Convert BigQuery field validation from a regex to hand-written code

This is entirely for the sake of performance. Benchmark results on my machine:

netcoreapp2.0:
- Short: 180ns => 9ns
- Long: 939ns => 96ns

netcoreapp3.1:
- Short: 159ns => 8ns
- Long: 887ns => 79ns

net461:
- Short: 197ns => 8ns
- Long: 1425ns => 86ns

Fixes #4975

apis/Google.Cloud.BigQuery.V2/Google.Cloud.BigQuery.V2.Benchmarks/Google.Cloud.BigQuery.V2.Benchmarks.csproj

@@ -1,16 +1,15 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.0;net461</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netcoreapp2.0</TargetFrameworks>
+    <TargetFrameworks>netcoreapp2.0;netcoreapp3.1;net461</TargetFrameworks>
     <OutputType>Exe</OutputType>
     <IsPackable>False</IsPackable>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <LangVersion>7.2</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.10.14" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.12.1" />
     <ProjectReference Include="../Google.Cloud.BigQuery.V2/Google.Cloud.BigQuery.V2.csproj" />
   </ItemGroup>
 

apis/Google.Cloud.BigQuery.V2/Google.Cloud.BigQuery.V2.Benchmarks/TableSchemaBuilderBenchmark.cs

@@ -0,0 +1,30 @@
+// Copyright 2020 Google LLC
+// 
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// 
+//     https://www.apache.org/licenses/LICENSE-2.0
+// 
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using BenchmarkDotNet.Attributes;
+
+namespace Google.Cloud.BigQuery.V2.Benchmarks
+{
+    public class TableSchemaBuilderBenchmark
+    {
+        private const string ShortFieldName = "abc";
+        private const string LongFieldName = "abcdefghijklmnopqrstuvwxyz1234567890_ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+        [Benchmark]
+        public void ValidateFieldName_Short() => TableSchemaBuilder.ValidateFieldName(ShortFieldName, "param");
+
+        [Benchmark]
+        public void ValidateFieldName_Long() => TableSchemaBuilder.ValidateFieldName(LongFieldName, "param");
+    }
+}

apis/Google.Cloud.BigQuery.V2/Google.Cloud.BigQuery.V2/AssemblyInfo.cs

@@ -13,4 +13,5 @@
 // limitations under the License.
 using System.Runtime.CompilerServices;
 
-[assembly: InternalsVisibleTo("Google.Cloud.BigQuery.V2.Tests,PublicKey=0024000004800000940000000602000000240000525341310004000001000100afab79952ee22215f12b4e09337e65509c943fbc22d7006bc371d581d0f0ebf0da5d8039aab2607fb68a138a5d80a71bc02b7ebf586dbe1f2493c0ab20423ababfd15ce74d2264a6b37745f3658f016abaad662182aaef634a60f1346fcc45343acab5b6781535a3134818e13fac895a6c106c0480e34bbb06cb123e5583d8d2")]
+[assembly: InternalsVisibleTo("Google.Cloud.BigQuery.V2.Tests,PublicKey=0024000004800000940000000602000000240000525341310004000001000100afab79952ee22215f12b4e09337e65509c943fbc22d7006bc371d581d0f0ebf0da5d8039aab2607fb68a138a5d80a71bc02b7ebf586dbe1f2493c0ab20423ababfd15ce74d2264a6b37745f3658f016abaad662182aaef634a60f1346fcc45343acab5b6781535a3134818e13fac895a6c106c0480e34bbb06cb123e5583d8d2")]
+[assembly: InternalsVisibleTo("Google.Cloud.BigQuery.V2.Benchmarks,PublicKey=0024000004800000940000000602000000240000525341310004000001000100afab79952ee22215f12b4e09337e65509c943fbc22d7006bc371d581d0f0ebf0da5d8039aab2607fb68a138a5d80a71bc02b7ebf586dbe1f2493c0ab20423ababfd15ce74d2264a6b37745f3658f016abaad662182aaef634a60f1346fcc45343acab5b6781535a3134818e13fac895a6c106c0480e34bbb06cb123e5583d8d2")]

apis/Google.Cloud.BigQuery.V2/Google.Cloud.BigQuery.V2/BigQueryNumeric.cs

@@ -36,6 +36,7 @@ public struct BigQueryNumeric : IEquatable<BigQueryNumeric>, IComparable<BigQuer
 
         private static readonly BigInteger s_integerScalingFactor = new BigInteger(1_000_000_000L);
         // TODO: Don't require a 0 before the decimal point.
+        // TODO: Replace with manual validation if we find this is a performance bottleneck (as it was with field name validation).
         private static readonly Regex s_validation = new Regex(@"^-?[0-9]+\.?[0-9]*$");
 
         // Note: the following properties must be declared *after* s_maxValue and s_minValue. Initialization order matters.

apis/Google.Cloud.BigQuery.V2/Google.Cloud.BigQuery.V2/TableSchemaBuilder.cs

@@ -31,11 +31,6 @@ namespace Google.Cloud.BigQuery.V2
     /// </remarks>
     public sealed class TableSchemaBuilder : IEnumerable
     {
-        // From BigQuery documentation:
-        // The name must contain only letters (a-z, A-Z), numbers (0-9),
-        // or underscores (_), and must start with a letter or underscore.
-        // The maximum length is 128 characters.
-        private static readonly Regex s_fieldNamePattern = new Regex("^[a-zA-Z_][a-zA-Z0-9_]{0,127}$");
         private readonly List<TableFieldSchema> _fields = new List<TableFieldSchema>();
 
         /// <summary>
@@ -76,7 +71,38 @@ public void Add(string name, BigQueryDbType type, BigQueryFieldMode mode = BigQu
         internal static void ValidateFieldName(string name, string paramName)
         {
             GaxPreconditions.CheckNotNull(name, paramName);
-            GaxPreconditions.CheckArgument(s_fieldNamePattern.IsMatch(name), paramName, "Invalid field name '{0}'", name);
+
+            GaxPreconditions.CheckArgument(IsValidFieldName(name), paramName, "Invalid field name '{0}'", name);
+        }
+
+        // From BigQuery documentation:
+        // The name must contain only letters (a-z, A-Z), numbers (0-9),
+        // or underscores (_), and must start with a letter or underscore.
+        // The maximum length is 128 characters.
+        // This was originally a regular expression, but the manual code is very significantly faster.
+        // (Roughly 10x faster with the benchmarks I've run.)
+        private static bool IsValidFieldName(string name)
+        {
+            if (name.Length < 1 || name.Length > 128)
+            {
+                return false;
+            }
+            char first = name[0];
+            bool validFirst = (first >= 'a' && first <= 'z') || (first >= 'A' && first <= 'Z') || first == '_';
+            if (!validFirst)
+            {
+                return false;
+            }
+            for (int i = 1; i < name.Length; i++)
+            {
+                char c = name[i];
+                bool valid = (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '_';
+                if (!valid)
+                {
+                    return false;
+                }
+            }
+            return true;
         }
 
         /// <summary>