docid deltas while indexing

storing deltas is especially helpful for repetitive data like logs.
In those cases, recording a doc on a term costed 4 bytes instead of 1
byte now.

HDFS Indexing 1.1GB Total memory consumption:
Before:  760 MB
Now:     590 MB

src/postings/recorder.rs

@@ -105,8 +105,14 @@ impl Recorder for DocIdRecorder {
 
     #[inline]
     fn new_doc(&mut self, doc: DocId, arena: &mut MemoryArena) {
+        let delta = if self.current_doc == u32::MAX {
+            doc
+        } else {
+            doc - self.current_doc
+        };
+
         self.current_doc = doc;
-        self.stack.writer(arena).write_u32_vint(doc);
+        self.stack.writer(arena).write_u32_vint(delta);
     }
 
     #[inline]
@@ -125,18 +131,22 @@ impl Recorder for DocIdRecorder {
         let (buffer, doc_ids) = buffer_lender.lend_all();
         self.stack.read_to_end(arena, buffer);
         // TODO avoid reading twice.
+        let mut prev_doc = 0;
         if let Some(doc_id_map) = doc_id_map {
-            doc_ids.extend(
-                VInt32Reader::new(&buffer[..])
-                    .map(|old_doc_id| doc_id_map.get_new_doc_id(old_doc_id)),
-            );
+            doc_ids.extend(VInt32Reader::new(&buffer[..]).map(|delta_doc_id| {
+                let old_doc_id = prev_doc + delta_doc_id;
+                prev_doc = old_doc_id;
+                doc_id_map.get_new_doc_id(old_doc_id)
+            }));
             doc_ids.sort_unstable();
 
             for doc in doc_ids {
                 serializer.write_doc(*doc, 0u32, &[][..]);
             }
         } else {
-            for doc in VInt32Reader::new(&buffer[..]) {
+            for delta_doc_id in VInt32Reader::new(&buffer[..]) {
+                let doc = prev_doc + delta_doc_id;
+                prev_doc = doc;
                 serializer.write_doc(doc, 0u32, &[][..]);
             }
         }
@@ -164,9 +174,14 @@ impl Recorder for TermFrequencyRecorder {
 
     #[inline]
     fn new_doc(&mut self, doc: DocId, arena: &mut MemoryArena) {
+        let delta = if self.current_doc == u32::MAX {
+            doc
+        } else {
+            doc - self.current_doc
+        };
         self.term_doc_freq += 1;
         self.current_doc = doc;
-        self.stack.writer(arena).write_u32_vint(doc);
+        self.stack.writer(arena).write_u32_vint(delta);
     }
 
     #[inline]
@@ -191,21 +206,26 @@ impl Recorder for TermFrequencyRecorder {
         let buffer = buffer_lender.lend_u8();
         self.stack.read_to_end(arena, buffer);
         let mut u32_it = VInt32Reader::new(&buffer[..]);
+        let mut prev_doc = 0;
         if let Some(doc_id_map) = doc_id_map {
             let mut doc_id_and_tf = vec![];
-            while let Some(old_doc_id) = u32_it.next() {
+            while let Some(delta_doc_id) = u32_it.next() {
+                let doc_id = prev_doc + delta_doc_id;
+                prev_doc = doc_id;
                 let term_freq = u32_it.next().unwrap_or(self.current_tf);
-                doc_id_and_tf.push((doc_id_map.get_new_doc_id(old_doc_id), term_freq));
+                doc_id_and_tf.push((doc_id_map.get_new_doc_id(doc_id), term_freq));
             }
             doc_id_and_tf.sort_unstable_by_key(|&(doc_id, _)| doc_id);
 
             for (doc_id, tf) in doc_id_and_tf {
                 serializer.write_doc(doc_id, tf, &[][..]);
             }
         } else {
-            while let Some(doc) = u32_it.next() {
+            while let Some(delta_doc_id) = u32_it.next() {
+                let doc_id = prev_doc + delta_doc_id;
+                prev_doc = doc_id;
                 let term_freq = u32_it.next().unwrap_or(self.current_tf);
-                serializer.write_doc(doc, term_freq, &[][..]);
+                serializer.write_doc(doc_id, term_freq, &[][..]);
             }
         }
     }
@@ -241,9 +261,14 @@ impl Recorder for TfAndPositionRecorder {
 
     #[inline]
     fn new_doc(&mut self, doc: DocId, arena: &mut MemoryArena) {
+        let delta = if self.current_doc == u32::MAX {
+            doc
+        } else {
+            doc - self.current_doc
+        };
         self.current_doc = doc;
         self.term_doc_freq += 1u32;
-        self.stack.writer(arena).write_u32_vint(doc);
+        self.stack.writer(arena).write_u32_vint(delta);
     }
 
     #[inline]
@@ -269,7 +294,10 @@ impl Recorder for TfAndPositionRecorder {
         self.stack.read_to_end(arena, buffer_u8);
         let mut u32_it = VInt32Reader::new(&buffer_u8[..]);
         let mut doc_id_and_positions = vec![];
-        while let Some(doc) = u32_it.next() {
+        let mut prev_doc = 0;
+        while let Some(delta_doc_id) = u32_it.next() {
+            let doc_id = prev_doc + delta_doc_id;
+            prev_doc = doc_id;
             let mut prev_position_plus_one = 1u32;
             buffer_positions.clear();
             loop {
@@ -287,9 +315,9 @@ impl Recorder for TfAndPositionRecorder {
             if let Some(doc_id_map) = doc_id_map {
                 // this simple variant to remap may consume to much memory
                 doc_id_and_positions
-                    .push((doc_id_map.get_new_doc_id(doc), buffer_positions.to_vec()));
+                    .push((doc_id_map.get_new_doc_id(doc_id), buffer_positions.to_vec()));
             } else {
-                serializer.write_doc(doc, buffer_positions.len() as u32, buffer_positions);
+                serializer.write_doc(doc_id, buffer_positions.len() as u32, buffer_positions);
             }
         }
         if doc_id_map.is_some() {