Normalize some variable names and reuse existing logic

closes https://github.com/official-stockfish/Stockfish/pull/5346

No functional change

src/search.cpp

@@ -137,10 +137,10 @@ void update_all_stats(const Position& pos,
 
 Search::Worker::Worker(SharedState&                    sharedState,
                        std::unique_ptr<ISearchManager> sm,
-                       size_t                          thread_id,
+                       size_t                          threadId,
                        NumaReplicatedAccessToken       token) :
     // Unpack the SharedState struct into member variables
-    thread_idx(thread_id),
+    threadIdx(threadId),
     numaAccessToken(token),
     manager(std::move(sm)),
     options(sharedState.options),

src/search.h

@@ -244,7 +244,7 @@ class Worker {
     // It searches from the root position and outputs the "bestmove".
     void start_searching();
 
-    bool is_mainthread() const { return thread_idx == 0; }
+    bool is_mainthread() const { return threadIdx == 0; }
 
     // Public because they need to be updatable by the stats
     CounterMoveHistory    counterMoves;
@@ -270,7 +270,7 @@ class Worker {
     // Get a pointer to the search manager, only allowed to be called by the
     // main thread.
     SearchManager* main_manager() const {
-        assert(thread_idx == 0);
+        assert(threadIdx == 0);
         return static_cast<SearchManager*>(manager.get());
     }
 
@@ -291,7 +291,7 @@ class Worker {
     Depth     rootDepth, completedDepth;
     Value     rootDelta;
 
-    size_t                    thread_idx;
+    size_t                    threadIdx;
     NumaReplicatedAccessToken numaAccessToken;
 
     // Reductions lookup table initialized at startup

src/thread.cpp

@@ -127,9 +127,7 @@ void Thread::idle_loop() {
     }
 }
 
-Search::SearchManager* ThreadPool::main_manager() {
-    return static_cast<Search::SearchManager*>(main_thread()->worker.get()->manager.get());
-}
+Search::SearchManager* ThreadPool::main_manager() { return main_thread()->worker->main_manager(); }
 
 uint64_t ThreadPool::nodes_searched() const { return accumulate(&Search::Worker::nodes); }
 uint64_t ThreadPool::tb_hits() const { return accumulate(&Search::Worker::tbHits); }