lightningd: extract routehint selection code.

We're going to want this for bolt13 formation as well.

As a result of reworking the logic into "candidate selection" then
"route hint selection", we need to change the way round-robin works.
We use a simple incrementing index now.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/routehint.h

@@ -0,0 +1,40 @@
+/* Code for routehints to be inserted into invoices and offers */
+#ifndef LIGHTNING_LIGHTNINGD_ROUTEHINT_H
+#define LIGHTNING_LIGHTNINGD_ROUTEHINT_H
+#include "config.h"
+#include <ccan/short_types/short_types.h>
+#include <ccan/tal/tal.h>
+#include <common/amount.h>
+#include <stdbool.h>
+
+struct lightningd;
+struct short_channel_id;
+
+struct routehint_candidate {
+	struct route_info *r;
+	struct channel *c;
+	struct amount_msat capacity;
+};
+
+/**
+ * routehint_candidates - get possible incoming channels for routehinting.
+ * @ctx: tal context to allocate return off
+ * @ld: lightningd
+ * @incoming_channels_reply: reply from gossipd get_incoming_channels
+ * @expose_all_private: consider private channels too (otherwise iff no public)
+ * @hints: only consider these channels (if !expose_all_private).
+ * @none_public: set to true if we used private channels because none were public.
+ * @deadends: set to true if we found a dead-end channel.
+ * @amount_offline: amount we didn't consider due to offline channels.
+ */
+struct routehint_candidate *
+routehint_candidates(const tal_t *ctx,
+		     struct lightningd *ld,
+		     const u8 *incoming_channels_reply,
+		     bool expose_all_private,
+		     const struct short_channel_id *hints,
+		     bool *none_public,
+		     bool *deadends,
+		     struct amount_msat *amount_offline);
+
+#endif /* LIGHTNING_LIGHTNINGD_ROUTEHINT_H */