module Data.Text.ParagraphLayout.Run (Run(..), spanToRuns)
where

import Data.List.NonEmpty (NonEmpty((:|)))
import Data.Text (Text)
import Data.Text.Glyphize (Direction(..))
import qualified Data.Text.ICU.Char as ICUChar
import Data.Text.Script (charScript)
import Data.Text.Zipper

import Data.Text.ParagraphLayout.ResolvedSpan

type ScriptCode = String

-- Each span can be broken into one or more runs by Balkón.
--
-- Each run could have a different script, language, or direction.
--
data Run = Run
    { runText :: Text
    , runDirection :: Maybe Direction
    , runScript :: Maybe ScriptCode
    }
    deriving (Eq, Show)

type ProtoRun = (Zipper, Maybe Direction, ScriptCode)

-- Represents a zipper that can advance by at least one character.
data ZipperChoice = ZipperChoice
    { nextChar :: Char
    , continuingRun :: Zipper
    -- ^ The zipper will advance over the next character,
    -- merging it with all preceding characters.
    , startingNewRun :: Zipper
    -- ^ The zipper will forget all preceding characters and then advance over
    -- the next character, making it the first character in a new run of text.
    }

considerNext :: Zipper -> Maybe ZipperChoice
considerNext z = case next z of
    Nothing -> Nothing
    Just c -> Just ZipperChoice
        { nextChar = c
        , continuingRun = step z
        , startingNewRun = step $ start $ following z
        }

data Merged a = Incompatible | Merged a

spanToRuns :: ResolvedSpan -> [Run]
spanToRuns s = map run $ protoRuns zipper
    where
        zipper = start $ spanText s
        run (z, d, sc) = Run
            { runText = preceding z
            , runDirection = d
            , runScript = Just sc
            }

protoRuns :: Zipper -> [ProtoRun]
protoRuns z = reverse $ protoRuns' z []

protoRuns' :: Zipper -> [ProtoRun] -> [ProtoRun]
protoRuns' curZipper curRuns = case considerNext curZipper of
    Nothing -> curRuns
    Just choice ->
        let headRun@(nextZipper, _, _) :| tailRuns = foldRun choice curRuns
        in protoRuns' nextZipper (headRun:tailRuns)

foldRun :: ZipperChoice -> [ProtoRun] -> NonEmpty ProtoRun

-- If there are no runs, create a new run with a single character.
foldRun x [] = (continuingRun x, d, s) :| []
    where
        d = charDirection (nextChar x)
        s = charScript (nextChar x)

foldRun x (previousRun@(_, d1, s1) : tailRuns) =
    case (mergeDirections d1 d2, mergeScripts s1 s2) of
        (Merged d, Merged s) -> (continuingRun x, d, s) :| tailRuns
        _ -> (startingNewRun x, d2, s2) :| previousRun : tailRuns
    where
        d2 = charDirection (nextChar x)
        s2 = charScript (nextChar x)

-- Simplified detection of text direction for unidirectional text.
mergeDirections :: Maybe Direction -> Maybe Direction -> Merged (Maybe Direction)
mergeDirections Nothing Nothing = Merged Nothing
mergeDirections (Just d1) Nothing = Merged (Just d1)
mergeDirections Nothing (Just d2) = Merged (Just d2)
mergeDirections (Just d1) (Just d2)
    | d1 == d2 = Merged (Just d1)
    | otherwise = Incompatible

-- TODO: Implement proper inheritance rules.
mergeScripts :: ScriptCode -> ScriptCode -> Merged ScriptCode
mergeScripts "Zyyy" s2 = Merged s2
mergeScripts s1 "Zyyy" = Merged s1
mergeScripts s1 "Zinh" = Merged s1
mergeScripts s1 s2
    | s1 == s2 = Merged s1
    | otherwise = Incompatible

-- TODO: Use the BiDi algorithm to support bidirectional text.
charDirection :: Char -> Maybe Direction
charDirection c = case ICUChar.direction c of
    ICUChar.LeftToRight -> Just DirLTR
    ICUChar.RightToLeft -> Just DirRTL
    ICUChar.RightToLeftArabic -> Just DirRTL
    _ -> Nothing