Keep per-test coverage with --coverage (roadmap step 4)feature/per-test-coverage

--coverage=DIR runs every attempt in a process of its own and keeps the
coverage of each test, for binaries built with LLVM source-based
coverage (Clang: -fprofile-instr-generate -fcoverage-mapping) or gcov
(GCC: --coverage).

How it works:

- The scheduler gives each attempt a fresh worker, as --isolate does,
  and sets LLVM_PROFILE_FILE and GCOV_PREFIX for it: the profile the
  process writes at exit is then the coverage of that one test, in
  DIR/profiles/t<index>-a<attempt>.profraw or a .gcov directory. Each
  runtime reads its own variable, so no symbol of either runtime is
  looked up: weak references would not pull them out of their static
  archives, and Mach-O does not allow undefined weak symbols.
- A process that runs other tests first (bisection) writes beside the
  profiles, never into one. The attempt records its profile, also in
  the JSON results.
- The profiles of an earlier run are removed when the run starts,
  because gcov adds to the .gcda files it finds.
- After the run, CoverageReport reads the last profile of every test:
  llvm-profdata and llvm-cov (PATH, xcrun on macOS, or
  CATCH3_LLVM_PROFDATA / CATCH3_LLVM_COV) export LCOV, gcov (or
  CATCH3_GCOV) writes JSON next to a copy of the .gcno. The result is
  DIR/coverage.json, the files, lines and branches each test reached,
  and DIR/coverage.lcov for the whole run. Without the tools the
  profiles stay and the run says so.
- --coverage with --in-process or --break is refused.

Tests: option parsing, LCOV reading and writing, gcov JSON, and an
integration test on CoverageFixture, a binary CMake builds with the
coverage flags of Clang or GCC: two tests reach different lines, each
map holds only its own, the LCOV report holds both, and a stale profile
of an earlier run is gone. Locally all 84 CTest tests pass with Clang,
and the coverage tests pass with GCC (built with
CATCH3_ENABLE_WERROR=OFF: Catch3 does not build with GCC's warnings as
errors, before and after this change).

CI: the Linux jobs install LLVM's coverage tools, a new job checks the
gcov path with GCC, and CATCH3_COVERAGE_REQUIRE_MAP turns a missing map
into a failure on Linux and macOS instead of a skip. MSVC has no
source-based coverage, so its builds skip those tests; the roadmap
lists step 4 as in progress until Windows with clang-cl is tested.

23 files changed, 1160 insertions, 11 deletions

diff --git a/.github/workflows/linux-builds.yml b/.github/workflows/linux-builds.yml
index a9c38c66..02ee8d67 100644
--- a/.github/workflows/linux-builds.yml
+++ b/.github/workflows/linux-builds.yml
@@ -32,10 +32,10 @@ jobs:
       with:
         persist-credentials: false
 
-    - name: Install Clang, CMake and Ninja
+    - name: Install Clang, LLVM's coverage tools, CMake and Ninja
       run: |
         sudo apt-get update
-        sudo apt-get install -y clang cmake ninja-build python3
+        sudo apt-get install -y clang llvm cmake ninja-build python3
 
     - name: Configure
       run: |
@@ -51,6 +51,8 @@ jobs:
 
     - name: Test
       run: ctest --test-dir build -j
+      env:
+        CATCH3_COVERAGE_REQUIRE_MAP: 1
 
     - name: Install, and find the installed package
       run: |
@@ -75,10 +77,10 @@ jobs:
       with:
         persist-credentials: false
 
-    - name: Install Clang, CMake and Ninja
+    - name: Install Clang, LLVM's coverage tools, CMake and Ninja
       run: |
         sudo apt-get update
-        sudo apt-get install -y clang cmake ninja-build python3
+        sudo apt-get install -y clang llvm cmake ninja-build python3
 
     - name: Configure with the sanitizers
       env:
@@ -111,8 +113,8 @@ jobs:
     steps:
     # Git comes first: without it the checkout falls back to an archive of
     # the repository, which is not a checkout the tests can ask questions of.
-    - name: Install Git, Clang, CMake and Ninja
-      run: dnf install -y git clang gcc-c++ cmake ninja-build python3
+    - name: Install Git, Clang, LLVM's coverage tools, CMake and Ninja
+      run: dnf install -y git clang llvm gcc-c++ cmake ninja-build python3
 
     - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
       with:
@@ -132,3 +134,41 @@ jobs:
 
     - name: Test
       run: ctest --test-dir build -j
+      env:
+        CATCH3_COVERAGE_REQUIRE_MAP: 1
+
+  # Catch3 is built with Clang everywhere; GCC's stricter warnings are not
+  # errors here. The job checks per-test coverage in gcov's format, which
+  # only GCC writes.
+  gcc-coverage:
+    name: Ubuntu 26.04, GCC, per-test coverage with gcov
+    runs-on: ubuntu-26.04
+    timeout-minutes: 30
+
+    steps:
+    - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
+      with:
+        persist-credentials: false
+
+    - name: Install GCC, CMake and Ninja
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y g++ cmake ninja-build python3
+
+    - name: Configure
+      run: |
+        cmake --preset basic-tests -GNinja \
+              -DCMAKE_BUILD_TYPE=Debug \
+              -DCMAKE_CXX_COMPILER=g++ \
+              -DCMAKE_CXX_STANDARD=17 \
+              -DCATCH3_ENABLE_WERROR=OFF
+
+    - name: Build
+      run: cmake --build build --target ProgmasoftSelfTest
+
+    - name: Test per-test coverage
+      env:
+        CATCH3_RUNNER_FIXTURE: build/tests/RunnerFixture
+        CATCH3_COVERAGE_FIXTURE: build/tests/CoverageFixture
+        CATCH3_COVERAGE_REQUIRE_MAP: 1
+      run: build/tests/ProgmasoftSelfTest --no-history "[coverage]"
diff --git a/.github/workflows/mac-builds.yml b/.github/workflows/mac-builds.yml
index c115bf00..4044db9b 100644
--- a/.github/workflows/mac-builds.yml
+++ b/.github/workflows/mac-builds.yml
@@ -44,3 +44,5 @@ jobs:
 
     - name: Test
       run: ctest --test-dir build -j
+      env:
+        CATCH3_COVERAGE_REQUIRE_MAP: 1
diff --git a/docs/command-line.md b/docs/command-line.md
index 236128e1..ccca35ff 100644
--- a/docs/command-line.md
+++ b/docs/command-line.md
@@ -120,6 +120,7 @@ Without a spec, every test runs except hidden ones, whose tags start with
 | `--junit=FILE` | write a JUnit XML report |
 | `--json=FILE` | write a JSON report with every attempt and its analysis |
 | `--trace=FILE` | write a trace for `chrome://tracing` or Perfetto |
+| `--coverage=DIR` | run every test in a process of its own and keep its coverage in DIR, with a map of what each test reached; see [Per-test coverage](progmasoft-runner.md#per-test-coverage) |
 | `--slowest[=N]` | list the N slowest tests (default 10) |
 | `--premature-exit-guard-file=FILE` | create FILE, private to the user, when the run starts and remove it when the run ends normally |
 
diff --git a/docs/progmasoft-roadmap.md b/docs/progmasoft-roadmap.md
index 81a8956f..1f61be79 100644
--- a/docs/progmasoft-roadmap.md
+++ b/docs/progmasoft-roadmap.md
@@ -22,7 +22,7 @@ The steps are ordered by what each one builds on.
 | 1 | Native property testing: choice-sequence shrinking, combinators, state machines, statistics, targeted search, example database, differential and metamorphic oracles | done; see [Native property testing](progmasoft-properties.md#top) |
 | 2 | The Catch3 runner | done; see [The Catch3 runner](progmasoft-runner.md#top) |
 | 3 | Mocking | done; see [Mocking](progmasoft-mocking.md#top) |
-| 4 | Per-test code coverage | planned |
+| 4 | Per-test code coverage | in progress: Linux and macOS, with Clang and GCC; see [Per-test coverage](progmasoft-runner.md#per-test-coverage). Windows with clang-cl is next |
 | 5 | Change-impact test selection and test prioritization | planned |
 | 6 | Mutation testing | planned |
 | 7 | Coverage-, mutation- and history-guided test generation | planned |
diff --git a/docs/progmasoft-runner.md b/docs/progmasoft-runner.md
index 19d1e06d..cb5f29c5 100644
--- a/docs/progmasoft-runner.md
+++ b/docs/progmasoft-runner.md
@@ -12,6 +12,7 @@
 [Temporary files](#temporary-files)<br>
 [What a failure report says](#what-a-failure-report-says)<br>
 [Reports and traces](#reports-and-traces)<br>
+[Per-test coverage](#per-test-coverage)<br>
 [Formats other tools read](#formats-other-tools-read)<br>
 [Project configuration](#project-configuration)<br>
 [Bazel and CTest](#bazel-and-ctest)<br>
@@ -332,6 +333,65 @@ replay line runs the failure again as it happened.
   attempt and section.
 
 
+## Per-test coverage
+
+`--coverage=DIR` runs every test in a process of its own and keeps what each
+one reached. Build the tests with coverage instrumentation:
+
+* Clang: `-fprofile-instr-generate -fcoverage-mapping`, LLVM source-based
+  coverage;
+* GCC: `--coverage`, gcov's format.
+
+```text
+$ ./tests --coverage=coverage
+Catch3: 120 test(s), up to 8 worker(s), seed 2153251184
+note: --coverage: 120 test profile(s); what each test reached is in coverage/coverage.json, the run combined in coverage/coverage.lcov
+
+120 test(s): 120 passed
+```
+
+Each attempt gets a process of its own, so the profile its process writes at
+exit is the coverage of that one test: `DIR/profiles/t<index>-a<attempt>`
+with `.profraw` for LLVM, or a `.gcov` directory below which gcov writes its
+`.gcda` files under their absolute paths. The runner sets `LLVM_PROFILE_FILE`
+and `GCOV_PREFIX` for the process; each runtime reads its own. A process
+that runs other tests first, to find a test that leaves state behind, writes
+beside the profiles, never into one. The profiles of an earlier run in
+`DIR/profiles` are removed when the run starts, because gcov adds to the
+data it finds.
+
+After the run, the runner reads the profile of the last attempt of every
+test:
+
+* `DIR/coverage.json` holds, for each test, its outcome, its profile and the
+  files it reached, with the lines it ran and, per line, how many of its
+  branches it took:
+
+  ```json
+  { "version": 1, "format": "llvm", "binary": "./tests",
+    "tests": [ { "name": "Adds one", "outcome": "passed",
+                 "profile": "coverage/profiles/t0-a1.profraw",
+                 "files": [ { "file": "/src/calc.cpp", "lines": [ 12, 13 ],
+                              "branches": [ { "line": 12, "taken": 1, "total": 2 } ] } ] } ] }
+  ```
+
+* `DIR/coverage.lcov` is the coverage of the whole run as an LCOV tracefile,
+  which `genhtml`, Codecov and editors read.
+
+LLVM profiles are read with `llvm-profdata` and `llvm-cov`, gcov data with
+`gcov`. They are looked up on `PATH`, through `xcrun` on macOS, or named by
+`CATCH3_LLVM_PROFDATA`, `CATCH3_LLVM_COV` and `CATCH3_GCOV`. Without them the
+profiles stay in `DIR/profiles` and the run says so. A test that crashes or
+is stopped at its time limit leaves no profile. The runner's own process is
+instrumented too and writes its profile where any instrumented program does,
+`default.profraw` in the working directory unless `LLVM_PROFILE_FILE` says
+otherwise; it holds no test.
+
+`--coverage` cannot be used with `--in-process` or `--break`, which run the
+tests in one process. Per-test coverage is tested on Linux and macOS; Windows
+builds with clang-cl are next.
+
+
 ## Formats other tools read
 
 These formats carry a `format` and a `version`. Within a version they only
diff --git a/premake5.lua b/premake5.lua
index d4c55c9e..def0463c 100644
--- a/premake5.lua
+++ b/premake5.lua
@@ -92,6 +92,7 @@ project "ProgmasoftPropertyTests"
         "tests/Progmasoft/RunnerConfig.tests.cpp",
         "tests/Progmasoft/RunnerAnalysis.tests.cpp",
         "tests/Progmasoft/RunnerImport.tests.cpp",
+        "tests/Progmasoft/RunnerCoverage.tests.cpp",
         "tests/Progmasoft/RunnerOutput.tests.cpp",
         "tests/Progmasoft/Snapshot.tests.cpp",
         "tests/Progmasoft/Stateful.tests.cpp",
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 43754fff..fac2fb62 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -301,6 +301,7 @@ set(PROGMASOFT_RUNNER_FILES
   "${CATCH_DIR}/src/Progmasoft/Runner/Execution.hpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Expected.hpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Files.hpp"
+  "${CATCH_DIR}/src/Progmasoft/Runner/CoverageReport.hpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Git.hpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Hash.hpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/History.hpp"
@@ -320,6 +321,7 @@ set(PROGMASOFT_RUNNER_FILES
   "${CATCH_DIR}/src/Progmasoft/Runner/Console.cpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Execution.cpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Files.cpp"
+  "${CATCH_DIR}/src/Progmasoft/Runner/CoverageReport.cpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Git.cpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/Hash.cpp"
   "${CATCH_DIR}/src/Progmasoft/Runner/History.cpp"
diff --git a/src/Progmasoft/Catch3/Results.hpp b/src/Progmasoft/Catch3/Results.hpp
index 4c6bb1ce..bff2a72e 100644
--- a/src/Progmasoft/Catch3/Results.hpp
+++ b/src/Progmasoft/Catch3/Results.hpp
@@ -153,6 +153,9 @@ namespace Progmasoft::Catch3
         /// The directory the test had for its temporary files; kept when the
         /// attempt failed, so its contents can be inspected.
         std::string temporaryDirectory;
+        /// In a coverage run, where the profile of this attempt went,
+        /// without its extension (`.profraw` or `.gcov`).
+        std::string coverageProfile;
     };
 
     /// One test of the run and everything the runner learned about it.
diff --git a/src/Progmasoft/Catch3/meson.build b/src/Progmasoft/Catch3/meson.build
index 8420003b..4bbab28e 100644
--- a/src/Progmasoft/Catch3/meson.build
+++ b/src/Progmasoft/Catch3/meson.build
@@ -288,6 +288,7 @@ runner_sources = files(
   '../Runner/Console.cpp',
   '../Runner/Execution.cpp',
   '../Runner/Files.cpp',
+  '../Runner/CoverageReport.cpp',
   '../Runner/Git.cpp',
   '../Runner/Hash.cpp',
   '../Runner/History.cpp',
diff --git a/src/Progmasoft/Runner/CoverageReport.cpp b/src/Progmasoft/Runner/CoverageReport.cpp
new file mode 100644
index 00000000..c443c158
--- /dev/null
+++ b/src/Progmasoft/Runner/CoverageReport.cpp
@@ -0,0 +1,594 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+#include <algorithm>
+#include <cstdlib>
+#include <optional>
+#include <string_view>
+#include <system_error>
+
+#include "CoverageReport.hpp"
+#include "Files.hpp"
+#include "Json.hpp"
+#include "Process.hpp"
+
+namespace Progmasoft::Catch3::Runner
+{
+    namespace
+    {
+        /// Reads a non-negative decimal number; nothing for anything else,
+        /// such as the `-` LCOV writes for a branch never reached.
+        std::optional<std::uint64_t>
+        Number(std::string_view text)
+        {
+            if (text.empty() || text.size() > 19)
+            {
+                return std::nullopt;
+            }
+            std::uint64_t value = 0;
+            for (const char character : text)
+            {
+                if (character < '0' || character > '9')
+                {
+                    return std::nullopt;
+                }
+                value
+                    = value * 10 + static_cast<std::uint64_t>(character - '0');
+            }
+            return value;
+        }
+
+        std::vector<std::string_view>
+        Split(std::string_view text, char separator)
+        {
+            std::vector<std::string_view> parts;
+            std::size_t start = 0;
+            while (true)
+            {
+                const auto end = text.find(separator, start);
+                parts.push_back(text.substr(start, end - start));
+                if (end == std::string_view::npos)
+                {
+                    return parts;
+                }
+                start = end + 1;
+            }
+        }
+
+        /// A tool of a coverage format: the program, or the command that
+        /// runs it, such as `xcrun llvm-cov` on macOS.
+        using Tool = std::vector<std::string>;
+
+        std::optional<Tool>
+        FindTool(const char *variable, const std::string &name)
+        {
+            std::vector<Tool> candidates;
+            if (auto configured = ReadEnvironment(variable);
+                !configured.empty())
+            {
+                candidates.push_back({ std::move(configured) });
+            }
+            candidates.push_back({ name });
+#if defined(__APPLE__)
+            candidates.push_back({ "xcrun", name });
+#endif
+            for (auto &candidate : candidates)
+            {
+                ChildOptions options;
+                options.arguments = candidate;
+                options.arguments.emplace_back("--version");
+                options.searchPath = true;
+                if (auto run = RunAndCapture(options);
+                    run && run->status.Succeeded())
+                {
+                    return std::move(candidate);
+                }
+            }
+            return std::nullopt;
+        }
+
+        /// Runs `tool` with `arguments`; its output, or why it failed.
+        Expected<std::string>
+        RunTool(const Tool &tool,
+                const std::vector<std::string> &arguments,
+                const std::filesystem::path &directory = {})
+        {
+            ChildOptions options;
+            options.arguments = tool;
+            options.arguments.insert(options.arguments.end(),
+                                     arguments.begin(),
+                                     arguments.end());
+            options.searchPath = true;
+            options.workingDirectory = directory;
+            auto run = RunAndCapture(options);
+            if (!run)
+            {
+                return run.GetError();
+            }
+            if (!run->status.Succeeded())
+            {
+                std::string message = run->errors;
+                while (!message.empty()
+                       && (message.back() == '\n' || message.back() == '\r'))
+                {
+                    message.pop_back();
+                }
+                return Error{ tool.back() + " failed: " + message };
+            }
+            return std::move(run->output);
+        }
+
+        void
+        Add(CoverageMap &total, const CoverageMap &part)
+        {
+            for (const auto &[file, coverage] : part)
+            {
+                FileCoverage &into = total[file];
+                for (const auto &[line, count] : coverage.lines)
+                {
+                    into.lines[line] += count;
+                }
+                for (const auto &[branch, count] : coverage.branches)
+                {
+                    into.branches[branch] += count;
+                }
+            }
+        }
+
+        /// What one test reached: the lines it ran and, per line, how many
+        /// of its branches it took.
+        JsonValue
+        ReachedJson(const CoverageMap &map)
+        {
+            JsonValue files = JsonValue::Array();
+            for (const auto &[file, coverage] : map)
+            {
+                JsonValue lines = JsonValue::Array();
+                for (const auto &[line, count] : coverage.lines)
+                {
+                    if (count > 0)
+                    {
+                        lines.Append(line);
+                    }
+                }
+                JsonValue branches = JsonValue::Array();
+                std::map<std::size_t, std::pair<std::size_t, std::size_t>>
+                    perLine;
+                for (const auto &[branch, count] : coverage.branches)
+                {
+                    auto &[taken, total] = perLine[branch.first];
+                    ++total;
+                    if (count > 0)
+                    {
+                        ++taken;
+                    }
+                }
+                for (const auto &[line, counts] : perLine)
+                {
+                    if (counts.first == 0)
+                    {
+                        continue;
+                    }
+                    branches.Append(
+                        JsonValue::Object({ { "line", line },
+                                            { "taken", counts.first },
+                                            { "total", counts.second } }));
+                }
+                if (lines.Items().empty())
+                {
+                    continue;
+                }
+                files.Append(
+                    JsonValue::Object({ { "file", file },
+                                        { "lines", std::move(lines) },
+                                        { "branches", std::move(branches) } }));
+            }
+            return files;
+        }
+
+        /// The final attempt of a test that left a profile.
+        const Attempt *
+        ProfiledAttempt(const TestResult &test)
+        {
+            for (auto attempt = test.attempts.rbegin();
+                 attempt != test.attempts.rend();
+                 ++attempt)
+            {
+                if (!attempt->coverageProfile.empty())
+                {
+                    return &*attempt;
+                }
+            }
+            return nullptr;
+        }
+
+        /// The .gcda files gcov wrote below `prefix`, each with the path the
+        /// object was built at, where its .gcno is.
+        std::vector<std::pair<std::filesystem::path, std::filesystem::path>>
+        GcdaFiles(const std::filesystem::path &prefix)
+        {
+            std::vector<std::pair<std::filesystem::path, std::filesystem::path>>
+                files;
+            std::error_code error;
+            for (auto entry
+                 = std::filesystem::recursive_directory_iterator(prefix, error);
+                 !error
+                 && entry != std::filesystem::recursive_directory_iterator();
+                 entry.increment(error))
+            {
+                if (!entry->is_regular_file(error)
+                    || entry->path().extension() != ".gcda")
+                {
+                    continue;
+                }
+                // GCOV_PREFIX is put before the absolute path of the object.
+                const auto relative
+                    = std::filesystem::relative(entry->path(), prefix, error);
+                std::filesystem::path original
+                    = std::filesystem::path{ "/" } / relative;
+                files.emplace_back(entry->path(), original);
+            }
+            std::sort(files.begin(), files.end());
+            return files;
+        }
+
+        Expected<CoverageMap>
+        GcovCoverage(const Tool &gcov, const std::filesystem::path &prefix)
+        {
+            CoverageMap map;
+            for (const auto &[gcda, original] : GcdaFiles(prefix))
+            {
+                auto gcno = original;
+                gcno.replace_extension(".gcno");
+                auto beside = gcda;
+                beside.replace_extension(".gcno");
+                std::error_code error;
+                std::filesystem::copy_file(
+                    gcno,
+                    beside,
+                    std::filesystem::copy_options::overwrite_existing,
+                    error);
+                if (error)
+                {
+                    return Error{ "the notes file " + gcno.string()
+                                  + " could not be read: " + error.message() };
+                }
+                auto json = RunTool(
+                    gcov,
+                    { "--json-format", "--stdout", gcda.filename().string() },
+                    gcda.parent_path());
+                if (!json)
+                {
+                    return json.GetError();
+                }
+                // One JSON document per data file, one per line.
+                for (const auto line : Split(*json, '\n'))
+                {
+                    if (!line.empty())
+                    {
+                        AddGcovJson(std::string{ line }, map);
+                    }
+                }
+            }
+            return map;
+        }
+    } // namespace
+
+    CoverageMap
+    ParseLcov(const std::string &text)
+    {
+        CoverageMap map;
+        FileCoverage *current = nullptr;
+        for (auto line : Split(text, '\n'))
+        {
+            if (!line.empty() && line.back() == '\r')
+            {
+                line.remove_suffix(1);
+            }
+            if (line.substr(0, 3) == "SF:")
+            {
+                current = &map[std::string{ line.substr(3) }];
+            }
+            else if (line == "end_of_record")
+            {
+                current = nullptr;
+            }
+            else if (current != nullptr && line.substr(0, 3) == "DA:")
+            {
+                const auto fields = Split(line.substr(3), ',');
+                const auto number = Number(fields[0]);
+                const auto count
+                    = fields.size() > 1 ? Number(fields[1]) : std::nullopt;
+                if (number && count)
+                {
+                    current->lines[*number] += *count;
+                }
+            }
+            else if (current != nullptr && line.substr(0, 5) == "BRDA:")
+            {
+                const auto fields = Split(line.substr(5), ',');
+                if (fields.size() != 4)
+                {
+                    continue;
+                }
+                const auto number = Number(fields[0]);
+                if (!number)
+                {
+                    continue;
+                }
+                // The branches of a line are numbered in the order they come;
+                // `-` counts as never taken.
+                std::size_t index = 0;
+                for (auto it = current->branches.lower_bound({ *number, 0 });
+                     it != current->branches.end()
+                     && it->first.first == *number;
+                     ++it)
+                {
+                    ++index;
+                }
+                current->branches[{ *number, index }]
+                    += Number(fields[3]).value_or(0);
+            }
+        }
+        return map;
+    }
+
+    void
+    AddGcovJson(const std::string &text, CoverageMap &map)
+    {
+        auto document = ParseJson(text);
+        if (!document)
+        {
+            return;
+        }
+        for (const auto &file : (*document)["files"].Items())
+        {
+            FileCoverage &coverage = map[file["file"].AsString()];
+            for (const auto &line : file["lines"].Items())
+            {
+                const auto number = static_cast<std::size_t>(
+                    line["line_number"].AsUnsigned());
+                coverage.lines[number] += line["count"].AsUnsigned();
+                std::size_t index = 0;
+                for (const auto &branch : line["branches"].Items())
+                {
+                    coverage.branches[{ number, index++ }]
+                        += branch["count"].AsUnsigned();
+                }
+            }
+        }
+    }
+
+    std::string
+    ToLcov(const CoverageMap &map)
+    {
+        std::string text;
+        for (const auto &[file, coverage] : map)
+        {
+            text += "SF:" + file + "\n";
+            std::size_t linesHit = 0;
+            for (const auto &[line, count] : coverage.lines)
+            {
+                text += "DA:" + std::to_string(line) + ","
+                        + std::to_string(count) + "\n";
+                linesHit += count > 0 ? 1 : 0;
+            }
+            std::size_t branchesHit = 0;
+            for (const auto &[branch, count] : coverage.branches)
+            {
+                text += "BRDA:" + std::to_string(branch.first) + ",0,"
+                        + std::to_string(branch.second) + ","
+                        + (count > 0 ? std::to_string(count) : "-") + "\n";
+                branchesHit += count > 0 ? 1 : 0;
+            }
+            text += "LF:" + std::to_string(coverage.lines.size()) + "\n";
+            text += "LH:" + std::to_string(linesHit) + "\n";
+            text += "BRF:" + std::to_string(coverage.branches.size()) + "\n";
+            text += "BRH:" + std::to_string(branchesHit) + "\n";
+            text += "end_of_record\n";
+        }
+        return text;
+    }
+
+    Status
+    PrepareCoverageDirectory(const std::filesystem::path &directory)
+    {
+        std::error_code error;
+        std::filesystem::remove_all(directory / "profiles", error);
+        if (error)
+        {
+            return Error{ "the profiles of an earlier run in "
+                          + directory.string()
+                          + " could not be removed: " + error.message() };
+        }
+        std::filesystem::create_directories(directory / "profiles", error);
+        if (error)
+        {
+            return Error{ "the coverage directory " + directory.string()
+                          + " could not be created: " + error.message() };
+        }
+        return Status::Ok();
+    }
+
+    std::vector<std::string>
+    WriteCoverageReports(const std::filesystem::path &directory,
+                         const RunResult &run,
+                         const std::string &program)
+    {
+        std::vector<std::string> notes;
+        std::size_t llvmProfiles = 0;
+        std::size_t gcovProfiles = 0;
+        for (const auto &test : run.tests)
+        {
+            if (const Attempt *attempt = ProfiledAttempt(test))
+            {
+                std::error_code error;
+                llvmProfiles += std::filesystem::exists(attempt->coverageProfile
+                                                            + ".profraw",
+                                                        error)
+                                    ? 1
+                                    : 0;
+                gcovProfiles += std::filesystem::exists(attempt->coverageProfile
+                                                            + ".gcov",
+