module FOV.Shadow where

import Data.Ratio
import Data.Set as S

import FOV.Common
import Geometry
import Level

-- Recursive Shadow Casting.

-- | A restrictive variant of Recursive Shadow Casting FOV with infinite range.
-- It's not designed for dungeons with diagonal walls, so they block visibility,
-- though they don't block movement. Such cases appear in the game only
-- when two corridors touch diagonally by accident and on the random pillars
-- levels.

-- | The current state of a scan is kept in a variable of Maybe Rational.
-- If Just something, we're in a visible interval. If Nothing, we're in
-- a shadowed interval.
scan :: ((Distance, Progress) -> Loc) -> LMap -> Distance -> Interval -> Set Loc
scan tr l d (s,e) =
    let ps = downBias (s * fromIntegral d)   -- minimal progress to check
        pe = upBias (e * fromIntegral d)     -- maximal progress to check
        st = if open (l `at` tr (d,ps)) then Just s   -- start in light
                                        else Nothing  -- start in shadow
    in
        -- trace (show (d,s,e,ps,pe)) $
        S.union (S.fromList [tr (d,p) | p <- [ps..pe]]) (scan' st ps pe)
  where
    scan' :: Maybe Rational -> Progress -> Progress -> Set Loc
    -- scan' st ps pe
    --   | trace (show (st,ps,pe)) False = undefined
    scan' (Just s) ps pe
      | s  >= e  = S.empty               -- empty interval
      | ps > pe  = scan tr l (d+1) (s,e) -- reached end, scan next
      | closed (l `at` tr (d,ps)) =
                   let ne = (fromIntegral ps - (1%2)) / (fromIntegral d + (1%2))
                   in  scan tr l (d+1) (s,ne) `S.union` scan' Nothing (ps+1) pe
                                      -- entering shadow
      | otherwise = scan' (Just s) (ps+1) pe
                                      -- continue in light
    scan' Nothing ps pe
      | ps > pe  = S.empty            -- reached end while in shadow
      | open (l `at` tr (d,ps)) =
                   let ns = (fromIntegral ps - (1%2)) / (fromIntegral d - (1%2))
                   in  scan' (Just ns) (ps+1) pe
                                      -- moving out of shadow
      | otherwise = scan' Nothing (ps+1) pe
                                      -- continue in shadow

downBias, upBias :: (Integral a, Integral b) => Ratio a -> b
downBias x = round (x - 1 % (denominator x * 3))
upBias   x = round (x + 1 % (denominator x * 3))