diff --git a/roaring/roaring.go b/roaring/roaring.go
index 3dc643f2c..310ab78cb 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -1329,17 +1329,17 @@ func (c *container) bitmapToRun() {
 
 // arrayToRun converts from array format to RLE format
 func (c *container) arrayToRun() {
-	c.runs = make([]interval32, 0)
+	c.runs = make([]interval32, 0, c.n) // what capacity to use?
 	start := c.array[0]
-	for i, v := range c.array {
-		if v - c.array[i-1] > 1 {
+	for i, v := range c.array[1:] {
+		if v - c.array[i] > 1 {
 			// if current-previous > 1, one run ends and another begins
-			c.runs = append(c.runs, interval32{start, c.array[i-1]})
+			c.runs = append(c.runs, interval32{start, c.array[i]})
 			start = v
 		}
 	}
 	// append final run
-	c.runs = append(c.runs, interval32{start, c.array[i]})
+	c.runs = append(c.runs, interval32{start, c.array[c.n-1]})
 	c.array = nil
 	c.mapped = false
 }
diff --git a/roaring/roaring_internal_test.go b/roaring/roaring_internal_test.go
index 186e014bd..4424969ad 100644
--- a/roaring/roaring_internal_test.go
+++ b/roaring/roaring_internal_test.go
@@ -731,6 +731,36 @@ func TestBitmapSetRange(t *testing.T) {
 	}
 }
 
+func TestArrayToRun(t *testing.T) {
+	a := &container{}
+	tests := []struct {
+		array []uint32
+		exp   []interval32
+	}{
+		{
+			array: []uint32{0},
+			exp:   []interval32{{start: 0, last: 0}},
+		},
+		{
+			array: []uint32{0, 1, 2, 3, 4},
+			exp:   []interval32{{start: 0, last: 4}},
+		},
+		{
+			array: []uint32{2, 5, 6, 7, 13, 14, 17},
+			exp:   []interval32{{start: 2, last: 2}, {start: 5, last: 7}, {start: 13, last: 14}, {start: 17, last: 17}},
+		},
+	}
+
+	for i, test := range tests {
+		a.array = test.array
+		a.n = len(test.array)
+		a.arrayToRun()
+		if !reflect.DeepEqual(a.runs, test.exp) {
+			t.Fatalf("test #%v expected %v, but got %v", i, test.exp, a.runs)
+		}
+	}
+}
+
 func TestBitmapZeroRange(t *testing.T) {
 	c := &container{bitmap: make([]uint64, bitmapN)}
 	tests := []struct {
@@ -811,5 +841,34 @@ func TestUnionBitmapRun(t *testing.T) {
 			a.bitmap[i] = 0
 		}
 	}
-
+}
+
+func TestRunToArray(t *testing.T) {
+	a := &container{}
+	tests := []struct {
+		runs []interval32
+		exp  []uint32
+	}{
+		{
+			runs: []interval32{{start: 0, last: 0}},
+			exp:  []uint32{0},
+		},
+		{
+			runs: []interval32{{start: 0, last: 4}},
+			exp:  []uint32{0, 1, 2, 3, 4},
+		},
+		{
+			runs: []interval32{{start: 2, last: 2}, {start: 5, last: 7}, {start: 13, last: 14}, {start: 17, last: 17}},
+			exp:  []uint32{2, 5, 6, 7, 13, 14, 17},
+		},
+	}
+
+	for i, test := range tests {
+		a.runs = test.runs
+		a.n = len(test.exp)
+		a.runToArray()
+		if !reflect.DeepEqual(a.array, test.exp) {
+			t.Fatalf("test #%v expected %v, but got %v", i, test.exp, a.array)
+		}
+	}
 }