correction: introduce CorrectableError trait

This is just a helper trait to describe all the errors in the library
from which we can get an "invalid residue". The residues contain all the
information needed for error correction.

src/primitives/correction.rs

@@ -6,6 +6,12 @@
 //! equation to identify the error values, in a BCH-encoded string.
 //!
 
+use crate::primitives::decode::{
+    CheckedHrpstringError, ChecksumError, InvalidResidueError, SegwitHrpstringError,
+};
+#[cfg(feature = "alloc")]
+use crate::DecodeError;
+
 /// **One more than** the maximum length (in characters) of a checksum which
 /// can be error-corrected without an allocator.
 ///
@@ -33,3 +39,68 @@
 // this to 16, or maybe even higher. But we will wait for implementors who
 // complain.
 pub const NO_ALLOC_MAX_LENGTH: usize = 7;
+
+/// Trait describing an error for which an error correction algorithm is applicable.
+///
+/// Essentially, this trait represents an error which contains an [`InvalidResidueError`]
+/// variant.
+pub trait CorrectableError {
+    /// Given a decoding error, if this is a "checksum failed" error, extract
+    /// that specific error type.
+    ///
+    /// There are many ways in which decoding a checksummed string might fail.
+    /// If the string was well-formed in all respects except that the final
+    /// checksum characters appear to be wrong, it is possible to run an
+    /// error correction algorithm to attempt to extract errors.
+    ///
+    /// In all other cases we do not have enough information to do correction.
+    ///
+    /// This is the function that implementors should implement.
+    fn residue_error(&self) -> Option<&InvalidResidueError>;
+}
+
+impl CorrectableError for InvalidResidueError {
+    fn residue_error(&self) -> Option<&InvalidResidueError> { Some(self) }
+}
+
+impl CorrectableError for ChecksumError {
+    fn residue_error(&self) -> Option<&InvalidResidueError> {
+        match self {
+            ChecksumError::InvalidResidue(ref e) => Some(e),
+            _ => None,
+        }
+    }
+}
+
+impl CorrectableError for SegwitHrpstringError {
+    fn residue_error(&self) -> Option<&InvalidResidueError> {
+        match self {
+            SegwitHrpstringError::Checksum(ref e) => e.residue_error(),
+            _ => None,
+        }
+    }
+}
+
+impl CorrectableError for CheckedHrpstringError {
+    fn residue_error(&self) -> Option<&InvalidResidueError> {
+        match self {
+            CheckedHrpstringError::Checksum(ref e) => e.residue_error(),
+            _ => None,
+        }
+    }
+}
+
+#[cfg(feature = "alloc")]
+impl CorrectableError for crate::segwit::DecodeError {
+    fn residue_error(&self) -> Option<&InvalidResidueError> { self.0.residue_error() }
+}
+
+#[cfg(feature = "alloc")]
+impl CorrectableError for DecodeError {
+    fn residue_error(&self) -> Option<&InvalidResidueError> {
+        match self {
+            DecodeError::Checksum(ref e) => e.residue_error(),
+            _ => None,
+        }
+    }
+}