Refactor CSV and Excel enumerators for improved performance and clarity

Directory.Build.props

@@ -1,5 +1,9 @@
 <Project>
   <PropertyGroup>
+    <LangVersion>latest</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <Deterministic>true</Deterministic>
     <AnalysisLevel>latest</AnalysisLevel>
     <AnalysisMode>All</AnalysisMode>
     <!-- <TreatWarningsAsErrors>true</TreatWarningsAsErrors> -->
@@ -19,8 +23,6 @@
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)'=='Release'">
     <Optimize>true</Optimize>
-    <DebugType>none</DebugType>
-    <DebugSymbols>false</DebugSymbols>
     <DefineConstants>$(DefineConstants.Replace("DEBUG;", ""))</DefineConstants>
     <DefineConstants>$(DefineConstants.Replace("TRACE;", ""))</DefineConstants>
   </PropertyGroup>
@@ -82,4 +84,4 @@
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
   </ItemGroup>
-</Project>
+</Project>

src/ExcelReader.Core/ExcelEpoch.cs

@@ -0,0 +1,39 @@
+namespace ExcelReader.Core
+{
+    // Excel reserves serial 60 for the fictitious 1900-02-29; OADate does not. Keep this conversion
+    // shared so the binary writers and the public Cell date reader cannot drift at the boundary.
+    internal static class ExcelEpoch
+    {
+        internal static double SerialToOADate(double serial, bool date1904)
+        {
+            if (date1904)
+            {
+                return serial + 1462.0;
+            }
+            return serial switch
+            {
+                < 60.0 => serial + 1.0,
+                60.0 => 60.0,
+                _ => serial,
+            };
+        }
+
+        internal static double OADateToSerial(double oadate, bool date1904)
+        {
+            double serial = oadate;
+            if (date1904)
+            {
+                serial -= 1462.0;
+            }
+            else if (oadate < 61.0)
+            {
+                serial--;
+            }
+            if (serial < 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(oadate), "Dates before the workbook epoch cannot be written to Excel.");
+            }
+            return serial;
+        }
+    }
+}

src/ExcelReader.Core/ExcelReader.Core.csproj

@@ -2,9 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>net10.0;net8.0</TargetFrameworks>
-    <LangVersion>latest</LangVersion>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
 
     <!-- Ship XML docs (members without a doc comment do not warn). -->
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
@@ -34,7 +32,6 @@
     <DebugType>portable</DebugType>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
-    <Deterministic>true</Deterministic>
     <ContinuousIntegrationBuild Condition="'$(GITHUB_ACTIONS)' == 'true'">true</ContinuousIntegrationBuild>
   </PropertyGroup>
 

src/ExcelReader.Core/Parser/ExcelParser.cs

@@ -33,10 +33,10 @@ public ExcelEnumerable<T> Parse(XlsxReader reader)
 
         [SuppressMessage("Usage", "VSTHRD200:Use \"Async\" suffix for async methods",
             Justification = "Synchronous entry point; the enumerable also implements IAsyncEnumerable, but ParseAsync is the async counterpart.")]
-        public XlsExcelEnumerable<T> Parse(XlsReader reader)
+        public ExcelEnumerable<T, XlsReader, XlsReader.Enumerator> Parse(XlsReader reader)
         {
             ArgumentNullException.ThrowIfNull(reader);
-            return new XlsExcelEnumerable<T>(reader, _config);
+            return new ExcelEnumerable<T, XlsReader, XlsReader.Enumerator>(reader, _config);
         }
 
         [SuppressMessage("Usage", "VSTHRD200:Use \"Async\" suffix for async methods",
@@ -75,10 +75,10 @@ public ExcelEnumerable<T> ParseAsync(XlsxReader reader, CancellationToken ct = d
             return new ExcelEnumerable<T>(reader, _config, ct);
         }
 
-        public XlsExcelEnumerable<T> ParseAsync(XlsReader reader, CancellationToken ct = default)
+        public ExcelEnumerable<T, XlsReader, XlsReader.Enumerator> ParseAsync(XlsReader reader, CancellationToken ct = default)
         {
             ArgumentNullException.ThrowIfNull(reader);
-            return new XlsExcelEnumerable<T>(reader, _config, ct);
+            return new ExcelEnumerable<T, XlsReader, XlsReader.Enumerator>(reader, _config, ct);
         }
 
         public ExcelEnumerable<T, XlsbReader, XlsbReader.Enumerator> ParseAsync(XlsbReader reader, CancellationToken ct = default)