// 用 volatile 以防止我们针对内存的操作被优化掉
// 因为我们是通过对内存地址 0xb8000 写入来操作 VGA Buffer 的
// 这实际上是通过内存操作的副作用来实现的
// 但由于我们从未访问过这段内存，所以它可能会在未来的 Rust 编译器中被优化掉
// 为了告诉编译器，这部分针对内存的操作是我们故意的，就需要用到 volatile crate
use core::fmt;
use lazy_static::lazy_static;
use spin::Mutex;
use volatile::Volatile;

#[allow(dead_code)] // 允许未使用的代码
#[derive(Debug, Clone, Copy, PartialEq, Eq)] // 启用 copy 语义
#[repr(u8)] // 每个 enum variant 都以 u8 存储
pub enum Color {
    Black = 0,
    Blue = 1,
    Green = 2,
    Cyan = 3,
    Red = 4,
    Magenta = 5,
    Brown = 6,
    LightGray = 7,
    DarkGray = 8,
    LightBlue = 9,
    LightGreen = 10,
    LightCyan = 11,
    LightRed = 12,
    Pink = 13,
    Yellow = 14,
    White = 15,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(transparent)] // 为了保证 ColorCode 和 u8 有同样的数据布局
struct ColorCode(u8);

impl ColorCode {
    fn new(foreground: Color, background: Color) -> ColorCode {
        ColorCode((background as u8) << 4 | (foreground as u8))
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(C)] // 使数据结构的布局和 C 一样，这样才能保证每个成员的顺序一致
struct ScreenChar {
    ascii_character: u8,
    color_code: ColorCode,
}

const BUFFER_HEIGHT: usize = 25;
const BUFFER_WIDTH: usize = 80;

#[repr(transparent)]
struct Buffer {
    chars: [[Volatile<ScreenChar>; BUFFER_WIDTH]; BUFFER_HEIGHT],
}

pub struct Writer {
    column_position: usize,      // 当前光标所在列
    color_code: ColorCode,       // 当前颜色代码
    buffer: &'static mut Buffer, // VGA Buffer 的引用，生命周期是 'static
}

impl Writer {
    pub fn write_byte(&mut self, byte: u8) {
        match byte {
            // 如果碰到 '\n' 则创建新行
            b'\n' => self.new_line(),
            byte => {
                // 如果列数超出限定的阈值，则创建新行
                if self.column_position >= BUFFER_WIDTH {
                    self.new_line();
                }
                let row = BUFFER_HEIGHT - 1;
                let col = self.column_position;
                let color_code = self.color_code;
                // 将 ScreenChar 写入到 Buffer 的相应位置
                self.buffer.chars[row][col].write(ScreenChar {
                    ascii_character: byte,
                    color_code,
                });
                self.column_position += 1;
            }
        }
    }
    fn new_line(&mut self) {
        // 将所有字符上移一行
        for row in 1..BUFFER_HEIGHT {
            for col in 0..BUFFER_WIDTH {
                let character = self.buffer.chars[row][col].read();
                self.buffer.chars[row - 1][col].write(character);
            }
        }
        // 清空最下面的一行
        self.clear_row(BUFFER_HEIGHT - 1);
        // 将当前列的位置设为 0
        self.column_position = 0;
    }
    fn clear_row(&mut self, row: usize) {
        let blank = ScreenChar {
            ascii_character: b' ',
            color_code: self.color_code,
        };
        for col in 0..BUFFER_WIDTH {
            self.buffer.chars[row][col].write(blank);
        }
    }
    pub fn write_string(&mut self, s: &str) {
        for byte in s.bytes() {
            // Rust 字符串是以 UTF-8 编码的，然而 VGA Buffer 只支持 ASCII字符，所以有可能存在无法打印的字符
            match byte {
                // 当在 ASCII 字符范围内时，打印它
                0x20..=0x7e | b'\n' => self.write_byte(byte),
                // 否则打印 ■
                _ => self.write_byte(0xfe),
            }
        }
    }
}

impl fmt::Write for Writer {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.write_string(s);
        Ok(())
    }
}

// 将 Writer 设置为静态以便调用
lazy_static! {  // 内置的 static 会用到标准库里的一些东西，所以这里用 lazy_static
    pub static ref WRITER: Mutex<Writer> = Mutex::new(Writer {  // 用互斥锁来防止冲突
        column_position: 0,
        color_code: ColorCode::new(Color::Yellow, Color::Black),
        buffer: unsafe { &mut *(0xb8000 as *mut Buffer) }
    });
}

// 将宏导出到 crate 的根，这样我们就可以用
// use crate::println
// 来使用它
#[macro_export]
macro_rules! print {    // print!()
    ($($arg:tt)*) => ($crate::vga_buffer::_print(format_args!($($arg)*)));
}

#[macro_export]
macro_rules! println {  // println!()
    () => ($crate::print!("\n"));
    ($($arg:tt)*) => ($crate::print!("{}\n", format_args!($($arg)*)));
}

// 由于宏需要从外部调用这个函数，所以它必须是公开的
// 用 #[doc(hidden)] 来阻止它生成文档
#[doc(hidden)]
pub fn _print(args: fmt::Arguments) {
    use core::fmt::Write;
    use x86_64::instructions::interrupts;

    interrupts::without_interrupts(|| {
        WRITER.lock().write_fmt(args).unwrap();
    });
}

#[test_case]
fn test_println_simple() {
    // 测试单行 println!
    println!("test_println_simple output");
}

#[test_case]
fn test_println_many() {
    // 测试多行 println!
    for _ in 0..10 {
        println!("test_println_many output");
    }
}

#[test_case]
fn test_println_output() {
    // 测试字符是否真的打印到了屏幕上
    use core::fmt::Write;
    use x86_64::instructions::interrupts;

    let s = "Some test string that fits on a single line";
    interrupts::without_interrupts(|| {
        let mut writer = WRITER.lock();
        writeln!(writer, "\n{}", s).expect("writeln failed");
        for (i, c) in s.chars().enumerate() {
            let screen_char = writer.buffer.chars[BUFFER_HEIGHT - 2][i].read();
            assert_eq!(char::from(screen_char.ascii_character), c);
        }
    });
}