perf: make orderedIDSet membership checks constant time (#5178)

orderedIDSet.add scanned the entire existing slice for every inserted ID,
so building a Collection was quadratic in the number of packages sharing an
index key. This is most visible in idsByType: package types are few, so that
index accumulates nearly every package in the SBOM into a single set, and
each Add rescans it.

Keep the ordered slice (ordering and dedup semantics are unchanged) and add
a hash index for membership checks. The index is only built once a set grows
beyond a small threshold, so the many single-element sets held by idsByName
and idsByPath do not pay for a map, while the large idsByType sets get
constant-time lookups.

BenchmarkCollectionAdd (Apple M2 Pro), packages sharing one type:

    packages   before        after       speedup
    1,000        1.37 ms      0.76 ms      1.8x
    10,000     100.31 ms      9.80 ms     10.2x
    50,000   2,423.79 ms     34.46 ms     70.3x

The tradeoff is memory: sets past the threshold allocate an index map,
about 14% more bytes at 50,000 packages, with allocation counts unchanged.

Signed-off-by: Oleksandr Vodotiiets <61548316+avodotiiets@users.noreply.github.com>

syft/pkg/collection.go

@@ -299,19 +299,56 @@ func (c *Collection) Sorted(types ...Type) (pkgs []Package) {
 	return pkgs
 }
 
+// orderedIDSetIndexThreshold is the number of elements beyond which an orderedIDSet maintains a hash
+// index for membership checks. Below this size a linear scan of the contiguous slice is competitive
+// with a map lookup and avoids allocating a map for the many small (often single-element) sets held
+// by the name and path indexes. Above it, the linear scan is what makes bulk insertion quadratic.
+const orderedIDSetIndexThreshold = 16
+
 type orderedIDSet struct {
 	slice []artifact.ID
+
+	// index is nil until the set grows beyond orderedIDSetIndexThreshold, after which it is kept in
+	// sync with slice and used to answer membership checks in constant time.
+	index map[artifact.ID]struct{}
+}
+
+func (s *orderedIDSet) contains(id artifact.ID) bool {
+	if s.index != nil {
+		_, exists := s.index[id]
+		return exists
+	}
+
+	for _, existingID := range s.slice {
+		if existingID == id {
+			return true
+		}
+	}
+
+	return false
 }
 
 func (s *orderedIDSet) add(ids ...artifact.ID) {
-loopNewIDs:
 	for _, newID := range ids {
-		for _, existingID := range s.slice {
-			if existingID == newID {
-				continue loopNewIDs
-			}
+		if s.contains(newID) {
+			continue
 		}
+
 		s.slice = append(s.slice, newID)
+
+		switch {
+		case s.index != nil:
+			s.index[newID] = struct{}{}
+		case len(s.slice) > orderedIDSetIndexThreshold:
+			s.buildIndex()
+		}
+	}
+}
+
+func (s *orderedIDSet) buildIndex() {
+	s.index = make(map[artifact.ID]struct{}, len(s.slice))
+	for _, id := range s.slice {
+		s.index[id] = struct{}{}
 	}
 }
 
@@ -319,6 +356,7 @@ func (s *orderedIDSet) delete(id artifact.ID) {
 	for i, existingID := range s.slice {
 		if existingID == id {
 			s.slice = append(s.slice[:i], s.slice[i+1:]...)
+			delete(s.index, id) // no-op when the index has not been built
 			return
 		}
 	}

syft/pkg/collection_test.go

@@ -2,6 +2,7 @@ package pkg
 
 import (
 	"context"
+	"fmt"
 	"testing"
 
 	"github.com/scylladb/go-set/strset"
@@ -482,3 +483,73 @@ func Test_idOrderedSet_add(t *testing.T) {
 		})
 	}
 }
+
+func Test_idOrderedSet_addBeyondIndexThreshold(t *testing.T) {
+	// exercise the indexed path: sets larger than orderedIDSetIndexThreshold must still deduplicate
+	// and retain insertion ordering
+	const count = orderedIDSetIndexThreshold * 4
+
+	var expected []artifact.ID
+	var input []artifact.ID
+	for i := 0; i < count; i++ {
+		id := artifact.ID(fmt.Sprintf("id-%d", i))
+		expected = append(expected, id)
+		input = append(input, id)
+	}
+	// add every element a second time; none of these should be appended
+	input = append(input, expected...)
+
+	var s orderedIDSet
+	s.add(input...)
+
+	assert.Equal(t, expected, s.slice)
+	require.NotNil(t, s.index, "index should be built once the set grows beyond the threshold")
+	assert.Len(t, s.index, len(expected), "index and slice should agree")
+}
+
+func Test_idOrderedSet_deleteKeepsIndexInSync(t *testing.T) {
+	const count = orderedIDSetIndexThreshold * 2
+
+	var s orderedIDSet
+	for i := 0; i < count; i++ {
+		s.add(artifact.ID(fmt.Sprintf("id-%d", i)))
+	}
+	require.NotNil(t, s.index)
+
+	removed := artifact.ID("id-0")
+	s.delete(removed)
+
+	assert.Len(t, s.slice, count-1)
+	assert.NotContains(t, s.slice, removed)
+	assert.False(t, s.contains(removed), "deleted element must not remain in the index")
+
+	// re-adding must work (it would be skipped if the index still held a stale entry)
+	s.add(removed)
+	assert.Equal(t, removed, s.slice[len(s.slice)-1])
+}
+
+func BenchmarkCollectionAdd(b *testing.B) {
+	// packages sharing a type all land in the same idsByType set, which is the case that previously
+	// made insertion quadratic
+	for _, count := range []int{1_000, 10_000, 50_000} {
+		pkgs := make([]Package, count)
+		for i := range pkgs {
+			pkgs[i] = Package{
+				Name:    fmt.Sprintf("package-%d", i),
+				Version: "1.0.0",
+				Type:    NpmPkg,
+			}
+			pkgs[i].SetID()
+		}
+
+		b.Run(fmt.Sprintf("packages=%d", count), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				c := NewCollection()
+				for _, p := range pkgs {
+					c.Add(p)
+				}
+			}
+		})
+	}
+}