//! VGA text mode driver.
//!
//! In text mode the screen is a 25x80 grid of 2-byte cells memory-mapped at
//! physical address 0xb8000: byte 0 = ASCII char, byte 1 = colour attribute
//! (low nibble foreground, high nibble background).
//! The bootloader identity-maps that address, so we can write to it directly.

use core::fmt;
use lazy_static::lazy_static;
use spin::Mutex;
use volatile::Volatile;

#[allow(dead_code)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
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,
}

/// Foreground + background packed into one attribute byte.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(transparent)]
struct ColorCode(u8);

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

/// One on-screen cell. `repr(C)` guarantees the char-then-colour layout.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(C)]
struct ScreenChar {
    ascii_character: u8,
    color_code: ColorCode,
}

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

/// The whole screen. `Volatile` tells the compiler these writes have side
/// effects (they change what's on the monitor) so it must not drop them.
#[repr(transparent)]
struct Buffer {
    chars: [[Volatile<ScreenChar>; BUFFER_WIDTH]; BUFFER_HEIGHT],
}

/// Always writes on the bottom row and scrolls everything up on newline.
pub struct Writer {
    column_position: usize,
    color_code: ColorCode,
    buffer: &'static mut Buffer,
}

impl Writer {
    pub fn write_byte(&mut self, byte: u8) {
        match byte {
            b'\n' => self.new_line(),
            byte => {
                if self.column_position >= BUFFER_WIDTH {
                    self.new_line();
                }
                let row = BUFFER_HEIGHT - 1;
                let col = self.column_position;
                self.buffer.chars[row][col].write(ScreenChar {
                    ascii_character: byte,
                    color_code: self.color_code,
                });
                self.column_position += 1;
            }
        }
    }

    pub fn write_string(&mut self, s: &str) {
        for byte in s.bytes() {
            match byte {
                // printable ASCII or newline
                0x20..=0x7e | b'\n' => self.write_byte(byte),
                // VGA only knows code page 437; show anything else as a block
                _ => self.write_byte(0xfe),
            }
        }
    }

    fn new_line(&mut self) {
        for row in 1..BUFFER_HEIGHT {
            for col in 0..BUFFER_WIDTH {
                let c = self.buffer.chars[row][col].read();
                self.buffer.chars[row - 1][col].write(c);
            }
        }
        self.clear_row(BUFFER_HEIGHT - 1);
        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);
        }
    }
}

/// Lets us use Rust's formatting machinery (`write!`, `{}`) with the Writer.
impl fmt::Write for Writer {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.write_string(s);
        Ok(())
    }
}

lazy_static! {
    /// Global writer. The raw pointer cast to 0xb8000 is the one `unsafe`
    /// spot: we promise that address really is the VGA buffer.
    pub static ref WRITER: Mutex<Writer> = Mutex::new(Writer {
        column_position: 0,
        color_code: ColorCode::new(Color::LightGreen, Color::Black),
        buffer: unsafe { &mut *(0xb8000 as *mut Buffer) },
    });
}

/// Write formatted text at a fixed screen position (row, col) in black on
/// light grey, without moving the normal cursor. Used for status lines.
/// Text that runs past the end of the row is cut off.
pub fn print_at(row: usize, col: usize, args: fmt::Arguments) {
    /// A tiny fmt::Write target that writes cells left to right on one row.
    struct At<'a> {
        writer: &'a mut Writer,
        row: usize,
        col: usize,
    }
    impl fmt::Write for At<'_> {
        fn write_str(&mut self, s: &str) -> fmt::Result {
            let color_code = ColorCode::new(Color::Black, Color::LightGray);
            for byte in s.bytes() {
                if self.col < BUFFER_WIDTH {
                    let ascii_character = byte;
                    self.writer.buffer.chars[self.row][self.col]
                        .write(ScreenChar { ascii_character, color_code });
                    self.col += 1;
                }
            }
            Ok(())
        }
    }
    use core::fmt::Write;
    x86_64::instructions::interrupts::without_interrupts(|| {
        let mut writer = WRITER.lock();
        let _ = At { writer: &mut writer, row, col }.write_fmt(args);
    });
}

/// Draw " uptime: N s " right-aligned on the top row. Called from the timer
/// interrupt handler about once a second.
pub fn show_uptime(seconds: u64) {
    print_at(0, BUFFER_WIDTH - 15, format_args!(" uptime: {:>4}s ", seconds));
}

/// Erase the character before the cursor on the bottom row (Backspace).
/// Stops at the start of the line (no going back to the previous row).
pub fn backspace() {
    x86_64::instructions::interrupts::without_interrupts(|| {
        let mut writer = WRITER.lock();
        if writer.column_position > 0 {
            writer.column_position -= 1;
            let col = writer.column_position;
            let blank = ScreenChar { ascii_character: b' ', color_code: writer.color_code };
            writer.buffer.chars[BUFFER_HEIGHT - 1][col].write(blank);
        }
    });
}

#[doc(hidden)]
pub fn _print(args: fmt::Arguments) {
    use core::fmt::Write;
    // Disable interrupts while we hold the lock. Otherwise an interrupt
    // handler that also prints would spin forever waiting for a lock held by
    // the code it interrupted (a deadlock).
    x86_64::instructions::interrupts::without_interrupts(|| {
        WRITER.lock().write_fmt(args).unwrap();
    });
}

/// `print!` for the VGA screen.
#[macro_export]
macro_rules! print {
    ($($arg:tt)*) => ($crate::vga_buffer::_print(format_args!($($arg)*)));
}

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

#[test_case]
fn test_println_simple() {
    println!("test_println_simple output");
}

#[test_case]
fn test_println_output() {
    // The printed line ends up on the second-to-last row (println adds "\n").
    use core::fmt::Write;
    let s = "Some test string that fits on a single line";
    // Hold the lock with interrupts off, so the timer can't redraw meanwhile.
    x86_64::instructions::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);
        }
    });
}