Writing of chunk index (#279)

git-chunk/src/file/write.rs

@@ -87,20 +87,48 @@ impl Index {
     }
     /// Plan to write a new chunk as part of the index when [`write_to()`][Index::write_to()] is called.
     pub fn plan_chunk(&mut self, chunk: crate::Id, exact_size_on_disk: u64) {
-        self.will_write = true;
+        assert!(self.will_write, "BUG: create the index with `for_writing()`");
+        assert!(
+            !self.chunks.iter().any(|e| e.kind == chunk),
+            "BUG: must not add chunk of same kind twice: {:?}",
+            std::str::from_utf8(&chunk)
+        );
         self.chunks.push(Entry {
             kind: chunk,
             offset: 0..exact_size_on_disk,
         })
     }
 
+    /// Return the amount of chunks we currently know.
+    pub fn num_chunks(&self) -> usize {
+        self.chunks.len()
+    }
+
     /// After [planning all chunks][Index::plan_chunk()] call this method with the destination to write the chunks to.
     /// Use the [Chunk] writer to write each chunk in order.
-    pub fn into_write<W>(self, out: W) -> std::io::Result<Chunk<W>>
+    /// `current_offset` is the byte position at which `out` will continue writing.
+    pub fn into_write<W>(self, mut out: W, mut current_offset: usize) -> std::io::Result<Chunk<W>>
     where
         W: std::io::Write,
     {
-        // TODO: write index
+        assert!(
+            self.will_write,
+            "BUG: create the index with `for_writing()`, cannot write decoded indices"
+        );
+        /// First chunk starts past the table of contents
+        let mut current_offset = (current_offset + Self::size_for_entries(self.num_chunks())) as u64;
+
+        for entry in &self.chunks {
+            out.write_all(&entry.kind)?;
+            out.write_all(&current_offset.to_be_bytes())?;
+
+            current_offset += entry.offset.end;
+        }
+
+        // sentinel to mark end of chunks
+        out.write_all(&0u32.to_be_bytes())?;
+        out.write_all(&current_offset.to_be_bytes())?;
+
         Ok(Chunk::new(out, self.chunks.into()))
     }
 }