Keep sequence projection storage proportional to assigned items

internal/engine/sequence_projection_test.go

@@ -0,0 +1,49 @@
+package engine
+
+import "testing"
+
+func TestSparseSequenceProjectionIndexesOnlyAssignedPositions(t *testing.T) {
+	index := newSequenceItemProjectionIndex([]sequenceItemProjection{
+		{inputIndex: 0, outputIndex: 9000}, {inputIndex: 3000, outputIndex: 2},
+	})
+	if len(index.outputByInput) != 2 || len(index.inputByOutput) != 2 {
+		t.Fatal("sparse projection retained unassigned positions")
+	}
+	for input, output := range map[int]int{0: 9000, 3000: 2} {
+		if got, found := index.outputForInput(input); !found || got != output {
+			t.Fatalf("input %d = %d, %t", input, got, found)
+		}
+		if got, found := index.inputForOutput(output); !found || got != input {
+			t.Fatalf("output %d = %d, %t", output, got, found)
+		}
+	}
+	for _, position := range []int{-1, 1, 10000} {
+		if _, found := index.outputForInput(position); found {
+			t.Fatalf("unassigned input %d exists", position)
+		}
+		if _, found := index.inputForOutput(position); found {
+			t.Fatalf("unassigned output %d exists", position)
+		}
+	}
+}
+
+func TestContiguousSequenceRangesKeepArithmeticLookup(t *testing.T) {
+	ranges := make([]sequenceRange, 128)
+	for position := range ranges {
+		ranges[position] = sequenceRange{sourceIndex: position, path: Keys("items"), start: position, end: position + 1}
+	}
+	for _, assigned := range indexSequenceRanges(ranges) {
+		if len(assigned.projection.outputByInput) != 0 || len(assigned.projection.inputByOutput) != 0 {
+			t.Fatal("contiguous range materialized a projection index")
+		}
+		if output, found := assigned.outputForInput(0); !found || output != assigned.start {
+			t.Fatal("arithmetic forward lookup lost its offset")
+		}
+		if input, found := assigned.inputForOutput(assigned.start); !found || input != 0 {
+			t.Fatal("arithmetic reverse lookup lost its offset")
+		}
+		if _, found := assigned.inputForOutput(assigned.end); found {
+			t.Fatal("range includes its exclusive end")
+		}
+	}
+}