main: add very basic command line interface

There are three commands: one to write the default config, one to write
the embedded udev rules file, and one to actually run the program.

I might reformat the help text at some point. It's not very nice as-is.

src/main.zig

@@ -8,21 +8,69 @@ const udev = @import("udev_rules");
 
 const log = std.log.scoped(.main);
 
+fn printStderr(comptime fmt: []const u8, args: anytype) void {
+    std.debug.print(fmt ++ "\n", args);
+}
+
+const commands = .{
+    .install_udev = "install-udev-rules",
+    .write_config = "write-default-config",
+    .run = "run",
+};
+
+const command_help = .{
+    .install_udev = "[udev rules.d path]: Install the built-in LabJack u12 udev rules file. May require sudo privileges. Linux only.",
+    .write_config = "[path]: write the default configuration to a json file.",
+    .run = "[config path]: run the rotctl interface with the provided config.",
+};
+
 pub fn main() !u8 {
     var gpa = std.heap.GeneralPurposeAllocator(.{}){};
     defer _ = gpa.deinit();
     const allocator = gpa.allocator();
 
+    const args = std.process.argsAlloc(allocator) catch {
+        printStderr("Couldn't allocate arguments array", .{});
+        return 1;
+    };
+    defer std.process.argsFree(allocator, args);
+
+    if (args.len < 2) {
+        printHelp();
+        return 1;
+    }
+
+    if (std.mem.eql(u8, args[1], commands.install_udev)) {
+        if (args.len > 3) {
+            printHelp();
+            return 1;
+        }
+
+        return installUdevRules(if (args.len == 3) args[2] else null);
+    } else if (std.mem.eql(u8, args[1], commands.write_config)) {
+        if (args.len > 3) {
+            printHelp();
+            return 1;
+        }
+
+        Config.loadDefault(allocator);
+        return writeDefaultConfig(if (args.len == 3) args[2] else null);
+    } else if (std.mem.eql(u8, args[1], commands.run)) {
+        if (args.len > 3) {
+            printHelp();
+            return 1;
+        }
         blk: {
-        const conf_file = std.fs.cwd().openFile("yaes.json", .{}) catch {
-            log.warn("Could not load config file yaes.json. Using default config.", .{});
+            const confpath = if (args.len == 3) args[2] else "yaes.json";
+            const conf_file = std.fs.cwd().openFile(confpath, .{}) catch {
+                log.warn("Could not load config file '{s}'. Using default config.", .{confpath});
                 Config.loadDefault(allocator);
                 break :blk;
             };
             defer conf_file.close();
 
-        Config.load(allocator, conf_file.reader()) catch {
-            log.err("Could not parse config file yaes.json. Good luck figuring out why.", .{});
+            Config.load(allocator, conf_file.reader()) catch |err| {
+                log.err("Could not parse config file '{s}': {s}.", .{ confpath, @errorName(err) });
                 return 1;
             };
         }
@@ -37,4 +85,76 @@ pub fn main() !u8 {
         };
 
         return 0;
+    } else {
+        printHelp();
+        return 1;
+    }
+}
+
+fn installUdevRules(outpath: ?[]const u8) u8 {
+    const rules_path = outpath orelse "/etc/udev/rules.d";
+    var rules_d = std.fs.cwd().openDir(rules_path, .{}) catch |err| {
+        printStderr(
+            "could not open udev rules path '{s}': {s}",
+            .{ rules_path, @errorName(err) },
+        );
+        return 1;
+    };
+    defer rules_d.close();
+
+    rules_d.writeFile(.{ .sub_path = udev.rules_filename, .data = udev.rules }) catch |err| {
+        printStderr(
+            "could not write rules file '{s}{s}{s}': {s}",
+            .{
+                rules_path,
+                if (rules_path.len == 0)
+                    "./"
+                else if (rules_path[rules_path.len - 1] == '/')
+                    ""
+                else
+                    "/",
+                udev.rules_filename,
+                @errorName(err),
+            },
+        );
+        return 1;
+    };
+
+    return 0;
+}
+
+fn writeDefaultConfig(outarg: ?[]const u8) u8 {
+    const outpath = outarg orelse "yaes.json";
+
+    const outfile = std.fs.cwd().createFile(outpath, .{}) catch |err| {
+        printStderr("Could not write config file '{s}': {s}", .{ outpath, @errorName(err) });
+        return 1;
+    };
+    defer outfile.close();
+
+    std.json.stringify(Config.global.*, .{ .whitespace = .indent_4 }, outfile.writer()) catch |err| {
+        printStderr("Could not serialize config file '{s}': {s}", .{ outpath, @errorName(err) });
+        return 1;
+    };
+
+    printStderr("config written to {s}", .{outpath});
+    return 0;
+}
+
+fn printHelp() void {
+    printStderr(
+        \\Usage: yaes [command] [args...]
+        \\
+        \\  Control a Yaesu G-5500DC rotator through a LabJack U12 using the hamlib TCP interface.
+        \\
+        \\Available commands:
+        \\
+    , .{});
+
+    inline for (@typeInfo(@TypeOf(commands)).Struct.fields) |field| {
+        printStderr(
+            "  {s} {s}",
+            .{ @field(commands, field.name), @field(command_help, field.name) },
+        );
+    }
 }