First attempt at implementing VLQ encoding, fails for large negative numbers when Int > 32 bits.

compiler/src/Generate/VLQ.hs

@@ -2,6 +2,55 @@ module Generate.VLQ
   ( encode
   ) where
 
+import Data.Bits ((.|.), (.&.))
+import Data.Bits qualified as Bit
+import Data.List qualified as List
+import Data.Map (Map, (!))
+import Data.Map qualified as Map
+import Data.Function ((&))
+import Data.Foldable.WithIndex (ifoldr)
+
+
+{- Ported from the Elm package Janiczek/elm-vlq
+-}
+
 encode :: Int -> String
-encode _num = 
-  ""
+encode num = 
+  let
+    numWithSign =
+      if num < 0
+        then (negate num `Bit.shiftL` 1) .|. 1
+        else num `Bit.shiftL` 1
+  in
+  encodeHelp numWithSign ""
+
+encodeHelp :: Int -> String -> String
+encodeHelp num acc =
+  let
+      clamped =
+          num .&. 31
+
+      newNum =
+          num `Bit.shiftR` 5
+
+      newClamped =
+          if newNum > 0 then
+              clamped .|. 32
+
+          else
+              clamped
+
+      newAcc =
+          base64Table ! newClamped : acc
+  in
+  if newNum > 0 then
+      encodeHelp newNum newAcc
+
+  else
+      List.reverse newAcc
+
+
+base64Table :: Map Int Char
+base64Table =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='"
+        & ifoldr Map.insert Map.empty

gren.cabal

@@ -225,7 +225,8 @@ Common gren-common
         utf8-string,
         vector,
         text >= 2 && < 3,
-        base64-bytestring >= 1.2 && < 2
+        base64-bytestring >= 1.2 && < 2,
+        indexed-traversable
 
 Executable gren
     Import:

tests/Generate/VLQSpec.hs

@@ -16,4 +16,4 @@ spec = do
       encode 123456789 `shouldBe` "qxmvrH"
       -- limits:
       encode (-2147483648) `shouldBe` "B"
-      encode 2147483647 `shouldBe` "+/////D"
+      encode 2147483647 `shouldBe` "+/////D"