fuzz-tests: add test for amount-{sat, msat} arithmetic

diff --git a/tests/fuzz/fuzz-amount-arith.c b/tests/fuzz/fuzz-amount-arith.c
new file mode 100644
index 00000000..4fb0b720
--- /dev/null
+++ b/tests/fuzz/fuzz-amount-arith.c
@@ -0,0 +1,237 @@
+#include "config.h"
+#include <assert.h>
+#include <common/amount.h>
+#include <math.h>
+#include <tests/fuzz/libfuzz.h>
+#include <wally_script.h>
+#include <wire/wire.h>
+
+void init(int *argc, char ***argv) {}
+
+#define MAX_SATS ((u64)WALLY_SATOSHI_PER_BTC * WALLY_BTC_MAX)
+#define MAX_MSATS (MAX_SATS * 1000)
+#define MAX_FEE_BASE 10000000
+#define MAX_FEE_PROP 1000000
+
+static struct amount_msat fromwire_amount_msat_bounded(const u8 **cursor, size_t *max)
+{
+	u64 amt = fromwire_u64(cursor, max) % MAX_MSATS;
+	return amount_msat(amt);
+}
+
+enum op {
+	OP_MSAT_ADD,
+	OP_MSAT_SUB,
+	OP_MSAT_MUL,
+	OP_MSAT_DIV,
+	OP_MSAT_RATIO,
+	OP_MSAT_RATIO_FLOOR,
+	OP_MSAT_RATIO_CEIL,
+	OP_MSAT_SCALE,
+	OP_MSAT_ADD_SAT,
+	OP_MSAT_SUB_SAT,
+	OP_SAT_ADD,
+	OP_SAT_SUB,
+	OP_SAT_MUL,
+	OP_SAT_DIV,
+	OP_SAT_SCALE,
+	OP_FEE,
+	OP_ADD_FEE,
+	OP_SUB_FEE,
+	OP_TX_FEE,
+	OP_FEERATE,
+	OP_COUNT
+};
+
+void run(const uint8_t *data, size_t size) {
+	u8 op;
+	u64 u64_param;
+	double f;
+	struct amount_msat a, b, out_ms;
+	struct amount_sat sa, sb, out_s;
+
+	a = fromwire_amount_msat_bounded(&data, &size);
+	b = fromwire_amount_msat_bounded(&data, &size);
+	sa = amount_msat_to_sat_round_down(a);
+	sb = amount_msat_to_sat_round_down(b);
+
+	u64_param = fromwire_u64(&data, &size);
+	op = fromwire_u8(&data, &size) % OP_COUNT;
+
+	if (size < sizeof(f))
+		return;
+	memcpy(&f, data, sizeof(f));
+	data += sizeof(f);
+
+	switch (op) {
+		case OP_MSAT_ADD:
+		{
+			if (amount_msat_add(&out_ms, a, b))
+				assert(out_ms.millisatoshis == a.millisatoshis + b.millisatoshis);
+			break;
+		}
+		case OP_MSAT_SUB:
+		{
+			if (amount_msat_sub(&out_ms, a, b))
+				assert(out_ms.millisatoshis + b.millisatoshis == a.millisatoshis);
+			break;
+		}
+		case OP_MSAT_MUL:
+		{
+			if (amount_msat_mul(&out_ms, a, u64_param))
+				assert(out_ms.millisatoshis == a.millisatoshis * u64_param);
+			break;
+		}
+		case OP_MSAT_DIV:
+		{
+			if (u64_param == 0)
+				break;
+			out_ms = amount_msat_div(a, u64_param);
+			assert(out_ms.millisatoshis == a.millisatoshis / u64_param);
+			break;
+		}
+		case OP_MSAT_RATIO:
+		{
+			if (b.millisatoshis == 0)
+				break;
+			double ratio = amount_msat_ratio(a, b);
+			double expected = (double)a.millisatoshis / b.millisatoshis;
+			assert(ratio == expected);
+			break;
+		}
+		case OP_MSAT_RATIO_FLOOR:
+		{
+			if (b.millisatoshis == 0)
+				break;
+			u64 floor = amount_msat_ratio_floor(a, b);
+			assert(floor == a.millisatoshis / b.millisatoshis);
+			break;
+		}
+		case OP_MSAT_RATIO_CEIL:
+		{
+			if (b.millisatoshis == 0)
+				break;
+			u64 ceil = amount_msat_ratio_ceil(a, b);
+			u64 quotient = a.millisatoshis / b.millisatoshis;
+			u64 remainder = a.millisatoshis % b.millisatoshis;
+
+			assert(ceil == quotient + (remainder != 0));
+			break;
+		}
+		case OP_MSAT_SCALE:
+		{
+			if (amount_msat_scale(&out_ms, a, f)) {
+				double expect = (double)a.millisatoshis * f;
+				assert(fabs((double)out_ms.millisatoshis - expect) < 1.0);
+			}
+			break;
+		}
+		case OP_MSAT_ADD_SAT:
+		{
+			if (amount_msat_add_sat(&out_ms, a, sa))
+				assert(out_ms.millisatoshis == sa.satoshis * MSAT_PER_SAT + a.millisatoshis);
+			break;
+		}
+		case OP_MSAT_SUB_SAT:
+		{
+			if (amount_msat_sub_sat(&out_ms, a, sa))
+				assert(out_ms.millisatoshis + sa.satoshis * MSAT_PER_SAT == a.millisatoshis);
+			break;
+		}
+		case OP_SAT_ADD:
+		{
+			if (amount_sat_add(&out_s, sa, sb))
+				assert(out_s.satoshis == sa.satoshis + sb.satoshis);
+			break;
+		}
+		case OP_SAT_SUB:
+		{
+			if (amount_sat_sub(&out_s, sa, sb))
+				assert(out_s.satoshis + sb.satoshis == sa.satoshis);
+			break;
+		}
+		case OP_SAT_MUL:
+		{
+			if (amount_sat_mul(&out_s, sa, u64_param))
+				assert(out_s.satoshis == sa.satoshis * u64_param);
+			break;
+		}
+		case OP_SAT_DIV:
+		{
+			if (u64_param == 0)
+				break;
+			out_s = amount_sat_div(sa, u64_param);
+			assert(out_s.satoshis == sa.satoshis / u64_param);
+			break;
+		}
+		case OP_SAT_SCALE:
+		{
+			if (amount_sat_scale(&out_s, sa, f)) {
+			double expect = sa.satoshis * f;
+			assert(fabs((double)out_s.satoshis - expect) < 1.0);
+		}
+		break;
+		}
+		case OP_FEE:
+		{
+			if (amount_msat_fee(&out_ms, a, (u32)a.millisatoshis,
+							(u32)b.millisatoshis))
+				assert(out_ms.millisatoshis >= (u32)a.millisatoshis);
+			break;
+		}
+		case OP_ADD_FEE:
+		{
+			u32 fee_base = (u32)a.millisatoshis % MAX_FEE_BASE;
+			u32 fee_prop = (u32)b.millisatoshis % MAX_FEE_PROP;
+			struct amount_msat original = a;
+			struct amount_msat fee;
+
+			if (amount_msat_fee(&fee, original, fee_base, fee_prop)) {
+				struct amount_msat total;
+				assert(amount_msat_add(&total, original, fee));
+				assert(total.millisatoshis == original.millisatoshis + fee.millisatoshis);
+			}
+			break;
+		}
+		case OP_SUB_FEE:
+		{
+			u32 fee_base = (u32)a.millisatoshis % MAX_FEE_BASE;
+			u32 fee_prop = (u32)b.millisatoshis % MAX_FEE_PROP;
+			struct amount_msat input = a;
+			struct amount_msat output = amount_msat_sub_fee(input, fee_base, fee_prop);
+			struct amount_msat fee;
+
+			if (amount_msat_fee(&fee, output, fee_base, fee_prop)) {
+				struct amount_msat sum;
+				if (amount_msat_add(&sum, output, fee))
+					assert(amount_msat_less_eq(sum, input));
+			}
+			break;
+		}
+		case OP_TX_FEE:
+		{
+			u32 fee_per_kw = (u32)a.millisatoshis;
+			/* weights > 2^32 are not real tx and hence, discarded */
+			size_t weight = (size_t)(b.millisatoshis & UINT32_MAX);
+
+			struct amount_sat fee = amount_tx_fee(fee_per_kw, weight);
+			u64 expected = (fee_per_kw * weight) / MSAT_PER_SAT;
+			assert(fee.satoshis == expected);
+			break;
+		}
+		case OP_FEERATE:
+		{
+			struct amount_sat fee = amount_msat_to_sat_round_down(a);
+			size_t weight = (size_t)b.millisatoshis;
+			u32 feerate;
+
+			if (weight && amount_feerate(&feerate, fee, weight)) {
+				u64 expected = (fee.satoshis * MSAT_PER_SAT) / weight;
+				assert(feerate == expected);
+			}
+			break;
+		}
+		default:
+			assert(false && "Unknown operation!");
+	}
+}