New duplicate filter sink

include/spdlog/sinks/dup_filter_sink.h

+// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+
+#pragma once
+
+#include "dist_sink.h"
+#include "spdlog/details/null_mutex.h"
+#include "spdlog/details/log_msg.h"
+
+#include <string>
+#include <chrono>
+
+// Duplicate remove sink.
+// Filter the message if previous one is identical and less than max max_skip_duration have passed
+//
+// Example:
+//     auto d = std::make_shared<dup_filter_sink_st>(std::chrono::seconds(5));
+//     d->add_sink(std::make_shared<stdout_color_sink_mt>());
+//     spdlog::logger l("logger", d);
+//     l.info("Hello");
+//     l.info("Hello");
+//     l.info("Hello");
+//     l.info("Different Hello");
+//
+// Will produce:
+//       [2019-06-25 17:50:56.511] [logger] [info] Hello
+//       [2019-06-25 17:50:56.512] [logger] [info] 3 duplicate messages..
+//       [2019-06-25 17:50:56.512] [logger] [info] Different Hello
+
+
+
+namespace spdlog {
+namespace sinks {
+template<typename Mutex>
+class dup_filter_sink : public dist_sink<Mutex>
+{
+public:
+    template<class Rep, class Period>
+    explicit dup_filter_sink(std::chrono::duration<Rep, Period> max_ignore_duration)
+        : max_skip_duration_{max_ignore_duration}
+        , last_msg_time_{log_clock::now()}
+        , skip_counter_{0}
+    {}
+
+protected:
+    std::chrono::microseconds max_skip_duration_;
+    log_clock::time_point last_msg_time_;
+    size_t skip_counter_;
+    std::string last_msg_payload_;
+
+    void sink_it_(const details::log_msg &msg) override
+    {
+        auto msg_time = msg.time;
+        auto delta_time = msg_time - last_msg_time_;
+        last_msg_time_ = msg_time;
+
+        if (delta_time < max_skip_duration_ && last_msg_payload_ == msg.payload)
+        {
+            skip_counter_++;
+            return;
+        }
+
+        // got different message from the previous
+        if (skip_counter_ > 0)
+        {
+            fmt::basic_memory_buffer<char, 80> buf;
+            fmt::format_to(buf, "{} duplicate messages..", skip_counter_);
+            details::log_msg skipped_msg{msg.logger_name, msg.level, string_view_t{buf.data(), buf.size()}};
+
+            // log the skip notification
+            for (auto &sink : dist_sink<Mutex>::sinks_)
+            {
+                if (sink->should_log(msg.level))
+                {
+                    sink->log(skipped_msg);
+                }
+            }
+            skip_counter_ = 0;
+        }
+
+        last_msg_payload_.assign(msg.payload.data(), msg.payload.data() + msg.payload.size());
+        dist_sink<Mutex>::sink_it_(msg);
+    }
+};
+
+using dup_filter_sink_mt = dup_filter_sink<std::mutex>;
+using dup_filter_sink_st = dup_filter_sink<details::null_mutex>;
+} // namespace sinks
+} // namespace spdlog