Handle the line prefix correctly

CHANGELOG.md

@@ -7,7 +7,9 @@
 ### Changed
 
 ### Fixed
+
 - Wrong default HIndent configuration in [`README.md`] ([#750]).
+- Fix the bug of panicking when the given source code has CPP lines and space-prefixed lines ([#780]).
 
 ### Removed
 
@@ -363,6 +365,7 @@ This version is accidentally pushlished, and is the same as 5.3.3.
 [@uhbif19]: https://github.com/uhbif19
 [@toku-sa-n]: https://github.com/toku-sa-n
 
+[#780]: https://github.com/mihaimaruseac/hindent/pull/780
 [#750]: https://github.com/mihaimaruseac/hindent/pull/750
 [#742]: https://github.com/mihaimaruseac/hindent/pull/742
 [#741]: https://github.com/mihaimaruseac/hindent/pull/741

TESTS.md

@@ -3411,6 +3411,24 @@ isDebug = False
 #endif
 ```
 
+Conditionals inside a `where` with empty lines and CPP
+
+```haskell
+-- https://github.com/mihaimaruseac/hindent/issues/779
+foo = bar + baz
+  where
+
+#if 0
+    bar = 1
+
+    baz = 1
+#else
+    bar = 2
+
+    baz = 2
+#endif
+```
+
 Macro definitions (`#define`)
 
 ```haskell

src/HIndent.hs

@@ -122,7 +122,11 @@ reformat config mexts mfilepath rawCode =
     processBlock (HaskellSource yPos text) =
       let ls = S8.lines text
           prefix = findPrefix ls
-          code = unlines' (map (stripPrefix prefix) ls)
+          code = unlines' (map stripPrefixIfNotNull ls)
+          stripPrefixIfNotNull s =
+            if S.null s
+              then s
+              else stripPrefix prefix s
        in case parseModule mfilepath allExts (UTF8.toString code) of
             POk _ m ->
               Right

src/HIndent/Printer.hs

@@ -13,7 +13,7 @@ module HIndent.Printer
 
 import Control.Applicative
 import Control.Monad
-import Control.Monad.State.Strict 
+import Control.Monad.State.Strict
 import Data.ByteString.Builder
 import Data.Int (Int64)
 import HIndent.Config