Fix the panic in handleModification()

Fixes #4

cmd/hercules/main.go

@@ -144,6 +144,11 @@ func main() {
 			var val int64
 			if i < len(status) {
 				val = status[i]
+				// not sure why this sometimes happens...
+				// TODO(vmarkovtsev): find the root cause of tiny negative balances
+				if val < 0 {
+					val = 0
+				}
 			}
 			fmt.Printf("%[1]*[2]d ", width, val)
 		}

file.go

@@ -125,12 +125,12 @@ func (file *File) Update(time int, pos int, ins_length int, del_length int) {
 	if ins_length > 0 && (origin.value != time || origin.key == pos) {
 		// insert our new interval
 		if iter.Item().value == time {
-			if iter.Prev().Item().value != time {
+			prev := iter.Prev()
+			if prev.Item().value != time {
 				iter.Item().key = pos
 			} else {
-				next_iter := iter.Next()
 				tree.DeleteWithIterator(iter)
-				iter = next_iter
+				iter = prev
 			}
 			origin.value = time // cancels the insertion after applying the delta
 		} else {

file_test.go

@@ -317,3 +317,31 @@ func TestBug3(t *testing.T) {
 	assert.Equal(t, "0 0\n1 -1\n", dump)
 	assert.Equal(t, int64(1), status[0])
 }
+
+func TestBug4(t *testing.T) {
+	status := map[int]int64{}
+	file := NewFile(0, 10, status)
+	file.Update(125, 0, 20, 9)
+	file.Update(125, 0, 20, 20)
+	file.Update(166, 12, 1, 1)
+	file.Update(214, 2, 1, 1)
+	file.Update(214, 4, 9, 0)
+	file.Update(214, 27, 1, 1)
+	file.Update(215, 3, 1, 1)
+	file.Update(215, 13, 1, 1)
+	file.Update(215, 17, 1, 1)
+	file.Update(215, 19, 5, 0)
+	file.Update(215, 25, 0, 1)
+	file.Update(215, 31, 6, 1)
+	file.Update(215, 27, 15, 0)
+	file.Update(215, 2, 25, 4)
+	file.Update(215, 28, 1, 1)
+	file.Update(215, 30, 7, 2)
+	file.Update(215, 38, 1, 0)
+	file.Update(215, 40, 4, 2)
+	file.Update(215, 46, 1, 0)
+	file.Update(215, 49, 1, 0)
+	file.Update(215, 52, 2, 6)
+	dump := file.Dump()
+	assert.Equal(t, "0 125\n2 215\n48 125\n50 215\n69 125\n73 215\n79 125\n80 0\n81 -1\n", dump)
+}