-- | Shaping for a paragraph of plain, unidirectional text using a single font.
--
-- The input text must be encoded as UTF-8 in a contiguous byte array.
--
-- Positions and distances are represented as 32-bit integers. Their unit must
-- be defined by the caller, who must calculate the desired dimensions of the
-- EM square of the input font and set them using @hb_font_set_scale()@. For
-- example, if @1em = 20px@, if the output pixels are square, and if the output
-- coordinates are in 1/64ths of a pixel, you should set both the @x_scale@ and
-- the @y_scale@ to @1280@.
module Data.Text.ParagraphLayout.Plain
    (LineHeight(..)
    ,Paragraph(..)
    ,ParagraphLayout(..)
    ,ParagraphOptions(..)
    ,Rect(..)
    ,Span(..)
    ,SpanLayout(..)
    ,exampleParagraph
    ,layoutPlain
    )
where

import Data.Int (Int32)
import Data.Text (pack)
import Data.Text.Array (Array)
import Data.Text.Foreign (I8)
import Data.Text.Glyphize (Font, GlyphInfo, GlyphPos)
import Data.Text.Internal (Text(Text))

import Data.Text.ParagraphLayout.Rect

-- | Text to be laid out as a paragraph.
--
-- May be divided into any number of neighbouring spans, each of which will
-- have its own layout rectangle(s) calculated.
data Paragraph = Paragraph

    Array
    -- ^ A byte array containing the whole text to be laid out, in UTF-8.

    I8
    -- ^ Byte offset of the first span.
    -- Any characters preceding this offset will not be shaped, but may still
    -- be used to influence the shape of neighbouring characters.

    [Span]
    -- ^ Parts of the text to be laid out, in logical order.
    -- The offset plus total length of all spans must not exceed array bounds.
    -- Any characters following the last span will not be shaped, but may still
    -- be used to influence the shape of neighbouring characters.

    ParagraphOptions
    -- ^ Properties applying to the paragraph as a whole.

data ParagraphOptions = ParagraphOptions
    { paragraphFont :: Font
    , paragraphLineHeight :: LineHeight
    , paragraphMaxWidth :: Int32
    }

data LineHeight

    = Absolute Int32
    -- ^ Set line height independently of the font.

    | Relative Float
    -- ^ Set line height as a multiplier of the font's built-in value.

data Span = Span

    { spanLength :: I8
    -- ^ Byte offset to the next span or the end of the paragraph text.

    , spanLanguage :: String
    -- ^ Used for selecting the appropriate glyphs and line breaking rules.

    }

-- | The resulting layout of the whole paragraph.
data ParagraphLayout = ParagraphLayout
    { paragraphRect :: Rect Int32
    , spanLayouts :: [SpanLayout]
    }
    deriving (Eq, Read, Show)

-- | The resulting layout of each span, which may include multiple bounding
-- boxes if broken over multiple lines.
data SpanLayout = SpanLayout [Box]
    deriving (Eq, Read, Show)

type Box =
    ( Rect Int32
    -- ^ Rectangle containing all glyph advances in this box. This is the space
    -- that the glyphs "take up" and is probably what you want to use for
    -- detecting position-based events such as mouse clicks.
    --
    -- Beware that actual glyphs will not be drawn exactly to the borders of
    -- this rectangle -- they may be offset inwards and they can also extend
    -- outwards!
    --
    -- These are not the typographic bounding boxes that you use for determining
    -- the area to draw on -- you need FreeType or a similar library for that.
    --
    -- The origin coordinates are relative to the paragraph.
    --
    -- The sizes can be positive or negative, depending on the text direction.
    --
    -- X coordinates increase from left to right.
    -- Y coordinates increase from bottom to top.
    , [(GlyphInfo, GlyphPos)]
    )

-- | Interface for basic plain text layout.
--
-- The entire paragraph will be assumed to have the same text direction and
-- will be shaped using a single font, aligned to the left for LTR text or to
-- the right for RTL text.
layoutPlain :: Paragraph -> ParagraphLayout
-- Stub implementation to make this a valid Haskell source.
-- Of course, this will eventually be replaced by an actual implementation. :)
layoutPlain (Paragraph _ _ spans _)
    = ParagraphLayout (Rect 0 0 0 0) (map (\_ -> SpanLayout []) spans)

exampleArray :: Array
exampleOffset :: Int
(Text exampleArray exampleOffset _) = pack "Tak jsem tady, 世界!"

exampleParagraph :: Font -> Paragraph
exampleParagraph font = Paragraph
    exampleArray
    (fromIntegral exampleOffset + 4)
    [Span 11 "cs" -- this will contain the text "jsem tady, "
    ,Span 7 "ja" -- this will contain the text "世界!"
    ]
    (ParagraphOptions font (Relative 1.5) 20000)