libcamera: utils: Add map_keys() function

Add a map_keys() function to the utils namespace to extract keys from a
map.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[Niklas: change return type to std::vector instead of std::set]
Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

test/utils.cpp

@@ -6,6 +6,7 @@
  */
 
 #include <iostream>
+#include <map>
 #include <sstream>
 #include <string>
 #include <vector>
@@ -144,6 +145,27 @@ protected:
 		if (TestPass != testDirname())
 			return TestFail;
 
+
+		/* utils::map_keys() test. */
+		const std::map<std::string, unsigned int> map{
+			{ "zero", 0 },
+			{ "one", 1 },
+			{ "two", 2 },
+		};
+		std::vector<std::string> expectedKeys{
+			"zero",
+			"one",
+			"two",
+		};
+
+		std::sort(expectedKeys.begin(), expectedKeys.end());
+
+		const std::vector<std::string> keys = utils::map_keys(map);
+		if (keys != expectedKeys) {
+			cerr << "utils::map_keys() test failed" << endl;
+			return TestFail;
+		}
+
 		return TestPass;
 	}
 };