Added explicit `CompareFn` type for comparator function.

Changed return type of comparator function from `i32` to `std::cmp::Ordering`.

src/comparator.rs

@@ -13,15 +13,18 @@
 // limitations under the License.
 //
 
+use std::cmp::Ordering;
 use std::ffi::CString;
 use std::mem;
 use std::slice;
 
 use libc::{c_char, c_int, c_void, size_t};
 
+pub type CompareFn = fn(&[u8], &[u8]) -> Ordering;
+
 pub struct ComparatorCallback {
     pub name: CString,
-    pub f: fn(&[u8], &[u8]) -> i32,
+    pub f: CompareFn,
 }
 
 pub unsafe extern "C" fn destructor_callback(raw_cb: *mut c_void) {
@@ -43,5 +46,9 @@ pub unsafe extern "C" fn compare_callback(raw_cb: *mut c_void,
     let cb: &mut ComparatorCallback = &mut *(raw_cb as *mut ComparatorCallback);
     let a: &[u8] = slice::from_raw_parts(a_raw as *const u8, a_len as usize);
     let b: &[u8] = slice::from_raw_parts(b_raw as *const u8, b_len as usize);
-    (cb.f)(a, b)
+    match (cb.f)(a, b) {
+        Ordering::Less => -1,
+        Ordering::Equal => 0,
+        Ordering::Greater => 1,
+    }
 }

src/rocksdb_options.rs

@@ -19,7 +19,7 @@ use std::mem;
 use libc::{self, c_int, c_uchar, c_uint, c_void, size_t, uint64_t};
 
 use {BlockBasedOptions, Options, WriteOptions};
-use comparator::{self, ComparatorCallback};
+use comparator::{self, ComparatorCallback, CompareFn};
 use ffi;
 use merge_operator::{self, MergeFn, MergeOperatorCallback, full_merge_callback,
                      partial_merge_callback};
@@ -220,7 +220,7 @@ impl Options {
     /// The client must ensure that the comparator supplied here has the same
     /// name and orders keys *exactly* the same as the comparator provided to
     /// previous open calls on the same DB.
-    pub fn set_comparator(&mut self, name: &str, compare_fn: fn(&[u8], &[u8]) -> i32) {
+    pub fn set_comparator(&mut self, name: &str, compare_fn: CompareFn) {
         let cb = Box::new(ComparatorCallback {
             name: CString::new(name.as_bytes()).unwrap(),
             f: compare_fn,
@@ -236,7 +236,7 @@ impl Options {
     }
 
     #[deprecated(since = "0.5.0", note = "add_comparator has been renamed to set_comparator")]
-    pub fn add_comparator(&mut self, name: &str, compare_fn: fn(&[u8], &[u8]) -> i32) {
+    pub fn add_comparator(&mut self, name: &str, compare_fn: CompareFn) {
         self.set_comparator(name, compare_fn);
     }