path: root/tools/bench_main.zig

//! `p4-bench`: what the board's serial link can actually carry, verified byte for byte.
//!
//! Two modes, because there are two different questions and conflating them is how this port got
//! optimised by guesswork so far.
//!
//! **`--link` (default) needs `examples/uartperf.zig` flashed.** It measures the CEILING: the wire
//! and the UART driver with nothing else running. Every number is checksummed - the board reports a
//! CRC over exactly the bytes it received, and the host compares it against a CRC over exactly the
//! bytes it sent. An unverified throughput figure is a guess about how fast data was corrupted,
//! and on this UART the failure that matters is a silent RX overrun, which a byte count cannot see.
//!
//! **`--editor` needs the editor flashed.** It measures how much of that ceiling pardes uses, by
//! typing at rising rates until it falls behind. There is no protocol available here - the board is
//! running an editor, and its answer to a keystroke is a screen update - so this half is timing
//! only, and it is honest about that.
//!
//! The uplink figure is measured to `tcdrain`, not to the last `write`. A write returns once the
//! kernel has accepted the bytes, which at 115200 is long before they are on the wire; timing to the
//! write would report the speed of memcpy into a tty buffer.

const std = @import("std");
const serial = @import("serial.zig");
const proto = @import("perfproto");
const rtt = @import("rtt.zig");

const Mode = enum {
    /// Verified bulk throughput and latency against `examples/uartperf.zig`. The ceiling.
    link,
    /// Keystroke latency against the editor, plus the rate ladder.
    editor,
    /// A named experiment: one controlled variable, many trials, machine-readable.
    sweep,
};

/// Which variable an experiment varies. One per run, because the point is a controlled variable and
/// a session that changed two things at once would not answer either question.
const Sweep = enum {
    /// Screen area, over an in-band resize. Tests whether per-keystroke cost is paid per CELL.
    geometry,
    /// Characters already in the line before the measured keystroke. Tests whether an edit is O(n)
    /// in the buffer - `modal.spliceAlloc` copies the whole content per keystroke, so it should be.
    length,
    /// One-off operations at a fixed geometry: motions, an insert, and a forced full repaint.
    ops,
    /// WHERE in a fixed line the edit happens. The length sweep shows cost rising with the line,
    /// but a line's length and the cursor's column grow together while typing, so that experiment
    /// cannot tell "the document is big" from "the cursor is far along it". This one holds the
    /// document constant at one long line and moves only the column, which separates them: a cost
    /// that follows the column is a walk from the start of the line (grapheme/width iteration), and
    /// a cost that does not is proportional to the document itself.
    position,
};

/// Widths and signed integers do not mix in Zig 0.16: `printIntAny` emits an explicit `+` for any
/// non-negative SIGNED value whenever a width is given (`std/Io/Writer.zig:1548-1559` - the plus is
/// omitted only when `width` is null or zero). Every number here is a duration or a count that
/// cannot be negative, so they are printed as unsigned and the tables line up.
fn pos(x: i64) u64 {
    return @intCast(@max(0, x));
}

const Options = struct {
    port: []const u8 = "/dev/ttyUSB0",
    baud: serial.Baud = .b115200,
    mode: Mode = .link,
    /// Payload bytes per direction for the bulk tests. 64 KiB is ~5.7 s each way at 115200 - long
    /// enough that start-up transients do not dominate, short enough to rerun after every change.
    bulk: u32 = 64 * 1024,
    /// Round trips for the latency figure.
    samples: u32 = 32,
    no_reset: bool = false,
    which: Sweep = .geometry,
    /// Trials per condition. Medians over an odd count, so the reported value is a real sample and
    /// not an average smeared across a transient.
    repeat: u32 = 5,
    /// Emit one CSV row per trial instead of a table. Raw trials, not summaries: the analysis should
    /// be able to see the spread and recompute any statistic, and a tool that only prints medians
    /// has thrown that away.
    csv: bool = false,
    /// Stamped into every CSV row, so a file of results records the build it came from rather than
    /// relying on the order the runs happened in.
    label: []const u8 = "-",
};

pub fn main(init: std.process.Init.Minimal) void {
    var o: Options = .{};
    var it: std.process.Args.Iterator = .init(init.args);
    _ = it.skip();
    while (it.next()) |a| {
        if (eql(a, "-h") or eql(a, "--help")) return usage();
        if (eql(a, "--no-reset")) {
            o.no_reset = true;
            continue;
        }
        if (eql(a, "--link")) {
            o.mode = .link;
            continue;
        }
        if (eql(a, "--editor")) {
            o.mode = .editor;
            continue;
        }
        if (eql(a, "--csv")) {
            o.csv = true;
            continue;
        }
        const val = it.next() orelse fatal("that flag needs a value");
        if (eql(a, "--port")) {
            o.port = val;
        } else if (eql(a, "--baud")) {
            const rate = std.fmt.parseInt(u32, val, 10) catch fatal("--baud must be a number");
            o.baud = std.enums.fromInt(serial.Baud, rate) orelse fatal("unsupported baud");
        } else if (eql(a, "--bulk")) {
            o.bulk = std.fmt.parseInt(u32, val, 10) catch fatal("--bulk must be a number");
        } else if (eql(a, "--samples")) {
            o.samples = std.fmt.parseInt(u32, val, 10) catch fatal("--samples must be a number");
        } else if (eql(a, "--repeat")) {
            o.repeat = std.fmt.parseInt(u32, val, 10) catch fatal("--repeat must be a number");
        } else if (eql(a, "--label")) {
            o.label = val;
        } else if (eql(a, "--sweep")) {
            o.mode = .sweep;
            o.which = std.meta.stringToEnum(Sweep, val) orelse
                fatal("--sweep takes geometry, length or ops");
        } else fatal("unrecognised argument; try --help");
    }
    run(o) catch |err| switch (err) {
        error.AccessDenied => {
            out("p4-bench: cannot open the port: AccessDenied\n\n");
            out(serial.access_denied_help);
            out("\n");
            std.process.exit(1);
        },
        error.NoMarker => fatal(
            \\the board never printed its readiness marker after reset.
            \\
            \\  Expected `MARK UARTPERF_READY` within 25 s. Flash the responder:
            \\      zig build flash -Dapp=examples/uartperf.zig
        ),
        error.NoResponder => fatal(
            \\the board is not answering the measurement protocol.
            \\
            \\  Flash the responder first:
            \\      zig build flash -Dapp=examples/uartperf.zig
            \\  Or measure the editor instead:
            \\      p4-bench --editor
        ),
        error.NoEditor => fatal(
            \\the board never reached the editor (no `MARK PARDES_READY` within 25 s).
            \\
            \\      zig build flash -Dpardes
        ),
        else => {
            var b: [128]u8 = undefined;
            fatal(std.fmt.bufPrint(&b, "{s}", .{@errorName(err)}) catch "failed");
        },
    };
}

/// Accumulates bytes off the wire and hands back whole frames. A frame split across reads is the
/// normal case on a serial line, so the buffer is the struct rather than a local.
const Frames = struct {
    buf: [proto.header_len + proto.max_payload]u8 = undefined,
    len: usize = 0,
    /// Bytes discarded while resynchronising. Nonzero means the stream contained something that was
    /// not a frame, which is itself a finding.
    junk: u32 = 0,
    /// Length of the frame handed out by the last `take`, still occupying the head of the buffer.
    /// `commit` is what removes it, so a caller may borrow a payload across the call that produced
    /// it and no further.
    pending: usize = 0,

    const Frame = struct { op: proto.Op, payload: []const u8 };

    /// The next whole frame, or null on timeout. `payload` borrows the buffer and is invalidated by
    /// the following call.
    fn next(f: *Frames, port: *serial.Port, timeout_us: i64) !?Frame {
        const deadline = nowUs(port) + timeout_us;
        while (true) {
            // Serve from what is already buffered before touching the wire: a single read can
            // deliver several frames, and re-polling between them would add latency that is not
            // the board's.
            if (f.take()) |fr| return fr;
            if (nowUs(port) >= deadline) return null;
            if (f.len == f.buf.len) {
                // Full and still not a frame: the buffer holds only junk. Drop one byte so the
                // resynchronising scan can advance.
                f.drop(1);
                continue;
            }
            const n = try port.readTimeout(f.buf[f.len..], 2);
            f.len += n;
        }
    }

    fn take(f: *Frames) ?Frame {
        while (f.len > 0) {
            const header = proto.parseHeader(f.buf[0..f.len]) catch {
                f.resync();
                continue;
            } orelse return null;
            const total = proto.header_len + @as(usize, header.len);
            if (f.len < total) return null;
            const payload = f.buf[proto.header_len..total];
            if (proto.crc(payload) != header.crc) {
                f.resync();
                continue;
            }
            f.pending = total;
            return .{ .op = header.op, .payload = payload };
        }
        return null;
    }

    /// Advance to the next plausible frame start. Dropping ONE byte per call was the obvious
    /// spelling and it is far too slow to be correct here: after a reset the buffer holds ~1.4 KB of
    /// bootloader log, and one byte discarded per poll took longer than the handshake timeout, so a
    /// working board looked like a missing one. Scanning to the next `P` covers the whole run of
    /// junk in one step.
    fn resync(f: *Frames) void {
        f.junk += 1;
        const next_magic = std.mem.indexOfScalarPos(u8, f.buf[0..f.len], 1, proto.magic[0]) orelse f.len;
        f.drop(next_magic);
    }

    fn drop(f: *Frames, n: usize) void {
        const k = @min(n, f.len);
        std.mem.copyForwards(u8, f.buf[0 .. f.len - k], f.buf[k..f.len]);
        f.len -= k;
    }

    fn commit(f: *Frames) void {
        if (f.pending > 0) {
            f.drop(f.pending);
            f.pending = 0;
        }
    }
};

fn run(o: Options) !void {
    var port = try serial.Port.open(o.port, o.baud);
    defer port.close();

    var r: Report = .{};
    // No banner in CSV mode: a file of results should be parseable by anything that reads CSV, and
    // a human-readable header line at the top of it is not. The condition is stamped into every row
    // by `--label` instead, which survives concatenation of several runs.
    if (!o.csv) {
        r.print("p4-bench  {s} @ {d} baud  wire capacity {d} B/s each way\n\n", .{
            o.port, o.baud.rate(), port.capacity(),
        });
        r.flush();
    }

    if (!o.no_reset) try port.resetToRun(.{});

    switch (o.mode) {
        .link => try link(&port, o, &r),
        .editor => try editor(&port, o, &r),
        .sweep => try sweep(&port, o, &r),
    }
}

/// Put the editor in a known state: reached, first frame drawn, insert mode on.
///
/// Every experiment starts here, and it matters that it is the same every time. `rtt.roundTrip`
/// with an empty stimulus is used as a settle: it sends nothing and returns when the wire has been
/// quiet, which is exactly "wait for the board to stop talking".
fn ready(port: *serial.Port, o: Options) !void {
    if (!o.no_reset) try waitFor(port, "MARK PARDES_READY", 25_000);
    _ = try rtt.roundTrip(port, "", 3_000_000, 300_000);
    try port.write("i");
    _ = try rtt.roundTrip(port, "", 400_000, 250_000);
}

/// One controlled-variable experiment, emitted as raw trials.
///
/// The measured quantity is always the same - the round trip of ONE inserted character - so that
/// conditions are comparable. Only the condition changes.
fn sweep(port: *serial.Port, o: Options, r: *Report) !void {
    try ready(port, o);
    if (o.csv) r.print("label,experiment,cols,rows,length,op,rep,rtt_us,settle_us,bytes\n", .{});

    switch (o.which) {
        // AREA. If a keystroke's cost is paid per cell, halving the rows should roughly halve the
        // compute. If the cost is per EDIT, geometry will barely move it. The editor clamps itself
        // to 40x12, so these are all reachable and 40x12 is the ceiling rather than a midpoint.
        .geometry => {
            for ([_][2]u16{ .{ 40, 12 }, .{ 40, 8 }, .{ 40, 6 }, .{ 30, 12 }, .{ 20, 12 }, .{ 20, 6 } }) |g| {
                var buf: [32]u8 = undefined;
                const resize = std.fmt.bufPrint(&buf, "\x1b[48;{d};{d};0;0t", .{ g[1], g[0] }) catch continue;
                try port.write(resize);
                // A resize is a full repaint; let it finish so it is not measured as a keystroke.
                _ = try rtt.roundTrip(port, "", 3_000_000, 400_000);
                try trials(port, o, r, .{ .cols = g[0], .rows = g[1], .op = "insert" });
            }
        },
        // LENGTH. `modal.spliceAlloc` allocates and copies the whole buffer for every edit, so the
        // per-keystroke cost should rise with the line. This is the experiment that decides whether
        // the ~14 ms is a fixed overhead or a function of the document.
        .length => {
            var at: u32 = 0;
            for ([_]u32{ 0, 20, 40, 80, 160, 320, 640 }) |target| {
                // Type up to the target WITHOUT measuring, so the measured keystroke always sees a
                // line of exactly `target` characters before it.
                // Primed in small batches rather than one round trip per character: the priming is
                // not the measurement, and a round trip each cost 0.3 s, which made the 160-character
                // condition take minutes. Eight at a time is 8 B on a wire with a 128-byte FIFO, so
                // nothing can be lost, and one settle per batch keeps the board from queueing.
                while (at < target) {
                    const batch: u32 = @min(8, target - at);
                    var fill: [8]u8 = @splat('y');
                    try port.write(fill[0..batch]);
                    _ = try rtt.roundTrip(port, "", 2_000_000, 120_000);
                    at += batch;
                }
                try trials(port, o, r, .{ .length = target, .op = "insert" });
            }
        },
        // OPS. Not a sweep of a number but of a KIND, to separate "an edit" from "a motion" from
        // "everything changed". Repeated, because the one-shot table showed 40-byte motions costing
        // the same round trip as 81-byte inserts and that needs more than one sample to assert.
        .ops => {
            try port.write("\x1b"); // motions must be motions
            _ = try rtt.roundTrip(port, "", 400_000, 250_000);
            for ([_]struct { name: []const u8, keys: []const u8 }{
                .{ .name = "motion_h", .keys = "h" },
                .{ .name = "motion_l", .keys = "l" },
                .{ .name = "line_start", .keys = "0" },
                .{ .name = "line_end", .keys = "$" },
                .{ .name = "insert_esc", .keys = "ix\x1b" },
                .{ .name = "repaint_39", .keys = "\x1b[48;12;39;0;0t" },
                .{ .name = "repaint_40", .keys = "\x1b[48;12;40;0;0t" },
            }) |p| {
                try trials(port, o, r, .{ .op = p.name, .keys = p.keys });
            }
        },
        // POSITION. One 320-character line, built once, then the cursor is parked at three places
        // in it and the SAME single-character insert is timed at each. The document never changes,
        // so anything that moves is a function of where the cursor is, not of how much text exists.
        .position => {
            var built: u32 = 0;
            while (built < 320) {
                const batch: u32 = @min(8, 320 - built);
                var fill: [8]u8 = @splat('y');
                try port.write(fill[0..batch]);
                _ = try rtt.roundTrip(port, "", 2_000_000, 120_000);
                built += batch;
            }
            // Leave insert mode so `0`, `$` and `h` are motions, then for each position re-enter
            // insert exactly at it. `i` inserts before the cursor, so the column IS the parked one.
            try port.write("\x1b");
            _ = try rtt.roundTrip(port, "", 400_000, 250_000);
            for ([_]struct { name: []const u8, go: []const u8, col: u32 }{
                .{ .name = "col_end", .go = "$", .col = 320 },
                .{ .name = "col_start", .go = "0", .col = 0 },
                .{ .name = "col_end_again", .go = "$", .col = 320 },
            }) |p| {
                try port.write(p.go);
                _ = try rtt.roundTrip(port, "", 400_000, 250_000);
                try port.write("i");
                _ = try rtt.roundTrip(port, "", 400_000, 250_000);
                try trials(port, o, r, .{ .length = p.col, .op = p.name });
                // Undo the insertions this condition made, so the next one starts from the same
                // document. Backspace as many times as there were trials.
                for (0..@min(o.repeat, 64)) |_| {
                    _ = try rtt.roundTrip(port, "\x7f", 2_000_000, 120_000);
                }
                try port.write("\x1b");
                _ = try rtt.roundTrip(port, "", 400_000, 250_000);
            }
        },
    }
}

const Condition = struct {
    cols: u16 = 0,
    rows: u16 = 0,
    length: u32 = 0,
    op: []const u8,
    /// The stimulus. Defaults to one inserted character, which is the comparable unit.
    keys: []const u8 = "x",
};

/// `o.repeat` trials of one condition. Raw rows in CSV mode; median in table mode.
fn trials(port: *serial.Port, o: Options, r: *Report, c: Condition) !void {
    var rtts: [64]i64 = undefined;
    var got: u32 = 0;
    var lost: u32 = 0;
    var last_bytes: usize = 0;
    var last_settle: i64 = 0;
    const n = @min(o.repeat, 64);
    for (0..n) |rep| {
        const s = try rtt.roundTrip(port, c.keys, 3_000_000, 250_000);
        if (s) |v| {
            if (got < 64) {
                rtts[got] = v.rtt_us;
                got += 1;
            }
            last_bytes = v.bytes;
            last_settle = v.settle_us;
            if (o.csv) r.print("{s},{s},{d},{d},{d},{s},{d},{d},{d},{d}\n", .{
                o.label, @tagName(o.which), c.cols, c.rows, c.length, c.op,
                rep,     pos(v.rtt_us),     pos(v.settle_us), v.bytes,
            });
        } else lost += 1;
        r.flush();
    }
    if (o.csv) return;
    if (got == 0) {
        r.print("  {s:<12} {d:>3}x{d:<3} len {d:>4}   no response\n", .{ c.op, c.cols, c.rows, c.length });
        return;
    }
    std.mem.sort(i64, rtts[0..got], {}, std.sort.asc(i64));
    r.print("  {s:<12} {d:>3}x{d:<3} len {d:>4}   median {d:>7} us   spread {d:>6} us   {d:>5} B\n", .{
        c.op,             c.cols,          c.rows,       c.length,
        pos(rtts[got / 2]), pos(rtts[got - 1] - rtts[0]), last_bytes,
    });
    r.flush();
}

fn link(port: *serial.Port, o: Options, r: *Report) !void {
    if (!o.no_reset) {
        try waitFor(port, "MARK UARTPERF_READY", 25_000);
        // The bootloader's log is not ours and it is still arriving. Feeding ~1.4 KB of text to a
        // frame parser wastes the handshake window resynchronising through it.
        port.drain();
    }

    var frames: Frames = .{};

    // A ping proves the responder is there and the framing agrees, before anything is timed.
    //
    // Retried, because the first one after a reset can genuinely be lost: the board prints its
    // marker from `zig_main` and the host answers within microseconds, while the board is still
    // inside `write` pushing the rest of that string through a 128-byte FIFO. Its RX FIFO holds the
    // ping meanwhile, but a `source`-sized burst is not the only thing that can outlast one - and a
    // measuring instrument that fails on a startup race would be reporting its own bug as the
    // board's.
    {
        var buf: [proto.header_len + 8]u8 = undefined;
        const ping = proto.encode(&buf, .ping, "handshake"[0..8]);
        var tries: u32 = 0;
        while (true) : (tries += 1) {
            if (tries == 5) return error.NoResponder;
            try port.write(ping);
            const fr = (try frames.next(port, 500_000)) orelse continue;
            const ok = fr.op == .pong and std.mem.eql(u8, fr.payload, "handshake"[0..8]);
            frames.commit();
            if (ok) break;
        }
    }

    // ---- UPLINK: host -> board, verified by the board's CRC over what arrived.
    var chunk: [proto.max_payload]u8 = undefined;
    var frame: [proto.header_len + proto.max_payload]u8 = undefined;
    var sent: u32 = 0;
    var hash: std.hash.Crc32 = .init();
    const t_up = nowUs(port);
    while (sent < o.bulk) {
        const take: u32 = @min(@as(u32, proto.max_payload), o.bulk - sent);
        proto.fillPattern(chunk[0..take], sent);
        hash.update(chunk[0..take]);
        try port.write(proto.encode(&frame, .sink, chunk[0..take]));
        sent += take;
    }
    // A write returns once the kernel has the bytes, not once the wire does; without this the
    // uplink figure was 202% of the link's capacity.
    port.flushOutput();
    const up_us = @max(1, nowUs(port) - t_up);
    const want_crc = hash.final();

    try port.write(proto.encode(&frame, .report, ""));
    const up_stat = blk: {
        while (true) {
            const fr = (try frames.next(port, 3_000_000)) orelse return error.NoResponder;
            if (fr.op == .stat) {
                const s = proto.Stat.decode(fr.payload) orelse return error.NoResponder;
                frames.commit();
                break :blk s;
            }
            frames.commit();
        }
    };

    const up_ok = up_stat.bytes == sent and up_stat.crc == want_crc;
    r.print("  uplink    host -> board\n", .{});
    r.print("    {d} B in {d} us = {d} B/s ({d}% of wire)\n", .{
        sent, up_us, @divTrunc(@as(i64, sent) * 1_000_000, up_us),
        @divTrunc(@as(i64, sent) * 1_000_000 * 100, up_us * @as(i64, port.capacity())),
    });
    if (up_ok) {
        r.print("    VERIFIED  crc 0x{x:0>8} over {d} B\n", .{ up_stat.crc, up_stat.bytes });
    } else {
        r.print("    FAILED    board got {d} B crc 0x{x:0>8}; host sent {d} B crc 0x{x:0>8}", .{
            up_stat.bytes, up_stat.crc, sent, want_crc,
        });
        if (up_stat.bytes < sent) r.print("  <-- {d} B LOST", .{sent - up_stat.bytes});
        r.print("\n", .{});
    }
    if (up_stat.bad_frames > 0) r.print("    {d} frames arrived corrupt\n", .{up_stat.bad_frames});
    if (up_stat.tx_dropped > 0) r.print("    board dropped {d} B on transmit\n", .{up_stat.tx_dropped});
    r.flush();

    // ---- DOWNLINK: board -> host, verified by the host's CRC over what arrived.
    var req: [4]u8 = undefined;
    std.mem.writeInt(u32, &req, o.bulk, .little);
    const t_down = nowUs(port);
    try port.write(proto.encode(&frame, .source, &req));
    var got: u32 = 0;
    var down_hash: std.hash.Crc32 = .init();
    var down_stat: ?proto.Stat = null;
    var last = t_down;
    while (down_stat == null) {
        const fr = (try frames.next(port, 5_000_000)) orelse break;
        switch (fr.op) {
            .data => {
                got += @intCast(fr.payload.len);
                down_hash.update(fr.payload);
                last = nowUs(port);
            },
            .stat => down_stat = proto.Stat.decode(fr.payload),
            else => {},
        }
        frames.commit();
    }
    const down_us = @max(1, last - t_down);
    const mine = down_hash.final();

    r.print("\n  downlink  board -> host\n", .{});
    r.print("    {d} B in {d} us = {d} B/s ({d}% of wire)\n", .{
        got, down_us, @divTrunc(@as(i64, got) * 1_000_000, down_us),
        @divTrunc(@as(i64, got) * 1_000_000 * 100, down_us * @as(i64, port.capacity())),
    });
    if (down_stat) |s| {
        if (s.bytes == got and s.crc == mine) {
            r.print("    VERIFIED  crc 0x{x:0>8} over {d} B\n", .{ mine, got });
        } else {
            r.print("    FAILED    board sent {d} B crc 0x{x:0>8}; host got {d} B crc 0x{x:0>8}", .{
                s.bytes, s.crc, got, mine,
            });
            if (got < s.bytes) r.print("  <-- {d} B LOST", .{s.bytes - got});
            r.print("\n", .{});
        }
    } else r.print("    FAILED    no closing stat frame\n", .{});
    if (frames.junk > 0) r.print("    {d} resynchronisation events on the host\n", .{frames.junk});
    r.flush();

    // ---- LATENCY: a verified round trip, so a lost ping is distinguishable from a slow one.
    var min: i64 = std.math.maxInt(i64);
    var max: i64 = 0;
    var sum: i64 = 0;
    var ok: u32 = 0;
    var lost: u32 = 0;
    for (0..o.samples) |_| {
        const t0 = nowUs(port);
        try port.write(proto.encode(&frame, .ping, "ping"));
        var answered = false;
        while (try frames.next(port, 500_000)) |fr| {
            const was_pong = fr.op == .pong and std.mem.eql(u8, fr.payload, "ping");
            frames.commit();
            if (was_pong) {
                answered = true;
                break;
            }
        }
        if (!answered) {
            lost += 1;
            continue;
        }
        const dt = nowUs(port) - t0;
        min = @min(min, dt);
        max = @max(max, dt);
        sum += dt;
        ok += 1;
    }
    r.print("\n  round trip  13 B out, 13 B back\n", .{});
    if (ok > 0) {
        r.print("    min {d} us  mean {d} us  max {d} us  ({d} samples, {d} lost)\n", .{
            pos(min), pos(@divTrunc(sum, ok)), pos(max), o.samples, lost,
        });
        r.print("    wire floor for 26 B is {d} us; the rest is the board\n", .{
            @divTrunc(26 * 1_000_000, @as(i64, port.capacity())),
        });
    } else r.print("    every ping lost\n", .{});
    r.flush();
}

fn editor(port: *serial.Port, o: Options, r: *Report) !void {
    if (!o.no_reset) try waitFor(port, "MARK PARDES_READY", 25_000);
    // Settle the first full frame before anything is timed against it.
    _ = try rtt.roundTrip(port, "", 2_000_000, 300_000);

    // The editor is modal: a bare `x` would be a motion. One `i` makes every later `x` an edit,
    // which is the cheapest change that still forces a real render.
    try port.write("i");
    _ = try rtt.roundTrip(port, "", 300_000, 200_000);

    const base = try rtt.measure(port, .{ .samples = @min(o.samples, 16), .gap_us = 400_000 });
    if (base.median_us < 0) return error.NoEditor;
    r.print("  editor, uncontended at 2.5 keys/s\n", .{});
    r.print("    rtt median {d} us   settle {d} us   {d} B per keystroke\n\n", .{
        pos(base.median_us), pos(base.median_settle_us), base.median_bytes,
    });
    r.print("   keys/s   median rtt     settle    B/key   wire   lost   verdict\n", .{});
    r.print("  ---------------------------------------------------------------\n", .{});
    r.flush();

    const ceiling = base.median_us * 3;
    var best: i64 = -1;
    for ([_]i64{ 160, 120, 80, 60, 40, 30, 20, 15, 10 }) |gap_ms| {
        const s = try rtt.measure(port, .{
            .samples = @min(o.samples, 16),
            .gap_us = gap_ms * 1000,
            .timeout_us = 1_000_000,
        });
        const rate = @divTrunc(@as(i64, 1000), gap_ms);
        const pass = s.lost == 0 and s.median_us >= 0 and s.median_us <= ceiling;
        if (pass) best = rate;
        r.print("  {d:>7}  {d:>9} us  {d:>9} us  {d:>7}  {d:>4}%  {d:>5}   {s}\n", .{
            pos(rate),      pos(s.median_us), pos(s.median_settle_us),
            s.median_bytes, s.wire_percent,   s.lost,
            if (s.lost > 0) "LOST INPUT" else if (pass) "ok" else "behind",
        });
        r.flush();
        if (s.lost > 0) break;
    }
    r.print("\n", .{});
    if (best < 0) {
        r.print("  CEILING: under 6 keys/s - it kept up at no rate tried.\n", .{});
    } else {
        r.print("  CEILING: {d} keys/s sustained (median rtt within 3x of {d} us).\n", .{ best, base.median_us });
    }
    r.flush();

    // ONE-OFF COSTS. Typing turned out to be cheap, so the operations that are not typing are where
    // "too slow to use" has to live. Each is measured once, in the state the ladder left the buffer
    // in (a long line of `x`), and the interesting column is bytes: an operation that emits ~1.5 KB
    // has repainted the whole screen, and at this baud that is 130 ms of wire before anything else
    // can happen.
    try port.write("\x1b"); // out of insert mode; motions are motions again
    _ = try rtt.roundTrip(port, "", 400_000, 250_000);

    r.print("\n  one-off operations (bytes is the tell: ~1.5 KB is a full repaint)\n", .{});
    r.print("   operation            rtt      settle     bytes\n", .{});
    r.print("  ------------------------------------------------\n", .{});
    const probes = [_]struct { name: []const u8, keys: []const u8 }{
        .{ .name = "motion h", .keys = "h" },
        .{ .name = "motion l", .keys = "l" },
        .{ .name = "line start", .keys = "0" },
        .{ .name = "line end", .keys = "$" },
        .{ .name = "insert char", .keys = "ix\x1b" },
        // A geometry change is the one stimulus guaranteed to force a full repaint, so it
        // calibrates the column: whatever this costs is what "everything changed" costs.
        .{ .name = "resize 40->39", .keys = "\x1b[48;12;39;0;0t" },
        .{ .name = "resize 39->40", .keys = "\x1b[48;12;40;0;0t" },
    };
    for (probes) |p| {
        if (try rtt.roundTrip(port, p.keys, 3_000_000, 250_000)) |s| {
            r.print("   {s:<16} {d:>8} us {d:>9} us {d:>9}\n", .{
                p.name, pos(s.rtt_us), pos(s.settle_us), s.bytes,
            });
        } else {
            r.print("   {s:<16}   no response (the editor ignored it)\n", .{p.name});
        }
        r.flush();
    }
}

/// Wait for a plain text marker rather than a fixed delay: a slow boot should lengthen the run, not
/// silently start measuring a board that is still in its bootloader.
fn waitFor(port: *serial.Port, marker: []const u8, timeout_ms: i64) !void {
    var at: usize = 0;
    var buf: [1024]u8 = undefined;
    const deadline = port.nowMs() + timeout_ms;
    while (port.nowMs() < deadline) {
        const n = port.readTimeout(&buf, 200) catch 0;
        for (buf[0..n]) |b| {
            if (b == marker[at]) {
                at += 1;
                if (at == marker.len) return;
            } else at = if (b == marker[0]) 1 else 0;
        }
    }
    return error.NoMarker;
}

const Report = struct {
    buf: [4096]u8 = undefined,
    len: usize = 0,

    fn print(self: *Report, comptime fmt: []const u8, args: anytype) void {
        const s = std.fmt.bufPrint(self.buf[self.len..], fmt, args) catch return;
        self.len += s.len;
    }

    fn flush(self: *Report) void {
        out(self.buf[0..self.len]);
        self.len = 0;
    }
};

fn usage() void {
    out(
        \\p4-bench - measure the board's serial link, verified with a checksum
        \\
        \\  p4-bench [--link | --editor] [--port <path>] [--baud <rate>]
        \\           [--bulk <bytes>] [--samples <n>] [--no-reset]
        \\
        \\  --link     (default) bulk throughput each way plus round-trip latency, every byte
        \\             checksummed. Needs examples/uartperf.zig flashed.
        \\  --editor   type at rising rates against pardes and report the highest rate it keeps
        \\             up with. Needs the editor flashed.
        \\
    );
}

fn nowUs(port: *serial.Port) i64 {
    return std.Io.Timestamp.now(port.io, .boot).toMicroseconds();
}

const io = std.Io.Threaded.global_single_threaded.io();
const stdout: std.Io.File = .{ .handle = 1, .flags = .{ .nonblocking = false } };

fn out(s: []const u8) void {
    stdout.writeStreamingAll(io, s) catch {};
}

fn fatal(msg: []const u8) noreturn {
    var b: [512]u8 = undefined;
    out(std.fmt.bufPrint(&b, "p4-bench: {s}\n", .{msg}) catch "p4-bench: error\n");
    std.process.exit(1);
}

fn eql(a: []const u8, b: []const u8) bool {
    return std.mem.eql(u8, a, b);
}