const std = @import("std");

// req: cli/002
fn usage(file: std.Io.File, io: std.Io) !void {
    var buffer: [1024]u8 = undefined;
    var writer = file.writer(io, &buffer);
    try writer.interface.writeAll(
        \\usage: lines FILE START [END]
        \\
        \\Print a 1-based inclusive line range from FILE to stdout.
        \\START and END must be positive decimal integers; END defaults to START.
        \\No writes, no regex, no program language, no stdin prompts.
        \\
        \\output:
        \\  stdout: selected file lines only, with original line order preserved
        \\  stderr: usage and diagnostics only
        \\
        \\exit status:
        \\  0 on success, 1 on runtime/input errors, 2 on usage errors
        \\
        \\examples:
        \\  lines src/main.zig 40 80
        \\  lines README.md 1 20 | grep usage
        \\
    );
    try writer.interface.flush();
}

fn fail(io: std.Io, comptime fmt: []const u8, args: anytype) noreturn {
    var buffer: [512]u8 = undefined;
    var writer = std.Io.File.stderr().writer(io, &buffer);
    writer.interface.print(fmt, args) catch {};
    writer.interface.flush() catch {};
    std.process.exit(1);
}

fn parseLineNo(text: []const u8, name: []const u8, io: std.Io) u64 {
    const value = std.fmt.parseInt(u64, text, 10) catch fail(io, "lines: invalid {s}: {s}\n", .{ name, text });
    if (value == 0) fail(io, "lines: {s} must be positive\n", .{name});
    return value;
}

fn stdoutFailure(out: *std.Io.File.Writer, io: std.Io) noreturn {
    if (out.err) |err| switch (err) {
        error.BrokenPipe => std.process.exit(0),
        else => fail(io, "lines: stdout: {s}\n", .{@errorName(err)}),
    };
    fail(io, "lines: stdout: WriteFailed\n", .{});
}

fn writeStdout(out: *std.Io.File.Writer, io: std.Io, bytes: []const u8) void {
    out.interface.writeAll(bytes) catch stdoutFailure(out, io);
}

fn flushStdout(out: *std.Io.File.Writer, io: std.Io) void {
    out.interface.flush() catch stdoutFailure(out, io);
}

// req: cli/001
// req: robustness/002
// req: robustness/003
// req: performance/001
fn streamRange(file: std.Io.File, io: std.Io, start: u64, end: u64) void {
    var read_buffer: [64 * 1024]u8 = undefined;
    var reader_file = file.readerStreaming(io, &read_buffer);
    var chunk: [64 * 1024]u8 = undefined;

    var out_buffer: [64 * 1024]u8 = undefined;
    var stdout_file = std.Io.File.stdout().writerStreaming(io, &out_buffer);

    var line_no: u64 = 1;
    while (line_no <= end) {
        const n = reader_file.interface.readSliceShort(&chunk) catch |err| fail(io, "lines: read failed: {s}\n", .{@errorName(err)});
        if (n == 0) break;

        var pos: usize = 0;
        while (pos < n and line_no <= end) {
            const rest = chunk[pos..n];
            const line_end = if (std.mem.indexOfScalar(u8, rest, '\n')) |newline| pos + newline + 1 else n;
            if (line_no >= start) writeStdout(&stdout_file, io, chunk[pos..line_end]);
            if (line_end <= n and chunk[line_end - 1] == '\n') line_no += 1;
            pos = line_end;
        }
    }
    flushStdout(&stdout_file, io);
}

pub fn main(init: std.process.Init) !void {
    const allocator = init.gpa;
    var args = try std.process.Args.Iterator.initAllocator(init.minimal.args, allocator);
    defer args.deinit();
    _ = args.skip();
    const path = args.next() orelse {
        try usage(std.Io.File.stderr(), init.io);
        std.process.exit(2);
    };
    if (std.mem.eql(u8, path, "--help") or std.mem.eql(u8, path, "-h")) {
        try usage(std.Io.File.stdout(), init.io);
        return;
    }
    const start_text = args.next() orelse {
        try usage(std.Io.File.stderr(), init.io);
        std.process.exit(2);
    };
    const maybe_end_text = args.next();
    if (args.next() != null) {
        try usage(std.Io.File.stderr(), init.io);
        std.process.exit(2);
    }

    // req: cli/001
    // req: cli/001.e1
    // req: cli/003
    const start = parseLineNo(start_text, "START", init.io);
    const end = if (maybe_end_text) |end_text| parseLineNo(end_text, "END", init.io) else start;
    if (end < start) fail(init.io, "lines: END must be >= START\n", .{});

    const file = std.Io.Dir.cwd().openFile(init.io, path, .{ .mode = .read_only, .allow_directory = false }) catch |err| fail(init.io, "lines: {s}: {s}\n", .{ path, @errorName(err) });
    defer file.close(init.io);
    streamRange(file, init.io, start, end);
}