Move code to get latest unyanked version into fn

cf. #100

src/bin/add/fetch.rs

@@ -26,24 +26,24 @@ pub fn get_latest_version(crate_name: &str) -> Result<String, FetchVersionError>
     let crate_data = try!(fetch_cratesio(&format!("/crates/{}/versions", crate_name)));
     let crate_json = try!(Json::from_str(&crate_data));
 
+    read_latest_version(crate_json)
+}
+
+/// Read latest version from JSON structure
+///
+/// Assumes the version are sorted so that the first non-yanked version is the
+/// latest, and thus the one we want.
+fn read_latest_version(crate_json: Json) -> Result<String, FetchVersionError> {
     crate_json.as_object()
         .and_then(|c| c.get("versions"))
-        .and_then(|c| c.as_array())
-        .and_then(|c| {
-            for version in c {
-                let version_object = version.as_object();
-                let yanked = version_object
-                    .and_then(|v| v.get("yanked"))
-                    .and_then(|v| v.as_boolean());
-                if let Some(false) = yanked {
-                    return version_object;
-                }
-            }
-            return None;
-        })
+        .and_then(Json::as_array)
+        .and_then(|vs| vs.iter()
+            .filter_map(Json::as_object)
+            .find(|&v| !v.get("yanked").and_then(Json::as_boolean).unwrap_or(true))
+        )
         .and_then(|v| v.get("num"))
-        .and_then(|v| v.as_string())
-        .map(|v| v.to_owned())
+        .and_then(Json::as_string)
+        .map(str::to_owned)
         .ok_or(FetchVersionError::GetVersion)
 }