keys.hh: simplify empty clustering-key check

The exploded_clustering_prefix type has a convenient is_empty() method
and an even more convenient "operator bool" shortcut. Unfortunately,
the other clustering prefix types (clustering_key_prefix,
clustering_key_prefix_view) have, for historic reasons, an is_empty
method which takes a schema parameter. That also means they can't
have an "operator bool" shortcut.

But checking if a prefix doesn't really need the schema - all we need to
check is whether the byte representation is empty. The result is simpler
and more efficient code, and easier to use. It is also more consistent -
all clustering-key-related types will have an "operator bool" instead of
just some of them.

To avoid massive code changes, we leave a is_empty(schema) variant, which
simply calls is_empty(). There's already precedent for that - various
methods which have a variant taking schema (and ignoring it) and one
taking nothing.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20180521174220.13262-1-nyh@scylladb.com>

keys.hh

@@ -146,6 +146,19 @@ public:
     auto components(const schema& s) const {
         return components();
     }
+
+    bool is_empty() const {
+        return _bytes.empty();
+    }
+
+    explicit operator bool() const {
+        return !is_empty();
+    }
+
+    // For backward compatibility with existing code.
+    bool is_empty(const schema& s) const {
+        return is_empty();
+    }
 };
 
 template <typename TopLevel, typename TopLevelView>
@@ -304,8 +317,17 @@ public:
         return get_compound_type(s)->end(_bytes);
     }
 
+    bool is_empty() const {
+        return _bytes.empty();
+    }
+
+    explicit operator bool() const {
+        return !is_empty();
+    }
+
+    // For backward compatibility with existing code.
     bool is_empty(const schema& s) const {
-        return begin(s) == end(s);
+        return is_empty();
     }
 
     // Returns a range of bytes_view
@@ -520,10 +542,6 @@ public:
     bool is_full(const schema& s) const {
         return TopLevel::get_compound_type(s)->is_full(base::_bytes);
     }
-
-    bool is_empty(const schema& s) const {
-        return TopLevel::get_compound_type(s)->is_empty(base::_bytes);
-    }
 };
 
 template <typename TopLevel, typename TopLevelView, typename FullTopLevel>
@@ -542,10 +560,6 @@ public:
         return TopLevel::get_compound_type(s)->is_full(base::_bytes);
     }
 
-    bool is_empty(const schema& s) const {
-        return TopLevel::get_compound_type(s)->is_empty(base::_bytes);
-    }
-
     // Can be called only if is_full()
     FullTopLevel to_full(const schema& s) const {
         return FullTopLevel::from_exploded(s, base::explode(s));
@@ -803,4 +817,4 @@ struct appending_hash<clustering_key_prefix> {
     void operator()(Hasher& h, const clustering_key_prefix& ck, const schema& s) const {
         appending_hash<clustering_key_prefix_view>()(h, ck.view(), s);
     }
-};
+};