Geo3x3.fr

module Geo3x3 where

import Java.Lang (StringBuffer(append,toString))
import Prelude.PreludeBase(StringJ(charAt))
import Prelude.PreludeIO (Ref(put,get,modify))
import Prelude.Math (floor)
import Data.Char (digit)

type Lat = Double
type Lng = Double
type Level = Int
type Unit = Double

type Encoder s = ST s String
type Decoder s = ST s (Lat,Lng,Level,Unit)

encode :: Lat -> Lng -> Level -> String
encode lat lng level = ST.run (f lat lng level)
  where
    f :: Lat -> Lng -> Level -> Encoder s
    f lat lng level =
      if level < 1 then return ""
      else do
        refState <- Ref.new (0,0,0)
        refRes <- StringBuffer.new ""

        let (!c,!lng') = if lng >= 0
                         then ('E',lng)
                         else ('W',lng + 180)
            !lat' = lat + 90
            !unit = 180
          in do put refState (lat',lng',unit)
                append refRes c

        forM_ [1..level-1] $ \_ -> do
          (lat,lng,unit) <- get refState
          let !unit' = unit / 3
              x = fromIntegral $ Double.long $ floor $ lng / unit'
              y = fromIntegral $ Double.long $ floor $ lat / unit'
              !c = (from $ ord '0' + x + y * 3 + 1)::Char
              !lng' = lng - (fromIntegral x) * unit'
              !lat' = lat - (fromIntegral y) * unit'
            in do put refState (lat',lng',unit')
                  append refRes c

        toString refRes


decode :: String -> (Lat,Lng,Level,Unit)
decode code = ST.run (f code)
  where
    f :: String -> Decoder s
    f code =
      if length code == 0 then return (0,0,0,0)
      else do
          refState <- Ref.new (0,0,1,180)

          let (begin,isWest) =
                case charAt code 0 of
                  c | c == '-' || c == 'W' -> (1,True)
                  c | c == '+' || c == 'E' -> (1,False)
                  _ -> (0,False)
              clen = length code

          let loop = \i ->
                when (i < clen) $
                  let n = digit (charAt code i) 10
                    in when (1 <= n && n <= 9) $ do
                         modify refState $ \(lat,lng,level,unit) ->
                           let !unit' = unit / 3
                               (!lng',!lat') = let n' = n - 1
                                                 in (lng + (fromIntegral $ n' `mod` 3) * unit',
                                                     lat + (fromIntegral $ n' `div` 3) * unit')
                               !level' = level + 1
                             in (lat',lng',level',unit')
                         loop $ i + 1
            in loop begin

          modify refState $ \(lat,lng,level,unit) ->
            let !lat' = let lat' = lat + unit / 2 in lat' - 90
                !lng' = let lng' = lng + unit / 2 in if isWest then lng' -180 else lng'
              in (lat',lng',level,unit)

          get refState