Introduce constants checksumLength and maxEncodedStringLength.

Use these constants to avoid repeatedly hardcoding the same values
within functions.

lib/bech32/src/Codec/Binary/Bech32.hs

@@ -14,9 +14,11 @@
 
 module Codec.Binary.Bech32
     (
-      -- * Encoder & Decoder
+      -- * Encoding & Decoding
       encode
     , decode
+    , checksumLength
+    , maxEncodedStringLength
 
       -- * Human-Readable Part
     , HumanReadablePart
@@ -95,21 +97,31 @@ encode hrp@(HumanReadablePart hrpBytes) payload = do
     let payload' = payload5 ++ bech32CreateChecksum hrp payload5
     let rest = map (charset Arr.!) payload'
     let output = B8.map toLower hrpBytes <> B8.pack "1" <> B8.pack rest
-    guard (BS.length output <= 90)
+    guard (BS.length output <= maxEncodedStringLength)
     return output
 
 decode :: ByteString -> Maybe (HumanReadablePart, ByteString)
 decode bech32 = do
-    guard $ BS.length bech32 <= 90
+    guard $ BS.length bech32 <= maxEncodedStringLength
     guard $ B8.map toUpper bech32 == bech32 || B8.map toLower bech32 == bech32
     let (hrp, dat) = B8.breakEnd (== '1') $ B8.map toLower bech32
-    guard $ BS.length dat >= 6
+    guard $ BS.length dat >= checksumLength
     hrp' <- B8.stripSuffix (B8.pack "1") hrp >>= mkHumanReadablePart
     dat' <- mapM charsetMap $ B8.unpack dat
     guard $ bech32VerifyChecksum hrp' dat'
-    result <- toBase256 (take (BS.length dat - 6) dat')
+    result <- toBase256 (take (BS.length dat - checksumLength) dat')
     return (hrp', BS.pack result)
 
+-- | The length of the checksum portion of an encoded string, in bytes.
+checksumLength :: Int
+checksumLength = 6
+
+-- | The maximum length of an encoded string, in bytes. This length includes the
+--   human-readable part, the separator character, the encoded data portion,
+--   and the checksum.
+maxEncodedStringLength :: Int
+maxEncodedStringLength = 90
+
 {-------------------------------------------------------------------------------
                                    Internal
 -------------------------------------------------------------------------------}

lib/bech32/test/Codec/Binary/Bech32Spec.hs

@@ -68,10 +68,17 @@ spec = do
             Bech32.decode checksum `shouldSatisfy` isNothing
 
     describe "More Encoding/Decoding Cases" $ do
-        it "length > 90" $ do
-            let (Just hrp) = mkHumanReadablePart (B8.pack "ca")
-            Bech32.encode hrp (BS.pack (replicate 82 1))
+        it "length > maximum" $ do
+            let hrpUnpacked = "ca"
+            let hrpLength = length hrpUnpacked
+            let (Just hrp) = mkHumanReadablePart (B8.pack hrpUnpacked)
+            let separatorLength = 1
+            let maxDataLength =
+                    Bech32.maxEncodedStringLength
+                    - Bech32.checksumLength - separatorLength - hrpLength
+            Bech32.encode hrp (BS.pack (replicate (maxDataLength + 1) 1))
                 `shouldSatisfy` isNothing
+
         it "hrp lowercased" $ do
             let (Just hrp) = mkHumanReadablePart (B8.pack "HRP")
             Bech32.encode hrp mempty