Implement FNV-1a hash extension for String and update key hashing in BinaryTable

dart/lib/binary_table.dart

@@ -17,6 +17,7 @@ This file is part of the Binary Table package for Dart.
 
  */
 
+import 'dart:convert';
 import 'dart:io';
 import 'dart:typed_data';
 
@@ -453,6 +454,22 @@ extension FreeList on List<BT_FreeListEntry> {
 
 }
 
+extension fnv1a on String {
+
+  int get bt_hash {
+    List<int> bytes = utf8.encode(this);
+    int hash = 0xcbf29ce484222325; // FNV offset basis
+
+    for (int byte in bytes) {
+      hash ^= byte;
+      hash *= 0x100000001b3; // FNV prime
+    }
+
+    return hash;
+  }
+
+}
+
 class BinaryTable {
 
   RandomAccessFile _file;
@@ -732,7 +749,7 @@ class BinaryTable {
     antiFreeListScope(() {
       Map<int, BT_Pointer> addressTable = _addressTable;
 
-      int keyHash = key.hashCode;
+      int keyHash = key.bt_hash;
 
       if (addressTable.containsKey(keyHash)) {
         throw Exception('Key already exists');
@@ -757,7 +774,7 @@ class BinaryTable {
   operator [](String key) {
     Map<int, BT_Pointer> addressTable = _addressTable;
 
-    int keyHash = key.hashCode;
+    int keyHash = key.bt_hash;
 
     if (!addressTable.containsKey(keyHash)) {
       throw Exception('Key does not exist');
@@ -774,7 +791,7 @@ class BinaryTable {
     antiFreeListScope(() {
       Map<int, BT_Pointer> addressTable = _addressTable;
 
-      int keyHash = key.hashCode;
+      int keyHash = key.bt_hash;
 
       if (!addressTable.containsKey(keyHash)) {
         throw Exception('Key does not exist');