3 files changed, 108 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3d472c1..4e46b70 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,6 +22,7 @@ set(SOURCE_DIR ./src)
 set(CLIENT_SOURCE_DIR ./src/client)
 set(SOURCES
     ${SOURCE_DIR}/event.cpp
+    ${SOURCE_DIR}/filters.cpp
     ${SOURCE_DIR}/nostr_service.cpp
     ${CLIENT_SOURCE_DIR}/websocketpp_client.cpp
 )
diff --git a/include/nostr.hpp b/include/nostr.hpp
index ec8d1a8..ce25446 100644
--- a/include/nostr.hpp
+++ b/include/nostr.hpp
@@ -15,6 +15,7 @@
 namespace nostr
 {
 typedef std::vector<std::string> RelayList;
+typedef std::unordered_map<std::string, std::vector<std::string>> TagMap;
 
 // TODO: Add null checking to seralization and deserialization methods.
 /**
@@ -55,6 +56,38 @@ private:
     void validate();
 };
 
+/**
+ * @brief A set of filters for querying Nostr relays.
+ * @remark The `limit` field should always be included to keep the response size reasonable.  The
+ * `since` field is not required, and the `until` field will default to the present.  At least one
+ * of the other fields must be set for a valid filter.
+ */
+struct Filters
+{
+    std::vector<std::string> ids; ///< Event IDs.
+    std::vector<std::string> authors; ///< Event author npubs.
+    std::vector<int> kinds; ///< Kind numbers.
+    TagMap tags; ///< Tag names mapped to lists of tag values.
+    std::time_t since; ///< Unix timestamp.  Matching events must be newer than this.
+    std::time_t until; ///< Unix timestamp.  Matching events must be older than this.
+    int limit; ///< The maximum number of events the relay should return on the initial query.
+
+    /**
+     * @brief Serializes the filters to a JSON object.
+     * @returns A stringified JSON object representing the filters.
+     * @throws `std::invalid_argument` if the filter object is invalid.
+     */
+    std::string serialize();
+
+private:
+    /**
+     * @brief Validates the filters.
+     * @throws `std::invalid_argument` if the filter object is invalid.
+     * @remark The `until` field defaults to the present if it is not already set.
+     */
+    void validate();
+};
+
 class NostrService
 {
 public:
diff --git a/src/filters.cpp b/src/filters.cpp
new file mode 100644
index 0000000..78f3ce4
--- /dev/null
+++ b/src/filters.cpp
@@ -0,0 +1,74 @@
+#include <ctime>
+#include <sstream>
+#include <string>
+#include <nlohmann/json.hpp>
+
+#include "nostr.hpp"
+
+using nlohmann::json;
+using std::invalid_argument;
+using std::stringstream;
+using std::string;
+using std::time;
+
+namespace nostr
+{
+string Filters::serialize()
+{
+    try
+    {
+        this->validate();
+    } 
+    catch (const invalid_argument& e)
+    {
+        throw e;
+    }
+
+    json j = {
+        {"ids", this->ids},
+        {"authors", this->authors},
+        {"kinds", this->kinds},
+        {"since", this->since},
+        {"until", this->until},
+        {"limit", this->limit}
+    };
+    
+    for (auto& tag : this->tags)
+    {
+        stringstream jss;
+        jss << "#" << tag.first;
+        string js = jss.str();
+
+        j[js] = tag.second;
+    }
+
+    return j.dump();
+};
+
+void Filters::validate()
+{
+    bool hasLimit = this->limit > 0;
+    if (!hasLimit)
+    {
+        throw invalid_argument("Filters::validate: The limit must be greater than 0.");
+    }
+
+    bool hasUntil = this->until > 0;
+    if (!hasUntil)
+    {
+        this->until = time(nullptr);
+    }
+
+    bool hasIds = this->ids.size() > 0;
+    bool hasAuthors = this->authors.size() > 0;
+    bool hasKinds = this->kinds.size() > 0;
+    bool hasTags = this->tags.size() > 0;
+
+    bool hasFilter = hasIds || hasAuthors || hasKinds || hasTags;
+
+    if (!hasFilter)
+    {
+        throw invalid_argument("Filters::validate: At least one filter must be set.");
+    }
+};
+} // namespace nostr