdadi commented on code in PR #104:
URL: https://github.com/apache/datasketches-go/pull/104#discussion_r2703433246
##########
sampling/reservoir_items_sketch.go:
##########
@@ -115,6 +115,62 @@ func (s *ReservoirItemsSketch[T]) Reset() {
s.data = s.data[:0]
}
+// GetImplicitSampleWeight returns N/K when in sampling mode, or 1.0 in exact
mode.
+func (s *ReservoirItemsSketch[T]) GetImplicitSampleWeight() float64 {
+ if s.n < int64(s.k) {
+ return 1.0
+ }
+ return float64(s.n) / float64(s.k)
+}
+
+// Copy returns a deep copy of the sketch.
+func (s *ReservoirItemsSketch[T]) Copy() *ReservoirItemsSketch[T] {
+ dataCopy := make([]T, len(s.data))
+ copy(dataCopy, s.data)
+ return &ReservoirItemsSketch[T]{
+ k: s.k,
+ n: s.n,
+ data: dataCopy,
+ }
+}
+
+// DownsampledCopy returns a copy with a reduced reservoir size.
+// If newK >= current K, returns a regular copy.
+func (s *ReservoirItemsSketch[T]) DownsampledCopy(newK int)
*ReservoirItemsSketch[T] {
+ if newK >= s.k {
+ return s.Copy()
+ }
+
+ result, _ := NewReservoirItemsSketch[T](newK)
+
+ samples := s.Samples()
+ for _, item := range samples {
+ result.Update(item)
+ }
+
+ // Adjust N to preserve correct implicit weights
+ if result.n < s.n {
+ result.forceIncrementItemsSeen(s.n - result.n)
+ }
+
+ return result
+}
+
+// getValueAtPosition returns the item at the given position.
+func (s *ReservoirItemsSketch[T]) getValueAtPosition(pos int) T {
+ return s.data[pos]
+}
+
+// insertValueAtPosition replaces the item at the given position.
+func (s *ReservoirItemsSketch[T]) insertValueAtPosition(item T, pos int) {
+ s.data[pos] = item
+}
+
+// forceIncrementItemsSeen adds delta to the items seen count.
+func (s *ReservoirItemsSketch[T]) forceIncrementItemsSeen(delta int64) {
Review Comment:
I kept `forceIncrementItemsSeen` as-is because it only bumps n for implicit
weights and never writes into the reservoir, so it can’t overfill capacity.
Capacity validation now happens where it matters: `DownsampledCopy` returns an
error on bad newK, `UpdateFromRaw` rejects k<1 or len(items)>k/n, and
`UpdateSketch` propagates those errors/gadget creation failures.
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