From ce9a84f07bc4ee5124c87109729f1ccb2777851f Mon Sep 17 00:00:00 2001
From: Fred Yankowski <fcy@imsa.edu>
Date: Wed, 27 Apr 2022 09:24:44 -0500
Subject: [PATCH 1/2] add new `length` function for fast calculation of Fifo
 length

---
 src/Fifo.elm    | 42 +++++++++++++++++++++++++++++++-----------
 tests/Tests.elm | 18 ++++++++++++++++++
 2 files changed, 49 insertions(+), 11 deletions(-)

diff --git a/src/Fifo.elm b/src/Fifo.elm
index 9ece297..15c7cbb 100644
--- a/src/Fifo.elm
+++ b/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
diff --git a/tests/Tests.elm b/tests/Tests.elm
index 3220ce4..d1d6ddf 100644
--- a/tests/Tests.elm
+++ b/tests/Tests.elm
@@ -83,4 +83,22 @@ 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 remove" <|
+            \() ->
+                Fifo.fromList [ 1, 2, 3 ]
+                    |> Fifo.remove
+                    |> Tuple.second
+                    |> Fifo.length
+                    |> Expect.equal 2
         ]

From f5fab70bf75815b4666f59df88c22d0f08e858b9 Mon Sep 17 00:00:00 2001
From: Fred Yankowski <fcy@imsa.edu>
Date: Wed, 27 Apr 2022 09:48:42 -0500
Subject: [PATCH 2/2] test length after insert

---
 tests/Tests.elm | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tests/Tests.elm b/tests/Tests.elm
index d1d6ddf..a7dba83 100644
--- a/tests/Tests.elm
+++ b/tests/Tests.elm
@@ -93,7 +93,13 @@ all =
                 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 ]