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+
+' # CKY Algorithm for enumerating binary trees.
+
+
+' The CKY algorithm is one of the most celebrated algorithms in
+' Natural Language Processing. First described as early as 1961
+' it gives a dynamic programming formulation for counting binary
+' over a fixed length sequence.
+
+' https://en.wikipedia.org/wiki/CYK_algorithm
+
+' Historically the algorithm is typically descibed in terms of finding the '
+'All possible parse trees of a sentence under a Chomsky Normal Form
+' grammar' This allows for determining possible attachment of clauses in
+' sentences such as "John hit the ball".
+
+' ![](https://upload.wikimedia.org/wikipedia/en/4/4b/ParseTree.jpg)
+
+' Let us develop some notation for this example. Each of green symbols is
+' in the set of labels. For simplicity we can have 10 of them. 
+
+Label = Fin 10
+
+' In this parse, the label VP covers the span over 'hit the ball'
+
+Len = Fin 5
+sentence = ["John", "hit", "the", "ball", "eos"]
+           
+I = 1@Len
+J = 4@Len
+
+' We introduce a slice range function to allow us to pull out a slice of the sentence.
+
+-- Slice a range from a table. Used for viewing spans.
+def sliceRange (i:a) ?-> (j:a) ?-> (xs : a=>b) : (i..<j) => b = slice xs (ordinal i) (i..<j)
+res : (I..<J) => String = sliceRange sentence
+toList res
+
+
+' ## Index Helpers
+
+
+' In order to make our implementation of the CKY algorithm simpler we introduce
+ some basic index manipulation functions. 
+
+-- Changes type without changing position
+def rebase (i: a) ?-> (j: a) ?-> (x:(i<..)) : (j<..) =
+    ((ordinal x) - ((ordinal j) - (ordinal i)))@(j<..)
+
+K = 2@Len
+rebase (0@(K<..)) : (I<..)
+
+
+-- Cast based on ordinal value
+def cast (d:a) : m = (ordinal d)@_
+-- Shift over from a starting point
+def shift (j:a) ?-> (x: a) : (j<..) = cast x
+
+shift (1@_) : (I<..)
+
+-- Index arithmetic
+def start : a = 0 @ a
+def end : a = (size a - 1) @ a
+instance Add (Fin a)
+         add = \a b. ((ordinal a) + (ordinal b))@_
+         sub = \a b. ((ordinal a) - (ordinal b))@_
+         zero = start
+
+
+
+' ## Chart Manipulation
+
+' The modern incarnation of CKY abstracts the inference algorithm away from the
+  underlying grammar. The core focus of the algorithm is to enumerate all binary
+  trees. 
+
+' To do this we start with a dynamic programming chart.
+
+def Chart (a:Type) (b:Type) : Type = i:a => (i<..) => b
+def Params (a:Type) (b:Type) (labels:Type) : Type = labels => i:a => (i<..) => b
+def chart (ref:Ref h (Chart a b )) (i: a) (j: (i<..)) : Ref h b =
+    d = %indexRef ref i
+    d!j
+
+def cky [Add pos, Add semi, Mul semi] (weights' : Params pos semi labels) : (semi & Chart pos semi) =
+    -- Initialize the chart to all zeros
+    c_init : Chart pos semi  = for i. for j. zero
+    (first, last) = (start, end)
+    -- Sum out the labels
+    weights = for i j. sum for k. weights'.k.i.j
+    out = runState c_init $ \ c.
+      C = chart c
+
+      -- Enumerate over all spans d
+      -- Each of these needs to be done in order
+      for_ d.
+       boundary = last - d
+       v = case ordinal d == 0 of
+         -- Size 1 spans are mapped are initialized as 1
+         True -> one
+         False ->
+          -- Main loop. No writes
+          c' = get c
+          for i' : (..<boundary).
+           -- Calculate span (i, j)
+           i = %inject i'
+           j':(i<..) = shift d
+           j = %inject j'
+           w = weights.i.j'
+           -- Sum over k in i<..<j
+           sum for k' : (i<..<j).
+               k =  %inject k'
+               c'.i.(cast k') * c'.k.(rebase j') * w
+                          
+       -- Fill (i, j) in the chart
+       for i' : (..<boundary).
+          i = %inject i'
+          j':(i<..) = cast d
+          C i j' :=  v.i'
+       ()
+      get $ C first end
+    out
+
+
+' Run an example
+
+length = 10 
+WordPos = Fin 10
+Labels = Fin 10
+
+key =  (newKey 0)
+param : Params WordPos Float Labels = for i j k. rand (ixkey2 key i (j, k))
+(v, table) = cky param
+
+import plot
+
+' Todo : Want this to work
+-- :t (grad \p. log $ fst $ cky p) param 
+
+
+
+:t table
+
+-- :html matshow for i:WordPos. for j:WordPos. case (ordinal i) < (ordinal j) of
+--       True -> table.i.(shift (j - i- 1@_))
+--       False -> 0.0
+      
+