Extract some convenience parsing functions

lib/Graphics/Fountainhead/Parser.hs

@@ -33,6 +33,7 @@ module Graphics.Fountainhead.Parser
     , nameTableP
     , os2TableP
     , panoseP
+    , parseFontDirectory
     , parseTable
     , pascalStringP
     , postTableP
@@ -157,6 +158,29 @@ type Parser = Megaparsec.Parsec Void ByteString
 type ParseErrorBundle = Megaparsec.ParseErrorBundle ByteString Void
 type ParseState = Megaparsec.State ByteString Void
 
+-- | Does initial parsing and returns the font directory and parsing state
+-- that can be used to parse other tables in the font.
+--
+-- Font file name can be empty.
+parseFontDirectory
+    :: FilePath
+    -> ByteString
+    -> (ParseState, Either ParseErrorBundle FontDirectory)
+parseFontDirectory fontFile ttfContents =
+    let initialState = Megaparsec.State
+            { stateInput = ttfContents
+            , stateOffset = 0
+            , statePosState = Megaparsec.PosState
+                { pstateInput = ttfContents
+                , pstateOffset = 0
+                , pstateSourcePos = Megaparsec.initialPos fontFile
+                , pstateTabWidth = Megaparsec.defaultTabWidth
+                , pstateLinePrefix = ""
+                }
+            , stateParseErrors = []
+            }
+     in Megaparsec.runParser' fontDirectoryP initialState 
+
 -- * Font directory
 
 offsetSubtableP :: Parser OffsetSubtable