diff --git a/include/fractal.hpp b/include/fractal.hpp
index 05da93d..db0c1fd 100644
--- a/include/fractal.hpp
+++ b/include/fractal.hpp
@@ -168,7 +168,7 @@ public:
 	 * @exception @c mtl::invalid_argument if no groups were supplied, or an invalid group was encountered
 	 * @exception @c mtl::length_error if the maximum number of groups is reached
 	 */
-	weighted_group_id_t add_weighted_group(const etl::ivector<group_id_t>& groups);
+	weighted_group_id_t add_weighted_group(const etl::ivector<etl::pair<group_id_t, uint32_t>>& weights);
 
 	/**
 	 * @brief Checks if the maximum number of branch rules has been reached
diff --git a/src/fractal.cpp b/src/fractal.cpp
index b51b740..f61e44a 100644
--- a/src/fractal.cpp
+++ b/src/fractal.cpp
@@ -42,6 +42,30 @@ group_id_t ruleset_t::add_group_characteristic(const etl::ivector<token_id_t>& t
 	return m_group_characteristics.size() - 1;
 }
 
+weighted_group_id_t ruleset_t::add_weighted_group(const etl::ivector<etl::pair<group_id_t, uint32_t>>& weights) {
+	if (m_weighted_groups.full()) {
+		throw mtl::length_error();
+	}
+	if (weights.size() > g_max_wgroup_weights) {
+		throw mtl::length_error();
+	}
+
+	m_weighted_groups.push_back(weighted_group_t());
+	weighted_group_t& wgroup = m_weighted_groups.back();
+
+	for (const auto& weight : weights) {
+		if (!valid_group_characteristic(weight.first)) {
+			throw mtl::invalid_argument();
+		}
+
+		wgroup.m_groups.push_back(weight.first);
+		wgroup.m_weights.push_back(weight.second);
+		wgroup.m_weight_total += weight.second;
+	}
+
+	return m_weighted_groups.size() - 1;
+}
+
 #if 0
 token_id_t generator_t::add_token(token_type_e type, mtl::fixed value) {
 	log::debug << "Adding token with type " << (int)type << endl;