From 315d4d0aed4a0ed16308b5031b3594b7bd99fbcc Mon Sep 17 00:00:00 2001
From: Kishore Nallan <kishorenc@gmail.com>
Date: Sat, 19 Jun 2021 16:52:27 +0530
Subject: [PATCH] Fix intersection on empty posting lists.

---
 src/posting_list.cpp       | 29 +++++++++++++++++++++++++++++
 test/posting_list_test.cpp | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)

diff --git a/src/posting_list.cpp b/src/posting_list.cpp
index 83b02eda..9eda281e 100644
--- a/src/posting_list.cpp
+++ b/src/posting_list.cpp
@@ -613,6 +613,20 @@ void posting_list_t::merge(const std::vector<posting_list_t*>& posting_lists, st
 
 // Inspired by: https://stackoverflow.com/a/25509185/131050
 void posting_list_t::intersect(const std::vector<posting_list_t*>& posting_lists, std::vector<uint32_t>& result_ids) {
+    if(posting_lists.empty()) {
+        return;
+    }
+
+    if(posting_lists.size() == 1) {
+        auto it = posting_lists[0]->new_iterator();
+        while(it.valid()) {
+            result_ids.push_back(it.id());
+            it.next();
+        }
+
+        return ;
+    }
+
     auto its = std::vector<posting_list_t::iterator_t>();
     its.reserve(posting_lists.size());
 
@@ -650,6 +664,11 @@ void posting_list_t::intersect(const std::vector<posting_list_t*>& posting_lists
 bool posting_list_t::block_intersect(const std::vector<posting_list_t*>& posting_lists, const size_t batch_size,
                                      std::vector<posting_list_t::iterator_t>& its,
                                      result_iter_state_t& iter_state) {
+
+    if(posting_lists.empty()) {
+        return false;
+    }
+
     if(its.empty()) {
         its.reserve(posting_lists.size());
 
@@ -666,6 +685,16 @@ bool posting_list_t::block_intersect(const std::vector<posting_list_t*>& posting
     size_t num_lists = its.size();
 
     switch (num_lists) {
+        case 1:
+            while(its[0].valid()) {
+                iter_state.ids.push_back(its[0].id());
+                its[0].next();
+
+                if(iter_state.ids.size() == batch_size) {
+                    return its[0].valid();
+                }
+            }
+            break;
         case 2:
             while(!at_end2(its)) {
                 if(equals2(its)) {
diff --git a/test/posting_list_test.cpp b/test/posting_list_test.cpp
index 0644c0a2..ff10de7f 100644
--- a/test/posting_list_test.cpp
+++ b/test/posting_list_test.cpp
@@ -633,6 +633,38 @@ TEST(PostingListTest, IntersectionBasics) {
     ASSERT_FALSE(has_more);
     ASSERT_EQ(1, iter_state2.ids.size());
     ASSERT_EQ(20, iter_state2.ids[0]);
+
+    // single item itersection
+    std::vector<posting_list_t*> single_item_list = {&p1};
+    result_ids.clear();
+    posting_list_t::intersect(single_item_list, result_ids);
+    std::vector<uint32_t> expected_ids = {0, 2, 3, 20};
+    ASSERT_EQ(expected_ids.size(), result_ids.size());
+
+    for(size_t i = 0; i < expected_ids.size(); i++) {
+        ASSERT_EQ(expected_ids[i], result_ids[i]);
+    }
+
+    std::vector<posting_list_t::iterator_t> its3;
+    posting_list_t::result_iter_state_t iter_state3;
+    has_more = posting_list_t::block_intersect(single_item_list, 2, its3, iter_state3);
+    ASSERT_TRUE(has_more);
+    ASSERT_EQ(2, iter_state3.ids.size());
+    has_more = posting_list_t::block_intersect(single_item_list, 2, its3, iter_state3);
+    ASSERT_FALSE(has_more);
+    ASSERT_EQ(2, iter_state3.ids.size());
+
+    // empty intersection list
+    std::vector<posting_list_t*> empty_list;
+    result_ids.clear();
+    posting_list_t::intersect(empty_list, result_ids);
+    ASSERT_EQ(0, result_ids.size());
+
+    std::vector<posting_list_t::iterator_t> its4;
+    posting_list_t::result_iter_state_t iter_state4;
+    has_more = posting_list_t::block_intersect(empty_list, 1, its4, iter_state4);
+    ASSERT_FALSE(has_more);
+    ASSERT_EQ(0, iter_state4.ids.size());
 }
 
 TEST(PostingListTest, ResultsAndOffsetsBasics) {