gf-core/src/GF/Speech/TransformCFG.hs

----------------------------------------------------------------------
-- |
-- Module      : TransformCFG
-- Maintainer  : BB
-- Stability   : (stable)
-- Portability : (portable)
--
-- > CVS $Date: 2005/11/01 20:09:04 $ 
-- > CVS $Author: bringert $
-- > CVS $Revision: 1.24 $
--
--  This module does some useful transformations on CFGs.
--
-- FIXME: remove cycles
--
-- peb thinks: most of this module should be moved to GF.Conversion...
-----------------------------------------------------------------------------

-- FIXME: lots of this stuff is used by CFGToFiniteState, thus
-- the missing explicit expot list.
module GF.Speech.TransformCFG {- (CFRule_, CFRules, 
			       cfgToCFRules, getStartCat,
			       removeLeftRecursion,
			       removeEmptyCats, removeIdenticalRules) -} where

import GF.Conversion.Types
import GF.Data.Utilities
import GF.Formalism.CFG 
import GF.Formalism.Utilities (Symbol(..), mapSymbol, filterCats, symbol, 
			       NameProfile(..), name2fun)
import GF.Infra.Ident
import GF.Infra.Option
import GF.Infra.Print

import Control.Monad
import Control.Monad.State (State, get, put, evalState)
import Data.Map (Map)
import qualified Data.Map as Map
import Data.List
import Data.Maybe (fromMaybe)
import Data.Monoid (mconcat)
import Data.Set (Set)
import qualified Data.Set as Set


-- | not very nice to replace the structured CFCat type with a simple string
type CFRule_ = CFRule Cat_ Name Token
type Cat_ = String

type CFRules = [(Cat_,[CFRule_])]

cfgToCFRules :: CGrammar -> CFRules
cfgToCFRules cfg = groupProds [CFRule (catToString c) (map symb r) n | CFRule c r n <- cfg]
    where symb = mapSymbol catToString id
          -- symb (Cat c) = Cat (catToString c)
	  -- symb (Tok t) = Tok t
	  catToString = prt

getStartCat :: Options -> String
getStartCat opts = fromMaybe "S" (getOptVal opts gStartCat) ++ "{}.s"

-- | Group productions by their lhs categories
groupProds :: [CFRule_] -> CFRules
groupProds = Map.toList . Map.fromListWith (++) . map (\r -> (lhsCat r,[r]))

ungroupProds :: CFRules -> [CFRule_]
ungroupProds = concat . map snd


-- | Remove productions which use categories which have no productions
removeEmptyCats :: CFRules -> CFRules
removeEmptyCats = fix removeEmptyCats'
    where
    removeEmptyCats' :: CFRules -> CFRules
    removeEmptyCats' rs = k'
	where
	keep = filter (not . null . snd) rs
	allCats = nub [c | (_,r) <- rs, CFRule _ rhs _ <- r, Cat c <- rhs]
	emptyCats = filter (nothingOrNull . flip lookup rs) allCats
	k' = map (\ (c,xs) -> (c, filter (not . anyUsedBy emptyCats) xs)) keep

-- | Remove rules which have the same rhs. 
--   FIXME: this messes up probabilities, names and profiles
removeIdenticalRules :: CFRules -> CFRules
removeIdenticalRules g = [(c,sortNubBy cmpRules rs) | (c,rs) <- g]
    where 
    cmpRules (CFRule c1 ss1 _) (CFRule c2 ss2 _) = 
              mconcat [c1 `compare` c2, ss1 `compare` ss2]

-- Paull's algorithm, see
-- http://research.microsoft.com/users/bobmoore/naacl2k-proc-rev.pdf
removeLeftRecursion :: CFRules -> CFRules
removeLeftRecursion rs = removeDirectLeftRecursions $ map handleProds rs
    where 
    handleProds (c, r) = (c, concatMap handleProd r)
    handleProd (CFRule ai (Cat aj:alpha) n) | aj < ai  =
              -- FIXME: for non-recursive categories, this changes
	      -- the grammar unneccessarily, maybe we can use mutRecCats
	      -- to make this less invasive
              -- FIXME: this will give multiple rules with the same name,
	      -- which may mess up the probabilities.
             [CFRule ai (beta ++ alpha) n | CFRule _ beta _ <- lookup' aj rs]
    handleProd r = [r]

removeDirectLeftRecursions :: CFRules -> CFRules
removeDirectLeftRecursions = concat . flip evalState 0 . mapM removeDirectLeftRecursion

removeDirectLeftRecursion :: (Cat_,[CFRule_]) -- ^ All productions for a category
			  -> State Int CFRules
removeDirectLeftRecursion (a,rs) 
    | null dr = return [(a,rs)]
    | otherwise = 
        do
        a' <- fresh a
        let as = maybeEndWithA' nr
            is = [CFRule a' (tail r) n | CFRule _ r n <- dr]
            a's = maybeEndWithA' is
            -- the not null constraint here avoids creating new 
            -- left recursive (cyclic) rules.
            maybeEndWithA' xs = xs ++ [CFRule c (r++[Cat a']) n | CFRule c r n <- xs, 
                                                                  not (null r)] 
        return [(a, as), (a', a's)]
    where 
    (dr,nr) = partition isDirectLeftRecursive rs
    fresh x = do { n <- get; put (n+1); return $ x ++ "'" ++ show n }

isDirectLeftRecursive :: CFRule_ -> Bool
isDirectLeftRecursive (CFRule c (Cat c':_) _) = c == c'
isDirectLeftRecursive _ = False

removeCycles :: CFRules -> CFRules 
removeCycles = groupProds . removeCycles_ . ungroupProds
  where removeCycles_ rs = [r | r@(CFRule c rhs n) <- rs, rhs /= [Cat c]]

--
-- * CFG rule utilities
--

allCats :: CFRules -> [Cat_]
allCats = map fst

catRules :: CFRules -> Cat_ -> [CFRule_]
catRules rs c = fromMaybe [] (lookup c rs)

catSetRules :: CFRules -> [Cat_] -> [CFRule_]
catSetRules g s = concatMap (catRules g) s

lhsCat :: CFRule c n t -> c
lhsCat (CFRule c _ _) = c

ruleRhs :: CFRule c n t ->  [Symbol c t]
ruleRhs (CFRule _ ss _) = ss

ruleFun :: CFRule_ -> Fun
ruleFun (CFRule _ _ n) = name2fun n 

-- | Checks if a symbol is a non-terminal of one of the given categories.
catElem :: Symbol Cat_ t -> Set Cat_ -> Bool
catElem s cs = symbol (`Set.member` cs) (const False) s

-- | Check if any of the categories used on the right-hand side
--   are in the given list of categories.
anyUsedBy :: Eq c => [c] -> CFRule c n t -> Bool
anyUsedBy cs (CFRule _ ss _) = any (`elem` cs) (filterCats ss)

mkName :: String -> Name
mkName n = Name (IC n) []