A Stern Tree

The purpose of this post is to show how to generate a visual representation of the Stern-Brocot tree, like this: SBTree the rendering of which uses Northwood’s GoJS Javascript library.

To create to a rendered Stern-Brocot tree we need to:

Create a binary tree datatype with supporting functions.
Generate a specific type of binary tree – i.e. a Stern-Brocot tree.
Examine the GoJS model details and write some Haskell to map the binary tree to the Javascript.

Binary Trees

Yes, I could have found an existing Haskell library for binary trees – but really, where’s the fun in that?

A binary tree is a fairly simple recursive data structure that can be expressed as a sum data type:

data BTree a = Empty | BNode a (BTree a) (BTree a)

this indicates that a BTree of a is either Empty or is a BNode containing an a along with a left and right subtree. There are numerous useful operations that can be defined on a BTree – fmapping over it, traversing it in various orders – many of which are shown here.

--
--
module BinaryTrees where
import           Data.List      (sort)
import           Data.Tree
import           Data.Tree.View

data BTree a = Empty | BNode a (BTree a) (BTree a)

instance Functor BTree where
    fmap _ Empty                  = Empty
    fmap f (BNode v Empty Empty ) = BNode (f v) Empty Empty
    fmap f (BNode v l r )         = BNode (f v) (fmap f l) (fmap f r)

foldBTree :: (a -> b) ->  BTree a -> [b]
foldBTree _ Empty         = []
foldBTree f (BNode v l r) = f v : foldBTree f l ++ foldBTree f r

foldBTreeNodes :: (a -> BTree a -> BTree a -> b) ->  BTree a -> [b]
foldBTreeNodes _ Empty         = []
foldBTreeNodes f (BNode v l r) = f v l r : foldBTreeNodes f l ++ foldBTreeNodes f r

traverseDepthFirst :: BTree a -> [a]
traverseDepthFirst Empty        = []
traverseDepthFirst (BNode a l r) = a : traverseDepthFirst l ++ traverseDepthFirst r

traverseBreadthFirst :: BTree a -> [a]
traverseBreadthFirst tree = go [tree]
    where
        go [] = []
        go xs = fmap bNodeVal xs ++ go (concatMap lrSubTrees xs)
        bNodeVal   (BNode v _ _)         = v
        lrSubTrees (BNode _ Empty Empty) = []
        lrSubTrees (BNode _ Empty b)     = [b]
        lrSubTrees (BNode _ a Empty)     = [a]
        lrSubTrees (BNode _ a b)         = [a,b]



traverseInOrder :: BTree a -> [a]
traverseInOrder Empty         = []
traverseInOrder (BNode v l r) = traverseInOrder l ++ [v] ++ traverseInOrder r

traversePreOrder :: BTree a -> [a]
traversePreOrder Empty         = []
traversePreOrder (BNode v l r) = v : traversePreOrder l ++ traversePreOrder r

traversePostOrder :: BTree a -> [a]
traversePostOrder Empty         = []
traversePostOrder (BNode v l r) = traversePostOrder l ++ traversePostOrder r ++ [v]

depth :: BTree a -> Int
depth Empty         = 0
depth (BNode _ l r) = 1 +  max (depth l) (depth r)

width :: BTree a -> Int
width t = leftCount t + rightCount t where
    leftCount t = goLeft t 1 where
        goLeft (BNode _ Empty _) n = n
        goLeft (BNode _ l     _) n = goLeft l (n + 1)
    rightCount t = goRight t 1 where
        goRight (BNode _ _ Empty) n = n
        goRight (BNode _ _ r    ) n = goRight r (n + 1)

treeFromList :: (Ord a) => [a] -> BTree a
treeFromList lst = go (sort lst) where
        go [] = Empty
        go xs = BNode (xs !! half)
                  (go $ take half xs)
                  (go $ drop (half + 1) xs)
                where
                  len = length xs
                  half = len `div` 2

insert :: (Eq a, Ord a) => BTree a -> a -> BTree a
insert Empty x = BNode x Empty Empty
insert (BNode a left right) x
    | x == a = BNode a left right
    | x < a  = BNode a (insert left x) right
    | x > a  = BNode a left (insert right x)

module BinaryTrees where

import Data.List (sort)

import Data.Tree

import Data.Tree.View

data BTree a = Empty | BNode a (BTree a) (BTree a)

instance Functor BTree where

fmap _ Empty = Empty

fmap f (BNode v Empty Empty ) = BNode (f v) Empty Empty

fmap f (BNode v l r ) = BNode (f v) (fmap f l) (fmap f r)

foldBTree :: (a -> b) -> BTree a -> [b]

foldBTree _ Empty = []

foldBTree f (BNode v l r) = f v : foldBTree f l ++ foldBTree f r

foldBTreeNodes :: (a -> BTree a -> BTree a -> b) -> BTree a -> [b]

foldBTreeNodes _ Empty = []

foldBTreeNodes f (BNode v l r) = f v l r : foldBTreeNodes f l ++ foldBTreeNodes f r

traverseDepthFirst :: BTree a -> [a]

traverseDepthFirst Empty = []

traverseDepthFirst (BNode a l r) = a : traverseDepthFirst l ++ traverseDepthFirst r

traverseBreadthFirst :: BTree a -> [a]

traverseBreadthFirst tree = go [tree]

where

go [] = []

go xs = fmap bNodeVal xs ++ go (concatMap lrSubTrees xs)

bNodeVal (BNode v _ _) = v

lrSubTrees (BNode _ Empty Empty) = []

lrSubTrees (BNode _ Empty b) = [b]

lrSubTrees (BNode _ a Empty) = [a]

lrSubTrees (BNode _ a b) = [a,b]

traverseInOrder :: BTree a -> [a]

traverseInOrder Empty = []

traverseInOrder (BNode v l r) = traverseInOrder l ++ [v] ++ traverseInOrder r

traversePreOrder :: BTree a -> [a]

traversePreOrder Empty = []

traversePreOrder (BNode v l r) = v : traversePreOrder l ++ traversePreOrder r

traversePostOrder :: BTree a -> [a]

traversePostOrder Empty = []

traversePostOrder (BNode v l r) = traversePostOrder l ++ traversePostOrder r ++ [v]

depth :: BTree a -> Int

depth Empty = 0

depth (BNode _ l r) = 1 + max (depth l) (depth r)

width :: BTree a -> Int

width t = leftCount t + rightCount t where

leftCount t = goLeft t 1 where

goLeft (BNode _ Empty _) n = n

goLeft (BNode _ l _) n = goLeft l (n + 1)

rightCount t = goRight t 1 where

goRight (BNode _ _ Empty) n = n

goRight (BNode _ _ r ) n = goRight r (n + 1)

treeFromList :: (Ord a) => [a] -> BTree a

treeFromList lst = go (sort lst) where

go [] = Empty

go xs = BNode (xs !! half)

(go $ take half xs)

(go $ drop (half + 1) xs)

where

len = length xs

half = len `div` 2

insert :: (Eq a, Ord a) => BTree a -> a -> BTree a

insert Empty x = BNode x Empty Empty

insert (BNode a left right) x

| x == a = BNode a left right

| x < a = BNode a (insert left x) right

| x > a = BNode a left (insert right x)

And I think they’re fairly simple and self-explanatory.

Stern-Brocot Tree

Generation of the Stern-Brocot tree makes extensive use of the mediant of two fractions. The mediant is the ‘wrong’ way of adding two fractions. The numerators are added to become the new numerator and similarly for the denominator. i.e.

$\begin{align*} \frac{a}{b} + \frac{c}{d} = \frac{a + c}{b + d}\\ \\ \end{align*}$

Remembering our Fraction data type as data Fraction = F Integer Integer
we can define a Monoid on Fraction simply as

--
instance Monoid Fraction where
    mempty = F 0 0
    mappend (F a b) (F c d) = F (a + c) (b + d)

instance Monoid Fraction where

mempty = F 0 0

mappend (F a b) (F c d) = F (a + c) (b + d)

Trying this out in GHCI we have

--
λ-> (F 1 2) <> (F 3 4)
4/6
*Fractions
λ-> (F 3 6) <> (F 1  2)
4/8

λ-> (F 1 2) <> (F 3 4)

4/6

*Fractions

λ-> (F 3 6) <> (F 1 2)

4/8

The details of how the Stern-Brocot tree numbers are generated is discussed in some detail here, here and in Concrete Mathematics by Graham et al. In essence it is a matter of inserting mediants between successive pairs of fractions. To do that we recursively generate a binary tree using the fact that Fraction is a Monoid and that we combine Fractions (in this context) using the mediant function. To keep a reference to the ‘left’ and ‘right’ values the tree will have a triple of Fraction as the type. i.e.

--
-- nearest left, value, nearest right
type Data = (Fraction, Fraction, Fraction)

buildBrocTree :: Int -> BTree Data
buildBrocTree = build (BNode (F 0 1, F 1 1, F 1 0) Empty Empty) where
            build t 0  = t
            build (BNode nd@(fvw, fab, fxy) Empty Empty) n = build (BNode nd newLeft newRight) (n - 1) where
                         newLeft   = BNode (fvw, fvw <> fab, fab) Empty Empty
                         newRight  = BNode (fab, fab <> fxy, fxy) Empty Empty
            build (BNode nd l r) n = BNode nd (build l n) (build r n)

-- nearest left, value, nearest right

type Data = (Fraction, Fraction, Fraction)

buildBrocTree :: Int -> BTree Data

buildBrocTree = build (BNode (F 0 1, F 1 1, F 1 0) Empty Empty) where

build t 0 = t

build (BNode nd@(fvw, fab, fxy) Empty Empty) n = build (BNode nd newLeft newRight) (n - 1) where

newLeft = BNode (fvw, fvw <> fab, fab) Empty Empty

newRight = BNode (fab, fab <> fxy, fxy) Empty Empty

build (BNode nd l r) n = BNode nd (build l n) (build r n)

The above builds the tree to a given depth. We could also have an infinite tree and take what we need from it. i.e.

--
buildBrocTreeLazy :: BTree Data
buildBrocTreeLazy = build (BNode (F 0 1, F 1 1, F 1 0) Empty Empty) where
            build (BNode nd@(fvw, fab, fxy) Empty Empty) = build (BNode nd newLeft newRight) where
                        newLeft   = BNode (fvw, fvw <> fab, fab) Empty Empty
                        newRight  = BNode (fab, fab <> fxy, fxy) Empty Empty
            build (BNode nd l r)  = BNode nd (build l) (build r)

buildBrocTreeLazy :: BTree Data

buildBrocTreeLazy = build (BNode (F 0 1, F 1 1, F 1 0) Empty Empty) where

build (BNode nd@(fvw, fab, fxy) Empty Empty) = build (BNode nd newLeft newRight) where

newLeft = BNode (fvw, fvw <> fab, fab) Empty Empty

newRight = BNode (fab, fab <> fxy, fxy) Empty Empty

build (BNode nd l r) = BNode nd (build l) (build r)

Of course, in terms of types, what we really want is BTree Fraction rather than BTree Data and this is quite easy to do by fmapping over the tree and keeping only the middle Fraction from each triple. i.e.

--
-- match out the 'important' fraction.
fraction :: Data -> Fraction
fraction (_, fr, _) = fr

-- match out the 'important' fraction.

fraction :: Data -> Fraction

fraction (_, fr, _) = fr

and in GHCI

--
λ-> t = buildBrocTree 10
*RationalTrees
λ-> :t t
t :: BTree Data
*RationalTrees
λ-> tf = fmap fraction t
*RationalTrees
λ-> :t tf
tf :: BTree Fraction

λ-> t = buildBrocTree 10

*RationalTrees

λ-> :t t

t :: BTree Data

*RationalTrees

λ-> tf = fmap fraction t

*RationalTrees

λ-> :t tf

tf :: BTree Fraction

and we can also try one of the traverseXXX functions on binary trees – for example traversePreOrder tf gives:

[1/1, 1/2, 1/3, 1/4, 1/5, 1/6, 1/7, 1/8, 1/9, 1/10, 1/11, 2/19, 2/17, 3/26, 3/25, 2/15, 3/23, 4/31, 5/38, 3/22, 5/37, 4/29, 2/13, 3/20, 4/27, 5/34, 7/47, 5/33, 8/53, 7/46, 3/19, 5/32, 7/45, 8/51, 4/25, 7/44, 5/31, 2/11, 3/17, 4/23, 5/29, 6/35, 9/52, 7/40, 11/63, 10/57, 5/28, 8/45, 11/62, 13/73, 7/39, 12/67, 9/50, 3/16, 5/27, 7/38, 9/49, 12/65, 8/43, 13/70, 11/59, 4/21, 7/37, 10/53, 11/58, 5/26, 9/47, 6/31, 2/9, 3/14, 4/19, 5/24, 6/29, 7/34, 11/53, 9/43, 14/67, 13/62, 7/33, 11/52, 15/71, 18/85, 10/47, 17/80, 13/61, 5/23, 8/37, 11/51, 14/65, 19/88, 13/60, 21/97, 18/83, 7/32, 12/55, 17/78, 19/87, 9/41, 16/73, 11/50, 3/13, 5/22, 7/31, 9/40, 11/49, 16/71, 12/53, 19/84, 17/75, 8/35, 13/57, 18/79, 21/92, 11/48, 19/83, 14/61, 4/17, 7/30, 10/43, 13/56, 17/73, 11/47, 18/77, 15/64, 5/21, 9/38, 13/55, 14/59, 6/25, 11/46, 7/29, 2/7, 3/11, 4/15, 5/19, 6/23, 7/27, 8/31, 13/50, 11/42, 17/65, 16/61, 9/34, 14/53, 19/72, 23/87, 13/49, 22/83, 17/64, 7/26, 11/41, 15/56, 19/71, 26/97, 18/67, 29/108, 25/93, 10/37, 17/63, 24/89, 27/100, 13/48, 23/85, 16/59, 5/18, 8/29, 11/40, 14/51, 17/62, 25/91, 19/69, 30/109, 27/98, 13/47, 21/76, 29/105, 34/123, 18/65, 31/112, 23/83, 7/25, 12/43, 17/61, 22/79, 29/104, 19/68, 31/111, 26/93, 9/32, 16/57, 23/82, 25/89, 11/39, 20/71, 13/46, 3/10, 5/17, 7/24, 9/31, 11/38, 13/45, 20/69, 16/55, 25/86, 23/79, 12/41, 19/65, 26/89, 31/106, 17/58, 29/99, 22/75, 8/27, 13/44, 18/61, 23/78, 31/105, 21/71, 34/115, 29/98, 11/37, 19/64, 27/91, 30/101, 14/47, 25/84, 17/57, 4/13, 7/23, 10/33, 13/43, 16/53, 23/76, 17/56, 27/89, 24/79, 11/36, 18/59, 25/82, 29/95, 15/49, 26/85, 19/62, 5/16, 9/29, 13/42, 17/55, 22/71, 14/45, 23/74, 19/61, 6/19, 11/35, 16/51, 17/54, 7/22, 13/41, 8/25, 2/5, 3/8, 4/11, 5/14, 6/17, 7/20, 8/23, 9/26, 15/43, 13/37, 20/57, 19/54, 11/31, 17/48, 23/65, 28/79, 16/45, 27/76, 21/59, 9/25, 14/39, 19/53, 24/67, 33/92, 23/64, 37/103, 32/89, 13/36, 22/61, 31/86, 35/97, 17/47, 30/83, 21/58, 7/19, 11/30, 15/41, 19/52, 23/63, 34/93, 26/71, 41/112, 37/101, 18/49, 29/79, 40/109, 47/128, 25/68, 43/117, 32/87, 10/27, 17/46, 24/65, 31/84, 41/111, 27/73, 44/119, 37/100, 13/35, 23/62, 33/89, 36/97, 16/43, 29/78, 19/51, 5/13, 8/21, 11/29, 14/37, 17/45, 20/53, 31/82, 25/66, 39/103, 36/95, 19/50, 30/79, 41/108, 49/129, 27/71, 46/121, 35/92, 13/34, 21/55, 29/76, 37/97, 50/131, 34/89, 55/144, 47/123, 18/47, 31/81, 44/115, 49/128, 23/60, 41/107, 28/73, 7/18, 12/31, 17/44, 22/57, 27/70, 39/101, 29/75, 46/119, 41/106, 19/49, 31/80, 43/111, 50/129, 26/67, 45/116, 33/85, 9/23, 16/41, 23/59, 30/77, 39/100, 25/64, 41/105, 34/87, 11/28, 20/51, 29/74, 31/79, 13/33, 24/61, 15/38, 3/7, 5/12, 7/17, 9/22, 11/27, 13/32, 15/37, 24/59, 20/49, 31/76, 29/71, 16/39, 25/61, 34/83, 41/100, 23/56, 39/95, 30/73, 12/29, 19/46, 26/63, 33/80, 45/109, 31/75, 50/121, 43/104, 17/41, 29/70, 41/99, 46/111, 22/53, 39/94, 27/65, 8/19, 13/31, 18/43, 23/55, 28/67, 41/98, 31/74, 49/117, 44/105, 21/50, 34/81, 47/112, 55/131, 29/69, 50/119, 37/88, 11/26, 19/45, 27/64, 35/83, 46/109, 30/71, 49/116, 41/97, 14/33, 25/59, 36/85, 39/92, 17/40, 31/73, 20/47, 4/9, 7/16, 10/23, 13/30, 16/37, 19/44, 29/67, 23/53, 36/83, 33/76, 17/39, 27/62, 37/85, 44/101, 24/55, 41/94, 31/71, 11/25, 18/41, 25/57, 32/73, 43/98, 29/66, 47/107, 40/91, 15/34, 26/59, 37/84, 41/93, 19/43, 34/77, 23/52, 5/11, 9/20, 13/29, 17/38, 21/47, 30/67, 22/49, 35/78, 31/69, 14/31, 23/51, 32/71, 37/82, 19/42, 33/73, 24/53, 6/13, 11/24, 16/35, 21/46, 27/59, 17/37, 28/61, 23/50, 7/15, 13/28, 19/41, 20/43, 8/17, 15/32, 9/19, 2/3, 3/5, 4/7, 5/9, 6/11, 7/13, 8/15, 9/17, 10/19, 17/32, 15/28, 23/43, 22/41, 13/24, 20/37, 27/50, 33/61, 19/35, 32/59, 25/46, 11/20, 17/31, 23/42, 29/53, 40/73, 28/51, 45/82, 39/71, 16/29, 27/49, 38/69, 43/78, 21/38, 37/67, 26/47, 9/16, 14/25, 19/34, 24/43, 29/52, 43/77, 33/59, 52/93, 47/84, 23/41, 37/66, 51/91, 60/107, 32/57, 55/98, 41/73, 13/23, 22/39, 31/55, 40/71, 53/94, 35/62, 57/101, 48/85, 17/30, 30/53, 43/76, 47/83, 21/37, 38/67, 25/44, 7/12, 11/19, 15/26, 19/33, 23/40, 27/47, 42/73, 34/59, 53/92, 49/85, 26/45, 41/71, 56/97, 67/116, 37/64, 63/109, 48/83, 18/31, 29/50, 40/69, 51/88, 69/119, 47/81, 76/131, 65/112, 25/43, 43/74, 61/105, 68/117, 32/55, 57/98, 39/67, 10/17, 17/29, 24/41, 31/53, 38/65, 55/94, 41/70, 65/111, 58/99, 27/46, 44/75, 61/104, 71/121, 37/63, 64/109, 47/80, 13/22, 23/39, 33/56, 43/73, 56/95, 36/61, 59/100, 49/83, 16/27, 29/49, 42/71, 45/76, 19/32, 35/59, 22/37, 5/8, 8/13, 11/18, 14/23, 17/28, 20/33, 23/38, 37/61, 31/51, 48/79, 45/74, 25/41, 39/64, 53/87, 64/105, 36/59, 61/100, 47/77, 19/31, 30/49, 41/67, 52/85, 71/116, 49/80, 79/129, 68/111, 27/44, 46/75, 65/106, 73/119, 35/57, 62/101, 43/70, 13/21, 21/34, 29/47, 37/60, 45/73, 66/107, 50/81, 79/128, 71/115, 34/55, 55/89, 76/123, 89/144, 47/76, 81/131, 60/97, 18/29, 31/50, 44/71, 57/92, 75/121, 49/79, 80/129, 67/108, 23/37, 41/66, 59/95, 64/103, 28/45, 51/82, 33/53, 7/11, 12/19, 17/27, 22/35, 27/43, 32/51, 49/78, 39/62, 61/97, 56/89, 29/46, 46/73, 63/100, 75/119, 41/65, 70/111, 53/84, 19/30, 31/49, 43/68, 55/87, 74/117, 50/79, 81/128, 69/109, 26/41, 45/71, 64/101, 71/112, 33/52, 59/93, 40/63, 9/14, 16/25, 23/36, 30/47, 37/58, 53/83, 39/61, 62/97, 55/86, 25/39, 41/64, 57/89, 66/103, 34/53, 59/92, 43/67, 11/17, 20/31, 29/45, 38/59, 49/76, 31/48, 51/79, 42/65, 13/20, 24/37, 35/54, 37/57, 15/23, 28/43, 17/26, 3/4, 5/7, 7/10, 9/13, 11/16, 13/19, 15/22, 17/25, 28/41, 24/35, 37/54, 35/51, 20/29, 31/45, 42/61, 51/74, 29/42, 49/71, 38/55, 16/23, 25/36, 34/49, 43/62, 59/85, 41/59, 66/95, 57/82, 23/33, 39/56, 55/79, 62/89, 30/43, 53/76, 37/53, 12/17, 19/27, 26/37, 33/47, 40/57, 59/84, 45/64, 71/101, 64/91, 31/44, 50/71, 69/98, 81/115, 43/61, 74/105, 55/78, 17/24, 29/41, 41/58, 53/75, 70/99, 46/65, 75/106, 63/89, 22/31, 39/55, 56/79, 61/86, 27/38, 49/69, 32/45, 8/11, 13/18, 18/25, 23/32, 28/39, 33/46, 51/71, 41/57, 64/89, 59/82, 31/43, 49/68, 67/93, 80/111, 44/61, 75/104, 57/79, 21/29, 34/47, 47/65, 60/83, 81/112, 55/76, 89/123, 76/105, 29/40, 50/69, 71/98, 79/109, 37/51, 66/91, 45/62, 11/15, 19/26, 27/37, 35/48, 43/59, 62/85, 46/63, 73/100, 65/89, 30/41, 49/67, 68/93, 79/108, 41/56, 71/97, 52/71, 14/19, 25/34, 36/49, 47/64, 61/83, 39/53, 64/87, 53/72, 17/23, 31/42, 45/61, 48/65, 20/27, 37/50, 23/31, 4/5, 7/9, 10/13, 13/17, 16/21, 19/25, 22/29, 35/46, 29/38, 45/59, 42/55, 23/30, 36/47, 49/64, 59/77, 33/43, 56/73, 43/56, 17/22, 27/35, 37/48, 47/61, 64/83, 44/57, 71/92, 61/79, 24/31, 41/53, 58/75, 65/84, 31/40, 55/71, 38/49, 11/14, 18/23, 25/32, 32/41, 39/50, 57/73, 43/55, 68/87, 61/78, 29/37, 47/60, 65/83, 76/97, 40/51, 69/88, 51/65, 15/19, 26/33, 37/47, 48/61, 63/80, 41/52, 67/85, 56/71, 19/24, 34/43, 49/62, 53/67, 23/29, 42/53, 27/34, 5/6, 9/11, 13/16, 17/21, 21/26, 25/31, 38/47, 30/37, 47/58, 43/53, 22/27, 35/43, 48/59, 57/70, 31/38, 53/65, 40/49, 14/17, 23/28, 32/39, 41/50, 55/67, 37/45, 60/73, 51/62, 19/23, 33/40, 47/57, 52/63, 24/29, 43/52, 29/35, 6/7, 11/13, 16/19, 21/25, 26/31, 37/44, 27/32, 43/51, 38/45, 17/20, 28/33, 39/46, 45/53, 23/27, 40/47, 29/34, 7/8, 13/15, 19/22, 25/29, 32/37, 20/23, 33/38, 27/31, 8/9, 15/17, 22/25, 23/26, 9/10, 17/19, 10/11, 2/1, 3/2, 4/3, 5/4, 6/5, 7/6, 8/7, 9/8, 10/9, 11/10, 19/17, 17/15, 26/23, 25/22, 15/13, 23/20, 31/27, 38/33, 22/19, 37/32, 29/25, 13/11, 20/17, 27/23, 34/29, 47/40, 33/28, 53/45, 46/39, 19/16, 32/27, 45/38, 51/43, 25/21, 44/37, 31/26, 11/9, 17/14, 23/19, 29/24, 35/29, 52/43, 40/33, 63/52, 57/47, 28/23, 45/37, 62/51, 73/60, 39/32, 67/55, 50/41, 16/13, 27/22, 38/31, 49/40, 65/53, 43/35, 70/57, 59/48, 21/17, 37/30, 53/43, 58/47, 26/21, 47/38, 31/25, 9/7, 14/11, 19/15, 24/19, 29/23, 34/27, 53/42, 43/34, 67/53, 62/49, 33/26, 52/41, 71/56, 85/67, 47/37, 80/63, 61/48, 23/18, 37/29, 51/40, 65/51, 88/69, 60/47, 97/76, 83/65, 32/25, 55/43, 78/61, 87/68, 41/32, 73/57, 50/39, 13/10, 22/17, 31/24, 40/31, 49/38, 71/55, 53/41, 84/65, 75/58, 35/27, 57/44, 79/61, 92/71, 48/37, 83/64, 61/47, 17/13, 30/23, 43/33, 56/43, 73/56, 47/36, 77/59, 64/49, 21/16, 38/29, 55/42, 59/45, 25/19, 46/35, 29/22, 7/5, 11/8, 15/11, 19/14, 23/17, 27/20, 31/23, 50/37, 42/31, 65/48, 61/45, 34/25, 53/39, 72/53, 87/64, 49/36, 83/61, 64/47, 26/19, 41/30, 56/41, 71/52, 97/71, 67/49, 108/79, 93/68, 37/27, 63/46, 89/65, 100/73, 48/35, 85/62, 59/43, 18/13, 29/21, 40/29, 51/37, 62/45, 91/66, 69/50, 109/79, 98/71, 47/34, 76/55, 105/76, 123/89, 65/47, 112/81, 83/60, 25/18, 43/31, 61/44, 79/57, 104/75, 68/49, 111/80, 93/67, 32/23, 57/41, 82/59, 89/64, 39/28, 71/51, 46/33, 10/7, 17/12, 24/17, 31/22, 38/27, 45/32, 69/49, 55/39, 86/61, 79/56, 41/29, 65/46, 89/63, 106/75, 58/41, 99/70, 75/53, 27/19, 44/31, 61/43, 78/55, 105/74, 71/50, 115/81, 98/69, 37/26, 64/45, 91/64, 101/71, 47/33, 84/59, 57/40, 13/9, 23/16, 33/23, 43/30, 53/37, 76/53, 56/39, 89/62, 79/55, 36/25, 59/41, 82/57, 95/66, 49/34, 85/59, 62/43, 16/11, 29/20, 42/29, 55/38, 71/49, 45/31, 74/51, 61/42, 19/13, 35/24, 51/35, 54/37, 22/15, 41/28, 25/17, 5/3, 8/5, 11/7, 14/9, 17/11, 20/13, 23/15, 26/17, 43/28, 37/24, 57/37, 54/35, 31/20, 48/31, 65/42, 79/51, 45/29, 76/49, 59/38, 25/16, 39/25, 53/34, 67/43, 92/59, 64/41, 103/66, 89/57, 36/23, 61/39, 86/55, 97/62, 47/30, 83/53, 58/37, 19/12, 30/19, 41/26, 52/33, 63/40, 93/59, 71/45, 112/71, 101/64, 49/31, 79/50, 109/69, 128/81, 68/43, 117/74, 87/55, 27/17, 46/29, 65/41, 84/53, 111/70, 73/46, 119/75, 100/63, 35/22, 62/39, 89/56, 97/61, 43/27, 78/49, 51/32, 13/8, 21/13, 29/18, 37/23, 45/28, 53/33, 82/51, 66/41, 103/64, 95/59, 50/31, 79/49, 108/67, 129/80, 71/44, 121/75, 92/57, 34/21, 55/34, 76/47, 97/60, 131/81, 89/55, 144/89, 123/76, 47/29, 81/50, 115/71, 128/79, 60/37, 107/66, 73/45, 18/11, 31/19, 44/27, 57/35, 70/43, 101/62, 75/46, 119/73, 106/65, 49/30, 80/49, 111/68, 129/79, 67/41, 116/71, 85/52, 23/14, 41/25, 59/36, 77/47, 100/61, 64/39, 105/64, 87/53, 28/17, 51/31, 74/45, 79/48, 33/20, 61/37, 38/23, 7/4, 12/7, 17/10, 22/13, 27/16, 32/19, 37/22, 59/35, 49/29, 76/45, 71/42, 39/23, 61/36, 83/49, 100/59, 56/33, 95/56, 73/43, 29/17, 46/27, 63/37, 80/47, 109/64, 75/44, 121/71, 104/61, 41/24, 70/41, 99/58, 111/65, 53/31, 94/55, 65/38, 19/11, 31/18, 43/25, 55/32, 67/39, 98/57, 74/43, 117/68, 105/61, 50/29, 81/47, 112/65, 131/76, 69/40, 119/69, 88/51, 26/15, 45/26, 64/37, 83/48, 109/63, 71/41, 116/67, 97/56, 33/19, 59/34, 85/49, 92/53, 40/23, 73/42, 47/27, 9/5, 16/9, 23/13, 30/17, 37/21, 44/25, 67/38, 53/30, 83/47, 76/43, 39/22, 62/35, 85/48, 101/57, 55/31, 94/53, 71/40, 25/14, 41/23, 57/32, 73/41, 98/55, 66/37, 107/60, 91/51, 34/19, 59/33, 84/47, 93/52, 43/24, 77/43, 52/29, 11/6, 20/11, 29/16, 38/21, 47/26, 67/37, 49/27, 78/43, 69/38, 31/17, 51/28, 71/39, 82/45, 42/23, 73/40, 53/29, 13/7, 24/13, 35/19, 46/25, 59/32, 37/20, 61/33, 50/27, 15/8, 28/15, 41/22, 43/23, 17/9, 32/17, 19/10, 3/1, 5/2, 7/3, 9/4, 11/5, 13/6, 15/7, 17/8, 19/9, 32/15, 28/13, 43/20, 41/19, 24/11, 37/17, 50/23, 61/28, 35/16, 59/27, 46/21, 20/9, 31/14, 42/19, 53/24, 73/33, 51/23, 82/37, 71/32, 29/13, 49/22, 69/31, 78/35, 38/17, 67/30, 47/21, 16/7, 25/11, 34/15, 43/19, 52/23, 77/34, 59/26, 93/41, 84/37, 41/18, 66/29, 91/40, 107/47, 57/25, 98/43, 73/32, 23/10, 39/17, 55/24, 71/31, 94/41, 62/27, 101/44, 85/37, 30/13, 53/23, 76/33, 83/36, 37/16, 67/29, 44/19, 12/5, 19/8, 26/11, 33/14, 40/17, 47/20, 73/31, 59/25, 92/39, 85/36, 45/19, 71/30, 97/41, 116/49, 64/27, 109/46, 83/35, 31/13, 50/21, 69/29, 88/37, 119/50, 81/34, 131/55, 112/47, 43/18, 74/31, 105/44, 117/49, 55/23, 98/41, 67/28, 17/7, 29/12, 41/17, 53/22, 65/27, 94/39, 70/29, 111/46, 99/41, 46/19, 75/31, 104/43, 121/50, 63/26, 109/45, 80/33, 22/9, 39/16, 56/23, 73/30, 95/39, 61/25, 100/41, 83/34, 27/11, 49/20, 71/29, 76/31, 32/13, 59/24, 37/15, 8/3, 13/5, 18/7, 23/9, 28/11, 33/13, 38/15, 61/24, 51/20, 79/31, 74/29, 41/16, 64/25, 87/34, 105/41, 59/23, 100/39, 77/30, 31/12, 49/19, 67/26, 85/33, 116/45, 80/31, 129/50, 111/43, 44/17, 75/29, 106/41, 119/46, 57/22, 101/39, 70/27, 21/8, 34/13, 47/18, 60/23, 73/28, 107/41, 81/31, 128/49, 115/44, 55/21, 89/34, 123/47, 144/55, 76/29, 131/50, 97/37, 29/11, 50/19, 71/27, 92/35, 121/46, 79/30, 129/49, 108/41, 37/14, 66/25, 95/36, 103/39, 45/17, 82/31, 53/20, 11/4, 19/7, 27/10, 35/13, 43/16, 51/19, 78/29, 62/23, 97/36, 89/33, 46/17, 73/27, 100/37, 119/44, 65/24, 111/41, 84/31, 30/11, 49/18, 68/25, 87/32, 117/43, 79/29, 128/47, 109/40, 41/15, 71/26, 101/37, 112/41, 52/19, 93/34, 63/23, 14/5, 25/9, 36/13, 47/17, 58/21, 83/30, 61/22, 97/35, 86/31, 39/14, 64/23, 89/32, 103/37, 53/19, 92/33, 67/24, 17/6, 31/11, 45/16, 59/21, 76/27, 48/17, 79/28, 65/23, 20/7, 37/13, 54/19, 57/20, 23/8, 43/15, 26/9, 4/1, 7/2, 10/3, 13/4, 16/5, 19/6, 22/7, 25/8, 41/13, 35/11, 54/17, 51/16, 29/9, 45/14, 61/19, 74/23, 42/13, 71/22, 55/17, 23/7, 36/11, 49/15, 62/19, 85/26, 59/18, 95/29, 82/25, 33/10, 56/17, 79/24, 89/27, 43/13, 76/23, 53/16, 17/5, 27/8, 37/11, 47/14, 57/17, 84/25, 64/19, 101/30, 91/27, 44/13, 71/21, 98/29, 115/34, 61/18, 105/31, 78/23, 24/7, 41/12, 58/17, 75/22, 99/29, 65/19, 106/31, 89/26, 31/9, 55/16, 79/23, 86/25, 38/11, 69/20, 45/13, 11/3, 18/5, 25/7, 32/9, 39/11, 46/13, 71/20, 57/16, 89/25, 82/23, 43/12, 68/19, 93/26, 111/31, 61/17, 104/29, 79/22, 29/8, 47/13, 65/18, 83/23, 112/31, 76/21, 123/34, 105/29, 40/11, 69/19, 98/27, 109/30, 51/14, 91/25, 62/17, 15/4, 26/7, 37/10, 48/13, 59/16, 85/23, 63/17, 100/27, 89/24, 41/11, 67/18, 93/25, 108/29, 56/15, 97/26, 71/19, 19/5, 34/9, 49/13, 64/17, 83/22, 53/14, 87/23, 72/19, 23/6, 42/11, 61/16, 65/17, 27/7, 50/13, 31/8, 5/1, 9/2, 13/3, 17/4, 21/5, 25/6, 29/7, 46/11, 38/9, 59/14, 55/13, 30/7, 47/11, 64/15, 77/18, 43/10, 73/17, 56/13, 22/5, 35/8, 48/11, 61/14, 83/19, 57/13, 92/21, 79/18, 31/7, 53/12, 75/17, 84/19, 40/9, 71/16, 49/11, 14/3, 23/5, 32/7, 41/9, 50/11, 73/16, 55/12, 87/19, 78/17, 37/8, 60/13, 83/18, 97/21, 51/11, 88/19, 65/14, 19/4, 33/7, 47/10, 61/13, 80/17, 52/11, 85/18, 71/15, 24/5, 43/9, 62/13, 67/14, 29/6, 53/11, 34/7, 6/1, 11/2, 16/3, 21/4, 26/5, 31/6, 47/9, 37/7, 58/11, 53/10, 27/5, 43/8, 59/11, 70/13, 38/7, 65/12, 49/9, 17/3, 28/5, 39/7, 50/9, 67/12, 45/8, 73/13, 62/11, 23/4, 40/7, 57/10, 63/11, 29/5, 52/9, 35/6, 7/1, 13/2, 19/3, 25/4, 31/5, 44/7, 32/5, 51/8, 45/7, 20/3, 33/5, 46/7, 53/8, 27/4, 47/7, 34/5, 8/1, 15/2, 22/3, 29/4, 37/5, 23/3, 38/5, 31/4, 9/1, 17/2, 25/3, 26/3, 10/1, 19/2, 11/1]

Of course, a list of fractions like this is OK but with a bit of work the BTree Fraction can be rendered graphically.

GoJS

At the heart of rendering using Javascript/HTML is the notion of a TreeModel and TreeLayout as described in the GoJS documentation.

The Haskell code to handle the use of GoJS is here:

--
{-# LANGUAGE QuasiQuotes #-}
module JScript where
import           BinaryTrees
import           Fractions
import           RationalTrees
import           Text.RawString.QQ

topPage :: String
topPage = [r|
    <html>
    <head>
      <script src="https://cdnjs.cloudflare.com/ajax/libs/gojs/1.8.21/go-debug.js"></script>
      <script id="code">
      function init() {
        var $ = go.GraphObject.make;
        myDiagram = $(go.Diagram, "treeDiv",
        {
          initialAutoScale: go.Diagram.UniformToFill,
          layout: $(go.TreeLayout, { nodeSpacing: 5, layerSpacing: 30 })
        });
        var model = $(go.TreeModel);
        model.nodeDataArray =
        [
 |]

bottomPage :: String
bottomPage = [r|
        ];
        myDiagram.model = model;
      }
    </script>
   </head>
   <body onload="init()">
   <div id="tree">
   <!-- The DIV for the Diagram needs an explicit size or else we won't see anything.
        This also adds a border to help see the edges of the viewport. -->
   <div id="treeDiv" style="border: solid 1px black; width:100%; height:100%"></div>

 </div>
</body>
</html>
|]

fullPage :: (Show a) => BTree a ->  String
fullPage tr = concat [topPage, goJSModel . toNodeParentList $ tr, bottomPage]

-- a given node might/might not have children.
toNodeParentList :: BTree a -> [(a, Maybe a, Maybe a)]
toNodeParentList = foldBTreeNodes f where
        f v Empty          Empty          = (v, Nothing, Nothing)
        f v Empty          (BNode vr _ _) = (v, Nothing, Just vr)
        f v (BNode vl _ _) Empty          = (v, Just vl, Nothing)
        f v (BNode vl _ _) (BNode vr _ _) = (v, Just vl, Just vr)

shw :: (Show a) => a -> String
shw x = "'" ++ show x ++ "'"

goJSModel :: (Show a) => [(a, Maybe a, Maybe a)] -> String
goJSModel xs@((r, _, _):_) = foldr f (concat["{key:", shw r, "}"]) xs  where
    f  (w, Just x,  Just y)  ac =
        concat [ "{key:", shw x, ",parent:", shw w,"},",
        "{key:", shw y, ",parent:", shw w,"},", ac ]
    f  (w, Nothing, Just y)  ac =
        concat ["{key:", shw y, ",parent:", shw w,"},", ac ]
    f  (w, Just x,  Nothing) ac =
        concat [ "{key:", shw x, ",parent:", shw w,"},", "{key:", shw x,"}", ac ]
    f  (w, Nothing, Nothing) ac = ac

makeFractionTreeHTML :: FilePath -> BTree Fraction -> IO ()
makeFractionTreeHTML fname tr = writeFile fname (fullPage tr)

{-# LANGUAGE QuasiQuotes #-}

module JScript where

import BinaryTrees

import Fractions

import RationalTrees

import Text.RawString.QQ

topPage :: String

topPage = [r|

<html>

<head>

function init() {

var $ = go.GraphObject.make;

myDiagram = $(go.Diagram, "treeDiv",

{

initialAutoScale: go.Diagram.UniformToFill,

layout: $(go.TreeLayout, { nodeSpacing: 5, layerSpacing: 30 })

});

var model = $(go.TreeModel);

model.nodeDataArray =

[

bottomPage :: String

bottomPage = [r|

];

myDiagram.model = model;

}

</script>

</head>

<!-- The DIV for the Diagram needs an explicit size or else we won't see anything.

This also adds a border to help see the edges of the viewport. -->

</div>

</body>

</html>

fullPage :: (Show a) => BTree a -> String

fullPage tr = concat [topPage, goJSModel . toNodeParentList $ tr, bottomPage]

-- a given node might/might not have children.

toNodeParentList :: BTree a -> [(a, Maybe a, Maybe a)]

toNodeParentList = foldBTreeNodes f where

f v Empty Empty = (v, Nothing, Nothing)

f v Empty (BNode vr _ _) = (v, Nothing, Just vr)

f v (BNode vl _ _) Empty = (v, Just vl, Nothing)

f v (BNode vl _ _) (BNode vr _ _) = (v, Just vl, Just vr)

shw :: (Show a) => a -> String

shw x = "'" ++ show x ++ "'"

goJSModel :: (Show a) => [(a, Maybe a, Maybe a)] -> String

goJSModel xs@((r, _, _):_) = foldr f (concat["{key:", shw r, "}"]) xs where

f (w, Just x, Just y) ac =

concat [ "{key:", shw x, ",parent:", shw w,"},",

"{key:", shw y, ",parent:", shw w,"},", ac ]

f (w, Nothing, Just y) ac =

concat ["{key:", shw y, ",parent:", shw w,"},", ac ]

f (w, Just x, Nothing) ac =

concat [ "{key:", shw x, ",parent:", shw w,"},", "{key:", shw x,"}", ac ]

f (w, Nothing, Nothing) ac = ac

makeFractionTreeHTML :: FilePath -> BTree Fraction -> IO ()

makeFractionTreeHTML fname tr = writeFile fname (fullPage tr)

It uses QuasiQuotes to make the handling of text/html etc. simpler. The function fullPage combines the html head preamble with a GoJs compliant data model, expressed as a string, and then adds the body of the html along with the needed closing tags. The end result is a string of HTML and Javascript that will render the tree. The function toNodeParentList :: BTree a -> [(a, Maybe a, Maybe a)] morphs a BTree into a list of tuples that can be further processed to create data that is specific to GoJS. i.e. it is Javascript that contains parent-child relationships like this…

{key:’1/2′,parent:’1/1′},{key:’2/1′,parent:’1/1′},{key:’1/3′,parent:’1/2′}… etc.

and finally, to generate the page we have the function makeFractionTreeHTML :: FilePath -> BTree Fraction -> IO ()

We can now compose a few functions to generate the HTML/JS for a small Stern- Brocot tree.

makeFractionTreeHTML "tf3.html" . fmap fraction . buildBrocTree $ 3

which generates

    <html>
    <head>
      <script src="https://cdnjs.cloudflare.com/ajax/libs/gojs/1.8.21/go-debug.js"></script>
      <script id="code">
      function init() {
        var $ = go.GraphObject.make;
        myDiagram = $(go.Diagram, "treeDiv",
        {
          initialAutoScale: go.Diagram.UniformToFill,
          layout: $(go.TreeLayout, { nodeSpacing: 5, layerSpacing: 30 })
        });
        var model = $(go.TreeModel);
        model.nodeDataArray =
        [
 {key:'1/2',parent:'1/1'},{key:'2/1',parent:'1/1'},{key:'1/3',parent:'1/2'},
 {key:'2/3',parent:'1/2'},{key:'1/4',parent:'1/3'},{key:'2/5',parent:'1/3'},
 {key:'3/5',parent:'2/3'},{key:'3/4',parent:'2/3'},{key:'3/2',parent:'2/1'},
 {key:'3/1',parent:'2/1'},{key:'4/3',parent:'3/2'},{key:'5/3',parent:'3/2'},
 {key:'5/2',parent:'3/1'},{key:'4/1',parent:'3/1'},{key:'1/1'}
        ];
        myDiagram.model = model;
      }
    </script>
   </head>
   <body onload="init()">
   <div id="tree">
   <!-- The DIV for the Diagram needs an explicit size or else we won't see anything.
        This also adds a border to help see the edges of the viewport. -->
   <div id="treeDiv" style="border: solid 1px black; width:100%; height:100%"></div>

 </div>
</body>
</html>

<html>

<head>

<script src=

GitCommit

Just a code blog…

A Stern Tree

Binary Trees

Stern-Brocot Tree

GoJS