//! UART0 as the editor's terminal: bytes out, bytes in, and nothing else.
//!
//! This is the whole of the firmware's I/O. There is no framebuffer and no keyboard; the board
//! emits ANSI and consumes ANSI, and the terminal emulator on the far end of the CH340 does the
//! rest of the work - including answering the editor's own capability queries, which travel down
//! this wire like any other bytes.
//!
//! Deliberately not a `std.Io.Writer`. The ANSI encoding lives on the other side of the C ABI, next
//! to the vaxis that produces it (see `src/pardes/app.zig` for why the seam is there and not
//! elsewhere), so what crosses into this file is already a finished run of bytes. A writer here
//! would be a second buffer in front of one that already exists.
//!
//! Two decisions worth stating, because both are measurements rather than preferences.
//!
//! **Batched FIFO access.** The naive push is `while (txFree() == 0) {}` then `pushByte`, once per
//! byte: one MMIO read per byte at best, many while the FIFO is full. Reading `txFree` once and
//! then pushing that many cuts the status reads by up to the FIFO depth (128, `hal/uart.zig:52`).
//! At 115200 the wire costs ~86 us per byte and dwarfs either version, so today this is merely
//! free - and it stops being free the moment the divider is raised.
//!
//! **UART0's configuration is never touched.** Not the divider, not the format, not the pad
//! routing, and above all not `reset()`. The second-stage bootloader configured this block, and
//! `hal/uart.zig:195-211` records what happens if it is reset: UART_CLKDIV returns to its power-on
//! value, the console turns to garbage mid-sentence, and the board takes a watchdog reset with
//! nothing readable left to explain it. Everything here touches FIFO offset 0x000 and the status
//! register, and nothing else.

const hal = @import("hal");

/// UART0: the instance the CH340 is wired to, and the one the ROM and bootloader configured.
const uart0 = hal.uart.Uart.init(0);

/// Push `bytes` into the TX FIFO, blocking while it is full.
///
/// The spin is normally bounded by the wire - a full 128-byte FIFO drains in 11 ms at 115200 - and
/// dropping instead of waiting would truncate an escape sequence, leaving the host terminal in the
/// wrong colour for the rest of the session. So the wait is real backpressure.
///
/// But it is BOUNDED, for the reason `hal/uart.zig:182-186` gives about `update()`: a UART whose
/// core clock has been gated never makes progress, and "on a board with no debugger an infinite
/// spin is indistinguishable from a crash". That is not hypothetical here - it is how this port
/// spent an afternoon: output stopped mid-boot with no panic and no watchdog (the RTC watchdog
/// having been correctly disabled), which looked like a hang in whatever code came next rather than
/// a stalled transmitter. A bounded wait turns that into visibly dropped output plus a counter,
/// which is a diagnosis instead of a mystery.
///
/// The limit is per burst, not per call, and generous: 1,000,000 status reads is far longer than
/// any legitimate drain and still a fraction of a second.
pub fn write(bytes: []const u8) void {
    var rest = bytes;
    while (rest.len > 0) {
        // One status read per burst, not per byte.
        var room = uart0.txFree();
        var spins: u32 = 0;
        while (room == 0) {
            spins += 1;
            if (spins > 1_000_000) {
                dropped +%= @intCast(rest.len);
                return;
            }
            room = uart0.txFree();
        }
        const n = @min(room, rest.len);
        for (rest[0..n]) |b| uart0.pushByte(b);
        rest = rest[n..];
    }
}

/// Bytes abandoned because the transmitter stopped making progress. Nonzero means the console is
/// lying about what happened, so it is worth printing.
pub var dropped: u32 = 0;

/// One byte, for callers that must not touch `.rodata` to say anything - which during bring-up is
/// the difference between a diagnostic and a second copy of the bug being diagnosed.
pub fn writeByte(b: u8) void {
    var spins: u32 = 0;
    while (uart0.txFree() == 0) {
        spins += 1;
        if (spins > 1_000_000) {
            dropped +%= 1;
            return;
        }
    }
    uart0.pushByte(b);
}

/// Emit `n` bytes read from `addr` as two hex digits each, computing the digits arithmetically so
/// nothing here reads a lookup table. Used to answer "does a load from this address return what the
/// linker put there", which is not a question a string literal can be trusted to ask.
pub fn dumpHex(addr: u32, n: u32) void {
    const p: [*]const volatile u8 = @ptrFromInt(addr);
    var i: u32 = 0;
    while (i < n) : (i += 1) {
        const byte = p[i];
        for ([2]u8{ byte >> 4, byte & 0xf }) |nib| {
            writeByte(if (nib < 10) '0' + nib else 'a' + (nib - 10));
        }
    }
    writeByte('\r');
    writeByte('\n');
}

/// A u32 as eight hex digits, reading no memory at all.
pub fn dumpWord(v: u32) void {
    var shift: u5 = 28;
    while (true) {
        const nib: u8 = @intCast((v >> shift) & 0xf);
        writeByte(if (nib < 10) '0' + nib else 'a' + (nib - 10));
        if (shift == 0) break;
        shift -= 4;
    }
    writeByte('\r');
    writeByte('\n');
}

/// Move whatever the host has sent into `buf`, without waiting. Returns the count.
///
/// Non-blocking on purpose: the loop has a frame to render and a core to pump, and the editor must
/// not stall on a keystroke that may never come. `rxCount` is read once per call and the FIFO
/// drained to that mark, so a fast typist or a pasted buffer cannot hold the loop here.
pub fn read(buf: []u8) usize {
    const waiting = @min(uart0.rxCount(), buf.len);
    for (buf[0..waiting]) |*slot| slot.* = uart0.popByte();
    return waiting;
}

/// Discard anything already received, returning how much. Used once at startup: the host-side
/// bridge injects a window-size report before this program exists, and the bootloader's chatter has
/// already been echoed at the host. Neither is user input.
///
/// Pops rather than calling `resetRxFifo`, which is a CONF0_SYNC read-modify-write plus two commits
/// on the console UART - see this file's header.
pub fn drainInput() u32 {
    var discarded: u32 = 0;
    while (uart0.rxCount() > 0) : (discarded += 1) _ = uart0.popByte();
    return discarded;
}

/// The rate the hardware is actually producing, by reading its dividers back. Reported rather than
/// assumed: the host has to be opened at the same rate, and a mismatch shows up as garbage on the
/// screen rather than as an error anyone can act on.
pub fn baudrate() u32 {
    return uart0.baudrate(uart0.clockSource().nominalHz());
}