Support bech32 encoding without a checksum

BOLT 12 Offers uses bech32 encoding without a checksum since QR codes
already have a checksum. Add functions encode_without_checksum and
decode_without_checksum to support this use case.

Also, remove overall length check in decode since it is unnecessary.

src/lib.rs

@@ -182,12 +182,12 @@ impl<'a> Bech32Writer<'a> {
 
     /// Write out the checksum at the end. If this method isn't called this will happen on drop.
     pub fn finalize(mut self) -> fmt::Result {
-        self.inner_finalize()?;
+        self.write_checksum()?;
         mem::forget(self);
         Ok(())
     }
 
-    fn inner_finalize(&mut self) -> fmt::Result {
+    fn write_checksum(&mut self) -> fmt::Result {
         // Pad with 6 zeros
         for _ in 0..CHECKSUM_LENGTH {
             self.polymod_step(u5(0))
@@ -203,6 +203,7 @@ impl<'a> Bech32Writer<'a> {
         Ok(())
     }
 }
+
 impl<'a> WriteBase32 for Bech32Writer<'a> {
     type Err = fmt::Error;
 
@@ -215,7 +216,7 @@ impl<'a> WriteBase32 for Bech32Writer<'a> {
 
 impl<'a> Drop for Bech32Writer<'a> {
     fn drop(&mut self) {
-        self.inner_finalize()
+        self.write_checksum()
             .expect("Unhandled error writing the checksum on drop.")
     }
 }
@@ -421,6 +422,32 @@ pub fn encode_to_fmt<T: AsRef<[u5]>>(
     })?)
 }
 
+/// Encode a bech32 payload without a checksum to an [fmt::Write].
+/// This method is intended for implementing traits from [std::fmt].
+///
+/// # Errors
+/// * If [check_hrp] returns an error for the given HRP.
+/// * If `fmt` fails on write
+/// # Deviations from standard
+/// * No length limits are enforced for the data part
+pub fn encode_without_checksum_to_fmt<T: AsRef<[u5]>>(
+    fmt: &mut fmt::Write,
+    hrp: &str,
+    data: T,
+) -> Result<(), Error> {
+    let hrp = match check_hrp(hrp)? {
+        Case::Upper => Cow::Owned(hrp.to_lowercase()),
+        Case::Lower | Case::None => Cow::Borrowed(hrp),
+    };
+
+    fmt.write_str(&hrp)?;
+    fmt.write_char(SEP)?;
+    for b in data.as_ref() {
+        fmt.write_char(b.to_char())?;
+    }
+    Ok(())
+}
+
 /// Used for encode/decode operations for the two variants of Bech32
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 pub enum Variant {
@@ -463,15 +490,44 @@ pub fn encode<T: AsRef<[u5]>>(hrp: &str, data: T, variant: Variant) -> Result<St
     Ok(buf)
 }
 
+/// Encode a bech32 payload to string without the checksum.
+///
+/// # Errors
+/// * If [check_hrp] returns an error for the given HRP.
+/// # Deviations from standard
+/// * No length limits are enforced for the data part
+pub fn encode_without_checksum<T: AsRef<[u5]>>(hrp: &str, data: T) -> Result<String, Error> {
+    let mut buf = String::new();
+    encode_without_checksum_to_fmt(&mut buf, hrp, data)?;
+    Ok(buf)
+}
+
 /// Decode a bech32 string into the raw HRP and the data bytes.
 ///
-/// Returns the HRP in lowercase..
+/// Returns the HRP in lowercase, the data with the checksum removed, and the encoding.
 pub fn decode(s: &str) -> Result<(String, Vec<u5>, Variant), Error> {
-    // Ensure overall length is within bounds
-    if s.len() < CHECKSUM_LENGTH + 2 {
+    let (hrp_lower, mut data) = decode_without_checksum(s)?;
+    if data.len() < CHECKSUM_LENGTH {
         return Err(Error::InvalidLength);
     }
 
+    // Ensure checksum
+    match verify_checksum(hrp_lower.as_bytes(), &data) {
+        Some(variant) => {
+            // Remove checksum from data payload
+            let dbl: usize = data.len();
+            data.truncate(dbl - CHECKSUM_LENGTH);
+
+            Ok((hrp_lower, data, variant))
+        }
+        None => Err(Error::InvalidChecksum),
+    }
+}
+
+/// Decode a bech32 string into the raw HRP and the data bytes, assuming no checksum.
+///
+/// Returns the HRP in lowercase and the data with the checksum removed.
+pub fn decode_without_checksum(s: &str) -> Result<(String, Vec<u5>), Error> {
     // Split at separator and check for two pieces
     let (raw_hrp, raw_data) = match s.rfind(SEP) {
         None => return Err(Error::MissingSeparator),
@@ -480,9 +536,6 @@ pub fn decode(s: &str) -> Result<(String, Vec<u5>, Variant), Error> {
             (hrp, &data[1..])
         }
     };
-    if raw_data.len() < CHECKSUM_LENGTH {
-        return Err(Error::InvalidLength);
-    }
 
     let mut case = check_hrp(raw_hrp)?;
     let hrp_lower = match case {
@@ -492,7 +545,7 @@ pub fn decode(s: &str) -> Result<(String, Vec<u5>, Variant), Error> {
     };
 
     // Check data payload
-    let mut data = raw_data
+    let data = raw_data
         .chars()
         .map(|c| {
             // Only check if c is in the ASCII range, all invalid ASCII
@@ -527,17 +580,7 @@ pub fn decode(s: &str) -> Result<(String, Vec<u5>, Variant), Error> {
         })
         .collect::<Result<Vec<u5>, Error>>()?;
 
-    // Ensure checksum
-    match verify_checksum(hrp_lower.as_bytes(), &data) {
-        Some(variant) => {
-            // Remove checksum from data payload
-            let dbl: usize = data.len();
-            data.truncate(dbl - CHECKSUM_LENGTH);
-
-            Ok((hrp_lower, data, variant))
-        }
-        None => Err(Error::InvalidChecksum),
-    }
+    Ok((hrp_lower, data))
 }
 
 fn verify_checksum(hrp: &[u8], data: &[u5]) -> Option<Variant> {
@@ -801,6 +844,8 @@ mod tests {
                 Error::InvalidLength),
             ("1p2gdwpf",
                 Error::InvalidLength),
+            ("bc1p2",
+                Error::InvalidLength),
         );
         for p in pairs {
             let (s, expected_error) = p;
@@ -930,7 +975,7 @@ mod tests {
     }
 
     #[test]
-    fn writer() {
+    fn write_with_checksum() {
         let hrp = "lnbc";
         let data = "Hello World!".as_bytes().to_base32();
 
@@ -947,7 +992,23 @@ mod tests {
     }
 
     #[test]
-    fn write_on_drop() {
+    fn write_without_checksum() {
+        let hrp = "lnbc";
+        let data = "Hello World!".as_bytes().to_base32();
+
+        let mut written_str = String::new();
+        {
+            let mut writer = Bech32Writer::new(hrp, Variant::Bech32, &mut written_str).unwrap();
+            writer.write(&data).unwrap();
+        }
+
+        let encoded_str = encode_without_checksum(hrp, data).unwrap();
+
+        assert_eq!(encoded_str, written_str[..written_str.len() - CHECKSUM_LENGTH]);
+    }
+
+    #[test]
+    fn write_with_checksum_on_drop() {
         let hrp = "lntb";
         let data = "Hello World!".as_bytes().to_base32();