make DZG workaround more specific

In volkszaehler#132 and volkszaehler/vzlogger#533 it was reported that there
are DZG meters that fall into the range currently deemed broken
(all DZG meters with serial numbers before 6000 0000), but aren't
actually broken. The reason for this seemed to be that they're a
different type than mine that was broken.

Further inquiry with DZG revealed that indeed only DVS74 meters
are affected by the encoding issue, and then _those_ only for
serial numbers before 6000 0000. They also said that (currently)
the serial number ranges
 - 4200 0000 to 4899 9999, and
 - 5500 0000 to 5899 9999
are reserved for DVS74 meters.

As such, make the workaround detection more specific, and we no
longer need to have the check for 6000 0000 either, just need to
have a list of affected ranges. To achieve that, refactor the
code into a separate function that actually decodes the serial
number to an integer, so we can make the comparison more easily
and have the list of affected numbers more clearly in the code.

This should fix volkszaehler#132 since the serial number there starts with
4005, so it's clearly not in the affected ranges, as explained
by my contact at DZG. For volkszaehler/vzlogger#533 I don't have
the serial number, but it's the same type of meter as in volkszaehler#132.

sml/src/sml_list.c

@@ -112,16 +112,64 @@ sml_list *sml_list_init() {
 
 void sml_list_add(sml_list *list, sml_list *new_entry) { list->next = new_entry; }
 
+/* Note: this gets invoked for _every_ object seen */
+static bool detect_buggy_dzg_dvs74(sml_list *l)
+{
+#define DZG_SERIAL_LENGTH	10
+	static const unsigned char dzg_serial_oid[] = {1, 0, 96, 1, 0, 255};
+	static const unsigned char dzg_serial_start[] = { 0x0a, 0x01, 'D', 'Z', 'G' };
+	static const struct {
+		uint64_t start, end;
+	} broken_dvs74_ranges[] = {
+		{ .start = 42000000, .end = 48999999 },
+		{ .start = 55000000, .end = 58999999 },
+	};
+#define NUM_BROKEN_RANGES (sizeof(broken_dvs74_ranges)/sizeof(broken_dvs74_ranges[0]))
+	const unsigned char *val;
+	uint64_t serial = 0;
+	unsigned int i;
+
+	/* check that object ID is for the serial number */
+	if (!l->obj_name || l->obj_name->len != sizeof(dzg_serial_oid) ||
+	    memcmp(l->obj_name->str, dzg_serial_oid, sizeof(dzg_serial_oid)))
+		return false;
+
+	/* check that it's an octet string of the right length */
+	if (!l->value || l->value->type != SML_TYPE_OCTET_STRING ||
+	    l->value->data.bytes->len != DZG_SERIAL_LENGTH)
+		return false;
+
+	val = l->value->data.bytes->str;
+
+	/* check that it's a DZG meter at all */
+	if (memcmp(val, dzg_serial_start, sizeof(dzg_serial_start)))
+		return false;
+
+	/* convert remainder of serial number to integer */
+	for (i = sizeof(dzg_serial_start); i < DZG_SERIAL_LENGTH; i++) {
+		serial <<= 8;
+		serial |= val[i];
+	}
+
+	/*
+	 * Check that it's in one of the affected ranges,
+	 * which means it's a DVS74 with the bug
+	 * (DVS74 with serial >= 60000000 have the bug fixed)
+	 */
+	for (i = 0; i < NUM_BROKEN_RANGES; i++) {
+		if (serial >= broken_dvs74_ranges[i].start &&
+		    serial <= broken_dvs74_ranges[i].end)
+			return true;
+	}
+
+	return false;
+}
+
 struct workarounds {
-	unsigned int old_dzg_meter : 1;
+	unsigned int old_dzg_dvs74 : 1;
 };
 
 sml_list *sml_list_entry_parse(sml_buffer *buf, struct workarounds *workarounds) {
-	static const unsigned char dzg_serial_name[] = {1, 0, 96, 1, 0, 255};
-	static const unsigned char dzg_serial_start[] = {0x0a, 0x01, 'D', 'Z', 'G', 0x00};
-	// this is "1 DZG 00 60000000" in the hex encoding, see comment below
-	static const unsigned char dzg_serial_fixed[] =
-		{0x0a, 0x01, 'D', 'Z', 'G', 0x00, 0x03, 0x93, 0x87, 0x00};
 	static const unsigned char dzg_power_name[] = {1, 0, 16, 7, 0, 255};
 	u8 value_tl, value_len_more;
 	sml_list *l = NULL;
@@ -172,8 +220,9 @@ sml_list *sml_list_entry_parse(sml_buffer *buf, struct workarounds *workarounds)
 		goto error;
 
 	/*
-	 * Work around DZG meters before serial numbers starting with
-	 * 6 (in decimal) - they encode the consumption wrongly:
+	 * Work around DZG DVS74 meters in the serial number ranges listed
+	 * in detect_buggy_dzg_dvs74() above - they encode the consumption
+	 * incorrectly:
 	 * The value uses a scaler of -2, so e.g. 328.05 should be
 	 * encoded as an unsigned int with 2 bytes (called Unsigned16 in the standard):
 	 *   63 80 25 (0x8025 == 32805 corresponds to 328.05W)
@@ -197,14 +246,9 @@ sml_list *sml_list_entry_parse(sml_buffer *buf, struct workarounds *workarounds)
 	 * Note that this will NOT work if a meter outputs negative
 	 * values compressed as well - but mine doesn't.
 	 */
-	if (l->obj_name && l->obj_name->len == sizeof(dzg_serial_name) &&
-		memcmp(l->obj_name->str, dzg_serial_name, sizeof(dzg_serial_name)) == 0 && l->value &&
-		l->value->type == SML_TYPE_OCTET_STRING &&
-		l->value->data.bytes->len >= (int)sizeof(dzg_serial_start) &&
-		memcmp(l->value->data.bytes->str, dzg_serial_start, sizeof(dzg_serial_start)) == 0 &&
-		memcmp(l->value->data.bytes->str, dzg_serial_fixed, sizeof(dzg_serial_fixed)) < 0) {
-		workarounds->old_dzg_meter = 1;
-	} else if (workarounds->old_dzg_meter && l->obj_name &&
+	if (detect_buggy_dzg_dvs74(l)) {
+		workarounds->old_dzg_dvs74 = 1;
+	} else if (workarounds->old_dzg_dvs74 && l->obj_name &&
 			   l->obj_name->len == sizeof(dzg_power_name) &&
 			   memcmp(l->obj_name->str, dzg_power_name, sizeof(dzg_power_name)) == 0 && l->value &&
 			   (value_len_more == 1 || value_len_more == 2 || value_len_more == 3)) {