Add missing overlap at match on end of input extending into new window

fixes #42

src/lz77.rs

@@ -259,7 +259,6 @@ fn add_to_hash_table(
 /// `ProcessStatus::BufferFull($pos)` if the buffer is full after writing.
 ///
 /// `pos` should indicate the byte to start at in the next call to `process_chunk`,
-/// `is_hashed` should be set to true of the byte at pos has been added to the hash chain.
 macro_rules! write_literal {
     ($w:ident, $byte:expr, $pos:expr) => {
         let b_status = $w.write_literal($byte);
@@ -450,16 +449,19 @@ fn process_chunk_lazy(
                 state.current_distance = 0;
                 state.add = false;
 
-                // As this will be a 3-length match at the end of the input data, there can't be any
-                // overlap.
+                debug_assert!((position + prev_length as usize) >= end - 1);
+                // Needed to note overlap as we can end up with the last window containing the rest
+                // of the match.
+                overlap = (position + prev_length as usize).saturating_sub(end).saturating_sub(1);
+
                 // TODO: Not sure if we need to signal that the buffer is full here.
                 // It's only needed in the case of syncing.
                 if let BufferStatus::Full = b_status {
                     // TODO: These bytes should be hashed when doing a sync flush.
                     // This can't be done here as the new input data does not exist yet.
-                    return (0, buffer_full(end));
+                    return (overlap, buffer_full(end));
                 } else {
-                    return (0, ProcessStatus::Ok);
+                    return (overlap, ProcessStatus::Ok);
                 }
             };