lib/sorter: add `MapRead::keys_sorted_by_values` and the related comparator

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/lib/standard/collection/sorter.nit b/lib/standard/collection/sorter.nit
index 190f30a..00c45a3 100644 (file)
--- a/lib/standard/collection/sorter.nit
+++ b/lib/standard/collection/sorter.nit
@@ -267,6 +267,62 @@ redef class MapRead[K,V]
                comparator.sort(keys)
                return [for k in keys do self[k]]
        end
+
+       # Return an array of all keys sorted with their values using `comparator`.
+       #
+       # ~~~
+       # var map = new HashMap[String, Int]
+       # map["ten"] = 10
+       # map["two"] = 2
+       # map["one"] = 1
+       # assert map.keys_sorted_by_values(default_comparator) == ["one", "two", "ten"]
+       # assert map.keys_sorted_by_values(alpha_comparator) == ["one", "ten", "two"]
+       # ~~~
+       #
+       # See: `to_map_comparator` to get the comparator used internally.
+       fun keys_sorted_by_values(comparator: Comparator): Array[K]
+       do
+               var keys = self.keys.to_a
+               var map_cmp = to_map_comparator(comparator)
+               map_cmp.sort(keys)
+               return keys
+       end
+
+       # A comparator that compares things with their values in self.
+       #
+       # See `MapComparator` for details.
+       fun to_map_comparator(comparator: Comparator): MapComparator[K, V] do return new MapComparator[K,V](self, comparator)
+end
+
+# A comparator that compares things with their values in a map.
+#
+# ~~~
+# var map = new HashMap[String, Int]
+# map["ten"] = 10
+# map["two"] = 2
+# map["one"] = 1
+#
+# var map_cmp = map.to_map_comparator(default_comparator)
+# var a = ["ten", "one", "two"]
+# map_cmp.sort(a)
+# assert a == ["one", "two", "ten"]
+# map_cmp = map.to_map_comparator(alpha_comparator)
+# map_cmp.sort(a)
+# assert a == ["one", "ten", "two"]
+# ~~~
+class MapComparator[K,V]
+       super Comparator
+
+       # What is compared are the keys of the values
+       redef type COMPARED: K
+
+       # The map that associates compared elements to the value used to compare them
+       var map: MapRead[K,V]
+
+       # The comparator used to compare values
+       var comparator: Comparator
+
+       redef fun compare(a,b) do return comparator.compare(map[a], map[b])
 end
 
 # This comparator uses the operator `<=>` to compare objects.