compiler: reuse derived item properties from last compilation

A benchmark on an already bulit gallery with ~600 pictures shows a ~90% speedup:

Before:

  Time (mean ± σ):      2.879 s ±  0.125 s    [User: 14.686 s, System: 5.511 s]
  Range (min … max):    2.774 s …  3.203 s    10 runs

After:

  Time (mean ± σ):     289.5 ms ±  15.1 ms    [User: 596.1 ms, System: 359.3 ms]
  Range (min … max):   272.8 ms … 323.0 ms    10 runs

GitHub: closes #97

1 files changed, 34 insertions, 51 deletions

diff --git a/compiler/src/ItemProcessors.hs b/compiler/src/ItemProcessors.hs
index 209bc2a..f967954 100644
--- a/compiler/src/ItemProcessors.hs
+++ b/compiler/src/ItemProcessors.hs
@@ -19,17 +19,15 @@
 module ItemProcessors
   ( ItemProcessor
   , itemFileProcessor
-  , ThumbnailProcessor
   , thumbnailFileProcessor
   ) where
 
 
-import Data.Function ((&))
 import Data.Char (toLower)
 import System.FilePath (takeExtension)
 
 import Config (Resolution(..))
-import Resource (ItemProcessor, ThumbnailProcessor, Thumbnail(..), GalleryItemProps(..))
+import Resource (ItemProcessor, Thumbnail(..), GalleryItemProps(..))
 import Caching (Cache)
 import FileProcessors
 import Files
@@ -75,58 +73,43 @@ formatFromPath =
       _ -> Unknown
 
 
-type ItemFileProcessor =
-     FileName        -- ^ Input base path
-  -> FileName        -- ^ Output base path
-  -> FileName        -- ^ Output class (subdir)
-  -> ItemProcessor
+type ItemFileProcessor a =
+     FilePath -- ^ Filesystem input base path
+  -> FilePath -- ^ Filesystem output base path
+  -> FileName -- ^ Output class (subdir)
+  -> ItemProcessor a
 
-itemFileProcessor :: Maybe Resolution -> Cache -> ItemFileProcessor
-itemFileProcessor maxResolution cached inputBase outputBase resClass inputRes =
-      cached processor inPath outPath
-  >>  resourceAt outPath relOutPath
-  >>= descriptor outPath
-  where
-    relOutPath = resClass /> inputRes
-    inPath = localPath $ inputBase /> inputRes
-    outPath = localPath $ outputBase /> relOutPath
-    (processor, descriptor) = processorFor (formatFromPath inputRes) maxResolution
-
-    processorFor :: Format -> Maybe Resolution -> (FileProcessor, ItemDescriber)
-    processorFor PictureFormat (Just maxRes) = (resizePictureUpTo maxRes, getPictureProps)
-    processorFor PictureFormat Nothing = (copyFileProcessor, getPictureProps)
-    processorFor PlainTextFormat _ = (copyFileProcessor, const $ return . PlainText)
-    processorFor PortableDocumentFormat _ = (copyFileProcessor, const $ return . PDF)
-    processorFor VideoFormat _ = (copyFileProcessor, const $ return . Video)
-    processorFor AudioFormat _ = (copyFileProcessor, const $ return . Audio)
-    -- TODO: handle video reencoding and others?
-    processorFor Unknown _ = (copyFileProcessor, const $ return . Other)
 
+callFileProcessor :: (Path -> FileProcessor a) -> Cache a -> ItemFileProcessor a
+callFileProcessor processorProvider withCache inputBase outputBase resClass itemPath resPath =
+  withCache (processorProvider resPath)
+    itemPath
+    (resClass /> resPath)
+    (localPath $ inputBase /> resPath)
+    (localPath $ outputBase /> (resClass /> resPath))
 
-type ThumbnailFileProcessor =
-     FileName        -- ^ Input base path
-  -> FileName        -- ^ Output base path
-  -> FileName        -- ^ Output class (subdir)
-  -> ThumbnailProcessor
 
-thumbnailFileProcessor :: Resolution -> Cache -> ThumbnailFileProcessor
-thumbnailFileProcessor maxRes cached inputBase outputBase resClass inputRes =
-  cached <$> processorFor (formatFromPath inputRes)
-  & process
+itemFileProcessor :: Maybe Resolution -> Cache GalleryItemProps -> ItemFileProcessor GalleryItemProps
+itemFileProcessor maxResolution =
+  callFileProcessor (flip processorFor maxResolution . formatFromPath)
   where
-    relOutPath = resClass /> inputRes
-    inPath = localPath $ inputBase /> inputRes
-    outPath = localPath $ outputBase /> relOutPath
+    processorFor :: Format -> Maybe Resolution -> FileProcessor GalleryItemProps
+    processorFor PictureFormat (Just maxRes) =
+      transformThenDescribe (resizePictureUpTo maxRes) getPictureProps
+    processorFor PictureFormat Nothing =
+      transformThenDescribe copyFileProcessor getPictureProps
+    processorFor PlainTextFormat _ = copyResource PlainText
+    processorFor PortableDocumentFormat _ = copyResource PDF
+    processorFor VideoFormat _ = copyResource Video
+    processorFor AudioFormat _ = copyResource Audio
+    processorFor Unknown _ = copyResource Other
+    -- TODO: handle video reencoding and others?
 
-    process :: Maybe FileProcessor -> IO (Maybe Thumbnail)
-    process Nothing = return Nothing
-    process (Just proc) =
-      do
-        proc inPath outPath
-        resource <- resourceAt outPath relOutPath
-        resolution <- getImageResolution outPath
-        return $ Just $ Thumbnail resource resolution
 
-    processorFor :: Format -> Maybe FileProcessor
-    processorFor PictureFormat = Just $ resizePictureUpTo maxRes
-    processorFor _ = Nothing
+thumbnailFileProcessor :: Resolution -> Cache (Maybe Thumbnail) -> ItemFileProcessor (Maybe Thumbnail)
+thumbnailFileProcessor maxRes =
+  callFileProcessor (processorFor . formatFromPath)
+  where
+    processorFor :: Format -> FileProcessor (Maybe Thumbnail)
+    processorFor PictureFormat = transformThenDescribe (resizePictureUpTo maxRes) getThumbnailProps
+    processorFor _ = noopProcessor