re-implement differenceRunArray to not use iterator

hopefully less heap allocs and better perf now

roaring/roaring.go

@@ -2451,13 +2451,50 @@ func differenceBitmapRun(a, b *container) *container {
 	return output
 }
 
-// differenceRunArray computes the difference of an run from a array.
+// differenceRunArray subtracts the bits in an array container from a run
+// container.
 func differenceRunArray(a, b *container) *container {
 	if a.n == 0 || b.n == 0 {
 		return a.clone()
 	}
-	itr := newArrayIterator(b.array)
-	return differenceRunIterator(a, itr)
+	output := &container{runs: make([]interval16, 0, len(a.runs)), container_type: ContainerRun}
+
+	bidx := 0
+	vb := b.array[bidx]
+	for _, run := range a.runs {
+		start := run.start
+		for vb < run.start {
+			bidx++
+			if bidx >= len(b.array) {
+				break
+			}
+			vb = b.array[bidx]
+		}
+		for vb >= run.start && vb <= run.last {
+			if vb == start {
+				start++
+				bidx++
+				if bidx >= len(b.array) {
+					break
+				}
+				vb = b.array[bidx]
+				continue
+			}
+			output.runs = append(output.runs, interval16{start: start, last: vb - 1})
+			start = vb + 1
+			bidx++
+			if bidx >= len(b.array) {
+				break
+			}
+			vb = b.array[bidx]
+		}
+
+		if start <= run.last {
+			output.runs = append(output.runs, interval16{start: start, last: run.last})
+		}
+	}
+	output.Optimize()
+	return output
 }
 
 // differenceRunBitmap computes the difference of an run from a bitmap.

roaring/roaring_internal_test.go

@@ -1449,6 +1449,21 @@ func TestDifferenceRunArray(t *testing.T) {
 			array: []uint16{0, 9, 10, 11, 12, 13},
 			exp:   []interval16{{start: 1, last: 8}},
 		},
+		{
+			runs:  []interval16{{start: 1, last: 12}, {start: 14, last: 14}, {start: 18, last: 18}},
+			array: []uint16{0, 9, 10, 11, 12, 13, 14, 17},
+			exp:   []interval16{{start: 1, last: 8}, {start: 18, last: 18}},
+		},
+		{
+			runs:  []interval16{{start: 1, last: 12}, {start: 14, last: 14}, {start: 18, last: 18}},
+			array: []uint16{0, 9, 10, 11, 12, 13, 14, 17, 19},
+			exp:   []interval16{{start: 1, last: 8}, {start: 18, last: 18}},
+		},
+		{
+			runs:  []interval16{{start: 1, last: 12}, {start: 14, last: 17}, {start: 19, last: 28}},
+			array: []uint16{0, 9, 10, 11, 12, 13, 14, 17, 19, 25, 27},
+			exp:   []interval16{{start: 1, last: 8}, {start: 15, last: 16}, {start: 20, last: 24}, {start: 26, last: 26}, {start: 28, last: 28}},
+		},
 	}
 	for i, test := range tests {
 		a.runs = test.runs