module Data.Text.ParagraphLayout.Internal.VerticalOffsets
    ( VerticalOffsets (..)
    , alignLayoutTop
    , shift
    )
where

import Data.Int (Int32)

-- | Metrics used for vertical alignment of text fragments.
data VerticalOffsets = VerticalOffsets

    { layoutTop :: Int32
    -- ^ Y coordinate of the top edge of the fragment,
    -- including half-leading.

    , fontTop :: Int32
    -- ^ Y coordinate of the font's ascender.

    , baseline :: Int32
    -- ^ Y coordinate of the font's baseline.

    , fontBottom :: Int32
    -- ^ Y coordinate of the font's descender.

    , layoutBottom :: Int32
    -- ^ Y coordinate of the bottom edge of the fragment,
    -- including half-leading.

    }
    deriving (Eq, Show)

-- | Add a constant to each of the coordinates, effectively moving them
-- up by the given amount while preserving distances between them.
shift :: Int32 -> VerticalOffsets -> VerticalOffsets
shift d vo = vo
    { layoutTop = layoutTop vo + d
    , fontTop = fontTop vo + d
    , baseline = baseline vo + d
    , fontBottom = fontBottom vo + d
    , layoutBottom = layoutBottom vo + d
    }

-- | Set `layoutTop` to the given value and update all other coordinates
-- so that distances are preserved.
alignLayoutTop :: Int32 -> VerticalOffsets -> VerticalOffsets
alignLayoutTop x vo = shift (x - layoutTop vo) vo