common: hoist amount+currency parsing into common code.

diff --git a/common/iso4217.c b/common/iso4217.c
index a164e91c..85f5898e 100644
--- a/common/iso4217.c
+++ b/common/iso4217.c
@@ -1,7 +1,11 @@
 #include "config.h"
 #include <ccan/array_size/array_size.h>
 #include <ccan/mem/mem.h>
+#include <ccan/tal/str/str.h>
+#include <common/amount.h>
 #include <common/iso4217.h>
+#include <common/overflows.h>
+#include <common/utils.h>
 
 /* Wikipedia leads me to: https://www.currency-iso.org/en/home/tables/table-a1.html
 
@@ -202,3 +206,86 @@ const struct iso4217_name_and_divisor *find_iso4217(const utf8 *prefix,
 	}
 	return NULL;
 }
+
+static bool msat_or_any(const tal_t *ctx,
+			const char *buf,
+			size_t buflen,
+			u64 **amount)
+{
+	struct amount_msat msat;
+
+	if (memeqstr(buf, buflen, "any")) {
+		*amount = NULL;
+		return true;
+	}
+
+	if (!parse_amount_msat(&msat, buf, buflen))
+		return false;
+
+	*amount = tal_dup(ctx, u64, &msat.millisatoshis); /* Raw: parsing */
+	return true;
+}
+
+const char *parse_currency_amount(const tal_t *ctx,
+				  const char *buf,
+				  size_t buflen,
+				  const struct iso4217_name_and_divisor **isocode,
+				  u64 **amount)
+{
+	const char *dot;
+	size_t wholelen;
+	u64 cents;
+	u64 total;
+
+	if (msat_or_any(ctx, buf, buflen, amount)) {
+		*isocode = NULL;
+		return NULL;
+	}
+
+	/* BOLT #12:
+	 *
+	 * - MUST specify `offer_currency` `iso4217` as an ISO 4217 three-letter code.
+	 * - MUST specify `offer_amount` in the currency unit adjusted by the ISO 4217
+	 *   exponent (e.g. USD cents).
+	 */
+	if (buflen < ISO4217_NAMELEN)
+		return tal_fmt(ctx, "Not a number, and too short for currency");
+
+	*isocode = find_iso4217(buf + buflen - ISO4217_NAMELEN, ISO4217_NAMELEN);
+	if (!*isocode)
+		return tal_fmt(ctx, "Unknown currency suffix %.*s",
+			       ISO4217_NAMELEN,
+			       buf + buflen - ISO4217_NAMELEN);
+
+	buflen -= ISO4217_NAMELEN;
+	dot = memchr(buf, '.', buflen);
+	if (!dot) {
+		wholelen = buflen;
+		cents = 0;
+	} else {
+		const char *afterdot = dot + 1;
+		size_t partlen = buf + buflen - afterdot;
+		wholelen = dot - buf;
+		if (partlen != (*isocode)->minor_unit)
+			return tal_fmt(ctx, "Currency %s requires %u minor units",
+				       (*isocode)->name, (*isocode)->minor_unit);
+		if (!str_to_u64(afterdot, partlen, &cents))
+			return tal_fmt(ctx, "Bad minor units number");
+	}
+
+	if (!str_to_u64(buf, wholelen, &total))
+		return tal_fmt(ctx, "Not a valid number");
+
+	for (size_t i = 0; i < (*isocode)->minor_unit; i++) {
+		if (mul_overflows_u64(total, 10))
+			return tal_fmt(ctx, "excessively large value");
+		total *= 10;
+	}
+
+	if (add_overflows_u64(total, cents))
+		return tal_fmt(ctx, "excessively large value");
+
+	total += cents;
+	*amount = tal_dup(ctx, u64, &total);
+	return NULL;
+}
diff --git a/common/iso4217.h b/common/iso4217.h
index e57c5baa..a77a6d9f 100644
--- a/common/iso4217.h
+++ b/common/iso4217.h
@@ -18,4 +18,17 @@ struct iso4217_name_and_divisor {
 
 const struct iso4217_name_and_divisor *find_iso4217(const utf8 *prefix,
 						    size_t len);
+
+/**
+ * parse_currency_amount - convert msat amount, any, or currency amount.
+ *
+ * Returns error message or NULL.  On success:
+ * if *iso4217 == NULL: currency is BTC.  If *amount == NULL, "any".
+ * if *iso4217 != NULL: *amount is amount in cents in that currency, never NULL.
+ */
+const char *parse_currency_amount(const tal_t *ctx,
+				  const char *buf,
+				  size_t buflen,
+				  const struct iso4217_name_and_divisor **iso4217,
+				  u64 **amount);
 #endif /* LIGHTNING_COMMON_ISO4217_H */
diff --git a/plugins/offers_offer.c b/plugins/offers_offer.c
index 59570e26..77c3cf9b 100644
--- a/plugins/offers_offer.c
+++ b/plugins/offers_offer.c
@@ -13,87 +13,27 @@
 #include <plugins/offers.h>
 #include <plugins/offers_offer.h>
 
-static bool msat_or_any(const char *buffer,
-			const jsmntok_t *tok,
-			struct tlv_offer *offer)
-{
-	struct amount_msat msat;
-	if (json_tok_streq(buffer, tok, "any"))
-		return true;
-
-	if (!parse_amount_msat(&msat,
-			       buffer + tok->start, tok->end - tok->start))
-		return false;
-
-	offer->offer_amount = tal_dup(offer, u64,
-				&msat.millisatoshis); /* Raw: other currencies */
-	return true;
-}
-
 static struct command_result *param_amount(struct command *cmd,
 					   const char *name,
 					   const char *buffer,
 					   const jsmntok_t *tok,
 					   struct tlv_offer *offer)
 {
+	const char *err;
 	const struct iso4217_name_and_divisor *isocode;
-	jsmntok_t number, whole, frac;
-	u64 cents;
-
-	if (msat_or_any(buffer, tok, offer))
-		return NULL;
 
-	offer->offer_amount = tal(offer, u64);
-
-	/* BOLT #12:
-	 *
-	 * - MUST specify `offer_currency` `iso4217` as an ISO 4217 three-letter code.
-	 * - MUST specify `offer_amount` in the currency unit adjusted by the ISO 4217
-	 *   exponent (e.g. USD cents).
-	 */
-	if (tok->end - tok->start < ISO4217_NAMELEN)
-		return command_fail_badparam(cmd, name, buffer, tok,
-					     "should be 'any', msatoshis or <amount>[.<amount>]<ISO-4217>");
-
-	isocode = find_iso4217(buffer + tok->end - ISO4217_NAMELEN, ISO4217_NAMELEN);
-	if (!isocode)
-		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-				    "Unknown currency suffix %.*s",
-				    ISO4217_NAMELEN,
-				    buffer + tok->end - ISO4217_NAMELEN);
-
-	offer->offer_currency
-		= tal_dup_arr(offer, utf8, isocode->name, ISO4217_NAMELEN, 0);
-
-	number = *tok;
-	number.end -= ISO4217_NAMELEN;
-	if (!split_tok(buffer, &number, '.', &whole, &frac)) {
-		whole = number;
-		cents = 0;
-	} else {
-		if (frac.end - frac.start != isocode->minor_unit)
-			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-					    "Currency %s requires %u minor units",
-					    isocode->name, isocode->minor_unit);
-		if (!json_to_u64(buffer, &frac, &cents))
-			return command_fail_badparam(cmd, name, buffer,
-						     &number,
-						     "Bad minor units");
+	err = parse_currency_amount(offer,
+				    buffer + tok->start,
+				    tok->end - tok->start,
+				    &isocode,
+				    &offer->offer_amount);
+	if (err)
+		return command_fail_badparam(cmd, name, buffer, tok, err);
+
+	if (isocode) {
+		offer->offer_currency
+			= tal_dup_arr(offer, utf8, isocode->name, ISO4217_NAMELEN, 0);
 	}
-
-	if (!json_to_u64(buffer, &whole, offer->offer_amount))
-		return command_fail_badparam(cmd, name, buffer, tok,
-					     "should be 'any', msatoshis or <ISO-4217><amount>[.<amount>]");
-
-	for (size_t i = 0; i < isocode->minor_unit; i++) {
-		if (mul_overflows_u64(*offer->offer_amount, 10))
-			return command_fail_badparam(cmd, name, buffer,
-						     &whole,
-						     "excessively large value");
-		*offer->offer_amount *= 10;
-	}
-
-	*offer->offer_amount += cents;
 	return NULL;
 }