+ merged, err := v.fit(tree)
+ if err != nil {
+ return merged, fmt.Errorf("fit appended view: %w", err)
+ }
+
+ return merged, nil
+}
+
+// load keeps the node of line when it is built, and tells if it is.
+func (v *View) load(tree ViewNodeReader, line NodeKey) (bool, error) {
+ node, built, err := tree.Node(line.level, line.index)
+ if err != nil || !built {
+ return false, err
+ }
+
+ v.nodes[line] = node
+
+ return true, nil
+}
+
+// fit merges the most due pairs while a batch runs, and tells if a merge
+// changed the view.
+func (v *View) fit(tree ViewNodeReader) (bool, error) {
+ size := v.Size()
+ if size > v.high {
+ v.merging = true
+ }
+
+ merged := false
+ for v.merging && size > v.low {
+ best, err := v.mostDue(tree)
+ if err != nil || best < 0 {
+ if err != nil {
+ return merged, fmt.Errorf("choose merge candidate: %w", err)
+ }
+
+ return merged, nil
+ }
+
+ left, right := v.lines[best], v.lines[best+1]
+ parent := NodeKey{left.level + 1, left.index / 2}
+ _, err = v.load(tree, parent)
+ if err != nil {
+ return merged, fmt.Errorf("load merged parent: %w", err)
+ }
+
+ size += v.lineSize(parent) - v.lineSize(left) - v.lineSize(right)
+ delete(v.nodes, left)
+ delete(v.nodes, right)
+
+ v.lines[best] = parent
+ v.lines = append(v.lines[:best+1], v.lines[best+2:]...)
+ merged = true
+ }
+
+ v.merging = false
+
+ return merged, nil
+}
+
+// Size returns the sum of the line sizes.
+func (v *View) Size() int {
+ size := 0
+ for _, line := range v.lines {
+ size += v.lineSize(line)
+ }
+
+ return size
+}
+
+// mostDue returns the position of the left line of the most due sibling pair
+// whose parent is built, or -1 when there is no such pair.
+func (v *View) mostDue(tree ViewNodeReader) (int, error) {
+ best := -1
+ bestDue := 0.0
+ for n := 0; n+1 < len(v.lines); n++ {
+ left, right := v.lines[n], v.lines[n+1]
+ if left.index%2 != 0 || right.level != left.level || right.index != left.index+1 {
+ continue
+ }
+
+ last := nodeEnd(right.level, right.index) - 1
+ due := math.Ldexp(float64(v.end-last), -left.level)
+ if best >= 0 && due <= bestDue {
+ continue
+ }
+
+ built, err := tree.Built(left.level+1, left.index/2)
+ if err != nil {
+ return -1, fmt.Errorf("check parent node: %w", err)
+ }
+
+ if built {
+ best = n
+ bestDue = due
+ }
+ }
+
+ return best, nil
+}
+
+// lineSize returns the size of a view line.
+func (v *View) lineSize(line NodeKey) int {
+ node, built := v.nodes[line]
+ if !built {
+ return len(viewPending)
+ }
+
+ return node.Size