Don't copy LatestMap if merging a subset

extras/generate_latest_map

@@ -74,8 +74,9 @@ function generate_latest_map() {
         return len(m)
     }
 
-    // Merge produces a fresh ${latest_map_type} containing the keys from both inputs.
+    // Merge produces a ${latest_map_type} containing the keys from both inputs.
     // When both inputs contain the same key, the newer value is used.
+    // Tries to return one of its inputs, if that already holds the correct result.
     func (m ${latest_map_type}) Merge(n ${latest_map_type}) ${latest_map_type} {
         switch {
         case m == nil:
@@ -83,13 +84,33 @@ function generate_latest_map() {
         case n == nil:
             return m
         }
-        l := len(m)
-        if len(n) > l {
-            l = len(n)
+        if len(n) > len(m) {
+            m, n = n, m //swap so m is always at least as long as n
         }
-        out := make([]${entry_type}, 0, l)
 
         i, j := 0, 0
+    loop:
+        for i < len(m) {
+            switch {
+            case j >= len(n) || m[i].key < n[j].key:
+                i++
+            case m[i].key == n[j].key:
+                if m[i].Timestamp.Before(n[j].Timestamp) {
+                    break loop
+                }
+                i++
+                j++
+            default:
+                break loop
+            }
+        }
+        if i >= len(m) && j >= len(n) {
+            return m
+        }
+
+        out := make([]${entry_type}, i, len(m))
+        copy(out, m[:i])
+
         for i < len(m) {
             switch {
             case j >= len(n) || m[i].key < n[j].key: