// this is a minimal 16550 UART driver for COM1, this is mainly going
// to be used for debugging since its really simple to setup.
// Fun fact: i was drunk when i wrote this
//
// see: https://wiki.osdev.org/Serial_Ports

const COM1: u16 = 0x3F8;

inline fn outb(port: u16, val: u8) void {
    asm volatile ("outb %[val], %[port]"
        :
        : [val] "{al}" (val),
          [port] "N{dx}" (port),
    );
}

inline fn inb(port: u16) u8 {
    return asm volatile ("inb %[port], %[result]"
        : [result] "={al}" (-> u8),
        : [port] "N{dx}" (port),
    );
}

// 38400 buard
// 8 data bits
// no parity
// one stop bit
// FIFO on
// Idempotent so it should be relatively safe to call
// from the panic handler.
pub fn init() void {
    outb(COM1 + 1, 0x00); // no interrupts
    outb(COM1 + 3, 0x80); // DLAB on: the next two writes set the baud divisor
    outb(COM1 + 0, 0x03); // divisor 3 -> 38400 baud (low byte)
    outb(COM1 + 1, 0x00); // divisor high byte
    outb(COM1 + 3, 0x03); // DLAB off, 8N1
    outb(COM1 + 2, 0xC7); // enable + clear FIFOs, 14-byte trigger
    outb(COM1 + 4, 0x0B); // RTS/DSR set
}

// every byte we send to the port is also captured here so the revo
// console can render the same stream
var mirror: [1 << 15]u8 = undefined; // 32 KiB, power of two
var m_head: usize = 0; // next byte the console will render
var m_tail: usize = 0; // next slot putc will write

// COM1+5 (bit 5) = transmit holding register and
//                  spin until it's clear to send
fn putc(c: u8) void {
    while (inb(COM1 + 5) & 0x20 == 0) {}
    outb(COM1, c);
    mirror[m_tail & (mirror.len - 1)] = c;
    m_tail +%= 1;
}

// next un-rendered byte, or null if caught up. if the console fell far enough
// behind to lap the ring, drop the oldest bytes.
pub fn mirrorNext() ?u8 {
    if (m_head == m_tail) return null;
    if (m_tail -% m_head > mirror.len) m_head = m_tail -% mirror.len;
    const c = mirror[m_head & (mirror.len - 1)];
    m_head +%= 1;
    return c;
}

// a raw serial console needs \r\n, but revo (and most sane code) emits bare \n.
// inject the \r ourselves, skipping it when the \n already has one so existing
// "\r\n" strings don't turn into "\r\r\n".
pub fn write(s: []const u8) void {
    var prev: u8 = 0;
    for (s) |c| {
        if (c == '\n' and prev != '\r') putc('\r');
        putc(c);
        prev = c;
    }
}

// dump a u64 as decimal. handy for "usable: N MiB" type logging.
pub fn writeDec(value: u64) void {
    if (value == 0) return putc('0');
    var buf: [20]u8 = undefined;
    var i: usize = buf.len;
    var v = value;
    while (v > 0) {
        i -= 1;
        buf[i] = '0' + @as(u8, @intCast(v % 10));
        v /= 10;
    }
    write(buf[i..]);
}

// dump a u64 as 0x-prefixed 16-digit hex.
pub fn writeHex(value: u64) void {
    const digits = "0123456789abcdef";
    var buf: [18]u8 = undefined;
    buf[0] = '0';
    buf[1] = 'x';
    var i: usize = 0;
    while (i < 16) : (i += 1) {
        const shift: u6 = @intCast((15 - i) * 4);
        buf[2 + i] = digits[(value >> shift) & 0xF];
    }
    write(&buf);
}