Add json_map and json_set buildfile value types

These expose the std::map<json_value,json_value> and std::set<json_value>
types to buildfiles.

New functions:

  $size(<json-set>)
  $size(<json-map>)
  $keys(<json-map>)

Note that the $keys() function returns the list of map key as a json array.

For example:

m = [json_map] 2@([json] a@1 b@2) 1@([json] 1 2)
s = [json_set] ([json] x@1 y@2) ([json] a@1 b@2)

print ($m[2][b])              # 2
print ($s[([json] y@2 x@1)])  # true

1 files changed, 23 insertions, 0 deletions

diff --git a/libbuild2/functions-json.cxx b/libbuild2/functions-json.cxx
index 5715e13..e06d9a5 100644
--- a/libbuild2/functions-json.cxx
+++ b/libbuild2/functions-json.cxx
@@ -308,5 +308,28 @@ namespace build2
       }
     };
 #endif
+
+    // $size(<json-set>)
+    // $size(<json-map>)
+    //
+    // Return the number of elements in the sequence.
+    //
+    f["size"] += [] (set<json_value> v)             {return v.size ();};
+    f["size"] += [] (map<json_value, json_value> v) {return v.size ();};
+
+    // $keys(<json-map>)
+    //
+    // Return the list of keys in a json map as a json array.
+    //
+    // Note that the result is sorted in ascending order.
+    //
+    f["keys"] += [](map<json_value, json_value> v)
+    {
+      json_value r (json_type::array);
+      r.array.reserve (v.size ());
+      for (pair<const json_value, json_value>& p: v)
+        r.array.push_back (p.first); // @@ PERF: use C++17 map::extract() to steal.
+      return r;
+    };
   }
 }