Add snapshots and JUnit result serialization0.2.0

21 files changed, 1216 insertions, 17 deletions

diff --git a/MODULE.bazel b/MODULE.bazel
index 8bff2332..fcc80d58 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -1,4 +1,4 @@
-module(name = "catch3", version = "0.1.0")
+module(name = "catch3", version = "0.2.0")
 
 bazel_dep(name = "bazel_skylib", version = "1.9.0")
 bazel_dep(name = "rules_cc", version = "0.2.17")
diff --git a/docs/progmasoft-catch3.md b/docs/progmasoft-catch3.md
index 96989479..43a73f03 100644
--- a/docs/progmasoft-catch3.md
+++ b/docs/progmasoft-catch3.md
@@ -58,9 +58,48 @@ state-machine commands, parallel trials, and adaptive shrinking are deliberately
 not implied by this first module; they need their own deterministic contracts and
 tests before being added.
 
+## Snapshots
+
+`<Progmasoft/Catch3/Snapshot.hpp>` provides file-backed text snapshots through
+`CompareSnapshot(name, actual, options)`. The directory is supplied explicitly;
+names are restricted to portable ASCII filename characters and cannot contain
+path separators. CRLF and CR are normalized to LF by default, with no trimming
+or other content rewriting.
+
+Ordinary comparisons never write files. `SnapshotUpdateMode::kCreateMissing`
+creates only absent snapshots, while `kAlways` explicitly permits replacement.
+The result reports a match, create/update, mismatch with a one-based first
+difference location, invalid input, or I/O failure. `CATCH3_CHECK_SNAPSHOT`
+combines this result with Catch2's normal assertion reporting. Keep snapshots
+under a directory owned by the test suite and enable replacement only for an
+intentional update run.
+
+```cpp
+const Progmasoft::Catch3::SnapshotOptions options{
+    .Directory = "tests/snapshots",
+};
+CATCH3_CHECK_SNAPSHOT("rendered-report", RenderReport(), options);
+```
+
+## Structured results and XML
+
+`<Progmasoft/Catch3/Results.hpp>` defines a runner-neutral hierarchy of
+`TestRunResult`, `TestSuiteResult`, and `TestCaseResult`. `SummarizeTestRun`
+validates finite, non-negative durations and computes pass/failure/error/skip
+counts. `<Progmasoft/Catch3/XmlWriter.hpp>` serializes that model as deterministic
+JUnit-style XML through `JUnitXmlWriter::Write` or `ToString`. It escapes XML
+markup, replaces malformed UTF-8 and XML-forbidden control characters, and
+omits timestamps so identical results produce identical output. Catch2's own
+`--reporter xml` and `--reporter junit` remain the direct reporters for results
+produced by its runner; this writer is for Progmasoft's explicit result model.
+
+The snapshot and result/XML APIs have compiled C++ implementations in the
+`Progmasoft::Catch3` library target. Generator templates and convenience macros
+remain in headers where their types or call-site expansion require it.
+
 ## Build and integration
 
-The CMake build exposes `Progmasoft::Catch3`, a C++20 interface target linked to
+The CMake build exposes `Progmasoft::Catch3`, a compiled C++20 library linked to
 `Catch2::Catch2`. The existing Catch2 target retains its C++14 baseline. After
 installation, consumers can use:
 
diff --git a/docs/why-catch3.md b/docs/why-catch3.md
index 0b62402f..30dd5159 100644
--- a/docs/why-catch3.md
+++ b/docs/why-catch3.md
@@ -40,7 +40,10 @@ future Catch2 upstream change will be adopted unchanged or that all downstream
 projects have already been validated.
 
 The split also applies to implementation and packaging: Catch2 remains the
-engine dependency, while Progmasoft's header-only API is a separate target.
+engine dependency, while Progmasoft's compiled C++20 library is a separate
+target. Template-based generators and call-site assertion macros remain in
+headers; snapshot I/O, result aggregation, and XML serialization are compiled
+implementations.
 Keep tests for the two areas independently identifiable so additions do not
 silently alter the upstream compatibility baseline.
 
diff --git a/premake5.lua b/premake5.lua
index a03b8734..9c2115d2 100644
--- a/premake5.lua
+++ b/premake5.lua
@@ -54,6 +54,25 @@ project "Catch2WithMain"
     includedirs { "src", generated_include }
     links { "Catch2" }
 
+project "ProgmasoftCatch3"
+    kind "StaticLib"
+    language "C++"
+    cppdialect "C++20"
+    targetdir "build/premake/bin/%{cfg.buildcfg}"
+    objdir "build/premake/obj/ProgmasoftCatch3/%{cfg.buildcfg}"
+    files {
+        "src/Progmasoft/Results.cpp",
+        "src/Progmasoft/Results.hpp",
+        "src/Progmasoft/Snapshot.cpp",
+        "src/Progmasoft/Snapshot.hpp",
+        "src/Progmasoft/XmlWriter.cpp",
+        "src/Progmasoft/XmlWriter.hpp",
+        "src/Progmasoft/Catch3/**.hpp",
+        "src/Progmasoft/Catch3.hpp",
+    }
+    includedirs { "src", generated_include }
+    links { "Catch2" }
+
 project "ProgmasoftPropertyTests"
     kind "ConsoleApp"
     language "C++"
@@ -62,7 +81,9 @@ project "ProgmasoftPropertyTests"
     objdir "build/premake/obj/ProgmasoftPropertyTests/%{cfg.buildcfg}"
     files {
         "tests/Progmasoft/Property.tests.cpp",
+        "tests/Progmasoft/Results.tests.cpp",
+        "tests/Progmasoft/Snapshot.tests.cpp",
         "src/Progmasoft/**.hpp",
     }
     includedirs { "src", generated_include }
-    links { "Catch2WithMain", "Catch2" }
+    links { "ProgmasoftCatch3", "Catch2WithMain", "Catch2" }
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index cb8c339d..5eec18e9 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -360,18 +360,22 @@ if(CATCH_ENABLE_REPRODUCIBLE_BUILD)
 endif()
 add_library(Catch2::Catch2 ALIAS Catch2)
 
-# Progmasoft's C++20 additions are a separate target so the Catch2
-# compatibility engine can keep its existing, broader C++14 baseline.
-add_library(ProgmasoftCatch3 INTERFACE)
+# Progmasoft's compiled C++20 utilities are separate from Catch2's
+# compatibility engine, which retains its existing C++14 baseline.
+add_library(ProgmasoftCatch3 STATIC
+  "${CATCH_DIR}/src/Progmasoft/Results.cpp"
+  "${CATCH_DIR}/src/Progmasoft/Snapshot.cpp"
+  "${CATCH_DIR}/src/Progmasoft/XmlWriter.cpp"
+)
 add_library(Progmasoft::Catch3 ALIAS ProgmasoftCatch3)
 set_target_properties(ProgmasoftCatch3 PROPERTIES EXPORT_NAME Catch3)
-target_link_libraries(ProgmasoftCatch3 INTERFACE Catch2::Catch2)
+target_link_libraries(ProgmasoftCatch3 PUBLIC Catch2::Catch2)
 target_include_directories(ProgmasoftCatch3
-  INTERFACE
+  PUBLIC
     $<BUILD_INTERFACE:${CATCH_DIR}/src>
     $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
-target_compile_features(ProgmasoftCatch3 INTERFACE cxx_std_20)
+target_compile_features(ProgmasoftCatch3 PUBLIC cxx_std_20)
 
 if(ANDROID)
   target_link_libraries(Catch2 PRIVATE log)
diff --git a/src/Progmasoft/BUILD.bazel b/src/Progmasoft/BUILD.bazel
index 9927aafa..642bc158 100644
--- a/src/Progmasoft/BUILD.bazel
+++ b/src/Progmasoft/BUILD.bazel
@@ -6,6 +6,11 @@ package(default_visibility = ["//visibility:public"])
 
 cc_library(
     name = "catch3",
+    srcs = [
+        "Results.cpp",
+        "Snapshot.cpp",
+        "XmlWriter.cpp",
+    ],
     hdrs = [
         "Catch3.hpp",
         "Catch3/Assertions.hpp",
@@ -14,7 +19,14 @@ cc_library(
         "Catch3/Generators/Integer.hpp",
         "Catch3/Property.hpp",
         "Catch3/Random.hpp",
+        "Catch3/Results.hpp",
+        "Catch3/Snapshot.hpp",
+        "Catch3/XmlWriter.hpp",
     ],
+    copts = select({
+        "@platforms//os:windows": ["/std:c++20"],
+        "//conditions:default": ["-std=c++20"],
+    }),
     strip_include_prefix = "/src",
     deps = ["//:catch2"],
 )
diff --git a/src/Progmasoft/Catch3.hpp b/src/Progmasoft/Catch3.hpp
index 1ae73e53..d2d34e74 100644
--- a/src/Progmasoft/Catch3.hpp
+++ b/src/Progmasoft/Catch3.hpp
@@ -11,3 +11,6 @@
 #include <Progmasoft/Catch3/Generators/Integer.hpp>
 #include <Progmasoft/Catch3/Property.hpp>
 #include <Progmasoft/Catch3/Random.hpp>
+#include <Progmasoft/Catch3/Results.hpp>
+#include <Progmasoft/Catch3/Snapshot.hpp>
+#include <Progmasoft/Catch3/XmlWriter.hpp>
diff --git a/src/Progmasoft/Catch3/Assertions.hpp b/src/Progmasoft/Catch3/Assertions.hpp
index 19ac13ef..668c086d 100644
--- a/src/Progmasoft/Catch3/Assertions.hpp
+++ b/src/Progmasoft/Catch3/Assertions.hpp
@@ -6,6 +6,7 @@
 #include <catch2/catch_test_macros.hpp>
 
 #include <Progmasoft/Catch3/Property.hpp>
+#include <Progmasoft/Catch3/Snapshot.hpp>
 
 /// Runs a property and reports its replay seed and minimized counterexample.
 #define CATCH3_CHECK_PROPERTY( ... )                            \
@@ -15,3 +16,12 @@
         INFO( catch3PropertyResult.Describe() );                \
         CHECK( catch3PropertyResult.Succeeded() );              \
     } while ( false )
+
+/// Compares a value with a named snapshot and reports the first difference.
+#define CATCH3_CHECK_SNAPSHOT( name, actual, options )                      \
+    do {                                                                    \
+        const auto catch3SnapshotResult =                                   \
+            ::Progmasoft::Catch3::CompareSnapshot( name, actual, options ); \
+        INFO( catch3SnapshotResult.Message );                               \
+        CHECK( catch3SnapshotResult.Succeeded() );                          \
+    } while ( false )
diff --git a/src/Progmasoft/Catch3/Results.hpp b/src/Progmasoft/Catch3/Results.hpp
new file mode 100644
index 00000000..b0b69b55
--- /dev/null
+++ b/src/Progmasoft/Catch3/Results.hpp
@@ -0,0 +1,57 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <vector>
+
+namespace Progmasoft::Catch3 {
+
+    enum class TestCaseStatus : unsigned char {
+        kPassed,
+        kFailed,
+        kErrored,
+        kSkipped,
+    };
+
+    /// Framework-neutral result for one completed test case.
+    struct TestCaseResult final {
+        std::string Name;
+        std::string ClassName;
+        std::string SourceFile;
+        TestCaseStatus Status = TestCaseStatus::kPassed;
+        double DurationSeconds = 0.0;
+        std::string Message;
+        std::string Details;
+        std::string StandardOutput;
+        std::string StandardError;
+    };
+
+    struct TestSuiteResult final {
+        std::string Name;
+        std::vector<TestCaseResult> TestCases;
+    };
+
+    struct TestRunResult final {
+        std::string Name;
+        std::vector<TestSuiteResult> TestSuites;
+    };
+
+    struct TestRunSummary final {
+        std::size_t Tests = 0;
+        std::size_t Failures = 0;
+        std::size_t Errors = 0;
+        std::size_t Skipped = 0;
+        double DurationSeconds = 0.0;
+
+        [[nodiscard]] std::size_t Passed() const noexcept;
+    };
+
+    /// Validates durations and statuses while aggregating nested test results.
+    [[nodiscard]] TestRunSummary
+    SummarizeTestSuite( const TestSuiteResult& result );
+    [[nodiscard]] TestRunSummary
+    SummarizeTestRun( const TestRunResult& result );
+
+} // namespace Progmasoft::Catch3
diff --git a/src/Progmasoft/Catch3/Snapshot.hpp b/src/Progmasoft/Catch3/Snapshot.hpp
new file mode 100644
index 00000000..09863c1c
--- /dev/null
+++ b/src/Progmasoft/Catch3/Snapshot.hpp
@@ -0,0 +1,55 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#pragma once
+
+#include <cstddef>
+#include <filesystem>
+#include <string>
+#include <string_view>
+
+namespace Progmasoft::Catch3 {
+
+    /// Updates are deliberately explicit so normal test runs never rewrite
+    /// files.
+    enum class SnapshotUpdateMode : unsigned char {
+        kNever,
+        kCreateMissing,
+        kAlways,
+    };
+
+    enum class SnapshotStatus : unsigned char {
+        kMatched,
+        kCreated,
+        kUpdated,
+        kMismatched,
+        kInvalidName,
+        kInvalidOptions,
+        kIoError,
+    };
+
+    struct SnapshotOptions final {
+        std::filesystem::path Directory;
+        SnapshotUpdateMode UpdateMode = SnapshotUpdateMode::kNever;
+        bool NormalizeLineEndings = true;
+    };
+
+    /// Structured comparison result; DifferenceLine/Column are one-based.
+    struct SnapshotResult final {
+        SnapshotStatus Status = SnapshotStatus::kInvalidOptions;
+        std::filesystem::path File;
+        std::string Expected;
+        std::string Actual;
+        std::string Message;
+        std::size_t DifferenceLine = 0;
+        std::size_t DifferenceColumn = 0;
+
+        [[nodiscard]] bool Succeeded() const noexcept;
+    };
+
+    /// Compare against `<Directory>/<name>.snap`; names cannot contain paths.
+    [[nodiscard]] SnapshotResult
+    CompareSnapshot( std::string_view name,
+                     std::string_view actual,
+                     const SnapshotOptions& options );
+
+} // namespace Progmasoft::Catch3
diff --git a/src/Progmasoft/Catch3/XmlWriter.hpp b/src/Progmasoft/Catch3/XmlWriter.hpp
new file mode 100644
index 00000000..3216b5ae
--- /dev/null
+++ b/src/Progmasoft/Catch3/XmlWriter.hpp
@@ -0,0 +1,20 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#pragma once
+
+#include <iosfwd>
+#include <string>
+
+namespace Progmasoft::Catch3 {
+
+    struct TestRunResult;
+
+    /// Emits deterministic JUnit-style XML without timestamps or global state.
+    class JUnitXmlWriter final {
+    public:
+        static void Write( const TestRunResult& result, std::ostream& output );
+        [[nodiscard]] static std::string
+        ToString( const TestRunResult& result );
+    };
+
+} // namespace Progmasoft::Catch3
diff --git a/src/Progmasoft/Results.cpp b/src/Progmasoft/Results.cpp
new file mode 100644
index 00000000..69a819a7
--- /dev/null
+++ b/src/Progmasoft/Results.cpp
@@ -0,0 +1,73 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#include <Progmasoft/Catch3/Results.hpp>
+
+#include <cmath>
+#include <limits>
+#include <stdexcept>
+
+namespace Progmasoft::Catch3 {
+
+    namespace {
+
+        void AccumulateCases( TestRunSummary& summary,
+                              const std::vector<TestCaseResult>& testCases ) {
+            for ( const TestCaseResult& testCase : testCases ) {
+                if ( !std::isfinite( testCase.DurationSeconds ) ||
+                     testCase.DurationSeconds < 0.0 ) {
+                    throw std::invalid_argument(
+                        "test duration must be finite and non-negative" );
+                }
+                if ( summary.Tests ==
+                     std::numeric_limits<std::size_t>::max() ) {
+                    throw std::overflow_error( "test count overflow" );
+                }
+
+                ++summary.Tests;
+                summary.DurationSeconds += testCase.DurationSeconds;
+                if ( !std::isfinite( summary.DurationSeconds ) ) {
+                    throw std::overflow_error( "test duration total overflow" );
+                }
+
+                switch ( testCase.Status ) {
+                case TestCaseStatus::kPassed:
+                    break;
+                case TestCaseStatus::kFailed:
+                    ++summary.Failures;
+                    break;
+                case TestCaseStatus::kErrored:
+                    ++summary.Errors;
+                    break;
+                case TestCaseStatus::kSkipped:
+                    ++summary.Skipped;
+                    break;
+                default:
+                    throw std::invalid_argument( "unknown test case status" );
+                }
+            }
+        }
+
+    } // namespace
+
+    std::size_t TestRunSummary::Passed() const noexcept {
+        if ( Failures > Tests || Errors > Tests - Failures ) { return 0; }
+        const std::size_t failuresAndErrors = Failures + Errors;
+        if ( Skipped > Tests - failuresAndErrors ) { return 0; }
+        return Tests - failuresAndErrors - Skipped;
+    }
+
+    TestRunSummary SummarizeTestSuite( const TestSuiteResult& result ) {
+        TestRunSummary summary;
+        AccumulateCases( summary, result.TestCases );
+        return summary;
+    }
+
+    TestRunSummary SummarizeTestRun( const TestRunResult& result ) {
+        TestRunSummary summary;
+        for ( const TestSuiteResult& suite : result.TestSuites ) {
+            AccumulateCases( summary, suite.TestCases );
+        }
+        return summary;
+    }
+
+} // namespace Progmasoft::Catch3
diff --git a/src/Progmasoft/Snapshot.cpp b/src/Progmasoft/Snapshot.cpp
new file mode 100644
index 00000000..7278eee3
--- /dev/null
+++ b/src/Progmasoft/Snapshot.cpp
@@ -0,0 +1,265 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#include <Progmasoft/Catch3/Snapshot.hpp>
+
+#include <atomic>
+#include <chrono>
+#include <cstdint>
+#include <fstream>
+#include <iterator>
+#include <system_error>
+
+#if defined( _WIN32 )
+#    if !defined( NOMINMAX )
+#        define NOMINMAX
+#    endif
+#    include <Windows.h>
+#endif
+#include <utility>
+
+namespace Progmasoft::Catch3 {
+
+    namespace {
+
+        bool IsSafeSnapshotName( std::string_view name ) noexcept {
+            if ( name.empty() || name == "." || name == ".." ) { return false; }
+            for ( const unsigned char character : name ) {
+                const bool isLetter =
+                    ( character >= 'a' && character <= 'z' ) ||
+                    ( character >= 'A' && character <= 'Z' );
+                const bool isDigit = character >= '0' && character <= '9';
+                if ( !isLetter && !isDigit && character != '_' &&
+                     character != '-' && character != '.' ) {
+                    return false;
+                }
+            }
+            return true;
+        }
+
+        std::string NormalizeSnapshotText( std::string_view text,
+                                           bool normalizeLineEndings ) {
+            if ( !normalizeLineEndings ) { return std::string( text ); }
+
+            std::string normalized;
+            normalized.reserve( text.size() );
+            for ( std::size_t index = 0; index < text.size(); ++index ) {
+                if ( text[index] == '\r' ) {
+                    if ( index + 1 < text.size() && text[index + 1] == '\n' ) {
+                        ++index;
+                    }
+                    normalized.push_back( '\n' );
+                } else {
+                    normalized.push_back( text[index] );
+                }
+            }
+            return normalized;
+        }
+
+        void LocateDifference( SnapshotResult& result ) noexcept {
+            const std::size_t commonLength =
+                result.Expected.size() < result.Actual.size()
+                    ? result.Expected.size()
+                    : result.Actual.size();
+            std::size_t index = 0;
+            while ( index < commonLength &&
+                    result.Expected[index] == result.Actual[index] ) {
+                ++index;
+            }
+
+            result.DifferenceLine = 1;
+            result.DifferenceColumn = 1;
+            for ( std::size_t offset = 0; offset < index; ++offset ) {
+                if ( result.Actual[offset] == '\n' ) {
+                    ++result.DifferenceLine;
+                    result.DifferenceColumn = 1;
+                } else {
+                    ++result.DifferenceColumn;
+                }
+            }
+        }
+
+        bool WriteAtomically( const std::filesystem::path& destination,
+                              std::string_view contents,
+                              bool replaceExisting,
+                              std::error_code& error ) {
+            // Write beside the destination so replacement stays on one volume
+            // and readers never observe a partially written snapshot.
+            static std::atomic<std::uint64_t> nextTemporaryFile{ 0 };
+            const auto clockValue =
+                std::chrono::steady_clock::now().time_since_epoch().count();
+            const auto sequence =
+                nextTemporaryFile.fetch_add( 1, std::memory_order_relaxed );
+            std::filesystem::path temporary = destination;
+            temporary += ".tmp." + std::to_string( clockValue ) + "." +
+                         std::to_string( sequence );
+
+            {
+                std::ofstream output( temporary,
+                                      std::ios::binary | std::ios::trunc );
+                if ( !output ) {
+                    error = std::make_error_code( std::errc::io_error );
+                    return false;
+                }
+                output.write( contents.data(),
+                              static_cast<std::streamsize>( contents.size() ) );
+                output.flush();
+                if ( !output ) {
+                    error = std::make_error_code( std::errc::io_error );
+                    output.close();
+                    std::error_code ignored;
+                    std::filesystem::remove( temporary, ignored );
+                    return false;
+                }
+            }
+
+            if ( replaceExisting ) {
+#if defined( _WIN32 )
+                if ( MoveFileExW( temporary.c_str(),
+                                  destination.c_str(),
+                                  MOVEFILE_REPLACE_EXISTING |
+                                      MOVEFILE_WRITE_THROUGH ) == 0 ) {
+                    error = std::error_code( static_cast<int>( GetLastError() ),
+                                             std::system_category() );
+                }
+#else
+                std::filesystem::rename( temporary, destination, error );
+#endif
+            } else {
+                // A hard-link create is atomic and refuses a concurrent file.
+                std::filesystem::create_hard_link(
+                    temporary, destination, error );
+                std::error_code ignored;
+                std::filesystem::remove( temporary, ignored );
+            }
+            if ( error ) {
+                std::error_code ignored;
+                std::filesystem::remove( temporary, ignored );
+                return false;
+            }
+            return true;
+        }
+
+    } // namespace
+
+    bool SnapshotResult::Succeeded() const noexcept {
+        return Status == SnapshotStatus::kMatched ||
+               Status == SnapshotStatus::kCreated ||
+               Status == SnapshotStatus::kUpdated;
+    }
+
+    SnapshotResult CompareSnapshot( std::string_view name,
+                                    std::string_view actual,
+                                    const SnapshotOptions& options ) {
+        SnapshotResult result;
+        if ( !IsSafeSnapshotName( name ) ) {
+            result.Status = SnapshotStatus::kInvalidName;
+            result.Message =
+                "snapshot name must be a non-empty ASCII filename component";
+            return result;
+        }
+        if ( options.Directory.empty() ||
+             ( options.UpdateMode != SnapshotUpdateMode::kNever &&
+               options.UpdateMode != SnapshotUpdateMode::kCreateMissing &&
+               options.UpdateMode != SnapshotUpdateMode::kAlways ) ) {
+            result.Status = SnapshotStatus::kInvalidOptions;
+            result.Message = "snapshot directory or update mode is invalid";
+            return result;
+        }
+
+        result.File = options.Directory / ( std::string( name ) + ".snap" );
+        result.Actual =
+            NormalizeSnapshotText( actual, options.NormalizeLineEndings );
+
+        std::error_code error;
+        const std::filesystem::file_status fileStatus =
+            std::filesystem::symlink_status( result.File, error );
+        if ( error && error != std::errc::no_such_file_or_directory ) {
+            result.Status = SnapshotStatus::kIoError;
+            result.Message =
+                "could not inspect snapshot file: " + error.message();
+            return result;
+        }
+
+        const bool exists = !error && fileStatus.type() !=
+                                          std::filesystem::file_type::not_found;
+        if ( exists && std::filesystem::is_symlink( fileStatus ) ) {
+            result.Status = SnapshotStatus::kIoError;
+            result.Message = "snapshot files must not be symbolic links";
+            return result;
+        }
+        if ( exists && !std::filesystem::is_regular_file( fileStatus ) ) {
+            result.Status = SnapshotStatus::kIoError;
+            result.Message = "snapshot path is not a regular file";
+            return result;
+        }
+
+        if ( !exists ) {
+            if ( options.UpdateMode == SnapshotUpdateMode::kNever ) {
+                result.Status = SnapshotStatus::kMismatched;
+                result.Message = "snapshot file does not exist";
+                LocateDifference( result );
+                return result;
+            }
+
+            std::filesystem::create_directories( options.Directory, error );
+            if ( error || !WriteAtomically(
+                              result.File, result.Actual, false, error ) ) {
+                result.Status = SnapshotStatus::kIoError;
+                result.Message =
+                    "could not create snapshot file: " + error.message();
+                return result;
+            }
+            result.Status = SnapshotStatus::kCreated;
+            result.Message = "snapshot created";
+            return result;
+        }
+
+        std::ifstream input( result.File, std::ios::binary );
+        if ( !input ) {
+            result.Status = SnapshotStatus::kIoError;
+            result.Message = "could not open snapshot file for reading";
+            return result;
+        }
+        const std::string fileContents{ std::istreambuf_iterator<char>( input ),
+                                        std::istreambuf_iterator<char>() };
+        if ( input.bad() ) {
+            result.Status = SnapshotStatus::kIoError;
+            result.Message = "could not read snapshot file";
+            return result;
+        }
+        input.close();
+        if ( input.fail() ) {
+            result.Status = SnapshotStatus::kIoError;
+            result.Message = "could not close snapshot file after reading";
+            return result;
+        }
+        result.Expected =
+            NormalizeSnapshotText( fileContents, options.NormalizeLineEndings );
+
+        if ( result.Expected == result.Actual ) {
+            result.Status = SnapshotStatus::kMatched;
+            result.Message = "snapshot matched";
+            return result;
+        }
+
+        LocateDifference( result );
+        if ( options.UpdateMode == SnapshotUpdateMode::kAlways ) {
+            if ( !WriteAtomically( result.File, result.Actual, true, error ) ) {
+                result.Status = SnapshotStatus::kIoError;
+                result.Message =
+                    "could not update snapshot file: " + error.message();
+                return result;
+            }
+            result.Status = SnapshotStatus::kUpdated;
+            result.Message = "snapshot updated";
+            return result;
+        }
+
+        result.Status = SnapshotStatus::kMismatched;
+        result.Message = "snapshot differs at line " +
+                         std::to_string( result.DifferenceLine ) + ", column " +
+                         std::to_string( result.DifferenceColumn );
+        return result;
+    }
+
+} // namespace Progmasoft::Catch3
diff --git a/src/Progmasoft/XmlWriter.cpp b/src/Progmasoft/XmlWriter.cpp
new file mode 100644
index 00000000..b3a5f9b1
--- /dev/null
+++ b/src/Progmasoft/XmlWriter.cpp
@@ -0,0 +1,274 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#include <Progmasoft/Catch3/Results.hpp>
+#include <Progmasoft/Catch3/XmlWriter.hpp>
+
+#include <cmath>
+#include <iomanip>
+#include <locale>
+#include <ostream>
+#include <sstream>
+#include <stdexcept>
+#include <string_view>
+
+namespace Progmasoft::Catch3 {
+
+    namespace {
+
+        struct DecodedCharacter final {
+            char32_t Value = 0;
+            std::size_t Length = 0;
+        };
+
+        DecodedCharacter DecodeUtf8( std::string_view text,
+                                     std::size_t offset ) noexcept {
+            const auto first = static_cast<unsigned char>( text[offset] );
+            if ( first < 0x80 ) { return { first, 1 }; }
+
+            std::size_t length = 0;
+            char32_t value = 0;
+            char32_t minimum = 0;
+            if ( first >= 0xC2 && first <= 0xDF ) {
+                length = 2;
+                value = first & 0x1F;
+                minimum = 0x80;
+            } else if ( first >= 0xE0 && first <= 0xEF ) {
+                length = 3;
+                value = first & 0x0F;
+                minimum = 0x800;
+            } else if ( first >= 0xF0 && first <= 0xF4 ) {
+                length = 4;
+                value = first & 0x07;
+                minimum = 0x10000;
+            } else {
+                return { 0xFFFD, 0 };
+            }
+
+            if ( offset + length > text.size() ) { return { 0xFFFD, 0 }; }
+            for ( std::size_t index = 1; index < length; ++index ) {
+                const auto continuation =
+                    static_cast<unsigned char>( text[offset + index] );
+                if ( ( continuation & 0xC0 ) != 0x80 ) { return { 0xFFFD, 0 }; }
+                value = static_cast<char32_t>( ( value << 6 ) |
+                                               ( continuation & 0x3F ) );
+            }
+            if ( value < minimum || value > 0x10FFFF ||
+                 ( value >= 0xD800 && value <= 0xDFFF ) ) {
+                return { 0xFFFD, 0 };
+            }
+            return { value, length };
+        }
+
+        bool IsXmlCharacter( char32_t value ) noexcept {