Got mat and rub working (I think)

elm.json

@@ -9,11 +9,12 @@
     ],
     "elm-version": "0.19.0 <= v < 0.20.0",
     "dependencies": {
+        "TSFoster/elm-bytes-extra": "1.3.0 <= v < 1.4.0",
         "elm/bytes": "1.0.8 <= v < 2.0.0",
-        "elm/core": "1.0.0 <= v < 1.0.5"
+        "elm/core": "1.0.0 <= v < 1.0.5",
+        "elm-community/list-extra": "8.7.0 <= v < 9.0.0"
     },
     "test-dependencies": {
-        "TSFoster/elm-bytes-extra": "1.3.0 <= v < 1.4.0",
         "elm-explorations/test": "1.0.0 <= v < 2.0.0"
     }
 }

src/BitWriter.elm

@@ -1,6 +1,11 @@
-module BitWriter exposing (..)
+module BitWriter exposing
+    ( BitWriter
+    , bit
+    , bits
+    , empty
+    , run
+    )
 
-import BitParser as BP
 import Bitwise
 import Bytes exposing (Bytes)
 import Bytes.Encode as BE
@@ -67,15 +72,12 @@ bits bs writer =
 
 
 
--- PERF: don't go through BitParser to not hold all bits in memory
-
-
-bytes : Bytes -> BitWriter -> BitWriter
-bytes bite writer =
-    case BP.run (BP.rawBits (Bytes.width bite * 8)) bite of
-        -- this is bad, but shouldn't happen
-        Nothing ->
-            writer
-
-        Just bs ->
-            bits bs writer
+-- -- PERF: don't go through BitParser to not hold all bits in memory
+-- bytes : Bytes -> BitWriter -> BitWriter
+-- bytes bite writer =
+--     case BP.run (BP.rawBits (Bytes.width bite * 8)) bite of
+--         -- this is bad, but shouldn't happen
+--         Nothing ->
+--             writer
+--         Just bs ->
+--             bits bs writer

src/Urbit.elm

@@ -1,9 +1,11 @@
-module Urbit exposing (..)
+module Urbit exposing (Noun(..), mat, rub)
 
 import BitParser as BP exposing (BitParser)
+import BitWriter as BW exposing (BitWriter)
+import Bitwise
 import Bytes exposing (Bytes)
-import Bytes.Decode as BD
-import Bytes.Encode as BE
+import Bytes.Extra as Bytes
+import List.Extra as List
 
 
 type Noun
@@ -11,13 +13,39 @@ type Noun
     | Atom Bytes
 
 
+isSig : Bytes -> Bool
+isSig bytes =
+    Bytes.toByteValues bytes |> List.all (\x -> x == 0)
+
+
+mat : Bytes -> BitWriter -> BitWriter
+mat bytes writer =
+    if isSig bytes then
+        writer |> BW.bit 1
+
+    else
+        let
+            bits =
+                bytesToBits bytes |> List.dropWhileRight (\x -> x == 0)
+
+            lengthBits =
+                bits |> List.length |> intToBits |> List.reverse |> List.drop 1 |> List.reverse
+        in
+        writer
+            |> BW.bit 0
+            |> BW.bits (List.repeat (List.length lengthBits) 0)
+            |> BW.bit 1
+            |> BW.bits lengthBits
+            |> BW.bits bits
+
+
 rub : BitParser Bytes
 rub =
     BP.bit
         |> BP.andThen
             (\zeroBit ->
                 if zeroBit == 1 then
-                    BP.succeed (BE.unsignedInt8 0 |> BE.encode)
+                    BP.succeed Bytes.empty
 
                 else
                     let
@@ -40,7 +68,7 @@ rub =
                                         (\preLengthRawBits ->
                                             let
                                                 length =
-                                                    BP.bitsToInt (1 :: preLengthRawBits)
+                                                    BP.bitsToInt (preLengthRawBits ++ [ 1 ])
                                             in
                                             BP.bits length
                                         )
@@ -48,7 +76,22 @@ rub =
             )
 
 
+bytesToBits : Bytes -> List Int
+bytesToBits bytes =
+    case
+        BP.run (BP.rawBits (Bytes.width bytes * 8)) bytes
+    of
+        Nothing ->
+            []
 
--- met : BitParser Bytes
--- met =
---     1
+        Just bits ->
+            bits
+
+
+intToBits : Int -> List Int
+intToBits n =
+    if n <= 0 then
+        []
+
+    else
+        Bitwise.and 1 n :: intToBits (Bitwise.shiftRightBy 1 n)

tests/Test/Bit.elm

@@ -1,13 +1,11 @@
-module Test.BitParser exposing (tests)
+module Test.Bit exposing (tests)
 
 import BitParser
 import BitWriter
-import Bytes exposing (Bytes)
-import Bytes.Encode as BE
-import Bytes.Extra
-import Expect exposing (Expectation)
-import Fuzz exposing (Fuzzer)
+import Bytes
+import Expect
 import Test exposing (..)
+import Test.Utils exposing (..)
 
 
 tests : Test
@@ -37,19 +35,3 @@ tests =
                 )
             ]
         ]
-
-
-bytes : Fuzzer Bytes
-bytes =
-    (Fuzz.intRange 0 255 |> Fuzz.list)
-        |> Fuzz.map (\l -> l |> List.map BE.unsignedInt8 |> BE.sequence |> BE.encode)
-
-
-bytesEq : Bytes -> Bytes -> Expectation
-bytesEq a b =
-    Expect.equal (Bytes.Extra.toByteValues a) (Bytes.Extra.toByteValues b)
-
-
-maybeBytesEq : Maybe Bytes -> Maybe Bytes -> Expectation
-maybeBytesEq a b =
-    Expect.equal (Maybe.map Bytes.Extra.toByteValues a) (Maybe.map Bytes.Extra.toByteValues b)

tests/Test/Urbit.elm

@@ -0,0 +1,40 @@
+module Test.Urbit exposing (tests)
+
+import BitParser
+import BitWriter
+import Bytes exposing (Bytes)
+import Bytes.Extra as Bytes
+import Fuzz exposing (Fuzzer)
+import List.Extra as List
+import Test exposing (..)
+import Test.Utils exposing (..)
+import Urbit exposing (..)
+
+
+tests : Test
+tests =
+    concat
+        [ fuzz atom
+            "mat <-> rub"
+            (\a ->
+                maybeBytesEq (Just a) (BitWriter.run (mat a BitWriter.empty) |> BitParser.run rub)
+            )
+        ]
+
+
+atom : Fuzzer Bytes
+atom =
+    bytes
+        |> Fuzz.map
+            (\a -> a |> Bytes.toByteValues |> List.dropWhileRight (\x -> x == 0) |> Bytes.fromByteValues)
+
+
+noun : Fuzzer Noun
+noun =
+    (\() ->
+        Fuzz.oneOf
+            [ bytes |> Fuzz.map Atom
+            , Fuzz.map2 (\a b -> Cell ( a, b )) noun noun
+            ]
+    )
+        ()

tests/Test/Utils.elm

@@ -0,0 +1,24 @@
+module Test.Utils exposing (..)
+
+import Bytes exposing (Bytes)
+import Bytes.Encode as BE
+import Bytes.Extra
+import Expect exposing (Expectation)
+import Fuzz exposing (Fuzzer)
+import Test exposing (..)
+
+
+bytes : Fuzzer Bytes
+bytes =
+    (Fuzz.intRange 0 255 |> Fuzz.list)
+        |> Fuzz.map (\l -> l |> List.map BE.unsignedInt8 |> BE.sequence |> BE.encode)
+
+
+bytesEq : Bytes -> Bytes -> Expectation
+bytesEq a b =
+    Expect.equal (Bytes.Extra.toByteValues a) (Bytes.Extra.toByteValues b)
+
+
+maybeBytesEq : Maybe Bytes -> Maybe Bytes -> Expectation
+maybeBytesEq a b =
+    Expect.equal (Maybe.map Bytes.Extra.toByteValues a) (Maybe.map Bytes.Extra.toByteValues b)