diff --git a/elm.json b/elm.json
index d5b89d2..a1c9977 100644
--- a/elm.json
+++ b/elm.json
@@ -15,6 +15,7 @@
         "Internal.Tools.Encode",
         "Internal.Tools.Hashdict",
         "Internal.Tools.Iddict",
+        "Internal.Tools.Mashdict",
         "Internal.Tools.Timestamp",
         "Internal.Tools.VersionControl",
         "Internal.Values.Context",
diff --git a/src/Internal/Tools/Mashdict.elm b/src/Internal/Tools/Mashdict.elm
new file mode 100644
index 0000000..22c27a8
--- /dev/null
+++ b/src/Internal/Tools/Mashdict.elm
@@ -0,0 +1,300 @@
+module Internal.Tools.Mashdict exposing
+    ( Mashdict
+    , empty, singleton, insert, remove, removeKey
+    , isEmpty, member, memberKey, get, size, isEqual
+    , keys, values, toList, fromList
+    , rehash, union
+    , encode, decoder, softDecoder
+    )
+
+{-|
+
+
+# Mashdict
+
+A **mashdict**, (short for "maybe mashdict") is a hashdict that uses a hash
+function that _maybe_ returns a value. In this case, the mashdict exclusively
+stores values for which the hashing algorithm returns a value, and it ignores
+the outcome for all other scenarios.
+
+In general, you are advised to learn more about the
+[Hashdict](Internal-Tools-Hashdict) before delving into the Mashdict.
+
+
+## Dictionaries
+
+@docs Mashdict
+
+
+## Build
+
+@docs empty, singleton, insert, remove, removeKey
+
+
+## Query
+
+@docs isEmpty, member, memberKey, get, size, isEqual
+
+
+## Lists
+
+@docs keys, values, toList, fromList
+
+
+## Transform
+
+@docs rehash, union
+
+
+## JSON coders
+
+@docs encode, decoder, softDecoder
+
+-}
+
+import FastDict as Dict exposing (Dict)
+import Json.Decode as D
+import Json.Encode as E
+
+
+{-| A dictionary of keys and values where each key is defined by its value, but
+a value is not always given. For example, this can be relevant when not all
+inserted values are relevant:
+
+    import Mashdict exposing (Mashdict)
+
+    users : Mashdict Event
+    users =
+        Mashdict.fromList .location
+            [ Event "Graduation party" 8 (Just "park")
+            , Event "National holiday" 17 Nothing
+            , Event "Local fair" 11 (Just "town square")
+            ]
+
+    -- National holiday will be ignored
+    -- because it does not hash
+    type alias Event =
+        { name : String
+        , participants : Int
+        , location : Maybe String
+        }
+
+In the example listed above, all events are stored by their specified location,
+which means that all you need to know is the value "park" to retrieve all the
+information about the event at the park. As a result of optimization, this means
+all values without a hash, are filtered out, as we can never query them.
+
+-}
+type Mashdict a
+    = Mashdict
+        { hash : a -> Maybe String
+        , values : Dict String a
+        }
+
+
+{-| Decode a mashdict from a JSON value. To create a mashdict, you are expected
+to insert a hash function. If the hash function doesn't properly hash the values
+as expected, the decoder will fail to decode the mashdict.
+-}
+decoder : (a -> Maybe String) -> D.Decoder a -> D.Decoder (Mashdict a)
+decoder f xDecoder =
+    D.keyValuePairs xDecoder
+        |> D.andThen
+            (\items ->
+                if List.all (\( hash, value ) -> f value == Just hash) items then
+                    items
+                        |> Dict.fromList
+                        |> (\d -> { hash = f, values = d })
+                        |> Mashdict
+                        |> D.succeed
+
+                else
+                    D.fail "Hash function fails to properly hash all values"
+            )
+
+
+{-| Create an empty mashdict.
+-}
+empty : (a -> Maybe String) -> Mashdict a
+empty hash =
+    Mashdict { hash = hash, values = Dict.empty }
+
+
+{-| Encode a Mashdict into a JSON value. Keep in mind that an Elm function
+cannot be universally converted to JSON, so it is up to you to preserve that
+hash function!
+-}
+encode : (a -> E.Value) -> Mashdict a -> E.Value
+encode encodeX (Mashdict h) =
+    h.values
+        |> Dict.toList
+        |> List.map (Tuple.mapSecond encodeX)
+        |> E.object
+
+
+{-| Convert an association list into a mashdict.
+-}
+fromList : (a -> Maybe String) -> List a -> Mashdict a
+fromList hash xs =
+    Mashdict
+        { hash = hash
+        , values =
+            xs
+                |> List.filterMap (\x -> hash x |> Maybe.map (\hx -> ( hx, x )))
+                |> Dict.fromList
+        }
+
+
+{-| Get the value associated with a hash. If the hash is not found, return
+`Nothing`. This is useful when you are not sure if a hash will be in the
+mashdict.
+-}
+get : String -> Mashdict a -> Maybe a
+get k (Mashdict h) =
+    Dict.get k h.values
+
+
+{-| Insert a value into a mashdict. The key is automatically generated by the
+hash function. If the function generates a collision, it replaces the existing
+value in the mashdict. If the function returns `Nothing`, the value isn't
+inserted and the original Mashdict is returned.
+-}
+insert : a -> Mashdict a -> Mashdict a
+insert v (Mashdict h) =
+    case h.hash v of
+        Just hash ->
+            Mashdict { h | values = Dict.insert hash v h.values }
+
+        Nothing ->
+            Mashdict h
+
+
+{-| Determine if a mashdict is empty.
+-}
+isEmpty : Mashdict a -> Bool
+isEmpty (Mashdict h) =
+    Dict.isEmpty h.values
+
+
+{-| Since the Hashdict contains a hash function, the == operator does not work
+simply. Instead, you should use the isEqual operator.
+-}
+isEqual : Mashdict a -> Mashdict a -> Bool
+isEqual h1 h2 =
+    toList h1 == toList h2
+
+
+{-| Get all of the hashes in a mashdict, sorted from lowest to highest.
+-}
+keys : Mashdict a -> List String
+keys (Mashdict h) =
+    Dict.keys h.values
+
+
+{-| Determine if a value's hash is in a mashdict.
+-}
+member : a -> Mashdict a -> Bool
+member value (Mashdict h) =
+    h.hash value
+        |> Maybe.map (\key -> Dict.member key h.values)
+        |> Maybe.withDefault False
+
+
+{-| Determine if a hash is in a mashdict.
+-}
+memberKey : String -> Mashdict a -> Bool
+memberKey key (Mashdict h) =
+    Dict.member key h.values
+
+
+{-| Remap a mashdict using a new hashing algorithm.
+-}
+rehash : (a -> Maybe String) -> Mashdict a -> Mashdict a
+rehash f (Mashdict h) =
+    Mashdict
+        { hash = f
+        , values =
+            h.values
+                |> Dict.values
+                |> List.filterMap
+                    (\v -> Maybe.map (\hash -> ( hash, v )) (f v))
+                |> Dict.fromList
+        }
+
+
+{-| Remove a value from a mashdict. If the value's hash is found, the key-value
+pair is removed. If the value's hash is not found, no changes are made.
+
+    hdict |> Mashdict.remove (Event "Graduation party" 8 (Just "park"))
+
+-}
+remove : a -> Mashdict a -> Mashdict a
+remove v (Mashdict h) =
+    case h.hash v of
+        Just hash ->
+            Mashdict { h | values = Dict.remove hash h.values }
+
+        Nothing ->
+            Mashdict h
+
+
+{-| Remove a key from a mashdict. If the key is not found, no changes are made.
+
+    hdict |> Mashdict.removeKey "park"
+
+-}
+removeKey : String -> Mashdict a -> Mashdict a
+removeKey k (Mashdict h) =
+    Mashdict { h | values = Dict.remove k h.values }
+
+
+{-| Create a mashdict with a single key-value pair.
+-}
+singleton : (a -> Maybe String) -> a -> Mashdict a
+singleton f v =
+    empty f |> insert v
+
+
+{-| Determine the number of values in a mashdict.
+-}
+size : Mashdict a -> Int
+size (Mashdict h) =
+    Dict.size h.values
+
+
+{-| Decode a mashdict from a JSON value. If you cannot deduce the originally
+used hash function, (or if you simply do not care) you can use this function to
+decode and rehash the Mashdict using your new hash function.
+-}
+softDecoder : (a -> Maybe String) -> D.Decoder a -> D.Decoder (Mashdict a)
+softDecoder f xDecoder =
+    D.keyValuePairs xDecoder
+        |> D.map (List.map Tuple.second >> fromList f)
+
+
+{-| Convert a mashdict into an association list of key-value pairs, sorted by
+keys.
+-}
+toList : Mashdict a -> List ( String, a )
+toList (Mashdict h) =
+    Dict.toList h.values
+
+
+{-| Combine two mashdicts under the hash function of the first. If there is a
+collision, preference is given to the first mashdict.
+-}
+union : Mashdict a -> Mashdict a -> Mashdict a
+union (Mashdict h1) hd2 =
+    case rehash h1.hash hd2 of
+        Mashdict h2 ->
+            Mashdict
+                { hash = h1.hash
+                , values = Dict.union h1.values h2.values
+                }
+
+
+{-| Get all values stored in the mashdict, in the order of their keys.
+-}
+values : Mashdict a -> List a
+values (Mashdict h) =
+    Dict.values h.values
diff --git a/tests/Test/Tools/Mashdict.elm b/tests/Test/Tools/Mashdict.elm
new file mode 100644
index 0000000..dfddc6c
--- /dev/null
+++ b/tests/Test/Tools/Mashdict.elm
@@ -0,0 +1,204 @@
+module Test.Tools.Mashdict exposing (..)
+
+import Expect
+import Fuzz exposing (Fuzzer)
+import Internal.Tools.Mashdict as Mashdict exposing (Mashdict)
+import Internal.Values.Event as Event
+import Json.Decode as D
+import Json.Encode as E
+import Test exposing (..)
+import Test.Values.Event as TestEvent
+
+
+fuzzer : (a -> Maybe String) -> Fuzzer a -> Fuzzer (Mashdict a)
+fuzzer toHash fuz =
+    Fuzz.map (Mashdict.fromList toHash) (Fuzz.list fuz)
+
+
+eventFuzzer : Fuzzer (Mashdict Event.Event)
+eventFuzzer =
+    fuzzer .stateKey TestEvent.fuzzer
+
+
+suite : Test
+suite =
+    describe "Mashdict"
+        [ describe "empty"
+            [ test "empty isEmpty"
+                (Mashdict.empty identity
+                    |> Mashdict.isEmpty
+                    |> Expect.equal True
+                    |> always
+                )
+            , fuzz TestEvent.fuzzer
+                "Nothing is member"
+                (\event ->
+                    Mashdict.empty .stateKey
+                        |> Mashdict.member event
+                        |> Expect.equal False
+                )
+            , fuzz Fuzz.string
+                "No key is member"
+                (\key ->
+                    Mashdict.empty identity
+                        |> Mashdict.memberKey key
+                        |> Expect.equal False
+                )
+            , fuzz Fuzz.string
+                "Get gets Nothing"
+                (\key ->
+                    Mashdict.empty identity
+                        |> Mashdict.get key
+                        |> Expect.equal Nothing
+                )
+            , test "Size is zero"
+                (Mashdict.empty identity
+                    |> Mashdict.size
+                    |> Expect.equal 0
+                    |> always
+                )
+            , test "No keys"
+                (Mashdict.empty identity
+                    |> Mashdict.keys
+                    |> Expect.equal []
+                    |> always
+                )
+            , test "No values"
+                (Mashdict.empty identity
+                    |> Mashdict.values
+                    |> Expect.equal []
+                    |> always
+                )
+            , test "To list is []"
+                (Mashdict.empty identity
+                    |> Mashdict.toList
+                    |> Expect.equal []
+                    |> always
+                )
+            , test "From list is empty"
+                ([]
+                    |> Mashdict.fromList (\x -> x)
+                    |> Mashdict.isEqual (Mashdict.empty identity)
+                    |> Expect.equal True
+                    |> always
+                )
+            , test "Empty + empty == empty"
+                (Mashdict.empty Maybe.Just
+                    |> Mashdict.union (Mashdict.empty Maybe.Just)
+                    |> Mashdict.isEqual (Mashdict.empty Maybe.Just)
+                    |> Expect.equal True
+                    |> always
+                )
+            , fuzz (Fuzz.intRange 0 10)
+                "JSON encode -> JSON decode"
+                (\indent ->
+                    Mashdict.empty Just
+                        |> Mashdict.encode E.string
+                        |> E.encode indent
+                        |> D.decodeString (Mashdict.decoder Just D.string)
+                        |> Result.map (Mashdict.isEqual (Mashdict.empty Just))
+                        |> Expect.equal (Ok True)
+                )
+            ]
+        , describe "singleton"
+            [ fuzz TestEvent.fuzzer
+                "singleton = empty + insert"
+                (\event ->
+                    Mashdict.empty .stateKey
+                        |> Mashdict.insert event
+                        |> Mashdict.isEqual (Mashdict.singleton .stateKey event)
+                        |> Expect.equal True
+                )
+            , fuzz TestEvent.fuzzer
+                "singleton - event = empty"
+                (\event ->
+                    Mashdict.singleton .stateKey event
+                        |> Mashdict.remove event
+                        |> Mashdict.isEqual (Mashdict.empty (always Nothing))
+                        |> Expect.equal True
+                )
+            , fuzz TestEvent.fuzzer
+                "singleton - event (key) = empty"
+                (\event ->
+                    case event.stateKey of
+                        Just key ->
+                            Mashdict.singleton .stateKey event
+                                |> Mashdict.removeKey key
+                                |> Mashdict.isEqual (Mashdict.empty .stateKey)
+                                |> Expect.equal True
+
+                        Nothing ->
+                            Expect.pass
+                )
+            , fuzz TestEvent.fuzzer
+                "Only isEmpty when not Nothing"
+                (\event ->
+                    Expect.equal
+                        (case event.stateKey of
+                            Just _ ->
+                                False
+
+                            Nothing ->
+                                True
+                        )
+                        (event
+                            |> Mashdict.singleton .stateKey
+                            |> Mashdict.isEmpty
+                        )
+                )
+            , fuzz TestEvent.fuzzer
+                "member"
+                (\event ->
+                    Expect.equal
+                        (case event.stateKey of
+                            Just _ ->
+                                True
+
+                            Nothing ->
+                                False
+                        )
+                        (Mashdict.singleton .stateKey event
+                            |> Mashdict.member event
+                        )
+                )
+            , fuzz2 TestEvent.fuzzer
+                Fuzz.string
+                "memberKey"
+                (\event rkey ->
+                    case event.stateKey of
+                        Just key ->
+                            Mashdict.singleton .stateKey event
+                                |> Mashdict.memberKey key
+                                |> Expect.equal True
+
+                        Nothing ->
+                            Mashdict.singleton .stateKey event
+                                |> Mashdict.memberKey rkey
+                                |> Expect.equal False
+                )
+            , fuzz TestEvent.fuzzer
+                "False memberKey"
+                (\event ->
+                    if event.stateKey == Just event.roomId then
+                        Expect.pass
+
+                    else
+                        Mashdict.singleton .stateKey event
+                            |> Mashdict.memberKey event.roomId
+                            |> Expect.equal False
+                )
+            ]
+        , describe "JSON"
+            [ fuzz2 eventFuzzer
+                (Fuzz.intRange 0 10)
+                "JSON encode -> JSON decode"
+                (\hashdict indent ->
+                    hashdict
+                        |> Mashdict.encode Event.encode
+                        |> E.encode indent
+                        |> D.decodeString (Mashdict.decoder .stateKey Event.decoder)
+                        |> Result.map Mashdict.toList
+                        |> Expect.equal (Ok <| Mashdict.toList hashdict)
+                )
+            ]
+        ]