From a00f5850eed4921aea7a0969bfdd3c15142e31fc Mon Sep 17 00:00:00 2001
From: Kerollmops <clement@meilisearch.com>
Date: Tue, 6 Oct 2020 14:52:05 +0200
Subject: [PATCH] Add support for placeholder search for empty queries

---
 public/script.js |  6 ++++++
 src/bin/serve.rs | 12 +++++++-----
 src/lib.rs       | 14 ++++++++++----
 src/search.rs    | 18 ++++++++++--------
 4 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/public/script.js b/public/script.js
index e8bac541d..6825c4d9b 100644
--- a/public/script.js
+++ b/public/script.js
@@ -74,3 +74,9 @@ $('#docs-count').text(function(index, text) {
 $('#db-size').text(function(index, text) {
   return filesize(parseInt(text))
 });
+
+// We trigger the input when we load the script, this way
+// we execute a placeholder search when the input is empty.
+$(window).on('load', function () {
+  $('#search').trigger('input');
+});
diff --git a/src/bin/serve.rs b/src/bin/serve.rs
index eebfa7a53..082f5fb86 100644
--- a/src/bin/serve.rs
+++ b/src/bin/serve.rs
@@ -172,7 +172,7 @@ async fn main() -> anyhow::Result<()> {
 
     #[derive(Deserialize)]
     struct QueryBody {
-        query: String,
+        query: Option<String>,
     }
 
     let env_cloned = env.clone();
@@ -184,10 +184,12 @@ async fn main() -> anyhow::Result<()> {
             let before_search = Instant::now();
             let rtxn = env_cloned.read_txn().unwrap();
 
-            let SearchResult { found_words, documents_ids } = index.search(&rtxn)
-                .query(query.query)
-                .execute()
-                .unwrap();
+            let mut search = index.search(&rtxn);
+            if let Some(query) = query.query {
+                search.query(query);
+            }
+
+            let SearchResult { found_words, documents_ids } = search.execute().unwrap();
 
             let body = match index.headers(&rtxn).unwrap() {
                 Some(headers) => {
diff --git a/src/lib.rs b/src/lib.rs
index bd707e422..a9ef09162 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,6 +14,7 @@ use csv::StringRecord;
 use fxhash::{FxHasher32, FxHasher64};
 use heed::types::*;
 use heed::{PolyDatabase, Database};
+use roaring::RoaringBitmap;
 
 pub use self::search::{Search, SearchResult};
 pub use self::criterion::{Criterion, default_criteria};
@@ -61,6 +62,10 @@ impl Index {
         })
     }
 
+    pub fn documents_ids(&self, rtxn: &heed::RoTxn) -> anyhow::Result<Option<RoaringBitmap>> {
+        Ok(self.main.get::<_, Str, RoaringBitmapCodec>(rtxn, DOCUMENTS_IDS_KEY)?)
+    }
+
     pub fn put_headers(&self, wtxn: &mut heed::RwTxn, headers: &StringRecord) -> heed::Result<()> {
         self.main.put::<_, Str, CsvStringRecordCodec>(wtxn, HEADERS_KEY, headers)
     }
@@ -114,10 +119,11 @@ impl Index {
     }
 
     /// Returns the number of documents indexed in the database.
-    pub fn number_of_documents<'t>(&self, rtxn: &'t heed::RoTxn) -> anyhow::Result<usize> {
-        let docids = self.main.get::<_, Str, RoaringBitmapCodec>(rtxn, DOCUMENTS_IDS_KEY)?
-            .with_context(|| format!("Could not find the list of documents ids"))?;
-        Ok(docids.len() as usize)
+    pub fn number_of_documents(&self, rtxn: &heed::RoTxn) -> anyhow::Result<usize> {
+        match self.documents_ids(rtxn)? {
+            Some(docids) => Ok(docids.len() as usize),
+            None => Ok(0),
+        }
     }
 
     pub fn search<'a>(&'a self, rtxn: &'a heed::RoTxn) -> Search<'a> {
diff --git a/src/search.rs b/src/search.rs
index ed4c80bcd..1e2329687 100644
--- a/src/search.rs
+++ b/src/search.rs
@@ -141,16 +141,18 @@ impl<'a> Search<'a> {
         };
 
         // Construct the DFAs related to the query words.
-        // TODO do a placeholder search when query string isn't present.
-        let dfas = match &self.query {
-            Some(q) => Self::generate_query_dfas(q),
-            None => return Ok(Default::default()),
+        let dfas = match self.query.as_deref().map(Self::generate_query_dfas) {
+            Some(dfas) if !dfas.is_empty() => dfas,
+            _ => {
+                // If the query is not set or results in no DFAs we return a placeholder.
+                let documents_ids = match self.index.documents_ids(self.rtxn)? {
+                    Some(docids) => docids.iter().take(limit).collect(),
+                    None => Vec::new(),
+                };
+                return Ok(SearchResult { documents_ids, ..Default::default() })
+            },
         };
 
-        if dfas.is_empty() {
-            return Ok(Default::default());
-        }
-
         let derived_words = self.fetch_words_docids(&fst, dfas)?;
         let candidates = Self::compute_candidates(&derived_words);