//! The CLI is defined by a set of commands with options and subcommands.
//! A command is a type where with special declarations and fields. If a
//! command has options, it needs to define an `option` field, that is a
//! struct where all fields are valid options, i.e. created with `Option`.
//! If a command has any subcommands a `Subcommands` type can be declared,
//! which needs to be a tagged union that contains a command type for each
//! supported subcommand. To provide sufficient help for each command a
//! command type must define a `help` declaration of type `Command_Help`.
//!
//! The basic structure of a command type:
//!
//!   struct {
//!     pub const help = Command_Help{ ... };
//!
//!     pub const Subcommands = union(enum) {
//!         another: Another_Command,
//!     };
//!
//!     options: struct{
//!         one: Option(u16, .{ ... }),
//!         two: Option([]const u8, .{ ... }),
//!     },
//!   }
//!
//! With all the setup above it is possible to "generate" a command help
//! during compile time. There are two versions for a help text: a short
//! and a long version. The short version displays the usage pattern of
//! the command whereas the long version displays a detailed help text.

const std = @import("std");
const assert = std.debug.assert;

var stdout_buf: [1024]u8 = undefined;
var stdout_printer: Printer = .init(std.fs.File.stdout().writer(&stdout_buf));
var stderr_printer: Printer = .init(std.fs.File.stderr().writer(&.{}));

pub var stdout = &stdout_printer;
pub var stderr = &stderr_printer;

pub const Command_Help = struct {
    pub const Section = struct {
        /// The heading of the section. This must be a single line with
        /// where each letter is uppercase.
        heading: []const u8,
        /// The text that should be displayed in the body of the section.
        ///
        /// Note: Single newlines will be replaced by spaces to format the
        /// help text according to the terminal width. Multiple consecutive
        /// newlines will be kept to keep paragraphs. If a line is indented
        /// it will be printed as separate line. This can be used to display
        /// lists or code, for example.
        body: []const u8,
    };

    // The fully qualified command name, which includes the executable
    // name and all the subcommands. All parts of the fully qualified
    // name must be divided by a single space only.
    //
    // Note: The last part is used to determining the subcommand during
    // argument parsing.
    name: []const u8,
    /// A synopsis of how to invoke the command. For each usage of the
    /// command a separate invocation can be provided. It should give
    /// a brief overview of how the command can be used.
    /// An invocation pattern should be formatted in manpage style
    /// (e.g. "program mycommand [-f|--foo]").
    ///
    /// Note: For better formatting a single pattern can be split into
    /// multiple lines.
    synopsis: []const []const u8,
    /// A short description of what the command does. In the long help
    /// it is displayed directly after the command name. In the short
    /// help it is used to give a brief description of the command.
    ///
    /// Note: This must be a single line of text.
    short_description: []const u8,
    /// A detailed description of the functioning of the command. This
    /// text is displayed in the long help. It should explain all the
    /// details of the command.
    ///
    /// Note: Single newlines will be replaced by spaces to format the
    /// help text according to the terminal width. Multiple consecutive
    /// newlines will be kept to keep paragraphs. If a line is indented
    /// it will be printed as separate line. This can be used to display
    /// lists or code, for example.
    long_description: []const u8,
    /// Extra sections that should be displayed at the bottom of the
    /// command help.
    extra_sections: []const Section = &.{},
};

pub fn Option_Help(comptime T: type) type {
    return struct {
        /// The long name of an option without the leading double-dash.
        ///
        /// Note: Only alphanumeric characters or dashes are allowed, but
        /// it must not start or end with a dash.
        name: []const u8,
        /// An option can have an optional short name which can be used as an
        /// alternative to the long name. A short name is a single character.
        ///
        /// Note: A short name must be alphanumeric.
        short_name: ?u8 = null,
        /// Every non-boolean option takes an argument as its value. To improve
        /// help message quality an argument must have a meaningful name that is
        /// displayed in the help text of the corresponding command.
        /// For boolean option on the other hand this field defaults to null and
        /// will be ignored when printing a help text.
        ///
        /// Note: This must be a single word, i.e. only characters, numbers,
        /// dashes, and underscores are allowed.
        argument_name: ?[]const u8 = null,
        /// A short description of what the option does. This text is displayed
        /// in the short help of the corresponding command. It should give a brief
        /// overview of what the option does.
        ///
        /// Note: This must be a single line of text.
        short_description: []const u8,
        /// A detailed description of the command option. This text is displayed
        /// in the long help of the corresponding command. It should explain all
        /// details of the option and can be as long as needed.
        ///
        /// Note: Single newlines will be replaced by spaces to format the
        /// help text according to the terminal width. Multiple consecutive
        /// newlines will be kept to keep paragraphs.
        long_description: []const u8,
        /// Default value that will be set if the user does not provide a specific
        /// value. If this field is null the option value will also be null.
        default_value: ?T = null,
    };
}

pub fn Option(comptime T: type, comptime opt_help: Option_Help(T)) type {
    comptime {
        const has_argname = opt_help.argument_name != null and opt_help.argument_name.?.len > 0;
        if (!has_argname and T != bool) {
            @compileError("non-boolean options must have an argument name");
        }

        switch (@typeInfo(T)) {
            .@"enum" => |info| {
                for (info.fields) |field| {
                    for (field.name) |ch| {
                        switch (ch) {
                            'a'...'z', 'A'...'Z', '0'...'9' => {},
                            else => @compileError("invalid enum '" ++ field.name ++ "' value in option " ++ opt_help.name),
                        }
                    }
                }
            },
            else => {},
        }
    }

    return struct {
        pub const Type = T;
        pub const help: Option_Help(T) = opt_help;

        const Self = @This();

        value: ?T,

        fn init(self: *Self) void {
            self.value = null;
        }

        pub fn value_or_default(self: Self) T {
            if (opt_help.default_value == null) {
                @compileError("option '" ++ opt_help.name ++ "' does not have default value");
            }
            return self.value orelse opt_help.default_value.?;
        }

        fn parse(self: *Self, name: []const u8, value: [:0]const u8) void {
            if (self.value != null) {
                fatal("duplicate option '{s}'", .{name});
            }

            self.value = switch (@typeInfo(T)) {
                .bool => val: {
                    if (value.len != 0) {
                        fatal("boolean option '{s}' does not take a value", .{name});
                    }
                    break :val true;
                },
                .int => val: {
                    if (value.len == 0) {
                        fatal("missing value for option '{s}'", .{name});
                    }
                    break :val std.fmt.parseInt(T, value, 10) catch |err| switch (err) {
                        error.InvalidCharacter => fatal("option '{s}' requires a numeric value", .{name}),
                        error.Overflow => fatal("option '{s}' exceeds the maximum numeric value", .{name}),
                    };
                },
                .@"enum" => |info| val: {
                    if (info.fields.len == 0) {
                        @compileError("enum cli option " ++ @typeName(T) ++ " does not have any values");
                    }
                    if (value.len == 0) {
                        fatal("missing value for option '{s}'", .{name});
                    }
                    break :val std.meta.stringToEnum(T, value) orelse {
                        fatal("invalid value for option '{s}' (use " ++ join_enum_values(T) ++ ")", .{name});
                    };
                },
                .pointer => |info| val: {
                    if (info.child != u8 and info.size != .Slice) {
                        @compileError("pointer cli option " ++ @typeName(T) ++ " not supported");
                    }
                    break :val value;
                },
                else => @compileError("cli option type " ++ @typeName(T) ++ " not supported"),
            };
        }
    };
}

pub const Args = struct {
    /// The command we are currently in or null if we reached the end or positional arguments.
    current_cmd: ?[]const u8,
    iter: std.process.ArgIterator,
    exe_name: []const u8,

    pub fn init(allocator: std.mem.Allocator, comptime exe_name: []const u8, program: anytype) !Args {
        comptime assert(exe_name.len > 0);

        var iter = try std.process.ArgIterator.initWithAllocator(allocator);
        errdefer iter.deinit();
        _ = iter.skip(); // skip executable name
        const cmd_info = Command_Info.from_command(switch (@typeInfo(@TypeOf(program))) {
            .pointer => |info| info.child,
            else => @compileError("expected pointer, got " ++ @typeName(@TypeOf(program))),
        });
        const current_cmd = parse_opts(&iter, cmd_info, program);
        return Args{
            .current_cmd = current_cmd,
            .iter = iter,
            .exe_name = exe_name,
        };
    }

    pub fn deinit(self: *Args, allocator: std.mem.Allocator) void {
        _ = allocator;
        self.iter.deinit();
    }

    pub fn next_subcommand(self: *Args, comptime Command: type) ?Command.Subcommands {
        const subcommands = comptime Command_Info.from_subcommand(Command);
        comptime assert(subcommands.len > 0);

        const arg = self.current_cmd orelse return null;
        inline for (subcommands) |subcmd| {
            if (std.mem.eql(u8, arg, subcmd.name)) {
                var cmd: subcmd.type = undefined;
                self.current_cmd = parse_opts(&self.iter, subcmd, &cmd);
                return @unionInit(Command.Subcommands, subcmd.tag.?, cmd);
            }
        }
        fatal_with_usage(Command, "unknown command '{s}' (see '{s} --help')", .{ arg, self.exe_name });
    }

    pub fn next_positional(self: *Args) ?[]const u8 {
        if (self.current_cmd) |cmd| {
            self.current_cmd = null;
            return cmd;
        }
        self.current_cmd = null;
        return self.iter.next();
    }

    /// Parse and fill all command options and return the current command.
    fn parse_opts(iter: *std.process.ArgIterator, comptime cmd_info: Command_Info, cmd: *cmd_info.type) ?[]const u8 {
        inline for (cmd_info.options) |opt| {
            @field(cmd.options, opt.field).init();
        }

        args_loop: while (iter.next()) |arg| {
            assert(arg.len > 0);
            if (arg.len < 2 or arg[0] != '-') {
                // We have either a single dash or a non-option here. Treat these as
                // a positional or subcommand (however the executor chooses to use it).
                return arg;
            }

            var opt_name: []const u8 = undefined;
            if (arg[1] != '-') {
                // We check for a short option name.
                // If we have a boolean flag or an option name longer than a
                // single character, we have both the name and the value in
                // a single argument. Otherwise we fetch the next argument.
                opt_name = arg[1..2];
                if (opt_name[0] == 'h') {
                    if (arg.len > 2) {
                        fatal_with_usage(cmd_info.type, "unknown option '{s}'", .{arg});
                    }
                    cmd_info.print_usage(stderr);
                    std.process.exit(0);
                }

                inline for (cmd_info.options) |opt| {
                    if (opt.short_name) |short_name| {
                        if (short_name == opt_name[0]) {
                            const value: [:0]const u8 = if (arg.len > 2 or opt.arg_name.len == 0) arg[2..] else next: {
                                const next_arg = iter.next();
                                if (next_arg == null or next_arg.?[0] == '-') {
                                    fatal("missing value for option '{c}'", .{opt_name[0]});
                                }
                                break :next next_arg.?;
                            };
                            @field(cmd.options, opt.field).parse(opt_name, value);
                            continue :args_loop;
                        }
                    }
                }
            } else if (arg.len > 2) {
                // We check for a long option name.
                // If we don't have the format `name=value`, we get the
                // next argument lazily, because for boolean flags we
                // don't need any extra argument.
                opt_name = arg[2..];
                var value: ?[:0]const u8 = null;
                if (std.mem.indexOfScalar(u8, arg, '=')) |eq| {
                    opt_name = arg[2..eq];
                    value = arg[eq + 1 ..];
                }

                if (std.mem.eql(u8, opt_name, "help")) {
                    cmd_info.print_help();
                }

                inline for (cmd_info.options) |opt| {
                    if (std.mem.eql(u8, opt_name, opt.long_name)) {
                        value = value orelse if (opt.arg_name.len == 0) "" else next_arg: {
                            const next = iter.next();
                            if (next == null or next.?[0] == '-') {
                                fatal("missing value for option '{s}'", .{opt_name});
                            }
                            break :next_arg next.?;
                        };
                        @field(cmd.options, opt.field).parse(opt_name, value.?);
                        continue :args_loop;
                    }
                }
            } else {
                // A single "--" was detected which means the rest of the arguments
                // are interpreted as positional. We are done parsing.
                return null;
            }

            fatal_with_usage(cmd_info.type, "unknown option '{s}'", .{opt_name});
        }

        return null;
    }
};

pub const Print_Options = struct {
    /// Indentation of the first line.
    margin: u16 = 0,
    /// Indentation of all lines except the first.
    hanging_indent: u16 = 0,
    /// The character by which the text should be split into multiple lines.
    line_split: u8 = ' ',
};

const Usage_Column = struct {
    cells: []const []const u8,
    width: u16,
};

pub const Printer = struct {
    writer: std.fs.File.Writer,
    max_width: u16,

    fn init(w: std.fs.File.Writer) Printer {
        return Printer{
            .writer = w,
            .max_width = 80,
        };
    }

    /// Drain the remaining buffered data and puts it to the sink.
    pub fn flush(self: *Printer) void {
        self.writer.interface.flush() catch {};
    }

    /// Write a value directly without splitting. This uses the formatting capabilities
    /// of `std.fmt`.
    pub fn write(self: *Printer, comptime fmt: []const u8, value: anytype) void {
        self.writer.interface.print(fmt, value) catch {};
    }

    /// Write a value directly without splitting and append a newline. This uses the
    /// formatting capabilities of `std.fmt`.
    pub fn write_line(self: *Printer, comptime fmt: []const u8, value: anytype) void {
        self.write(fmt ++ "\n", value);
    }

    /// Print the given text and split it into multiple lines if it exceeds
    /// a certain width. The splitting behaviour can be configured using the
    /// given print options.
    pub fn print(self: *Printer, text: []const u8, opts: Print_Options) void {
        if (text.len > 0) {
            const max_len_first = self.max_width - opts.margin;
            const max_len_others = self.max_width - opts.margin - opts.hanging_indent;

            self.put_indent(opts.margin);
            var rest = self.put_until_eol(text, max_len_first, opts.line_split);
            while (rest.len > 0) {
                self.put('\n');
                self.put_indent(opts.margin + opts.hanging_indent);
                rest = self.put_until_eol(rest, max_len_others, opts.line_split);
            }
        }
    }

    /// Same as `print`, but appends a newline at the end.
    pub fn print_line(self: *Printer, text: []const u8, opts: Print_Options) void {
        self.print(text, opts);
        self.put('\n');
    }

    /// Print a formatted text and split it into multiple lines if if exceeds
    /// a certain width. The splitting behaviour can be configured using the
    /// given print options.
    pub fn printf(self: *Printer, comptime format: []const u8, args: anytype, opts: Print_Options) void {
        if (@typeInfo(@TypeOf(args)).@"struct".fields.len == 0) {
            self.print(format, opts);
        } else {
            // TODO: Shall we estimate the buffer size depending on the
            // format string and arguments?
            var buf: [2048]u8 = undefined;
            const text = std.fmt.bufPrint(&buf, format, args) catch |err| switch (err) {
                error.NoSpaceLeft => dots: {
                    buf[buf.len - 3] = '.';
                    buf[buf.len - 2] = '.';
                    buf[buf.len - 1] = '.';
                    break :dots buf[0..];
                },
            };
            self.print(text, opts);
        }
    }

    /// Same as `printf`, but appends a newline at the end.
    pub fn printf_line(self: *Printer, comptime format: []const u8, args: anytype, opts: Print_Options) void {
        self.printf(format, args, opts);
        self.put('\n');
    }

    /// Print multiple lines of text. This essentially calls `print_line` for each
    /// given line of text.
    pub fn print_lines(self: *Printer, text: []const u8, opts: Print_Options) void {
        var rest = text;
        while (std.mem.indexOfScalar(u8, rest, '\n')) |eol| {
            self.print_line(rest[0..eol], opts);
            rest = rest[eol + 1 ..];
        }

        if (rest.len > 0) {
            self.print_line(rest, opts);
        }
    }

    fn put(self: *Printer, c: u8) void {
        self.writer.interface.writeByte(c) catch {};
    }

    fn put_str(self: *Printer, text: []const u8) void {
        self.writer.interface.writeAll(text) catch {};
    }

    fn print_usage_table(self: *Printer, comptime left_column: Usage_Column, comptime right_column: Usage_Column, opts: Print_Options) void {
        assert(opts.hanging_indent > 0);
        assert(left_column.cells.len == right_column.cells.len);

        const sep_len = 2;
        const max_len: usize = @intCast(@min(left_column.width + sep_len + right_column.width, self.max_width - opts.margin));

        if (left_column.width + sep_len > 2 * max_len / 3) {
            // There is very little space for the right column. So we write each column
            // to a separate line.
            const left_margin = opts.margin;
            const right_margin = opts.margin + opts.hanging_indent;
            for (left_column.cells, right_column.cells) |left, right| {
                self.print_line(left, .{ .margin = left_margin });
                self.print_line(right, .{ .margin = right_margin });
            }
        } else {
            // The left and the right column are on the same line. If the right column
            // gets too long, we print multiple lines for the right column.
            for (left_column.cells, right_column.cells) |left, right| {
                assert(left.len <= left_column.width);
                assert(right.len <= right_column.width);

                self.put_indent(opts.margin);
                self.put_str(left);
                self.put_indent(left_column.width - left.len + sep_len);

                const right_width = max_len - left_column.width - sep_len;
                var rest = self.put_until_eol(right, right_width, opts.line_split);
                while (rest.len > 0) {
                    self.put('\n');
                    self.put_indent(opts.margin + left_column.width + sep_len);
                    rest = self.put_until_eol(right, right_width, opts.line_split);
                }
                self.put('\n');
            }
        }
    }

    fn put_until_eol(self: *Printer, text: []const u8, line_len: usize, line_split: u8) []const u8 {
        const width = @min(text.len, line_len);
        var eol: usize = undefined;
        if (text.len <= width) {
            eol = text.len;
        } else if (text[width] == line_split) {
            eol = width;
        } else if (std.mem.lastIndexOfScalar(u8, text[0..width], line_split)) |last_split| {
            eol = last_split;
            if (std.mem.allEqual(u8, text[0..last_split], line_split)) {
                // There are leading line splits here, so we consider an indentation.
                // We want to keep indentation intact and print to the next split.
                eol = std.mem.indexOfScalarPos(u8, text, width, line_split) orelse text.len;
            }
        } else {
            // We don't have a split char in the line. So we extend the line
            // to the next space. This hopefully will not happen in the wild.
            eol = std.mem.indexOfScalarPos(u8, text, width, line_split) orelse text.len;
        }

        self.put_str(text[0..eol]);
        return if (eol < text.len) text[eol + 1 ..] else text[0..0];
    }

    fn put_indent(self: *Printer, n: usize) void {
        const buf = [_]u8{' '} ** 32;
        var remaining = n;
        while (remaining > buf.len) : (remaining -= buf.len) {
            self.put_str(&buf);
        }
        self.put_str(buf[0..remaining]);
    }
};

pub fn fatal(comptime format: []const u8, args: anytype) noreturn {
    stderr.printf_line(format, args, .{});
    stderr.flush();
    std.process.exit(1);
}

pub fn fatal_with_usage(comptime Command: type, comptime format: []const u8, args: anytype) noreturn {
    const info = Command_Info.from_command(Command);
    stderr.printf_line(format, args, .{});
    stderr.put('\n');
    info.print_usage(stderr);
    std.process.exit(1);
}

/// Print the usage of the given command.
/// This function automatically flushes.
pub fn print_usage(p: *Printer, comptime Command: type) void {
    Command_Info.from_command(Command).print_usage(p);
    p.flush();
}

/// Print the help text of the given command.
pub fn print_help(comptime Command: type) noreturn {
    Command_Info.from_command(Command).print_help();
}

pub fn ask(comptime prompt: []const u8, args: anytype, buf: []u8) ?[]const u8 {
    var stdin = std.fs.File.stdin().reader(buf);
    while (true) {
        stderr.printf(prompt, args, .{});
        stderr.flush();
        const line = stdin.interface.takeDelimiterExclusive('\n') catch |err| switch (err) {
            error.StreamTooLong => {
                stderr.printf_line("ERROR: The input exceeds the maximum of {} characters.", .{buf.len}, .{});
                continue;
            },
            else => unreachable,
        };

        const input = std.mem.trim(u8, line, " \t\r");
        return if (input.len == 0) null else input;
    }
}

pub fn ask_confirmation(comptime prompt: []const u8, args: anytype, comptime default_answer: enum { yes, no }) bool {
    comptime assert(prompt.len > 0 and prompt[prompt.len - 1] == '?');
    const yes = "yes";
    const no = "no";
    const default_ans = switch (default_answer) {
        .yes => yes,
        .no => no,
    };

    var confirmbuf: [16]u8 = undefined;
    return while (true) {
        const ans = ask(prompt ++ " (" ++ yes ++ "/" ++ no ++ ") [" ++ default_ans ++ "]: ", args, &confirmbuf) orelse default_ans;
        for (ans, 0..) |c, i| {
            confirmbuf[i] = std.ascii.toLower(c);
        }
        if (std.mem.eql(u8, ans, yes[0..1]) or std.mem.eql(u8, ans, yes)) break true;
        if (std.mem.eql(u8, ans, no[0..1]) or std.mem.eql(u8, ans, no)) break false;
    };
}

const Pager = struct {
    const Bold = struct {
        text: []const u8,
        dumb: bool,

        pub fn format(self: Bold, w: *std.io.Writer) std.io.Writer.Error!void {
            if (!self.dumb) try w.writeAll("\x1b[1m");
            try w.writeAll(self.text);
            if (!self.dumb) try w.writeAll("\x1b[m");
        }
    };

    pid: ?std.posix.pid_t,
    printer: Printer,
    dumb: bool,

    fn init(out_buf: []u8) Pager {
        if (!stdout.writer.file.isTty()) {
            return .{ .pid = null, .printer = stdout_printer, .dumb = true };
        }

        const fds = std.posix.pipe() catch return .{ .pid = null, .printer = stdout_printer, .dumb = true };
        const fdin = fds[0];
        const fdout = fds[1];

        const pid = std.posix.fork() catch {
            std.posix.close(fdin);
            std.posix.close(fdout);
            return .{ .pid = null, .printer = stdout_printer, .dumb = true };
        };

        if (pid == 0) {
            // pager process
            std.posix.close(fdout);
            std.posix.dup2(fdin, std.posix.STDIN_FILENO) catch unreachable;

            var env: [512:null]?[*:0]const u8 = undefined;
            var envlen: usize = 0;
            const environ = if (std.os.environ.len < 512) std.os.environ else std.os.environ[0..511];
            for (environ) |e| {
                env[envlen] = e;
                envlen += 1;
            }
            env[envlen] = null;

            // TODO: Allow environment PAGER to define custom pagers.
            const pager_argv: [*:null]const ?[*:0]const u8 = &.{ "less", "-R", null };

            std.posix.execvpeZ(pager_argv[0].?, pager_argv, &env) catch unreachable;
            std.process.exit(1);
        } else {
            // application process
            std.posix.close(fdin);
            var out: std.fs.File = .{ .handle = fdout };
            return .{
                .pid = pid,
                .printer = Printer.init(out.writer(out_buf)),
                .dumb = if (std.posix.getenv("TERM")) |term| std.mem.eql(u8, term, "dumb") else true,
            };
        }
    }

    fn deinit(self: *Pager) noreturn {
        self.printer.flush();
        if (self.pid) |pid| {
            self.printer.writer.file.close();
            _ = std.posix.waitpid(pid, 0);
        }
        std.process.exit(0);
    }

    fn bold(self: *Pager, text: []const u8) Bold {
        return .{ .text = text, .dumb = self.dumb };
    }
};

/// Information about a single command.
const Command_Info = struct {
    type: type,
    name: []const u8, // last part of the fully qualified name
    fq_name: []const u8, // fully qualified name separated by spaces
    synopsis: []const []const u8,
    short_description: []const u8,
    long_description: []const u8,
    options: []const Option_Info,
    tag: ?[]const u8, // The tag in the subcommands union.
    sections: []const Command_Help.Section,

    fn from_command(comptime Command: type) Command_Info {
        const help: Command_Help = comptime hlp: {
            if (!@hasDecl(Command, "help")) {
                @compileError("missing help declaration in command " ++ @typeName(Command));
            }
            if (@TypeOf(Command.help) != Command_Help) {
                @compileError("invalid help type in command " ++ @typeName(Command));
            }
            break :hlp Command.help;
        };

        const fq_name: []const u8 = comptime fqn: {
            if (help.name.len == 0) {
                @compileError("missing command name for " ++ @typeName(Command));
            }

            var first_non_space: usize = 0;
            while (first_non_space < help.name.len and help.name[first_non_space] == ' ') {
                first_non_space += 1;
            }

            var last_non_space = first_non_space;
            if (last_non_space < help.name.len) {
                var i = last_non_space;
                while (i < help.name.len) : (i += 1) {
                    switch (help.name[i]) {
                        ' ' => {},
                        'a'...'z', '0'...'9', '-' => last_non_space = i,
                        else => @compileError("invalid command name for " ++ @typeName(Command)),
                    }
                }
            }

            if (last_non_space < help.name.len) last_non_space += 1;
            break :fqn help.name[0..last_non_space];
        };

        const name: []const u8 = comptime nm: {
            if (std.mem.lastIndexOfScalar(u8, fq_name, ' ')) |space| {
                break :nm fq_name[space + 1 ..];
            } else {
                break :nm fq_name;
            }
        };

        const synopsis: []const []const u8 = comptime usage: {
            if (help.synopsis.len == 0) {
                @compileError("missing synopsis in command " ++ @typeName(Command));
            }
            for (help.synopsis) |usage| {
                if (usage.len == 0) {
                    @compileError("empty usage pattern in command " ++ @typeName(Command));
                }
            }
            break :usage help.synopsis;
        };

        const short_description: []const u8 = comptime descr: {
            if (help.short_description.len == 0) {
                @compileError("missing short help description in command " ++ @typeName(Command));
            }
            if (contains_eol(help.short_description)) {
                @compileError("short help description in command " ++ @typeName(Command) ++ " must be a single line");
            }
            break :descr sanitize_description(help.short_description);
        };

        const long_description: []const u8 = comptime descr: {
            if (help.long_description.len == 0) {
                @compileError("missing long help description in command " ++ @typeName(Command));
            }
            break :descr sanitize_description(help.long_description);
        };

        const options: []const Option_Info = comptime opts: {
            for (std.meta.fields(Command)) |field| {
                if (std.mem.eql(u8, field.name, "options")) {
                    break :opts Option_Info.from_options_field(Command, field);
                }
            }
            break :opts &.{};
        };

        const sections: [help.extra_sections.len]Command_Help.Section = comptime secs: {
            var extra_sections: [help.extra_sections.len]Command_Help.Section = undefined;
            for (help.extra_sections, 0..) |sec, i| {
                if (contains_eol(sec.heading)) {
                    @compileError("help section heading in command " ++ @typeName(Command) ++ " must be a single line");
                }
                if (!is_uppercase(sec.heading)) {
                    @compileError("help section heading in command " ++ @typeName(Command) ++ " must all be uppercase");
                }
                extra_sections[i] = .{
                    .heading = sec.heading,
                    .body = sanitize_description(sec.body),
                };
            }
            break :secs extra_sections;
        };

        return Command_Info{
            .type = Command,
            .name = name,
            .fq_name = fq_name,
            .synopsis = synopsis,
            .short_description = short_description,
            .long_description = long_description,
            .options = options,
            .tag = null,
            .sections = &sections,
        };
    }

    fn from_subcommand(comptime Command: type) []Command_Info {
        if (!@hasDecl(Command, "Subcommands")) {
            return &.{};
        }

        const union_info = switch (@typeInfo(Command.Subcommands)) {
            .@"union" => |u| u,
            else => @compileError(@typeName(Command.Subcommands) ++ " is not a union"),
        };
        assert(union_info.tag_type != null);

        var subcmds: [union_info.fields.len]Command_Info = undefined;
        for (union_info.fields, 0..) |field, i| {
            subcmds[i] = Command_Info.from_command(field.type);
            subcmds[i].tag = field.name;
        }
        return &subcmds;
    }

    /// Flushes the printer
    fn print_usage(comptime cmd: Command_Info, p: *Printer) void {
        const margin0 = 0;
        const margin1 = 4;
        const margin2 = 8;

        const Usage_Table = struct {
            fn column(comptime Info: type, comptime infos: []const Info, comptime extract: fn (comptime Info) []const u8) Usage_Column {
                const cells = comptime blk: {
                    var c: [infos.len][]const u8 = undefined;
                    for (infos, 0..) |info, i| {
                        c[i] = extract(info);
                    }
                    break :blk c;
                };
                const width = comptime blk: {
                    var w: usize = 0;
                    for (infos, 0..) |_, i| {
                        w = @max(w, cells[i].len);
                    }
                    break :blk w;
                };

                return .{ .cells = &cells, .width = width };
            }
        };

        // usage
        comptime assert(cmd.synopsis.len > 0);
        p.print_line("Usage:", .{ .margin = margin0 });
        inline for (cmd.synopsis) |usage| {
            p.print_line(usage, .{ .margin = margin1, .hanging_indent = margin2, .line_split = '\n' });
        }
        p.put('\n');

        // subcommands
        const subcmds = Command_Info.from_subcommand(cmd.type);
        if (subcmds.len > 0) {
            const name_col = comptime Usage_Table.column(Command_Info, subcmds, struct {
                fn extract(comptime subcmd: Command_Info) []const u8 {
                    return subcmd.name;
                }
            }.extract);

            const descr_col = comptime Usage_Table.column(Command_Info, subcmds, struct {
                fn extract(comptime subcmd: Command_Info) []const u8 {
                    return subcmd.short_description;
                }
            }.extract);

            p.print_line("Commands:", .{ .margin = margin0 });
            p.print_usage_table(name_col, descr_col, .{ .margin = margin1, .hanging_indent = margin2 });
            p.put('\n');
        }

        // options
        if (cmd.options.len > 0) {
            const title_col = comptime Usage_Table.column(Option_Info, cmd.options, struct {
                fn extract(comptime opt: Option_Info) []const u8 {
                    const argname = if (opt.arg_name.len == 0) "" else " <" ++ opt.arg_name ++ ">";
                    return if (opt.short_name) |short|
                        std.fmt.comptimePrint("-{c}, --{s}{s}", .{ short, opt.long_name, argname })
                    else
                        std.fmt.comptimePrint("    --{s}{s}", .{ opt.long_name, argname });
                }
            }.extract);

            const descr_col = comptime Usage_Table.column(Option_Info, cmd.options, struct {
                fn extract(comptime opt: Option_Info) []const u8 {
                    return opt.short_description;
                }
            }.extract);

            p.print_line("Options:", .{ .margin = margin0 });
            p.print_usage_table(title_col, descr_col, .{ .margin = margin1, .hanging_indent = margin2 });
            p.put('\n');
        }
        p.flush();
    }

    fn print_help(comptime cmd: Command_Info) noreturn {
        var out_buf: [1024]u8 = undefined;
        var pager = Pager.init(&out_buf);
        defer pager.deinit();

        var p = &pager.printer;
        const margin0 = 0;
        const margin1 = 4;
        const margin2 = 8;

        p.put('\n');

        // name
        p.printf_line("{f}", .{pager.bold("NAME")}, .{ .margin = margin0 });
        p.print_line(cmd.fq_name ++ " - " ++ cmd.short_description, .{ .margin = margin1 });
        p.put('\n');

        // synopsis
        p.printf_line("{f}", .{pager.bold("SYNOPSIS")}, .{ .margin = margin0 });
        inline for (cmd.synopsis) |usage| {
            p.print_line(usage, .{ .margin = margin1, .hanging_indent = margin2, .line_split = '\n' });
        }
        p.put('\n');

        // description
        p.printf_line("{f}", .{pager.bold("DESCRIPTION")}, .{ .margin = margin0 });
        p.print_lines(cmd.long_description, .{ .margin = margin1 });
        p.put('\n');

        // options
        if (cmd.options.len > 0) {
            p.printf_line("{f}", .{pager.bold("OPTIONS")}, .{ .margin = margin0 });
            inline for (cmd.options) |opt| {
                const argname = if (opt.arg_name.len == 0) "" else " <" ++ opt.arg_name ++ ">";
                if (opt.short_name) |short| {
                    p.printf_line("-{f}, --{f}{s}", .{ pager.bold(&[1]u8{short}), pager.bold(opt.long_name), argname }, .{ .margin = margin1 });
                } else {
                    p.printf_line("--{f}{s}", .{ pager.bold(opt.long_name), argname }, .{ .margin = margin1 });
                }
                p.print_lines(opt.long_description, .{ .margin = margin2 });
                p.put('\n');
            }
        }

        // subcommands
        const subcmds = Command_Info.from_subcommand(cmd.type);
        if (subcmds.len > 0) {
            p.printf_line("{f}", .{pager.bold("COMMANDS")}, .{ .margin = margin0 });
            inline for (subcmds) |subcmd| {
                p.printf_line("{f}", .{pager.bold(subcmd.name)}, .{ .margin = margin1 });
                p.print_lines(subcmd.short_description, .{ .margin = margin2 });
                p.put('\n');
            }
        }

        // sections
        inline for (cmd.sections) |section| {
            p.printf_line("{f}", .{pager.bold(section.heading)}, .{ .margin = margin0 });
            p.print_lines(section.body, .{ .margin = margin1 });
            p.put('\n');
        }
    }
};

/// Information about a single option. An option must contain a public declaration `help`
/// of type `Option_Help(T)`.
const Option_Info = struct {
    field: []const u8,
    short_name: ?u8,
    long_name: []const u8,
    arg_name: []const u8, // empty for boolean options
    short_description: []const u8,
    long_description: []const u8,

    fn from_options_field(comptime Command: type, comptime options_field: std.builtin.Type.StructField) []const Option_Info {
        const options = comptime blk: {
            const struct_info = switch (@typeInfo(options_field.type)) {
                .@"struct" => |s| s,
                else => @compileError("options for command " ++ @typeName(Command) ++ " must be a struct"),
            };

            var opts: [struct_info.fields.len]Option_Info = undefined;
            for (struct_info.fields, 0..) |field, i| {
                const Opt = field.type;
                const fq_fieldname = @typeName(Command) ++ "." ++ field.name;
                if (!@hasDecl(Opt, "help")) {
                    @compileError("missing help declaration in option " ++ fq_fieldname);
                }

                const help: Option_Help(Opt.Type) = hlp: {
                    if (@TypeOf(Opt.help) != Option_Help(Opt.Type)) {
                        @compileError("invalid help type in option " ++ fq_fieldname ++ ": " ++ @typeName(@TypeOf(Opt.help)) ++ " (expected " ++ @typeName(Option_Help(Opt.Type)) ++ ")");
                    }
                    break :hlp Opt.help;
                };

                const short_name: ?u8 = name: {
                    if (help.short_name) |ch| {
                        switch (ch) {
                            'a'...'z', 'A'...'Z', '0'...'9' => {},
                            else => @compileError("invalid short name for option " ++ fq_fieldname),
                        }
                    }

                    break :name help.short_name;
                };

                const long_name: []const u8 = name: {
                    if (std.mem.eql(u8, help.name, "help")) {
                        @compileError("option name 'help' is a reserved option name");
                    }
                    if (help.name.len == 0) {
                        @compileError("missing option name " ++ fq_fieldname);
                    }
                    if (contains_whitespace(help.name)) {
                        @compileError("invalid option name " ++ fq_fieldname);
                    }
                    break :name sanitize_name(help.name);
                };

                const arg_name: []const u8 = name: {
                    const arg = help.argument_name orelse break :name "";
                    if (contains_whitespace(arg)) {
                        @compileError("invalid argument name of option " ++ fq_fieldname);
                    }
                    break :name arg;
                };

                const short_description: []const u8 = descr: {
                    if (help.short_description.len == 0) {
                        @compileError("missing short help description in option " ++ fq_fieldname);
                    }
                    if (contains_eol(help.short_description)) {
                        @compileError("short help description in option " ++ fq_fieldname ++ " must be a single line");
                    }
                    break :descr help.short_description;
                };

                const long_description: []const u8 = descr: {
                    if (help.long_description.len == 0) {
                        @compileError("missing long help description in option " ++ fq_fieldname);
                    }
                    break :descr sanitize_description(help.long_description);
                };

                opts[i] = Option_Info{
                    .field = field.name,
                    .short_name = short_name,
                    .long_name = long_name,
                    .arg_name = arg_name,
                    .short_description = short_description,
                    .long_description = long_description,
                };
            }
            break :blk opts;
        };
        return &options;
    }

    fn contains_whitespace(comptime text: []const u8) bool {
        for (text) |c| {
            switch (c) {
                ' ', '\t', '\r', '\n' => return true,
                else => {},
            }
        }
        return false;
    }
};

fn sanitize_name(comptime name: []const u8) []const u8 {
    const sanitized = comptime blk: {
        var res: [name.len]u8 = undefined;
        for (name, 0..) |c, i| {
            res[i] = switch (c) {
                'a'...'z', 'A'...'Z', '0'...'9' => c,
                '_', '-' => '-',
                else => @compileError("invalid character in cli name '" ++ name ++ "'"),
            };
        }
        break :blk res;
    };
    return &sanitized;
}

fn sanitize_description(comptime text: []const u8) []const u8 {
    @setEvalBranchQuota(100_000);

    const sanitized = comptime blk: {
        var res: [text.len]u8 = undefined;
        var prev_was_eol: bool = false;
        for (text, 0..) |c, i| {
            var ch = c;
            switch (ch) {
                '\n' => {
                    if (prev_was_eol) {
                        res[i - 1] = '\n'; // preserve empty line
                    } else {
                        ch = ' ';
                    }
                    prev_was_eol = true;
                },
                '\t', ' ' => {
                    if (prev_was_eol) {
                        res[i - 1] = '\n';
                    } else {
                        ch = ' ';
                    }
                    prev_was_eol = false;
                },
                else => {
                    prev_was_eol = false;
                },
            }
            res[i] = ch;
        }
        break :blk res;
    };
    return &sanitized;
}

fn contains_eol(comptime text: []const u8) bool {
    for (text) |c| {
        switch (c) {
            '\r', '\n' => return true,
            else => {},
        }
    }
    return false;
}

fn is_uppercase(comptime text: []const u8) bool {
    for (text) |c| {
        if ('a' <= c and c <= 'z') return false;
    }
    return true;
}

fn join_enum_values(comptime E: type) []const u8 {
    const fields = std.meta.fields(E);
    return switch (fields.len) {
        0 => unreachable,
        1 => "'" ++ fields[0].name ++ "'",
        2 => "'" ++ fields[0].name ++ "' or '" ++ fields[1].name ++ "'",
        3 => str: {
            var text: []const u8 = "";
            for (fields[0 .. fields.len - 1]) |field| {
                text = text ++ "'" ++ field.name ++ "', ";
            }
            break :str text ++ "or '" ++ fields[fields.len - 1].name ++ "'";
        },
    };
}