Fix systemed#750: allow no more than 512 attribute names

This fixes two issues:

- use an unsigned type, so we can use the whole 9 bits and have 512
  keys, not 256
- fix the bounds check in AttributeKeyStore to reflex the lower
  threshold that was introduced in systemed#618

Hat tip @oobayly for reporting this.

Fixes systemed#750.

include/attribute_store.h

@@ -46,7 +46,7 @@ enum class AttributePairType: uint8_t { String = 0, Float = 1, Bool = 2 };
 // AttributePair is a key/value pair (with minzoom)
 #pragma pack(push, 1)
 struct AttributePair {
-	short keyIndex : 9;
+	unsigned short keyIndex : 9;
 	AttributePairType valueType : 2;
 	uint8_t minzoom : 5; // Support zooms from 0..31. In practice, we expect z16 to be the biggest minzoom.
 	union {

src/attribute_store.cpp

@@ -35,8 +35,8 @@ uint16_t AttributeKeyStore::key2index(const std::string& key) {
 	uint16_t newIndex = keys.size();
 
 	// This is very unlikely. We expect more like 50-100 keys.
-	if (newIndex >= 65535)
-		throw std::out_of_range("more than 65,536 unique keys");
+	if (newIndex >= 512)
+		throw std::out_of_range("more than 512 unique keys");
 
 	keys2indexSize++;
 	keys.push_back(key);

test/attribute_store.test.cpp

@@ -89,13 +89,38 @@ MU_TEST(test_attribute_store_reuses) {
 
 		mu_check(s1aIndex == s1bIndex);
 	}
+}
+
+MU_TEST(test_attribute_store_capacity) {
+	// We support a maximum of 511 attribute name, so confirm that we can roundtrip
+	// this value.
+	AttributePair pair(511, true, 0);
+	mu_check(pair.keyIndex == 511);
 
+	// Confirm that the attribute store will throw if we try to add more than 511 keys.
+	AttributeKeyStore keys;
+
+	for (int i = 1; i <= 511; i++) {
+		const uint16_t keyIndex = keys.key2index("key" + std::to_string(i));
+		mu_check(keyIndex == i);
+	}
+
+	// Trying to add a 512th key should throw
+	bool caughtException = false;
+
+	try {
+		keys.key2index("key512");
+	} catch (std::out_of_range) {
+		caughtException = true;
+	}
 
+	mu_check(caughtException == true);
 }
 
 MU_TEST_SUITE(test_suite_attribute_store) {
 	MU_RUN_TEST(test_attribute_store);
 	MU_RUN_TEST(test_attribute_store_reuses);
+	MU_RUN_TEST(test_attribute_store_capacity);
 }
 
 int main() {