Move our internal Callback_Registry away from a Vectors.Vector to a Hashed_Maps.Map

My previous assumptions about using a Vector as a sparse array turned
out to be incorrect. Whoopsies.

src/async-epoll.adb

@@ -4,7 +4,8 @@
 --
 
 
-private with Ada.Text_IO;
+private with Ada.Strings.Hash,
+            Ada.Text_IO;
 
 use Ada.Text_IO;
 
@@ -14,6 +15,7 @@ package body Async.Epoll is
                         Cb : in Callback_Tuple) is
         Event : aliased Epoll_Event;
         Descriptor : constant C.int := C.int (GNAT.Sockets.To_C (Cb.Socket));
+        Descriptor_Str : constant String := Natural'Image (Natural(Descriptor));
     begin
         Validate_Hub (This);
 
@@ -35,13 +37,13 @@ package body Async.Epoll is
                 raise Descriptor_Registration_Falied;
             end if;
 
-            This.Debug_Trace ("<Register>> Successfully added to the Epoll_Fd");
+            This.Debug_Trace ("<Register>> Successfully added descriptor:" &
+                                    Descriptor_Str & " to the Epoll_Fd");
         end;
 
-        This.Debug_Trace ("<Register>> Inserting descriptor:" &
-                                    Natural'Image (Natural (descriptor)));
+        This.Debug_Trace ("<Register>> Inserting descriptor:" & Descriptor_Str);
 
-        Callback_Registry.Insert (This.Callbacks, Natural(Descriptor), Cb);
+        Callback_Registry.Insert (This.Callbacks, Descriptor, Cb);
     end Register;
 
 
@@ -71,7 +73,7 @@ package body Async.Epoll is
                                                     Event.Data.Fd;
                             Cb : constant Callback_Tuple :=
                                                     Callback_Registry.Element (This.Callbacks,
-                                                                                Natural (Descriptor));
+                                                                                Descriptor);
                         begin
                             Cb.Callback.all (Cb.Socket, Cb.Context);
                         end;
@@ -120,4 +122,9 @@ package body Async.Epoll is
         Put_Line (Line);
     end Debug_Trace;
 
+    function Descriptor_Hash (Id : in C.int) return Hash_Type is
+    begin
+        return Ada.Strings.Hash (C.int'Image (Id));
+    end Descriptor_Hash;
+
 end Async.Epoll;

src/async-epoll.ads

@@ -2,7 +2,7 @@
 --  Main interface for dealing with epoll(7) from Ada
 --
 
-with Ada.Containers.Vectors,
+with Ada.Containers.Hashed_Maps,
         Interfaces.C,
         GNAT.Sockets;
 
@@ -24,8 +24,15 @@ package Async.Epoll is
         Context : Context_Type;
     end record;
 
+    function Descriptor_Hash (Id : in C.int) return Hash_Type;
+
     -- My_Callback.all (Fd);
-    package Callback_Registry is new Vectors (Natural, Callback_Tuple);
+    use Interfaces.C;
+    package Callback_Registry is new Hashed_Maps (Key_Type => C.int,
+                                            Element_Type => Callback_Tuple,
+                                            Hash => Descriptor_Hash,
+                                            Equivalent_Keys => "="
+                                            );
 
     type Hub is tagged private;
 
@@ -53,7 +60,7 @@ package Async.Epoll is
             Timeout : C.int := -1;
             Should_Continue : Boolean := True;
             Debug : Boolean := False;
-            Callbacks : Callback_Registry.Vector;
+            Callbacks : Callback_Registry.Map;
         end record;
 
         procedure Validate_Hub (H: in Hub);