Merge #664

664: Fix phrase search containing stop words r=ManyTheFish a=Samyak2

# Pull Request

This a WIP draft PR I wanted to create to let other potential contributors know that I'm working on this issue. I'll be completing this in a few hours from opening this.

## Related issue
Fixes #661 and towards fixing meilisearch/meilisearch#2905

## What does this PR do?
- [x] Change Phrase Operation to use a `Vec<Option<String>>` instead of `Vec<String>` where `None` corresponds to a stop word
- [x] Update all other uses of phrase operation
- [x] Update `resolve_phrase`
- [x] Update `create_primitive_query`?
- [x] Add test

## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [x] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?


Co-authored-by: Samyak S Sarnayak <samyak201@gmail.com>
Co-authored-by: Samyak Sarnayak <samyak201@gmail.com>

milli/src/search/criteria/attribute.rs

@@ -579,6 +579,7 @@ fn flatten_query_tree(query_tree: &Operation) -> FlattenedQueryTree {
             Phrase(words) => {
                 let queries = words
                     .iter()
+                    .filter_map(|w| w.as_ref())
                     .map(|word| vec![Query { prefix: false, kind: QueryKind::exact(word.clone()) }])
                     .collect();
                 vec![queries]

milli/src/search/criteria/exactness.rs

@@ -299,9 +299,11 @@ fn attribute_start_with_docids(
             }
             Phrase(phrase) => {
                 for word in phrase {
-                    let wc = ctx.word_position_docids(word, pos)?;
-                    if let Some(word_candidates) = wc {
-                        attribute_candidates_array.push(word_candidates);
+                    if let Some(word) = word {
+                        let wc = ctx.word_position_docids(word, pos)?;
+                        if let Some(word_candidates) = wc {
+                            attribute_candidates_array.push(word_candidates);
+                        }
                     }
                     pos += 1;
                 }
@@ -323,7 +325,7 @@ fn intersection_of(mut rbs: Vec<&RoaringBitmap>) -> RoaringBitmap {
 
 #[derive(Debug, Clone)]
 pub enum ExactQueryPart {
-    Phrase(Vec<String>),
+    Phrase(Vec<Option<String>>),
     Synonyms(Vec<String>),
 }
 

milli/src/search/criteria/mod.rs

@@ -418,15 +418,32 @@ pub fn resolve_query_tree(
     resolve_operation(ctx, query_tree, wdcache)
 }
 
-pub fn resolve_phrase(ctx: &dyn Context, phrase: &[String]) -> Result<RoaringBitmap> {
+pub fn resolve_phrase(ctx: &dyn Context, phrase: &[Option<String>]) -> Result<RoaringBitmap> {
     let mut candidates = RoaringBitmap::new();
     let mut first_iter = true;
     let winsize = phrase.len().min(3);
+
+    if phrase.is_empty() {
+        return Ok(candidates);
+    }
+
     for win in phrase.windows(winsize) {
         // Get all the documents with the matching distance for each word pairs.
         let mut bitmaps = Vec::with_capacity(winsize.pow(2));
-        for (offset, s1) in win.iter().enumerate() {
-            for (dist, s2) in win.iter().skip(offset + 1).enumerate() {
+        for (offset, s1) in win.iter().enumerate().filter_map(|(index, word)| {
+            if let Some(word) = word {
+                Some((index, word))
+            } else {
+                None
+            }
+        }) {
+            for (dist, s2) in win.iter().skip(offset + 1).enumerate().filter_map(|(index, word)| {
+                if let Some(word) = word {
+                    Some((index, word))
+                } else {
+                    None
+                }
+            }) {
                 if dist == 0 {
                     match ctx.word_pair_proximity_docids(s1, s2, 1)? {
                         Some(m) => bitmaps.push(m),

milli/src/search/criteria/proximity.rs

@@ -4,6 +4,7 @@ use std::mem::take;
 
 use log::debug;
 use roaring::RoaringBitmap;
+use slice_group_by::GroupBy;
 
 use super::{
     query_docids, query_pair_proximity_docids, resolve_phrase, resolve_query_tree, Context,
@@ -187,10 +188,15 @@ fn resolve_candidates<'t>(
             Phrase(words) => {
                 if proximity == 0 {
                     let most_left = words
-                        .first()
+                        .iter()
+                        .filter_map(|o| o.as_ref())
+                        .next()
                         .map(|w| Query { prefix: false, kind: QueryKind::exact(w.clone()) });
                     let most_right = words
-                        .last()
+                        .iter()
+                        .rev()
+                        .filter_map(|o| o.as_ref())
+                        .next()
                         .map(|w| Query { prefix: false, kind: QueryKind::exact(w.clone()) });
 
                     match (most_left, most_right) {
@@ -473,14 +479,34 @@ fn resolve_plane_sweep_candidates(
             }
             Phrase(words) => {
                 let mut groups_positions = Vec::with_capacity(words.len());
-                for word in words {
-                    let positions = match words_positions.get(word) {
-                        Some(positions) => positions.iter().map(|p| (p, 0, p)).collect(),
-                        None => return Ok(vec![]),
-                    };
-                    groups_positions.push(positions);
+
+                // group stop_words together.
+                for words in words.linear_group_by_key(Option::is_none) {
+                    // skip if it's a group of stop words.
+                    if matches!(words.first(), None | Some(None)) {
+                        continue;
+                    }
+                    // make a consecutive plane-sweep on the subgroup of words.
+                    let mut subgroup = Vec::with_capacity(words.len());
+                    for word in words.into_iter().map(|w| w.as_deref().unwrap()) {
+                        match words_positions.get(word) {
+                            Some(positions) => {
+                                subgroup.push(positions.iter().map(|p| (p, 0, p)).collect())
+                            }
+                            None => return Ok(vec![]),
+                        }
+                    }
+                    match subgroup.len() {
+                        0 => {}
+                        1 => groups_positions.push(subgroup.pop().unwrap()),
+                        _ => groups_positions.push(plane_sweep(subgroup, true)?),
+                    }
+                }
+                match groups_positions.len() {
+                    0 => vec![],
+                    1 => groups_positions.pop().unwrap(),
+                    _ => plane_sweep(groups_positions, false)?,
                 }
-                plane_sweep(groups_positions, true)?
             }
             Or(_, ops) => {
                 let mut result = Vec::new();

milli/src/search/criteria/typo.rs

@@ -9,6 +9,7 @@ use super::{
     query_docids, resolve_query_tree, Candidates, Context, Criterion, CriterionParameters,
     CriterionResult,
 };
+use crate::search::criteria::resolve_phrase;
 use crate::search::query_tree::{maximum_typo, Operation, Query, QueryKind};
 use crate::search::{word_derivations, WordDerivationsCache};
 use crate::Result;
@@ -256,27 +257,7 @@ fn resolve_candidates<'t>(
 
         match query_tree {
             And(ops) => mdfs(ctx, ops, number_typos, cache, wdcache),
-            Phrase(words) => {
-                let mut candidates = RoaringBitmap::new();
-                let mut first_loop = true;
-                for slice in words.windows(2) {
-                    let (left, right) = (&slice[0], &slice[1]);
-                    match ctx.word_pair_proximity_docids(left, right, 1)? {
-                        Some(pair_docids) => {
-                            if pair_docids.is_empty() {
-                                return Ok(RoaringBitmap::new());
-                            } else if first_loop {
-                                candidates = pair_docids;
-                                first_loop = false;
-                            } else {
-                                candidates &= pair_docids;
-                            }
-                        }
-                        None => return Ok(RoaringBitmap::new()),
-                    }
-                }
-                Ok(candidates)
-            }
+            Phrase(words) => resolve_phrase(ctx, words),
             Or(_, ops) => {
                 let mut candidates = RoaringBitmap::new();
                 for op in ops {

milli/src/search/query_tree.rs

@@ -4,7 +4,6 @@ use std::{fmt, mem};
 
 use charabia::classifier::ClassifiedTokenIter;
 use charabia::{SeparatorKind, TokenKind};
-use fst::Set;
 use roaring::RoaringBitmap;
 use slice_group_by::GroupBy;
 
@@ -18,8 +17,9 @@ type IsPrefix = bool;
 #[derive(Clone, PartialEq, Eq, Hash)]
 pub enum Operation {
     And(Vec<Operation>),
-    // serie of consecutive non prefix and exact words
-    Phrase(Vec<String>),
+    // series of consecutive non prefix and exact words
+    // `None` means a stop word.
+    Phrase(Vec<Option<String>>),
     Or(IsOptionalWord, Vec<Operation>),
     Query(Query),
 }
@@ -75,9 +75,13 @@ impl Operation {
         }
     }
 
-    fn phrase(mut words: Vec<String>) -> Self {
+    fn phrase(mut words: Vec<Option<String>>) -> Self {
         if words.len() == 1 {
-            Self::Query(Query { prefix: false, kind: QueryKind::exact(words.pop().unwrap()) })
+            if let Some(word) = words.pop().unwrap() {
+                Self::Query(Query { prefix: false, kind: QueryKind::exact(word) })
+            } else {
+                Self::Phrase(words)
+            }
         } else {
             Self::Phrase(words)
         }
@@ -264,8 +268,7 @@ impl<'a> QueryTreeBuilder<'a> {
         &self,
         query: ClassifiedTokenIter<A>,
     ) -> Result<Option<(Operation, PrimitiveQuery, MatchingWords)>> {
-        let stop_words = self.index.stop_words(self.rtxn)?;
-        let primitive_query = create_primitive_query(query, stop_words, self.words_limit);
+        let primitive_query = create_primitive_query(query, self.words_limit);
         if !primitive_query.is_empty() {
             let qt = create_query_tree(
                 self,
@@ -370,7 +373,10 @@ fn create_query_tree(
             PrimitiveQueryPart::Word(word, prefix) => {
                 let mut children = synonyms(ctx, &[&word])?.unwrap_or_default();
                 if let Some((left, right)) = split_best_frequency(ctx, &word)? {
-                    children.push(Operation::Phrase(vec![left.to_string(), right.to_string()]));
+                    children.push(Operation::Phrase(vec![
+                        Some(left.to_string()),
+                        Some(right.to_string()),
+                    ]));
                 }
                 let (word_len_one_typo, word_len_two_typo) = ctx.min_word_len_for_typo()?;
                 let exact_words = ctx.exact_words();
@@ -583,7 +589,11 @@ fn create_matching_words(
             PrimitiveQueryPart::Phrase(words) => {
                 let ids: Vec<_> =
                     (0..words.len()).into_iter().map(|i| id + i as PrimitiveWordId).collect();
-                let words = words.into_iter().map(|w| MatchingWord::new(w, 0, false)).collect();
+                let words = words
+                    .into_iter()
+                    .filter_map(|w| w)
+                    .map(|w| MatchingWord::new(w, 0, false))
+                    .collect();
                 matching_words.push((words, ids));
             }
         }
@@ -685,7 +695,7 @@ pub type PrimitiveQuery = Vec<PrimitiveQueryPart>;
 
 #[derive(Debug, Clone)]
 pub enum PrimitiveQueryPart {
-    Phrase(Vec<String>),
+    Phrase(Vec<Option<String>>),
     Word(String, IsPrefix),
 }
 
@@ -710,7 +720,6 @@ impl PrimitiveQueryPart {
 /// the primitive query is an intermediate state to build the query tree.
 fn create_primitive_query<A>(
     query: ClassifiedTokenIter<A>,
-    stop_words: Option<Set<&[u8]>>,
     words_limit: Option<usize>,
 ) -> PrimitiveQuery
 where
@@ -735,9 +744,14 @@ where
                 // 2. if the word is not the last token of the query and is not a stop_word we push it as a non-prefix word,
                 // 3. if the word is the last token of the query we push it as a prefix word.
                 if quoted {
-                    phrase.push(token.lemma().to_string());
+                    if let TokenKind::StopWord = token.kind {
+                        phrase.push(None)
+                    } else {
+                        phrase.push(Some(token.lemma().to_string()));
+                    }
                 } else if peekable.peek().is_some() {
-                    if !stop_words.as_ref().map_or(false, |swords| swords.contains(token.lemma())) {
+                    if let TokenKind::StopWord = token.kind {
+                    } else {
                         primitive_query
                             .push(PrimitiveQueryPart::Word(token.lemma().to_string(), false));
                     }
@@ -820,7 +834,7 @@ mod test {
             words_limit: Option<usize>,
             query: ClassifiedTokenIter<A>,
         ) -> Result<Option<(Operation, PrimitiveQuery)>> {
-            let primitive_query = create_primitive_query(query, None, words_limit);
+            let primitive_query = create_primitive_query(query, words_limit);
             if !primitive_query.is_empty() {
                 let qt = create_query_tree(
                     self,
@@ -1065,7 +1079,7 @@ mod test {
         OR
           AND
             OR
-              PHRASE ["word", "split"]
+              PHRASE [Some("word"), Some("split")]
               Tolerant { word: "wordsplit", max typo: 2 }
             Exact { word: "fish" }
           Tolerant { word: "wordsplitfish", max typo: 1 }
@@ -1084,7 +1098,7 @@ mod test {
 
         insta::assert_debug_snapshot!(query_tree, @r###"
         OR
-          PHRASE ["quickbrown", "fox"]
+          PHRASE [Some("quickbrown"), Some("fox")]
           PrefixTolerant { word: "quickbrownfox", max typo: 2 }
         "###);
     }
@@ -1101,7 +1115,7 @@ mod test {
 
         insta::assert_debug_snapshot!(query_tree, @r###"
         AND
-          PHRASE ["hey", "friends"]
+          PHRASE [Some("hey"), Some("friends")]
           Exact { word: "wooop" }
         "###);
     }
@@ -1138,8 +1152,8 @@ mod test {
 
         insta::assert_debug_snapshot!(query_tree, @r###"
         AND
-          PHRASE ["hey", "friends"]
-          PHRASE ["wooop", "wooop"]
+          PHRASE [Some("hey"), Some("friends")]
+          PHRASE [Some("wooop"), Some("wooop")]
         "###);
     }
 
@@ -1187,7 +1201,7 @@ mod test {
             .unwrap();
 
         insta::assert_debug_snapshot!(query_tree, @r###"
-        PHRASE ["hey", "my"]
+        PHRASE [Some("hey"), Some("my")]
         "###);
     }
 
@@ -1252,7 +1266,7 @@ mod test {
 
         insta::assert_debug_snapshot!(query_tree, @r###"
         AND
-          PHRASE ["hey", "my"]
+          PHRASE [Some("hey"), Some("my")]
           Exact { word: "good" }
         "###);
     }

milli/tests/search/mod.rs

@@ -15,6 +15,7 @@ use slice_group_by::GroupBy;
 mod distinct;
 mod facet_distribution;
 mod filters;
+mod phrase_search;
 mod query_criteria;
 mod sort;
 mod typo_tolerance;

milli/tests/search/phrase_search.rs

@@ -0,0 +1,56 @@
+use milli::update::{IndexerConfig, Settings};
+use milli::{Criterion, Index, Search, TermsMatchingStrategy};
+
+use crate::search::Criterion::{Attribute, Exactness, Proximity};
+
+fn set_stop_words(index: &Index, stop_words: &[&str]) {
+    let mut wtxn = index.write_txn().unwrap();
+    let config = IndexerConfig::default();
+
+    let mut builder = Settings::new(&mut wtxn, &index, &config);
+    let stop_words = stop_words.into_iter().map(|s| s.to_string()).collect();
+    builder.set_stop_words(stop_words);
+    builder.execute(|_| (), || false).unwrap();
+    wtxn.commit().unwrap();
+}
+
+fn test_phrase_search_with_stop_words_given_criteria(criteria: &[Criterion]) {
+    let index = super::setup_search_index_with_criteria(&criteria);
+
+    // Add stop_words
+    set_stop_words(&index, &["a", "an", "the", "of"]);
+
+    // Phrase search containing stop words
+    let txn = index.read_txn().unwrap();
+
+    let mut search = Search::new(&txn, &index);
+    search.query("\"the use of force\"");
+    search.limit(10);
+    search.authorize_typos(false);
+    search.terms_matching_strategy(TermsMatchingStrategy::All);
+
+    let result = search.execute().unwrap();
+    // 1 document should match
+    assert_eq!(result.documents_ids.len(), 1);
+
+    // test for a single stop word only, no other search terms
+    let mut search = Search::new(&txn, &index);
+    search.query("\"the\"");
+    search.limit(10);
+    search.authorize_typos(false);
+    search.terms_matching_strategy(TermsMatchingStrategy::All);
+    let result = search.execute().unwrap();
+    assert_eq!(result.documents_ids.len(), 0);
+}
+
+#[test]
+fn test_phrase_search_with_stop_words_no_criteria() {
+    let criteria = [];
+    test_phrase_search_with_stop_words_given_criteria(&criteria);
+}
+
+#[test]
+fn test_phrase_search_with_stop_words_all_criteria() {
+    let criteria = [Proximity, Attribute, Exactness];
+    test_phrase_search_with_stop_words_given_criteria(&criteria);
+}