add Fifo.length function

src/Fifo.elm

@@ -2,6 +2,7 @@ module Fifo exposing
     ( Fifo, empty, fromList
     , insert, remove
     , toList
+    , length
     )
 
 {-|
@@ -21,13 +22,18 @@ module Fifo exposing
 
 @docs toList
 
+
+# Inspecting
+
+@docs length
+
 -}
 
 
 {-| A FIFO containing items of type `a`.
 -}
 type Fifo a
-    = Fifo (List a) (List a)
+    = Fifo (List a) (List a) Int
 
 
 {-| Creates an empty Fifo.
@@ -38,7 +44,7 @@ type Fifo a
 -}
 empty : Fifo a
 empty =
-    Fifo [] []
+    Fifo [] [] 0
 
 
 {-| Inserts an item into a Fifo
@@ -50,8 +56,8 @@ empty =
 
 -}
 insert : a -> Fifo a -> Fifo a
-insert a (Fifo front back) =
-    Fifo front (a :: back)
+insert a (Fifo front back len) =
+    Fifo front (a :: back) (len + 1)
 
 
 {-| Removes the next (oldest) item from a Fifo, returning the item (if any), and the updated Fifo.
@@ -64,14 +70,14 @@ insert a (Fifo front back) =
 remove : Fifo a -> ( Maybe a, Fifo a )
 remove fifo =
     case fifo of
-        Fifo [] [] ->
+        Fifo [] [] _ ->
             ( Nothing, empty )
 
-        Fifo [] back ->
-            remove <| Fifo (List.reverse back) []
+        Fifo [] back len ->
+            remove <| Fifo (List.reverse back) [] len
 
-        Fifo (next :: rest) back ->
-            ( Just next, Fifo rest back )
+        Fifo (next :: rest) back len ->
+            ( Just next, Fifo rest back (len - 1) )
 
 
 {-| Creates a Fifo from a List.
@@ -84,7 +90,7 @@ remove fifo =
 -}
 fromList : List a -> Fifo a
 fromList list =
-    Fifo list []
+    Fifo list [] (List.length list)
 
 
 {-| Converts a Fifo to a List.
@@ -97,5 +103,19 @@ fromList list =
 
 -}
 toList : Fifo a -> List a
-toList (Fifo front back) =
+toList (Fifo front back _) =
     front ++ List.reverse back
+
+
+{-| Returns the number of items in the Fifo.
+
+    Fifo.fromList [1, 2, 3]
+    |> Fifo.remove
+    |> Tuple.second
+    |> Fifo.length
+        -- == 2
+
+-}
+length : Fifo a -> Int
+length (Fifo _ _ len) =
+    len

tests/Tests.elm

@@ -83,4 +83,28 @@ all =
                     |> Fifo.remove
                     |> Tuple.first
                     |> Expect.equal (Just 2)
+        , test "length" <|
+            \() ->
+                Fifo.fromList [ 1, 2, 3 ]
+                    |> Fifo.length
+                    |> Expect.equal 3
+        , test "length empty" <|
+            \() ->
+                Fifo.empty
+                    |> Fifo.length
+                    |> Expect.equal 0
+        , test "length insert" <|
+            \() ->
+                Fifo.empty
+                    |> Fifo.insert 42
+                    |> Fifo.insert 60
+                    |> Fifo.length
+                    |> Expect.equal 2
+        , test "length remove" <|
+            \() ->
+                Fifo.fromList [ 1, 2, 3 ]
+                    |> Fifo.remove
+                    |> Tuple.second
+                    |> Fifo.length
+                    |> Expect.equal 2
         ]