# Lisp Project of the Day

## cl-flat-tree

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# cl-flat-treedata-structures

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This system implements an interesting algorithm suitable for storing binary tries or transmitting them over the wire without overhead on storing pointers.

There is almost no information about this algorithm on the internet. The best description I found is:

https://datprotocol.github.io/book/ch01-01-flat-tree.html

Also, there are versions for Go, C, JS, Rust and Kotlin listed here:

https://github.com/mafintosh/flat-tree

Let's try to serialize a binary tree, represented by lists into a flat vector. First, we need to prepare a vector which size depends on the depth of the tree:

``````;; Here is a tree:
;;            /
;;       *
;;   +     15   100.0
;; A   B

POFTHEDAY> (defparameter *structure*
'(/ (* (+ a b)
15)
100.0))

POFTHEDAY> (defparameter *depth*
(rutils:tree-depth *structure*))

POFTHEDAY> *depth*
4

POFTHEDAY> (defparameter *size* (expt 2 *depth*))

POFTHEDAY> (defparameter *data*
(make-array (list (expt 2 *depth*))
:initial-element nil))
#(NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL)``````

Missing nodes are filled with nils.

Flat-tree is not a data-structure; it is an algorithm to calculate the tree node's index in the flat vector.

That is why we need to write a function which will take lists forming a binary tree and dump it into the vector as a "flat-tree":

``````POFTHEDAY> (defun fill-tree (vector obj depth offset)
(let ((index (flat-tree:index depth offset)))
(cond
((listp obj)
(setf (aref vector index)
(first obj))
(fill-tree vector (second obj)
(1- depth) 0)
(fill-tree vector (third obj)
(1- depth) 1))
;; If it is a symbol
(t
(setf (aref vector index)
obj))))
(values vector
obj))

POFTHEDAY> (fill-tree *data* *structure* 3 0)

;; The first value is a resulting vector
#(A + B * NIL 15 NIL / NIL NIL NIL 100.0 NIL NIL NIL NIL)

;; And the second is original data-structure
(/ (* (+ A B) 15) 100.0)``````

We also can define a function to print our tree in a readable form.

If you are more skilled in ASCII Art than me, maybe you can provide a more advanced version?

``````POFTHEDAY> (defun print-flat-tree (vector)
(loop for item across vector
for index upfrom 0
for as-str = (format nil "~S" item)
for depth = (flat-tree:depth index)
when item
do (format t "~A~%"

POFTHEDAY> (print-flat-tree *data*)
A
+
B
*
15
/
100.0``````

As a home task, I'll leave a function which restores the lisp data-structure from the vector. The solution should work with sparse tries like this one:

``````POFTHEDAY> read-flat-tree(#(A + B * NIL 15 NIL / NIL NIL NIL 100.0 NIL NIL NIL NIL))

(/ (* (+ A B)
15)
100.0)``````

Hope to see you tomorrow in the next #poftheday post!

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