diff --git a/src/roxy.rs b/src/roxy.rs
index e977a5d..e9a9dea 100644
--- a/src/roxy.rs
+++ b/src/roxy.rs
@@ -1,7 +1,7 @@
 use std::{
     fs::{self, File},
     io::{BufRead, BufReader, Read, Write},
-    path::Path,
+    path::{Path, PathBuf},
 };
 
 use crate::error::Error;
@@ -136,36 +136,37 @@ impl Roxy {
         File::create(path)
     }
 
+    fn to_formatted_path<P: AsRef<Path>>(path: &P) -> Result<PathBuf, Error> {
+        let path = path.as_ref();
+        match path.with_extension("").file_name() {
+            Some(name) if name == "index" => Ok(path.to_path_buf()),
+            Some(_) => Ok(path.with_extension("").join("index.html")),
+            None => Err(Error::from("invalid path \"{path:?}\"".to_string())),
+        }
+    }
+
     pub fn process_file<P: AsRef<Path>>(
         input: &P,
         output: P,
         parser: &mut Parser,
-    ) -> std::io::Result<()> {
+    ) -> Result<(), Error> {
         let buf = Self::parse(input, parser)?;
-        Self::mkdir_and_open(&output).and_then(|mut f| f.write_all(&buf))
+        Self::mkdir_and_open(&Self::to_formatted_path(&output)?)
+            .and_then(|mut f| f.write_all(&buf))
+            .map_err(Error::from)
     }
 }
 
 #[cfg(test)]
 mod tests {
-    use super::{Parse, Parser};
+    use super::Roxy;
 
-    struct TestParser;
-    impl TestParser {
-        pub fn new() -> Self {
-            Self
-        }
-    }
+    #[test]
+    fn formatted_path() {
+        let idx = Roxy::to_formatted_path(&"/test/index.html").unwrap();
+        let name = Roxy::to_formatted_path(&"/i-love/cats.html").unwrap();
 
-    impl Parse for TestParser {
-        fn parse(
-            &mut self,
-            path: &str,
-            src: &[u8],
-            dst: &mut Vec<u8>,
-        ) -> Result<(), crate::error::Error> {
-            Ok(())
-        }
+        assert_eq!(idx.to_str(), Some("/test/index.html"));
+        assert_eq!(name.to_str(), Some("/i-love/cats/index.html"));
     }
 }
-