askrene: refine: add helper function remove_flows

remove_flows is a helper function to remove flows from a set so that we
keep the number of flows limited.

Signed-off-by: Lagrang3 <[email protected]>

Author: Lagrang3

plugins/askrene/refine.c

@@ -632,3 +632,31 @@ double flows_probability(const tal_t *ctx, struct route_query *rq,
 	tal_free(working_ctx);
 	return probability;
 }
+
+bool remove_flows(const tal_t *ctx, struct flow ***flows, u32 n)
+{
+	const tal_t *working_ctx = tal(ctx, tal_t);
+	size_t *flows_index;
+
+	if (n == 0)
+		goto fail;
+	if (n > tal_count(*flows))
+		goto fail;
+
+	flows_index = tal_arrz(working_ctx, size_t, tal_count(*flows));
+	for (size_t i = 0; i < tal_count(*flows); i++) {
+		flows_index[i] = i;
+	}
+	asort(flows_index, tal_count(flows_index), revcmp_flows, *flows);
+	for (size_t count = tal_count(flows_index); n > 0; n--, count--) {
+		assert(count > 0);
+		tal_arr_remove(&flows_index, count - 1);
+	}
+	write_selected_flows(working_ctx, flows_index, flows);
+
+	tal_free(working_ctx);
+	return true;
+fail:
+	tal_free(working_ctx);
+	return false;
+}

plugins/askrene/refine.h

@@ -33,4 +33,8 @@ void squash_flows(const tal_t *ctx, struct route_query *rq,
 
 double flows_probability(const tal_t *ctx, struct route_query *rq,
 			 struct flow ***flows);
+
+/* Helper function: removes n flows from the set. It will remove those flows
+ * with the lowest amount values. */
+bool remove_flows(const tal_t *ctx, struct flow ***flows, u32 n);
 #endif /* LIGHTNING_PLUGINS_ASKRENE_REFINE_H */