feat: filter similar groups by file size ratio (80% default)

- Thumbnails vs originals no longer grouped as similar
- Added file_size to ImageEntry
- find_duplicate_groups now checks min size ratio for similar pairs
- New API: find_duplicate_groups_with_size_ratio for custom ratio
- New test: similar_groups_filtered_by_size_ratio
- 24 tests passing

src/lib.rs

@@ -13,6 +13,7 @@ pub struct ImageEntry {
     pub path: PathBuf,
     pub sha256: String,
     pub dhash: u64,
+    pub file_size: u64,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -68,6 +69,7 @@ pub fn scan_images(root: &Path) -> Result<Vec<ImageEntry>> {
             path: path.to_path_buf(),
             sha256,
             dhash,
+            file_size: bytes.len() as u64,
         });
     }
     Ok(out)
@@ -96,8 +98,15 @@ pub fn compute_dhash(img: &image::DynamicImage) -> u64 {
 pub fn hamming(a: u64, b: u64) -> u32 {
     (a ^ b).count_ones()
 }
-
 pub fn find_duplicate_groups(entries: &[ImageEntry], hamming_threshold: u32) -> Vec<DuplicateGroup> {
+    find_duplicate_groups_with_size_ratio(entries, hamming_threshold, 0.80)
+}
+
+pub fn find_duplicate_groups_with_size_ratio(
+    entries: &[ImageEntry],
+    hamming_threshold: u32,
+    min_size_ratio: f64,
+) -> Vec<DuplicateGroup> {
     let mut groups = Vec::new();
 
     let mut exact: HashMap<&str, Vec<PathBuf>> = HashMap::new();
@@ -138,7 +147,14 @@ pub fn find_duplicate_groups(entries: &[ImageEntry], hamming_threshold: u32) ->
                 continue;
             }
             if hamming(entries[i].dhash, entries[j].dhash) <= hamming_threshold {
-                union(&mut parent, i, j);
+                let (small, big) = if entries[i].file_size <= entries[j].file_size {
+                    (entries[i].file_size, entries[j].file_size)
+                } else {
+                    (entries[j].file_size, entries[i].file_size)
+                };
+                if big == 0 || (small as f64 / big as f64) >= min_size_ratio {
+                    union(&mut parent, i, j);
+                }
             }
         }
     }
@@ -245,6 +261,16 @@ mod tests {
             path: PathBuf::from(path),
             sha256: sha.to_string(),
             dhash,
+            file_size: 1000,
+        }
+    }
+
+    fn make_entry_sized(path: &str, sha: &str, dhash: u64, file_size: u64) -> ImageEntry {
+        ImageEntry {
+            path: PathBuf::from(path),
+            sha256: sha.to_string(),
+            dhash,
+            file_size,
         }
     }
 
@@ -303,4 +329,26 @@ mod tests {
         let groups7 = find_duplicate_groups(&entries, 7);
         assert!(!groups7.iter().any(|g| g.kind == DuplicateKind::Similar));
     }
+
+    #[test]
+    fn similar_groups_filtered_by_size_ratio() {
+        // Same dhash (hamming=0) but very different file sizes
+        let entries = vec![
+            make_entry_sized("big.jpg", "aaa", 0b0000_0000, 100_000),   // 100KB
+            make_entry_sized("tiny.jpg", "bbb", 0b0000_0001, 5_000),   // 5KB (5% of big)
+        ];
+        // Default 80% ratio should NOT group them
+        let groups = find_duplicate_groups(&entries, 8);
+        assert!(!groups.iter().any(|g| g.kind == DuplicateKind::Similar),
+            "files with very different sizes should not be grouped as similar");
+
+        // Same dhash, similar file sizes should group
+        let entries2 = vec![
+            make_entry_sized("a.jpg", "aaa", 0b0000_0000, 100_000),
+            make_entry_sized("b.jpg", "bbb", 0b0000_0001, 90_000),  // 90% of big
+        ];
+        let groups2 = find_duplicate_groups(&entries2, 8);
+        assert!(groups2.iter().any(|g| g.kind == DuplicateKind::Similar),
+            "files with similar sizes should be grouped");
+    }
 }