//! 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.
//!
//! Since stage 13 the same `Writer` can draw on the virtio-gpu framebuffer
//! instead (fbcon.rs): `use_framebuffer` switches over at boot when there
//! is a GPU; without one, this VGA text mode stays the console.

use crate::fbcon::{FbConsole, Rect};
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.
///
/// Two possible screens behind it: the VGA text buffer (always there in
/// QEMU, and what the test kernels use), or -- once virtio-gpu is up and
/// `use_framebuffer` was called -- the framebuffer console (fbcon.rs),
/// which has more and smaller cells. Everything below goes through
/// `put`/`scroll`/`width`/`height`, so it works the same on both.
pub struct Writer {
    column_position: usize,
    color_code: ColorCode,
    buffer: &'static mut Buffer,
    fb: Option<FbConsole>,
}

impl Writer {
    fn width(&self) -> usize {
        self.fb.as_ref().map_or(BUFFER_WIDTH, |f| f.cols())
    }

    fn height(&self) -> usize {
        self.fb.as_ref().map_or(BUFFER_HEIGHT, |f| f.rows())
    }

    /// Set one cell.
    fn put(&mut self, row: usize, col: usize, ascii_character: u8, color_code: ColorCode) {
        match &mut self.fb {
            Some(fb) => fb.put(row, col, ascii_character, color_code.0),
            None => self.buffer.chars[row][col].write(ScreenChar { ascii_character, color_code }),
        }
    }

    pub fn write_byte(&mut self, byte: u8) {
        match byte {
            b'\n' => self.new_line(),
            byte => {
                if self.column_position >= self.width() {
                    self.new_line();
                }
                let row = self.height() - 1;
                let col = self.column_position;
                self.put(row, col, byte, self.color_code);
                self.column_position += 1;
            }
        }
    }

    pub fn write_string(&mut self, s: &str) {
        for c in s.chars() {
            let byte = match (c, self.fb.is_some()) {
                ('\n', _) => b'\n',
                // printable ASCII is the same everywhere
                (' '..='~', _) => c as u8,
                // the framebuffer font has Latin-1 and a few more
                (c, true) => crate::font::glyph_index(c),
                // VGA only knows code page 437; show anything else as a block
                (_, false) => 0xfe,
            };
            self.write_byte(byte);
        }
    }

    fn new_line(&mut self) {
        match &mut self.fb {
            Some(fb) => fb.scroll_up(self.color_code.0),
            None => {
                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) {
        for col in 0..self.width() {
            self.put(row, col, b' ', self.color_code);
        }
    }

    /// After a print: framebuffer only -- draw the cursor and send the
    /// changed pixels to the screen. (VGA needs nothing: the card reads its
    /// buffer itself, 70 times a second.)
    fn finish(&mut self) {
        let (row, col) = (self.height() - 1, self.column_position);
        if let Some(fb) = &mut self.fb {
            fb.set_cursor(Some((row, col)));
            fb.flush();
        }
    }
}

/// 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) },
        fb: None,
    });
}

/// Switch the console to the framebuffer (called once virtio-gpu is up).
/// The text already on the VGA screen comes along, so the boot messages
/// stay visible. Returns the new size in characters.
pub fn use_framebuffer(fb: crate::virtio::gpu::FbInfo) -> (usize, usize) {
    x86_64::instructions::interrupts::without_interrupts(|| {
        let mut writer = WRITER.lock();
        let mut con = FbConsole::new(fb);
        let (cols, rows) = (con.cols(), con.rows());
        // VGA's 25 rows go to the bottom 25 rows of the new screen.
        for row in 0..BUFFER_HEIGHT.min(rows) {
            for col in 0..BUFFER_WIDTH.min(cols) {
                let c = writer.buffer.chars[row][col].read();
                con.put(rows - BUFFER_HEIGHT.min(rows) + row, col, c.ascii_character, c.color_code.0);
            }
        }
        writer.fb = Some(con);
        writer.finish();
        (cols, rows)
    })
}

/// Is the console on the framebuffer?
pub fn on_framebuffer() -> bool {
    x86_64::instructions::interrupts::without_interrupts(|| WRITER.lock().fb.is_some())
}

/// Confine the framebuffer console to a rectangle of the screen (the
/// `tile` demo puts it in one tile); a zero-sized rect = the whole screen.
/// Returns the new (columns, rows), or None on the VGA console.
pub fn set_viewport(mut view: Rect) -> Option<(usize, usize)> {
    x86_64::instructions::interrupts::without_interrupts(|| {
        let mut writer = WRITER.lock();
        let attr = writer.color_code.0;
        let fb = writer.fb.as_mut()?;
        if view.w == 0 || view.h == 0 {
            let info = crate::virtio::gpu::framebuffer()?;
            view = Rect { x: 0, y: 0, w: info.width, h: info.height };
        }
        fb.set_view(view, attr);
        let size = (fb.cols(), fb.rows());
        writer.column_position = writer.column_position.min(size.0);
        writer.finish();
        Some(size)
    })
}

/// The console's current rectangle in pixels (framebuffer only).
pub fn viewport() -> Option<Rect> {
    x86_64::instructions::interrupts::without_interrupts(|| WRITER.lock().fb.as_ref().map(|f| f.view()))
}

/// Text size of the console in characters (columns, rows).
pub fn size() -> (usize, usize) {
    x86_64::instructions::interrupts::without_interrupts(|| {
        let writer = WRITER.lock();
        (writer.width(), writer.height())
    })
}

/// 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 < self.writer.width() && self.row < self.writer.height() {
                    self.writer.put(self.row, self.col, byte, 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);
        if let Some(fb) = &mut writer.fb {
            fb.flush();
        }
    });
}

/// 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) {
    let width = size().0;
    print_at(0, width.saturating_sub(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 (row, col) = (writer.height() - 1, writer.column_position);
            let color_code = writer.color_code;
            writer.put(row, col, b' ', color_code);
        }
        writer.finish();
    });
}

/// Clear the screen (rows 1..; row 0 is the status line) and start
/// writing at the left of the bottom row again.
pub fn clear_screen() {
    x86_64::instructions::interrupts::without_interrupts(|| {
        let mut writer = WRITER.lock();
        for row in 1..writer.height() {
            writer.clear_row(row);
        }
        writer.column_position = 0;
        writer.finish();
    });
}

#[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(|| {
        let mut writer = WRITER.lock();
        writer.write_fmt(args).unwrap();
        writer.finish();
    });
}

/// `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);
        }
    });
}