Start Tazelish with its command line and version files

Tazelish installs Tazel and chooses which version of it runs, for those
who use Tazel alone. This first part holds everything that needs no
download:

- Versions: MAJOR.SEMIMAJOR.MINOR.PATCH without leading zeros,
  unstable, nightly, and YYYY-MM-DD-unstable or -nightly with a date
  the calendar has.
- The command line, read by hand into a typed command: install, use
  and default with --force before or after the version, self
  update|remove|purge, and help with or without a command. A wrong
  command line ends with exit code 2.
- tazelish.txt and config.txt: the nearest tazelish.txt in the current
  folder or above it comes first, then $HOME/.tazelish/config.txt or
  %USERPROFILE%\.tazelish\config.txt. use and default refuse to write
  over a file that exists unless --force is given.

install and self download from static.tazel.net, which this build does
not do; they say so and end with exit code 1.

C++17 with fmt, tested with Catch3, built by Bazel (clang-cl on
Windows, with the rules_cc patch the Tala repository carries). The code
throws no exceptions and builds with warnings as errors; the tests
leave out -Wconversion, which the headers of Catch3 do not pass. CI
tests on Linux, macOS and Windows.

1 files changed, 140 insertions, 0 deletions

diff --git a/Sources/Main.cpp b/Sources/Main.cpp
new file mode 100644
index 0000000..8ea8b89
--- /dev/null
+++ b/Sources/Main.cpp
@@ -0,0 +1,140 @@
+// SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+
+#include <cstdio>
+#include <filesystem>
+#include <fmt/format.h>
+#include <string_view>
+#include <vector>
+
+#include "Progmasoft/Tazelish/CommandLine.hpp"
+#include "Progmasoft/Tazelish/Help.hpp"
+#include "Progmasoft/Tazelish/VersionFiles.hpp"
+
+namespace
+{
+
+    namespace fs = std::filesystem;
+    namespace tazelish = Progmasoft::Tazelish;
+
+    constexpr int kSuccess = 0;
+    constexpr int kFailure = 1;
+    constexpr int kUsage = 2;
+
+    int
+    Report(const tazelish::FileError &error)
+    {
+        fmt::print(stderr,
+                   "error: {} {}\n",
+                   error.file.string(),
+                   error.message);
+        return kFailure;
+    }
+
+    int
+    Write(const fs::path &file, const tazelish::Version &version, bool force)
+    {
+        if (auto error = tazelish::WriteVersionFile(file, version, force))
+        {
+            return Report(*error);
+        }
+        fmt::print("Wrote {} to {}\n",
+                   tazelish::FormatVersion(version),
+                   file.string());
+        return kSuccess;
+    }
+
+    int
+    RunUse(const tazelish::Use &use)
+    {
+        return Write(fs::path(".") / fs::path(tazelish::kProjectFileName),
+                     use.version,
+                     use.force);
+    }
+
+    int
+    RunDefault(const tazelish::Default &choice)
+    {
+        const auto home = tazelish::HomeDirectory();
+        if (!home)
+        {
+#ifdef _WIN32
+            fmt::print(stderr, "error: USERPROFILE is not set\n");
+#else
+            fmt::print(stderr, "error: HOME is not set\n");
+#endif
+            return kFailure;
+        }
+        return Write(tazelish::ConfigFile(*home), choice.version, choice.force);
+    }
+
+    // Installing and updating download from static.tazel.net, which this build
+    // of Tazelish does not do.
+    int
+    RunDownloading(std::string_view command)
+    {
+        fmt::print(
+            stderr,
+            "error: tazelish {} downloads from static.tazel.net, and this "
+            "build of Tazelish does not download\n",
+            command);
+        return kFailure;
+    }
+
+    std::string_view
+    SelfName(tazelish::SelfAction action)
+    {
+        switch (action)
+        {
+            case tazelish::SelfAction::kUpdate:
+                return "self update";
+            case tazelish::SelfAction::kRemove:
+                return "self remove";
+            case tazelish::SelfAction::kPurge:
+                return "self purge";
+        }
+        return "self";
+    }
+
+    int
+    Run(const tazelish::Command &command)
+    {
+        if (const auto *help = std::get_if<tazelish::Help>(&command))
+        {
+            fmt::print("{}", tazelish::HelpText(help->topic));
+            return kSuccess;
+        }
+        if (const auto *use = std::get_if<tazelish::Use>(&command))
+        {
+            return RunUse(*use);
+        }
+        if (const auto *choice = std::get_if<tazelish::Default>(&command))
+        {
+            return RunDefault(*choice);
+        }
+        if (std::holds_alternative<tazelish::Install>(command))
+        {
+            return RunDownloading("install");
+        }
+        return RunDownloading(
+            SelfName(std::get<tazelish::Self>(command).action));
+    }
+
+} // namespace
+
+int
+main(int argc, char **argv)
+{
+    std::vector<std::string_view> arguments;
+    for (int index = 1; index < argc; ++index)
+    {
+        arguments.emplace_back(argv[index]);
+    }
+    const auto parsed = tazelish::ParseCommandLine(arguments);
+    if (const auto *rejection = std::get_if<tazelish::Rejection>(&parsed))
+    {
+        fmt::print(stderr, "error: {}\n", rejection->message);
+        return kUsage;
+    }
+    return Run(std::get<tazelish::Command>(parsed));
+}