Merge #4864

4864: Don't remove facet value when multiple original values map to the same normalized value r=ManyTheFish a=dureuill

# Pull Request

## Related issue

Fixes #4860 

> [!WARNING]  
> This PR contains a fix to the immediate issue, but it looks like the underlying data model is faulty: there is only one possible "original" value for each normalized value in a facet of a document, while because of array values (or manually written nested fields, if you're evil), it is technically possible to have multiple, distinct original values mapping to the same normalized value.

Co-authored-by: Louis Dureuil <louis@meilisearch.com>

milli/src/update/index_documents/extract/extract_fid_docid_facet_values.rs

@@ -9,7 +9,7 @@ use std::result::Result as StdResult;
 use bytemuck::bytes_of;
 use grenad::Sorter;
 use heed::BytesEncode;
-use itertools::{merge_join_by, EitherOrBoth};
+use itertools::{merge_join_by, EitherOrBoth, Itertools};
 use ordered_float::OrderedFloat;
 use roaring::RoaringBitmap;
 use serde_json::{from_slice, Value};
@@ -317,11 +317,15 @@ fn deladd_obkv_cbo_roaring_bitmaps(
 }
 
 /// Truncates a string to the biggest valid LMDB key size.
-fn truncate_string(s: String) -> String {
-    s.char_indices()
-        .take_while(|(idx, _)| idx + 4 < MAX_FACET_VALUE_LENGTH)
-        .map(|(_, c)| c)
-        .collect()
+fn truncate_str(s: &str) -> &str {
+    let index = s
+        .char_indices()
+        .map(|(idx, _)| idx)
+        .chain(std::iter::once(s.len()))
+        .take_while(|idx| idx <= &MAX_FACET_VALUE_LENGTH)
+        .last();
+
+    &s[..index.unwrap_or(0)]
 }
 
 /// Computes the diff between both Del and Add numbers and
@@ -401,37 +405,103 @@ where
     del_strings.dedup();
     add_strings.dedup();
 
+    let del_strings = del_strings.iter().chunk_by(|(normalized, _)| normalized);
+    let add_strings = add_strings.iter().chunk_by(|(normalized, _)| normalized);
+
     let merged_strings_iter = itertools::merge_join_by(
         del_strings.into_iter().filter(|(n, _)| !n.is_empty()),
         add_strings.into_iter().filter(|(n, _)| !n.is_empty()),
-        |del, add| del.cmp(add),
+        |(normalized_del, _), (normalized_add, _)| normalized_del.cmp(normalized_add),
     );
 
     // insert normalized and original facet string in sorter
     for eob in merged_strings_iter {
         key_buffer.truncate(TRUNCATE_SIZE);
+        let (side, normalized, original) = match eob {
+            EitherOrBoth::Both((normalized, del), (_, add)) => {
+                let merged_strings_iter =
+                    itertools::merge_join_by(del, add, |(_, original_del), (_, original_add)| {
+                        original_del.cmp(original_add)
+                    });
+
+                // FIXME: we're in a bit of a pickle here, because we're only saving **one** original value per side,
+                // but we possibly have multiple original values that changed in the case where the field is an
+                // array of multiple values that normalize to the same value.
+                // (e.g. "foo" = ["bar", "Bar", "bAr", "baR"]. I'm not judging why you would do that ¯\_(ツ)_/¯)
+                //
+                // We'll work best effort by ignoring when the same value appears in both sides, deleting the first
+                // value that is only in the old version, and adding the first value that is only in the new version
+                let mut obkv = KvWriterDelAdd::memory();
+                let mut del = None;
+                let mut add = None;
+                let mut both = None;
+
+                for eob in merged_strings_iter {
                     match eob {
-            EitherOrBoth::Both(_, _) => (), // no need to touch anything
-            EitherOrBoth::Left((normalized, original)) => {
-                let truncated = truncate_string(normalized);
+                        EitherOrBoth::Both((_normalized, original), _) => {
+                            both = match both {
+                                Some(both) => Some(both),
+                                None => Some(original),
+                            }
+                        }
+                        EitherOrBoth::Left((_normalized, original)) => {
+                            del = match del {
+                                Some(del) => Some(del),
+                                None => Some(original),
+                            };
+                        }
+                        EitherOrBoth::Right((_normalized, original)) => {
+                            add = match add {
+                                Some(add) => Some(add),
+                                None => Some(original),
+                            }
+                        }
+                    }
+                }
+
+                if let Some(del) = del {
+                    obkv.insert(DelAdd::Deletion, del)?;
+                }
+                if let Some(add) = add
+                    // prefer the newly added, but if there is none, keep a value in the list of values
+                    // since the normalized value appears both in old and new, we should never remove it.
+                    .or(both)
+                {
+                    obkv.insert(DelAdd::Addition, add)?;
+                }
+
+                let truncated = truncate_str(normalized);
+                key_buffer.extend_from_slice(truncated.as_bytes());
+
+                let bytes = obkv.into_inner()?;
+                fid_docid_facet_strings_sorter.insert(&key_buffer, bytes)?;
+                continue;
+            }
+            EitherOrBoth::Left((_normalized, mut original)) => {
+                // FIXME: we only consider the first value for the purpose of facet search
+                // another structure is needed, able to retain all originals associated with a normalized value.
+                let Some((normalized, original)) = original.next() else {
+                    continue;
+                };
+                (DelAdd::Deletion, normalized, original)
+            }
+            EitherOrBoth::Right((_normalized, mut original)) => {
+                // FIXME: we only consider the first value for the purpose of facet search
+                // another structure is needed, able to retain all originals associated with a normalized value.
+                let Some((normalized, original)) = original.next() else {
+                    continue;
+                };
+                (DelAdd::Addition, normalized, original)
+            }
+        };
+        let truncated = truncate_str(normalized);
         key_buffer.extend_from_slice(truncated.as_bytes());
 
         let mut obkv = KvWriterDelAdd::memory();
-                obkv.insert(DelAdd::Deletion, original)?;
+        obkv.insert(side, original)?;
         let bytes = obkv.into_inner()?;
         fid_docid_facet_strings_sorter.insert(&key_buffer, bytes)?;
     }
-            EitherOrBoth::Right((normalized, original)) => {
-                let truncated = truncate_string(normalized);
-                key_buffer.extend_from_slice(truncated.as_bytes());
-
-                let mut obkv = KvWriterDelAdd::memory();
-                obkv.insert(DelAdd::Addition, original)?;
-                let bytes = obkv.into_inner()?;
-                fid_docid_facet_strings_sorter.insert(&key_buffer, bytes)?;
-            }
-        }
-    }
 
     Ok(())
 }