{-# LANGUAGE TupleSections #-}

module Interpreter where

import Data.Function
import qualified Data.Map as M

{- VAM 2P, done the lazy way -}
data StrTable =
  StrTable Int (M.Map Int String)

data Datum
  = Atom Int -- unifies a constant
  | Struct (Int, Int) -- unifies a structure with arity
  deriving (Show)

data Instr
  = U Datum -- something unifiable
  | NoGoal -- trivial goal
  | Goal -- we start a new goal, set up backtracking etc
  | Call -- all seems okay, call the goal
  | LastCall -- tail call the goal
  deriving (Show)

type Code = [Instr]

type Alts = [(Code, Code)] -- clauses to try and the corresponding goals to restore

type Defs = M.Map (Int, Int) [Code]

data Interp =
  Interp
    { defs :: Defs -- global definitions for lookup
    , hed :: Code -- current head
    , gol :: Code -- current goal
    , stk :: [Alts] -- possible heads with stored original goal
    }
  deriving (Show)

prove :: Code -> Defs -> Either (Interp, String) Bool
prove g ds =
  let i0 = Interp ds g [LastCall] [[]]
      run (Left x) = x
      run (Right x) = run $ proveStep Right Left x
   in run (Right i0)

{- this gonna need Either String Bool for errors later -}
proveStep :: (Interp -> a) -> (Either (Interp, String) Bool -> a) -> Interp -> a
proveStep c f i = go i
  where
    ifail msg = f $ Left (i, msg)
    withDef f
      | Just d <- defs i M.!? f = ($ d)
      | otherwise = const $ ifail $ "no definition: " ++ show f
    {- Backtracking -}
    backtrack i@Interp {stk = ((s, gs):ss):sss} -- backtrack to next clause, restoring goal
     = c i {hed = s, gol = gs, stk = ss : sss}
    backtrack i@Interp {stk = []:ss@(_:_):sss} -- no next clause, pop stack and backtrack the caller clause
     =
      backtrack
        i {hed = error "failed hed", gol = error "failed gol", stk = ss : sss}
    backtrack i@Interp {stk = [[]]} = f (Right False)
    backtrack i@Interp {stk = []:[]:_} = ifail "broken stk" -- this should not happen
    {- Unification -}
    go i@Interp {hed = (U a:hs), gol = (U b:gs)} -- unify constants
     = unify a b
      where
        uok = c i {hed = hs, gol = gs}
        unify (Atom a) (Atom b)
          | a == b = uok
        unify (Struct a) (Struct b)
          | a == b = uok
        unify _ _ = backtrack i
    {- Resolution -}
    go i@Interp {hed = [NoGoal], gol = [LastCall], stk = [_]} = f (Right True) -- final success
    go i@Interp { hed = [NoGoal]
                , gol = [LastCall]
                , stk = _:((Goal:U (Struct f):gs, _):ss):sss
                } -- goal succeeded, continue with parent frame
     =
      withDef f $ \(hs:ohs) ->
        c i {hed = hs, gol = gs, stk = (map (, gs) ohs ++ ss) : sss}
    go i@Interp { hed = [NoGoal]
                , gol = (Call:Goal:U (Struct f):gs)
                , stk = ss:sss
                } -- next goal
     =
      withDef f $ \(hs:ohs) ->
        c i {hed = hs, gol = gs, stk = (map (, gs) ohs ++ ss) : sss}
    go i@Interp {hed = (Goal:U (Struct f):ngs), gol = (Call:gs), stk = ss:sss} -- normal call
     =
      withDef f $ \(hs:ohs) ->
        c
          i
            { hed = hs
            , gol = ngs
            , stk = (map (, ngs) ohs) : ((gs, error "gol no hed") : ss) : sss
            }
    go i@Interp {hed = (Goal:U (Struct f):ngs), gol = [LastCall], stk = _:sss} -- tail call
     =
      withDef f $ \(hs:ohs) ->
        c i {hed = hs, gol = ngs, stk = map (, ngs) ohs : sss}
    go _ = ifail "impossible instruction combo"