//! The board half of the link measurement: answer `tools/perfproto.zig` frames over UART0.
//!
//! This is the CEILING the editor is measured against. `p4-bench` against this firmware says what
//! the wire and the UART driver can do with nothing else running; `p4-bench` against the editor says
//! how much of that the editor manages to use. Optimising the editor without the first number is
//! guessing, because at 115200 baud a good deal of what feels slow is simply the wire, and no amount
//! of firmware work moves it.
//!
//! Three things it deliberately does NOT do, each of which would corrupt the number:
//!
//!   * **No `soc.rom.print`.** The mask ROM's `ets_printf` formats and then pushes one byte at a
//!     time, spinning on the FIFO for each - the exact cost this is trying to measure around. Every
//!     byte here goes through the same batched FIFO path `src/pardes/uart.zig` uses.
//!   * **No UART reconfiguration.** Not the divider, not the format, not `reset()`. The
//!     second-stage bootloader configured this block; `hal/uart.zig:195-211` records that resetting
//!     it returns UART_CLKDIV to its power-on value and takes the session with it.
//!   * **No allocation.** One parse buffer and one send buffer, both static, both sized by the
//!     protocol's own `max_payload`. A measurement that shared a heap with anything would measure
//!     the heap.
//!
//! The verification is the point. `sink` accumulates a CRC across every payload byte received and
//! `report` hands it back, so the host can prove that what arrived is what it sent - at this baud a
//! silent RX overrun is the failure mode that matters, and a byte count alone cannot see it.

const std = @import("std");
const hal = @import("hal");
const proto = @import("perfproto");

const uart0 = hal.uart.Uart.init(0);

/// Room for one whole frame. The protocol caps a payload at 1024 precisely so this can be static.
var rx: [proto.header_len + proto.max_payload]u8 = undefined;
var rx_len: usize = 0;

var tx: [proto.header_len + proto.max_payload]u8 = undefined;

/// The running `sink` accumulators, reported and reset by `report`.
var sunk_bytes: u32 = 0;
var sunk_crc: std.hash.Crc32 = undefined;
var bad_frames: u32 = 0;
var tx_dropped: u32 = 0;

/// Push bytes through the TX FIFO, reading the status once per burst rather than once per byte.
///
/// The spin is bounded because an unbounded one is indistinguishable from a hang on a board with no
/// debugger, and because this program's whole purpose is to report numbers: a wedged transmitter
/// that increments a counter can still be diagnosed, while one that spins forever cannot.
fn write(bytes: []const u8) void {
    var rest = bytes;
    while (rest.len > 0) {
        var room = uart0.txFree();
        var spins: u32 = 0;
        while (room == 0) {
            spins += 1;
            if (spins > 1_000_000) {
                tx_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..];
    }
}

fn send(op: proto.Op, payload: []const u8) void {
    write(proto.encode(&tx, op, payload));
}

fn sendStat() void {
    var buf: [proto.Stat.encoded_len]u8 = undefined;
    const s: proto.Stat = .{
        .bytes = sunk_bytes,
        .crc = sunk_crc.final(),
        .bad_frames = bad_frames,
        .tx_dropped = tx_dropped,
    };
    s.encode(&buf);
    send(.stat, &buf);
    sunk_bytes = 0;
    sunk_crc = .init();
    bad_frames = 0;
}

/// Stream `n` pattern bytes back as `data` frames, then a `stat` whose CRC covers all of them.
///
/// Filled a frame at a time from the shared generator rather than from a table: the host computes
/// the same sequence from the same function, so a disagreement is a real transport fault and not two
/// copies of a constant drifting apart.
fn source(n: u32) void {
    var chunk: [proto.max_payload]u8 = undefined;
    var sent: u32 = 0;
    var hash: std.hash.Crc32 = .init();
    while (sent < n) {
        const take: u32 = @min(@as(u32, proto.max_payload), n - sent);
        proto.fillPattern(chunk[0..take], sent);
        hash.update(chunk[0..take]);
        send(.data, chunk[0..take]);
        sent += take;
    }
    var buf: [proto.Stat.encoded_len]u8 = undefined;
    const s: proto.Stat = .{
        .bytes = sent,
        .crc = hash.final(),
        .bad_frames = bad_frames,
        .tx_dropped = tx_dropped,
    };
    s.encode(&buf);
    send(.stat, &buf);
}

/// Consume one complete frame from the head of `rx`. Returns the bytes consumed, or 0 when the
/// frame is not all here yet.
fn step() usize {
    const header = proto.parseHeader(rx[0..rx_len]) catch {
        // Lost sync. Drop ONE byte and let the next call try again from there: the magic is two
        // bytes, so resynchronising by scanning is the only correct recovery, and dropping the whole
        // buffer would discard a good frame that happened to follow a corrupt one.
        return 1;
    } orelse return 0;

    const total = proto.header_len + @as(usize, header.len);
    if (rx_len < total) return 0;
    const payload = rx[proto.header_len..total];

    if (proto.crc(payload) != header.crc) {
        // Corruption, not loss: the length was plausible and the bytes were not. Counted and
        // discarded, because acting on it would put the wrong answer in the host's hands.
        bad_frames +%= 1;
        return total;
    }

    switch (header.op) {
        .ping => send(.pong, payload),
        .sink => {
            sunk_bytes +%= header.len;
            sunk_crc.update(payload);
        },
        .report => sendStat(),
        .source => {
            const n = if (header.len >= 4) std.mem.readInt(u32, payload[0..4], .little) else 0;
            source(n);
        },
        // Replies are ours to send, never to receive. A reply arriving here means the host is
        // confused or the wire is looping back; count it rather than answering it.
        .pong, .stat, .data => bad_frames +%= 1,
    }
    return total;
}

export fn zig_main() noreturn {
    // FIRST, before any `.rodata` is touched - and the marker below IS `.rodata`. Without this the
    // bootloader's stale cache lines make that string read as machine code, the board emits noise,
    // and it looks exactly like a firmware that never started. Measured here before the call was
    // added: `\xefc\xff\xff\xd5\xb7...` instead of the marker.
    @import("soc").flushFlashCache();

    // The bootloader arms the RTC watchdog and expects the application to take it over. Nothing in
    // this repo ever did, so every image here was being reset on a ten-second cycle - invisible to
    // a program that prints once and spins, and fatal to one that must answer for a minute. It is
    // why this responder booted, printed its marker, and then went silent: `rst:0x10
    // (CHIP_LP_WDT_RESET)` in the next boot log, measured.
    _ = hal.rwdt.disable();
    sunk_crc = .init();

    // Announce readiness in plain text rather than as a frame: the host watches for this during
    // reset, while the bootloader's own chatter is still arriving and no frame parser is in sync.
    write("\r\nMARK UARTPERF_READY\r\n");

    while (true) {
        // Fill from the FIFO first and always, so the RX FIFO is never left to overflow while this
        // loop is busy elsewhere. 128 bytes at 115200 is 107 ms of slack and a `source` burst can
        // hold the transmitter far longer than that, which is exactly the hazard being measured on
        // the editor - so the measuring instrument must not have it.
        if (rx_len < rx.len) {
            const room = rx.len - rx_len;
            var got: usize = 0;
            while (got < room and uart0.rxCount() > 0) {
                rx[rx_len + got] = uart0.popByte();
                got += 1;
            }
            rx_len += got;
        }

        var off: usize = 0;
        while (off < rx_len) {
            const n = step();
            if (n == 0) break;
            off += n;
        }
        if (off > 0) {
            std.mem.copyForwards(u8, rx[0 .. rx_len - off], rx[off..rx_len]);
            rx_len -= off;
        }
    }
}

/// Reset entry, the same shape as every other application here: the bootloader hands over with an
/// unspecified stack pointer and the FPU off, and `.bss` is not cleared for us. The FPU bit matters
/// even in a program with no floats, because a generic `bufPrint` instantiation can reach std's
/// float formatting path - and `std.hash.Crc32`'s table generation is comptime, so nothing here
/// needs the FPU at run time, but nothing here is worth a trap either.
export fn _start() linksection(".text.entry") callconv(.naked) noreturn {
    asm volatile (
        \\ li t0, 1 << 13
        \\ csrs mstatus, t0
        \\ la sp, __stack_top
        \\ mv fp, sp
        \\ la t0, __bss_start
        \\ la t1, __bss_end
        \\ bgeu t0, t1, 2f
        \\1:
        \\ sw zero, 0(t0)
        \\ addi t0, t0, 4
        \\ bltu t0, t1, 1b
        \\2:
        \\ j zig_main
    );
}

/// A panic here would be a measurement that silently stopped, so it says so on the wire it was
/// measuring - through the ROM's printf, because a panic may well be the UART path itself failing.
pub const panic = std.debug.FullPanic(struct {
    fn call(msg: []const u8, _: ?usize) noreturn {
        @import("soc").rom.print("MARK UARTPERF_PANIC %s\r\n", .{msg.ptr});
        while (true) {}
    }
}.call);