WIP: r/demo/boards pagination

[jaekwon]

Investigating approach to use for pagination.
The current draft form works for next-pagination but doesn't really help skip forward or go backward.
So I'll be replacing "last" with offset, and make sure avl iteration can support offsets.

[jaekwon]

Decided to implement TraverseByOffset wtih similar reverse traverse feature.
Then this would be better suited for use for pagination in /r/boards.

[harry-hov]

I just took a look at TraverseByOffset. The naive way I can think of is:

// TraverseByOffset traverses all nodes, including inner nodes.
// A limit of math.MaxInt means no limit.
// XXX Actually implement this method signature by modifying the implementation below.
// XXX Then, make tests.
func (tree *Tree) TraverseByOffset(offset, limit int, ascending bool, leavesOnly bool, cb func(*Tree) bool) bool {
	count := 0
	start := offset
	end := offset + limit
	stop := false

	return tree.TraverseInRange("", "", ascending, leavesOnly, func(t *Tree) bool {
		count += 1
		if count <= start {
			return false
		}
		if count <= end {
			stop = cb(t)
			if stop {
				return stop
			}
		} else if count == end+1 {
			return true
		}
		return stop
	})
}

I'm still trying to figure out an efficient way to skip offset

[jaekwon]

Also relevant: #450

[moul]

#450 is merged.

If someone is working on this PR, please, rebase first.

Cc @harry-hov

[harry-hov]

This one doesn't uses TraverseInRange()

// TraverseByOffset traverses all nodes, including inner nodes.
// A limit of math.MaxInt means no limit.
// XXX Actually implement this method signature by modifying the implementation below.
// XXX Then, make tests.
func (node *Node) TraverseByOffset(offset, limit int, ascending bool, leavesOnly bool, cb func(*Node) bool) bool {
	return traverseByOffset(node, &offset, &limit, ascending, leavesOnly, cb)
}

func traverseByOffset(node *Node, offset, limit *int, ascending, leavesOnly bool, cb func(*Node) bool) bool {
	if node == nil {
		return false
	}

	var firstPass, secondPass *Node
	if ascending {
		firstPass = node.getLeftNode()
		secondPass = node.getRightNode()
	} else {
		firstPass = node.getRightNode()
		secondPass = node.getLeftNode()
	}

	if stop := traverseByOffset(firstPass, offset, limit, ascending, leavesOnly, cb); stop {
		return stop
	}
	if (!node.IsLeaf() && !leavesOnly) || node.IsLeaf() {
		if *offset > 0 {
			*offset -= 1
			return false
		}
		if *limit == 0 {
			return true
		}

		*limit -= 1
		if stop := cb(node); stop {
			return stop
		}
	}
	if stop := traverseByOffset(secondPass, offset, limit, ascending, leavesOnly, cb); stop {
		return stop
	}

	return false
}

I'm not sure if we can do this without a helper function.

---

PS: I also thought about writing an iterative approach. But needs a stack implementation for that. Maybe we can utilize this if needed #456