diff --git a/plugins/renepay/mcf.c b/plugins/renepay/mcf.c
index 2e2fdaffc..7d7da870f 100644
--- a/plugins/renepay/mcf.c
+++ b/plugins/renepay/mcf.c
@@ -256,6 +256,55 @@ static const s64 MU_MAX = 128;
  * I hope this will clarify my future self when I forget.
  *
  * */
+
+/*
+ * We want to use the whole number here for convenience, but
+ * we can't us a union, since bit order is implementation-defined and
+ * we want chanidx on the highest bits:
+ *
+ * [ 0       1 2     3            4 5 6 ... 31 ]
+ *   dual    part    chandir      chanidx
+ */
+struct arc {
+	u32 idx;
+};
+
+#define ARC_DUAL_BITOFF (0)
+#define ARC_PART_BITOFF (1)
+#define ARC_CHANDIR_BITOFF (1 + PARTS_BITS)
+#define ARC_CHANIDX_BITOFF (1 + PARTS_BITS + 1)
+#define ARC_CHANIDX_BITS  (32 - ARC_CHANIDX_BITOFF)
+
+static inline void arc_to_parts(struct arc arc,
+				u32 *chanidx,
+				int *chandir,
+				u32 *part,
+				bool *dual)
+{
+	if (chanidx)
+		*chanidx = (arc.idx >> ARC_CHANIDX_BITOFF);
+	if (chandir)
+		*chandir = (arc.idx >> ARC_CHANDIR_BITOFF) & 1;
+	if (part)
+		*part = (arc.idx >> ARC_PART_BITOFF) & ((1 << PARTS_BITS)-1);
+	if (dual)
+		*dual = (arc.idx >> ARC_DUAL_BITOFF) & 1;
+}
+
+static inline struct arc arc_from_parts(u32 chanidx, int chandir, u32 part, bool dual)
+{
+	struct arc arc;
+
+	assert(part < CHANNEL_PARTS);
+	assert(chandir == 0 || chandir == 1);
+	assert(chanidx < (1U << ARC_CHANIDX_BITS));
+	arc.idx = ((u32)dual << ARC_DUAL_BITOFF)
+		| (part << ARC_PART_BITOFF)
+		| ((u32)chandir << ARC_CHANDIR_BITOFF)
+		| (chanidx << ARC_CHANIDX_BITOFF);
+	return arc;
+}
+
 typedef union
 {
 	struct{
diff --git a/plugins/renepay/test/run-arc.c b/plugins/renepay/test/run-arc.c
new file mode 100644
index 000000000..09d2f675f
--- /dev/null
+++ b/plugins/renepay/test/run-arc.c
@@ -0,0 +1,114 @@
+#include "config.h"
+#include <stdio.h>
+#include <assert.h>
+#include <common/wireaddr.h>
+#include <common/bigsize.h>
+#include <common/channel_id.h>
+#include <common/setup.h>
+#include <common/utils.h>
+#include <common/node_id.h>
+#include <ccan/read_write_all/read_write_all.h>
+
+#include "../mcf.c"
+
+/* update-mocks isn't quiet smart enough for this, so place here */
+struct pay_plugin *const pay_plugin;
+
+/* AUTOGENERATED MOCKS START */
+/* Generated stub for flow_complete */
+void flow_complete(struct flow *flow UNNEEDED,
+		   const struct gossmap *gossmap UNNEEDED,
+		   struct chan_extra_map *chan_extra_map UNNEEDED,
+		   struct amount_msat delivered UNNEEDED)
+{ fprintf(stderr, "flow_complete called!\n"); abort(); }
+/* Generated stub for flow_set_fee */
+struct amount_msat flow_set_fee(struct flow **flows UNNEEDED)
+{ fprintf(stderr, "flow_set_fee called!\n"); abort(); }
+/* Generated stub for flow_set_probability */
+double flow_set_probability(
+		struct flow ** flows UNNEEDED,
+		const struct gossmap *const gossmap UNNEEDED,
+		struct chan_extra_map * chan_extra_map UNNEEDED)
+{ fprintf(stderr, "flow_set_probability called!\n"); abort(); }
+/* Generated stub for get_chan_extra_half_by_chan */
+struct chan_extra_half *get_chan_extra_half_by_chan(const struct gossmap *gossmap UNNEEDED,
+						    struct chan_extra_map *chan_extra_map UNNEEDED,
+						    const struct gossmap_chan *chan UNNEEDED,
+						    int dir UNNEEDED)
+{ fprintf(stderr, "get_chan_extra_half_by_chan called!\n"); abort(); }
+/* Generated stub for linear_fee_cost */
+s64 linear_fee_cost(
+		const struct gossmap_chan *c UNNEEDED,
+		const int dir UNNEEDED,
+		double base_fee_penalty UNNEEDED,
+		double delay_feefactor UNNEEDED)
+{ fprintf(stderr, "linear_fee_cost called!\n"); abort(); }
+/* AUTOGENERATED MOCKS END */
+
+int main(int argc, char *argv[])
+{
+	bool dual;
+	u32 part;
+	int chandir;
+	u32 chanidx;
+
+	common_setup(argv[0]);
+
+	for (int i = 0; i < 32; i++) {
+		for (int j = 0; j < 32; j++) {
+			for (int k = 0; k < 32; k++) {
+				struct arc a, a2;
+
+				a.idx = (1U << i) | (1U << j) | (1U << k);
+				arc_to_parts(a, &chanidx, &chandir, &part, &dual);
+				a2 = arc_from_parts(chanidx, chandir, part, dual);
+				assert(a.idx == a2.idx);
+			}
+		}
+	}
+
+	/* Test all chanidx */
+	for (int i = 0; i < (1U << ARC_CHANIDX_BITS); i++) {
+		struct arc a = arc_from_parts(i, chandir, part, dual);
+
+		arc_to_parts(a, &chanidx, NULL, NULL, NULL);
+		assert(chanidx == i);
+	}
+
+	/* Test both chandir */
+	for (int i = 0; i < 2; i++) {
+		struct arc a = arc_from_parts(chanidx, i, part, dual);
+
+		arc_to_parts(a, NULL, &chandir, NULL, NULL);
+		assert(chandir == i);
+	}
+
+	/* Test all parts */
+	for (int i = 0; i < CHANNEL_PARTS; i++) {
+		struct arc a = arc_from_parts(chanidx, chandir, i, dual);
+
+		arc_to_parts(a, NULL, NULL, &part, NULL);
+		assert(part == i);
+	}
+
+	/* Test both dual */
+	for (int i = 0; i < 2; i++) {
+		struct arc a = arc_from_parts(chanidx, chandir, part, i);
+
+		arc_to_parts(a, NULL, NULL, NULL, &dual);
+		assert(dual == i);
+
+/* This code not converted yet! */
+#if 0
+		assert(arc_is_dual(a) == dual);
+
+		a = arc_dual(a);
+		arc_to_parts(a, NULL, NULL, NULL, &dual);
+		assert(dual == !i);
+		assert(arc_is_dual(a) == dual);
+#endif
+	}
+
+	common_shutdown();
+}
+