Allow creating iterators over prefixes

Allow creating both db and column family iterators that are specific to
a key prefix, by setting the `prefix_same_as_start` read option when
creating the iterator.

Currently this only supports `Direction::Forward`, but it'd likely be
trivial to support `Backward` as well, by incrementing the given prefix
by one and seeking to the key before that key.

src/db.rs

@@ -872,6 +872,12 @@ impl DB {
         DBIterator::new(self, &opts, mode)
     }
 
+    pub fn prefix_iterator<'a>(&self, prefix: &'a [u8]) -> DBIterator {
+        let mut opts = ReadOptions::default();
+        opts.set_prefix_same_as_start(true);
+        DBIterator::new(self, &opts, IteratorMode::From(prefix, Direction::Forward))
+    }
+
     pub fn iterator_cf(
         &self,
         cf_handle: ColumnFamily,
@@ -881,6 +887,16 @@ impl DB {
         DBIterator::new_cf(self, cf_handle, &opts, mode)
     }
 
+    pub fn prefix_iterator_cf<'a>(
+        &self,
+        cf_handle: ColumnFamily,
+        prefix: &'a [u8]
+    ) -> Result<DBIterator, Error> {
+        let mut opts = ReadOptions::default();
+        opts.set_prefix_same_as_start(true);
+        DBIterator::new_cf(self, cf_handle, &opts, IteratorMode::From(prefix, Direction::Forward))
+    }
+
     pub fn raw_iterator(&self) -> DBRawIterator {
         let opts = ReadOptions::default();
         DBRawIterator::new(self, &opts)
@@ -1202,6 +1218,12 @@ impl ReadOptions {
             );
         }
     }
+
+    pub fn set_prefix_same_as_start(&mut self, v: bool) {
+        unsafe {
+            ffi::rocksdb_readoptions_set_prefix_same_as_start(self.inner, v as c_uchar)
+        }
+    }
 }
 
 impl Default for ReadOptions {

tests/test_iterator.rs

@@ -151,3 +151,35 @@ pub fn test_iterator() {
     let opts = Options::default();
     assert!(DB::destroy(&opts, path).is_ok());
 }
+
+fn key(k: &[u8]) -> Box<[u8]> { k.to_vec().into_boxed_slice() }
+
+#[test]
+pub fn test_prefix_iterator() {
+    let path = "_rust_rocksdb_prefixiteratortest";
+    {
+        let a1: Box<[u8]> = key(b"aaa1");
+        let a2: Box<[u8]> = key(b"aaa2");
+        let b1: Box<[u8]> = key(b"bbb1");
+        let b2: Box<[u8]> = key(b"bbb2");
+
+        let db = DB::open_default(path).unwrap();
+
+        assert!(db.put(&*a1, &*a1).is_ok());
+        assert!(db.put(&*a2, &*a2).is_ok());
+        assert!(db.put(&*b1, &*b1).is_ok());
+        assert!(db.put(&*b2, &*b2).is_ok());
+
+        {
+            let expected = vec![(cba(&a1), cba(&a1)), (cba(&a2), cba(&a2))];
+            let a_iterator = db.prefix_iterator(b"aaa");
+            assert_eq!(a_iterator.collect::<Vec<_>>(), expected)
+        }
+
+        {
+            let expected = vec![(cba(&b1), cba(&b1)), (cba(&b2), cba(&b2))];
+            let b_iterator = db.prefix_iterator(b"bbb");
+            assert_eq!(b_iterator.collect::<Vec<_>>(), expected)
+        }
+    }
+}