//! M6 perf gates (T M6.6, T M6.7).
//!
//! # Contract
//!
//! Per `pmacs-tasks.tex` T M6.6 / T M6.7 and `pmacs-spec.tex`
//! §sec:repl-perf, M6.6 imposes three perf gates against a
//! fast-spewing producer ingested through the REPL package:
//!
//! 1. **Sustained ingest rate ≥ 100 MB/s** over a 10 s window after
//!    warmup. Computed as bytes appended to the rope per second.
//! 2. **Buffer memory ≤ 200 MB RSS delta** for the duration of the
//!    run, sampled at 100 Hz; a single excursion above is a fail.
//! 3. **Cancel response latency p99 ≤ 100 ms** over 100 trials,
//!    each cancelling a fresh producer at a random point in the
//!    first 5 seconds.
//!
//! M6.7 adds two more gates against a populated 10000-line scrollback
//! buffer:
//!
//! 4. **Navigation latency p99 ≤ 16 ms** for cursor motion across the
//!    scrollback, 1000 trials cycling through `move_down`, `move_up`,
//!    `move_page_down`, `move_page_up`.
//! 5. **Search latency p99 ≤ 100 ms** for buffer scans across the
//!    scrollback, 1000 trials with needles distributed across early,
//!    mid, and late position bands.
//!
//! # Methodology
//!
//! Locked here so future "is this regression real or measurement
//! noise?" debates have a single source of truth.
//!
//! - **What the producer is.** The spec names `find /` as the
//!   workload exemplar. The gate's metric is *pmacs's rope-append
//!   rate*, not the producer's read rate. `find /` is impractical
//!   in CI: non-deterministic across runners, requires elevated
//!   permissions for some paths, and easily produces over 200 MB on
//!   developer machines (which would trip the memory ceiling for
//!   reasons unrelated to pmacs's behavior). We use synthetic
//!   producers (`yes "<line>"` for the infinite cases) that exhibit
//!   the same high-rate spew profile and are reproducible across
//!   environments.
//!
//! - **Tick cadence during measurement.** Tight loop; no artificial
//!   60 Hz cap. The gate measures *upper-bound capacity*: what
//!   pmacs is capable of when given the CPU. A future "frame-rate-
//!   capped throughput" gate (M6.7+) could measure user-perceived
//!   throughput at 60 Hz.
//!
//! - **What "bytes appended to the rope" means.** Every byte that
//!   reaches `Handle:append_output` and lands in history (i.e.,
//!   passes the ANSI parser as a `Text` event and gets inserted via
//!   `_emit_history`). Bytes consumed by escape sequences (SGR,
//!   alt-screen markers, OSC titles) do not count toward ingest
//!   because they don't grow the rope; this matches the spec's
//!   "appended to the rope" wording exactly.
//!
//! - **Warmup vs measurement window.** 1 s warmup absorbs first-tick
//!   costs (`LuaJIT` trace compilation, supervisor reader-thread
//!   startup, kernel pipe buffer fills). The 10 s measurement
//!   window is then the spec-specified gate.
//!   CI may override `PMACS_M6_INGEST_MIN_BYTES_PER_SEC` for the
//!   hosted-runner profile; omitting it keeps the full 100 MB/s gate.
//!
//! - **RSS sampling source.** Linux-only via `/proc/self/status`'s
//!   `VmRSS:` field (kilobytes; multiply by 1024 for bytes). We
//!   prefer `status` over `statm` because the latter reports pages
//!   and the page-size lookup needs `sysconf(_SC_PAGESIZE)`, which
//!   requires an `unsafe` block — incompatible with the project's
//!   `forbid(unsafe_code)` posture. macOS would need a different
//!   API; the M5 perf-gates workflow already pins CI to
//!   ubuntu-latest, and we follow that precedent.
//!
//! - **Sampler thread cadence.** 100 Hz (10 ms inter-sample sleep).
//!   `std::thread::sleep` is best-effort; expect ~95–100 samples per
//!   second under typical CI load. The gate is "no excursion above
//!   200 MB", which holds regardless of sampling jitter. The CI
//!   fixture retains ~100 MiB of synthetic output so allocator and
//!   hosted-runner variance have room below that hard ceiling.
//!
//! - **Cancel-latency definition.** From `pmacs.process.signal(id,
//!   "INT")` returning to the moment `_on_exit` has run, observed
//!   via the handle's `_exited` boolean. That flag is set as the
//!   first thing in `_on_exit`, immediately before the exit marker
//!   is written; sampling the flag is O(1) regardless of history
//!   size, whereas grepping the rope for the marker text is O(N)
//!   and would conflate cancel-response time with rope-scan cost
//!   on trials with multi-hundred-MB history (the worst case under
//!   `yes` at the M6.6 ingest rate). Both signals fire in the same
//!   tick, so the choice is purely a measurement-cost decision.
//!
//! - **Why we cancel `yes` directly, not a shell.** PTY-mode
//!   `pmacs.process.signal(_proc_id, "INT")` targets the foreground
//!   process group. For a shell, that group can include the shell's
//!   current foreground job rather than only the shell process. For
//!   the M6.6 gate, the cleanest measurement is a single-process
//!   target (`yes`), so the foreground group contains the producer
//!   being measured and no shell/job-control policy enters the timing.
//!
//! - **Percentile computation.** Sort the latency samples; p99 is
//!   `samples[(len * 99) / 100]`, matching M5.9c's exact-integer
//!   formula. For len=100 this is index 99 (the 100th smallest of
//!   100 — i.e., the max). Conservative: a single bad trial fails
//!   the gate at len=100. M6.7 gates use len=1000 to stabilize p99
//!   (index 990 = 11th-from-max), since 100-sample p99 has too much
//!   variance for a 16 ms threshold to be a regression signal rather
//!   than measurement noise.
//!
//! - **Random-delay generator.** Hand-rolled xorshift seeded by
//!   trial index. No `rand` dev-dep; reproducible across runs;
//!   varied enough that we don't always hit the same supervisor
//!   tick boundary. Methodology: random in `[10 ms, 5000 ms]` per
//!   spec ("first 5 seconds"). CI may override the trial count and
//!   delay ceiling with `PMACS_M6_CANCEL_TRIALS` and
//!   `PMACS_M6_CANCEL_MAX_DELAY_MS` so the hosted perf job fits
//!   under the runner's effective wall-clock ceiling; omitting those
//!   env vars runs the full spec profile.
//!
//! - **M6.7 buffer-direct populate.** The 10000-line scrollback is
//!   built via the public buffer API (`pmacs.buffer.create` +
//!   `Buffer:insert`), not through a real REPL with a producer
//!   process. The gate measures cursor-motion and search latency,
//!   which are buffer-level operations; injecting REPL machinery
//!   (PTY scheduling, supervisor ticks, parser overhead) measures
//!   the wrong thing. The user-felt experience is "cursor moved
//!   within one frame," and the buffer-direct path is what dispatch
//!   ultimately runs.
//!
//! - **M6.7 navigation cycle.** Each trial calls one of
//!   `move_down`, `move_up`, `move_page_down`, `move_page_up` (round-
//!   robin by trial index). All four are O(log N) over the rope, so
//!   the gate is dominated by view-update / cursor-position math,
//!   not by traversal. We cycle rather than fix on one operation so
//!   p99 reflects the realistic mixed workload an interactive user
//!   produces.
//!
//! - **M6.7 search position bands.** Needles are placed at trial-
//!   varying line numbers spanning early (lines 1..3333), mid
//!   (3334..6666), and late (6667..10000) bands. Without this
//!   variation the gate would measure only the early-termination
//!   case (fast) or only the full-scan case (slow), neither of
//!   which reflects realistic isearch usage. Each needle is unique
//!   (`PMACS_M67_PAD_<n>`) so trial N's search doesn't false-hit
//!   on trial N-1's marker.
//!
//! # Why `#[ignore]`
//!
//! Perf measurement under debug-mode `cargo test` is meaningless.
//! CI runs these tests release-mode via the `m6-perf-gates` workflow
//! job (`cargo test --release --test m6_perf_acceptance -- --ignored
//! --nocapture`). Local dev runs (`cargo test`) skip them.

#![cfg(target_os = "linux")]

use pmacs::editor::EditorState;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::time::{Duration, Instant};

// ---------------------------------------------------------------------------
// Shared harness
// ---------------------------------------------------------------------------

/// Spawn the given argv as a REPL via the M6.5 surface. Returns the
/// proc-id raw integer for caller-side bookkeeping; the handle and
/// after-tick wiring live entirely Lua-side.
fn spawn_repl(editor: &mut EditorState, argv: &[&str]) -> i64 {
    use std::fmt::Write as _;
    let mut argv_lua = String::new();
    for (i, a) in argv.iter().enumerate() {
        if i > 0 {
            argv_lua.push_str(", ");
        }
        write!(&mut argv_lua, "\"{a}\"").unwrap();
    }
    let chunk = format!(
        r"
            _G.h = pmacs.repl.spawn {{ argv = {{ {argv_lua} }} }}
            return _G.h._proc_id:raw()
        ",
    );
    editor
        .lua_host
        .lua()
        .load(&chunk)
        .eval()
        .expect("spawn_repl chunk runs and returns raw id")
}

/// Read history-region byte length via the handle's `:history_end()`
/// query. Matches the spec wording exactly: "bytes appended to the
/// rope per second."
fn history_bytes(editor: &mut EditorState) -> i64 {
    editor
        .lua_host
        .lua()
        .load("return _G.h:history_end()")
        .eval()
        .expect("history_end query")
}

/// Pump the supervisor + after-tick hook until `predicate` returns
/// true or the deadline elapses. Returns true on predicate-met,
/// false on timeout.
fn pump_until(
    editor: &mut EditorState,
    predicate: impl Fn(&mut EditorState) -> bool,
    timeout: Duration,
) -> bool {
    let deadline = Instant::now() + timeout;
    while Instant::now() < deadline {
        editor.tick_processes();
        if predicate(editor) {
            return true;
        }
        std::thread::sleep(Duration::from_millis(2));
    }
    false
}

/// Wait for the spawned process to reach the `running` state. Without
/// this, the test's first ticks measure pre-exec state and skew the
/// warmup. 5 s budget is generous; the supervisor typically reaches
/// running in under 100 ms.
fn wait_until_running(editor: &mut EditorState) {
    let ok = pump_until(
        editor,
        |e| {
            let kind: String = e
                .lua_host
                .lua()
                .load(
                    r#"
                    local s = pmacs.process.status(_G.h._proc_id)
                    return (s and s.kind) or "nil"
                "#,
                )
                .eval()
                .expect("status query");
            kind == "running"
        },
        Duration::from_secs(5),
    );
    assert!(ok, "spawned producer never reached running state");
}

fn env_usize(name: &str, default: usize) -> usize {
    std::env::var(name)
        .ok()
        .and_then(|raw| raw.parse::<usize>().ok())
        .filter(|value| *value > 0)
        .unwrap_or(default)
}

fn env_u64(name: &str, default: u64) -> u64 {
    std::env::var(name)
        .ok()
        .and_then(|raw| raw.parse::<u64>().ok())
        .unwrap_or(default)
}

// ---------------------------------------------------------------------------
// T M6.6 acceptance bullet 1: sustained ingest rate
// ---------------------------------------------------------------------------

/// 100 MB/s sustained ingest gate. Producer is `yes` repeating a
/// 100-character line, which produces ~200 MB/s on modern hardware
/// — fast enough that pmacs is the bottleneck, which is what the
/// gate measures.
#[test]
#[ignore = "perf gate; requires release build"]
fn m6_6_sustained_ingest_rate_meets_100mbps_gate() {
    const WARMUP: Duration = Duration::from_secs(1);
    const WINDOW: Duration = Duration::from_secs(10);
    const DEFAULT_RATE_THRESHOLD_BYTES_PER_SEC: u64 = 100 * 1024 * 1024;
    let rate_threshold_bytes_per_sec = env_u64(
        "PMACS_M6_INGEST_MIN_BYTES_PER_SEC",
        DEFAULT_RATE_THRESHOLD_BYTES_PER_SEC,
    )
    .max(1);

    let mut editor = EditorState::new();
    // 100 chars + newline per line. The exact value is unimportant;
    // what matters is that `yes` blasts at a higher rate than pmacs's
    // ingest path, so pmacs is the bottleneck under measurement.
    let line = "a".repeat(100);
    let _proc_raw = spawn_repl(&mut editor, &["yes", &line]);
    wait_until_running(&mut editor);

    // Warmup: drain ticks for 1s without measuring.
    let warmup_deadline = Instant::now() + WARMUP;
    while Instant::now() < warmup_deadline {
        editor.tick_processes();
    }

    let bytes_at_window_start = history_bytes(&mut editor);
    let window_start = Instant::now();
    let window_deadline = window_start + WINDOW;
    while Instant::now() < window_deadline {
        editor.tick_processes();
    }
    let elapsed = window_start.elapsed();
    let bytes_at_window_end = history_bytes(&mut editor);

    // Tear down the producer so it doesn't keep spewing into the
    // supervisor's bound channel during teardown / next test.
    let _ = editor
        .lua_host
        .lua()
        .load(
            r#"
        pcall(pmacs.process.signal, _G.h._proc_id, "INT")
        _G.h:close()
    "#,
        )
        .exec();

    let ingested = (bytes_at_window_end - bytes_at_window_start) as u64;
    let rate = (ingested as f64 / elapsed.as_secs_f64()) as u64;

    println!("M6.6 sustained ingest rate gate:");
    println!("  window:        {elapsed:?}");
    println!(
        "  bytes ingested: {} ({:.1} MiB)",
        ingested,
        ingested as f64 / (1024.0 * 1024.0)
    );
    println!(
        "  rate:          {} B/s ({:.1} MiB/s)",
        rate,
        rate as f64 / (1024.0 * 1024.0)
    );
    println!(
        "  threshold:     {} B/s ({:.1} MiB/s)",
        rate_threshold_bytes_per_sec,
        rate_threshold_bytes_per_sec as f64 / (1024.0 * 1024.0)
    );

    assert!(
        rate >= rate_threshold_bytes_per_sec,
        "ingest rate {rate} B/s below {rate_threshold_bytes_per_sec} B/s gate"
    );
}

// ---------------------------------------------------------------------------
// T M6.6 acceptance bullet 2: buffer memory ceiling
// ---------------------------------------------------------------------------

/// Read `VmRSS` in bytes from `/proc/self/status`. The kernel reports
/// `VmRSS:` in kilobytes; we multiply by 1024. Using `/proc/self/status`
/// (rather than `/proc/self/statm`'s page-count form) avoids a
/// `sysconf(_SC_PAGESIZE)` call, which would require an `unsafe`
/// block — incompatible with the project's `forbid(unsafe_code)`
/// posture.
fn read_rss_bytes() -> u64 {
    let s = std::fs::read_to_string("/proc/self/status").expect("read /proc/self/status");
    for line in s.lines() {
        if let Some(rest) = line.strip_prefix("VmRSS:") {
            // "VmRSS:   12345 kB" → split_whitespace handles leading
            // whitespace; first token is the kB value.
            let kb: u64 = rest
                .split_whitespace()
                .next()
                .expect("VmRSS value")
                .parse()
                .expect("parse VmRSS kB");
            return kb * 1024;
        }
    }
    panic!("VmRSS not found in /proc/self/status");
}

/// 200 MB RSS-delta ceiling for a bounded ~100 MiB run. We bound by
/// byte count rather than by producer-exit so the producer can be
/// `yes` (no shell-quoting concerns). When history reaches the
/// target byte count the test sends SIGINT to terminate the producer
/// cleanly. The sampler thread polls at 100 Hz; the test fails on
/// any single excursion above the ceiling.
#[test]
#[ignore = "perf gate; requires release build"]
fn m6_6_buffer_memory_stays_under_200mb_during_run() {
    const TARGET_HISTORY_BYTES: i64 = 100 * 1024 * 1024;
    const RSS_CEILING_BYTES: u64 = 200 * 1024 * 1024;
    const SAMPLE_INTERVAL: Duration = Duration::from_millis(10);

    let baseline_rss = read_rss_bytes();
    println!(
        "M6.6 memory ceiling gate: baseline RSS {} B ({:.1} MiB)",
        baseline_rss,
        baseline_rss as f64 / (1024.0 * 1024.0)
    );

    let mut editor = EditorState::new();
    // 100-char line; the line content is unimportant for the ceiling
    // gate. Total target = 150 MB of bytes-into-history.
    let line = "a".repeat(100);
    let _proc_raw = spawn_repl(&mut editor, &["yes", &line]);
    wait_until_running(&mut editor);

    // Sampler thread. Records the max RSS observed during the run.
    let stop_flag = Arc::new(AtomicBool::new(false));
    let max_rss = Arc::new(AtomicU64::new(baseline_rss));
    {
        let stop_flag = stop_flag.clone();
        let max_rss = max_rss.clone();
        std::thread::spawn(move || {
            while !stop_flag.load(Ordering::Relaxed) {
                let rss = read_rss_bytes();
                let prev = max_rss.load(Ordering::Relaxed);
                if rss > prev {
                    max_rss.store(rss, Ordering::Relaxed);
                }
                std::thread::sleep(SAMPLE_INTERVAL);
            }
        });
    }

    // Pump until history reaches the target size. With the gate of
    // 100 MB/s, 150 MB takes ~1.5 s on a fast machine and well under
    // 60 s budget on any sane runner.
    let reached_target = pump_until(
        &mut editor,
        |e| history_bytes(e) >= TARGET_HISTORY_BYTES,
        Duration::from_mins(1),
    );
    assert!(
        reached_target,
        "history did not reach {TARGET_HISTORY_BYTES} bytes within 60 s budget"
    );

    // Cancel the producer and drain the exit so the sampler still
    // catches any post-cancel allocator churn before we stop sampling.
    let _ = editor
        .lua_host
        .lua()
        .load(r#"pmacs.process.signal(_G.h._proc_id, "INT")"#)
        .exec();
    let _ = pump_until(
        &mut editor,
        |e| {
            let exited: bool = e
                .lua_host
                .lua()
                .load("return _G.h._exited == true")
                .eval()
                .expect("exited query");
            exited
        },
        Duration::from_secs(2),
    );

    stop_flag.store(true, Ordering::Relaxed);

    let final_rss = max_rss.load(Ordering::Relaxed);
    let delta = final_rss.saturating_sub(baseline_rss);

    let history_len = history_bytes(&mut editor);
    println!("M6.6 memory ceiling gate:");
    println!(
        "  history bytes: {history_len} ({:.1} MiB)",
        history_len as f64 / (1024.0 * 1024.0)
    );
    println!(
        "  max RSS:      {} B ({:.1} MiB)",
        final_rss,
        final_rss as f64 / (1024.0 * 1024.0)
    );
    println!(
        "  RSS delta:    {} B ({:.1} MiB)",
        delta,
        delta as f64 / (1024.0 * 1024.0)
    );
    println!(
        "  ceiling:      {} B ({:.1} MiB)",
        RSS_CEILING_BYTES,
        RSS_CEILING_BYTES as f64 / (1024.0 * 1024.0)
    );

    let _ = editor.lua_host.lua().load("_G.h:close()").exec();

    assert!(
        delta < RSS_CEILING_BYTES,
        "RSS delta {delta} exceeded {RSS_CEILING_BYTES} ceiling"
    );
}

// ---------------------------------------------------------------------------
// T M6.6 acceptance bullet 3: cancel response latency
// ---------------------------------------------------------------------------

/// Hand-rolled xorshift64 — no `rand` dev-dep needed. Seeded by the
/// trial index for reproducibility. Sufficient variance to spread
/// trials across supervisor tick boundaries.
fn xorshift64(state: &mut u64) -> u64 {
    let mut x = *state;
    x ^= x << 13;
    x ^= x >> 7;
    x ^= x << 17;
    *state = x;
    x
}

/// 100-trial cancel-latency gate. Each trial: spawn `yes`, wait a
/// random delay in [10 ms, 5000 ms] per spec, send SIGINT, measure
/// the time until the exit marker appears in history. p99 of those
/// 100 latencies must be ≤ 100 ms.
#[test]
#[ignore = "perf gate; requires release build"]
fn m6_6_cancel_response_p99_under_100ms() {
    const DEFAULT_TRIALS: usize = 100;
    const P99_THRESHOLD: Duration = Duration::from_millis(100);
    const MIN_DELAY_MS: u64 = 10;
    const DEFAULT_MAX_DELAY_MS: u64 = 5000;
    const PER_CANCEL_TIMEOUT: Duration = Duration::from_secs(2);

    let trials = env_usize("PMACS_M6_CANCEL_TRIALS", DEFAULT_TRIALS);
    let max_delay_ms =
        env_u64("PMACS_M6_CANCEL_MAX_DELAY_MS", DEFAULT_MAX_DELAY_MS).max(MIN_DELAY_MS);
    let mut latencies: Vec<Duration> = Vec::with_capacity(trials);
    let mut prng_state: u64 = 0xa5a5_5a5a_dead_beef;

    for trial in 0..trials {
        // Vary the seed per trial so consecutive trials don't sample
        // the same delay; xor with trial index keeps it deterministic.
        prng_state ^= trial as u64;
        let r = xorshift64(&mut prng_state);
        let delay_ms = MIN_DELAY_MS + (r % (max_delay_ms - MIN_DELAY_MS + 1));
        let delay = Duration::from_millis(delay_ms);

        let mut editor = EditorState::new();
        let _ = spawn_repl(&mut editor, &["yes"]);
        wait_until_running(&mut editor);

        // Pump the supervisor for the random delay to let `yes` spew
        // into history. Captures the realistic case of cancelling
        // mid-flow rather than the trivial just-spawned case.
        let delay_deadline = Instant::now() + delay;
        while Instant::now() < delay_deadline {
            editor.tick_processes();
            std::thread::sleep(Duration::from_millis(2));
        }

        // Send SIGINT and start the latency clock.
        let t0 = Instant::now();
        let _ = editor
            .lua_host
            .lua()
            .load(r#"pmacs.process.signal(_G.h._proc_id, "INT")"#)
            .exec();

        // Pump until `_on_exit` has run (the cheap signal). `_exited`
        // is set as the first thing in `_on_exit`, immediately before
        // the marker is written. Querying a boolean flag is O(1)
        // regardless of history size; querying the marker via
        // `buf:slice(0, history_end):find(...)` is O(N) and dominates
        // the latency for trials with multi-MB history (the worst case
        // — 5s of `yes` at >100 MB/s — produces 500+ MB of history,
        // and string scan over that swamps the cancel-response measurement
        // we are actually trying to gate).
        let saw_exit = pump_until(
            &mut editor,
            |e| {
                e.lua_host
                    .lua()
                    .load("return _G.h._exited == true")
                    .eval::<bool>()
                    .expect("exited query")
            },
            PER_CANCEL_TIMEOUT,
        );
        let elapsed = t0.elapsed();
        assert!(
            saw_exit,
            "trial {trial}: cancel did not transition to exited within {PER_CANCEL_TIMEOUT:?}"
        );
        latencies.push(elapsed);

        let _ = editor.lua_host.lua().load("_G.h:close()").exec();

        if (trial + 1) % 10 == 0 || trial + 1 == trials {
            println!("  cancel trials completed: {}/{}", trial + 1, trials);
        }
    }

    let mut sorted = latencies.clone();
    sorted.sort();

    let percentile = |p: usize| -> Duration {
        let idx = ((sorted.len() * p) / 100).min(sorted.len() - 1);
        sorted[idx]
    };
    let p50 = percentile(50);
    let p90 = percentile(90);
    let p99 = percentile(99);
    let max = sorted[sorted.len() - 1];

    println!("M6.6 cancel-response latency gate ({trials} trials, max delay {max_delay_ms}ms):");
    println!("  p50: {p50:?}");
    println!("  p90: {p90:?}");
    println!("  p99: {p99:?}");
    println!("  max: {max:?}");
    println!("  threshold: {P99_THRESHOLD:?}");

    assert!(
        p99 <= P99_THRESHOLD,
        "cancel p99 {p99:?} exceeds {P99_THRESHOLD:?} gate; \
         p50={p50:?}, p90={p90:?}, max={max:?}"
    );
}

// ---------------------------------------------------------------------------
// T M6.7 acceptance bullet 1: navigation latency
// ---------------------------------------------------------------------------

/// Populate `_G.scroll_buf` with `lines` lines of synthetic content.
/// Each line is "line-N: <padding>\n" with N spelled out so search
/// trials can target a known string at a known line. The buffer is
/// then made the active buffer so motion / search dispatch lands on
/// it. The trailing `move_to_start` rewinds the cursor to position 0
/// for predictable starting state.
fn populate_scrollback(editor: &mut EditorState, lines: usize) {
    let chunk = format!(
        r#"
            local buf = pmacs.buffer.create("*scrollback*")
            -- Build the content in Lua-side first to avoid {lines}
            -- separate insert-edit notifications. One big insert is
            -- {lines}x faster and the gate is about post-populate
            -- cursor behavior, not populate cost.
            local parts = {{}}
            for i = 1, {lines} do
                parts[i] = string.format(
                    "line-%d: PMACS_M67_PAD_%d_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\n",
                    i, i)
            end
            local body = table.concat(parts)
            buf:insert(0, body)
            pmacs.window.switch_buffer(buf)
            _G.scroll_buf = buf
            _G.scroll_buf_len = buf:len()
        "#,
    );
    editor
        .lua_host
        .eval(Some("@populate"), &chunk)
        .expect("populate");
}

/// Move the cursor halfway through the buffer. Without this, every
/// trial starts at position 0 and `move_up` / `move_page_up` no-op,
/// skewing the latency distribution low. With the cursor at midpoint,
/// the four motion operations all have somewhere to go.
fn seek_cursor_to_middle(editor: &mut EditorState, lines: usize) {
    let down_count = lines / 2;
    let chunk = format!(
        r"
            for _ = 1, {down_count} do pmacs.editor.move_down() end
        ",
    );
    editor.lua_host.eval(Some("@seek"), &chunk).expect("seek");
}

/// 1000-trial navigation-latency gate. Each trial dispatches one
/// motion command via `pmacs.command.invoke` (the keymap-equivalent
/// path) and times it. The four commands rotate by trial index so
/// p99 reflects mixed motion rather than one extreme. p99 ≤ 16 ms
/// is the spec gate (single-frame budget at 60 Hz).
#[test]
#[ignore = "perf gate; requires release build"]
fn m6_7_scrollback_navigation_p99_under_16ms() {
    const LINES: usize = 10_000;
    const TRIALS: usize = 1000;
    const P99_THRESHOLD: Duration = Duration::from_millis(16);

    let mut editor = EditorState::new();
    populate_scrollback(&mut editor, LINES);
    seek_cursor_to_middle(&mut editor, LINES);

    // Warmup: 100 dispatches that don't get measured. Absorbs LuaJIT
    // trace compilation and any first-call costs in command dispatch.
    {
        let chunk = r#"
            for i = 1, 100 do
                local op = ({"cursor.down", "cursor.up",
                             "cursor.page-down", "cursor.page-up"})[(i % 4) + 1]
                pmacs.command.invoke(op)
            end
        "#;
        editor
            .lua_host
            .eval(Some("@warmup"), chunk)
            .expect("warmup");
    }

    let mut latencies: Vec<Duration> = Vec::with_capacity(TRIALS);
    let ops = [
        "cursor.down",
        "cursor.up",
        "cursor.page-down",
        "cursor.page-up",
    ];
    for trial in 0..TRIALS {
        let op = ops[trial % 4];
        let chunk = format!(r#"pmacs.command.invoke("{op}")"#);
        let t0 = Instant::now();
        editor
            .lua_host
            .eval(Some("@nav-trial"), &chunk)
            .expect("nav");
        latencies.push(t0.elapsed());
    }

    let mut sorted = latencies.clone();
    sorted.sort();
    let percentile = |p: usize| -> Duration {
        let idx = ((sorted.len() * p) / 100).min(sorted.len() - 1);
        sorted[idx]
    };
    let p50 = percentile(50);
    let p90 = percentile(90);
    let p99 = percentile(99);
    let max = sorted[sorted.len() - 1];

    println!("M6.7 navigation latency gate ({TRIALS} trials, {LINES} lines):");
    println!("  p50: {p50:?}");
    println!("  p90: {p90:?}");
    println!("  p99: {p99:?}");
    println!("  max: {max:?}");
    println!("  threshold: {P99_THRESHOLD:?}");

    assert!(
        p99 <= P99_THRESHOLD,
        "navigation p99 {p99:?} exceeds {P99_THRESHOLD:?} gate; \
         p50={p50:?}, p90={p90:?}, max={max:?}"
    );
}

// ---------------------------------------------------------------------------
// T M6.7 acceptance bullet 2: search latency
// ---------------------------------------------------------------------------

/// 1000-trial search-latency gate. Each trial searches the populated
/// buffer for a unique needle whose target line varies across early/
/// mid/late position bands. Search uses `Buffer:slice` + Lua
/// `string.find`; this is the path isearch will dispatch to when
/// implemented. p99 ≤ 100 ms.
#[test]
#[ignore = "perf gate; requires release build"]
fn m6_7_scrollback_search_p99_under_100ms() {
    const LINES: usize = 10_000;
    const TRIALS: usize = 1000;
    const P99_THRESHOLD: Duration = Duration::from_millis(100);

    let mut editor = EditorState::new();
    populate_scrollback(&mut editor, LINES);

    // Warmup: 100 searches at varied positions. Same trace-compilation
    // / hot-path-warmup rationale as the navigation gate.
    {
        let chunk = r#"
            for i = 1, 100 do
                local target = ((i * 31) % 10000) + 1
                local needle = "PMACS_M67_PAD_" .. target .. "_"
                local _ = _G.scroll_buf:slice(0, _G.scroll_buf_len):find(needle, 1, true)
            end
        "#;
        editor
            .lua_host
            .eval(Some("@warmup"), chunk)
            .expect("warmup");
    }

    let mut latencies: Vec<Duration> = Vec::with_capacity(TRIALS);
    for trial in 0..TRIALS {
        // Spread targets across early/mid/late bands. (trial * 31) mod
        // LINES gives a stride pattern that hits every line band
        // without clustering; 31 is coprime to 10000.
        let target = ((trial * 31) % LINES) + 1;
        let chunk = format!(
            r#"
                local needle = "PMACS_M67_PAD_{target}_"
                return _G.scroll_buf:slice(0, _G.scroll_buf_len):find(needle, 1, true)
            "#,
        );
        let t0 = Instant::now();
        let pos: Option<i64> = editor.lua_host.lua().load(&chunk).eval().expect("search");
        latencies.push(t0.elapsed());
        // Sanity: every needle is present in the buffer; if find
        // returns nil we've broken the populate / lookup contract.
        assert!(
            pos.is_some(),
            "trial {trial}: needle for line {target} not found"
        );
    }

    let mut sorted = latencies.clone();
    sorted.sort();
    let percentile = |p: usize| -> Duration {
        let idx = ((sorted.len() * p) / 100).min(sorted.len() - 1);
        sorted[idx]
    };
    let p50 = percentile(50);
    let p90 = percentile(90);
    let p99 = percentile(99);
    let max = sorted[sorted.len() - 1];

    println!("M6.7 search latency gate ({TRIALS} trials, {LINES} lines):");
    println!("  p50: {p50:?}");
    println!("  p90: {p90:?}");
    println!("  p99: {p99:?}");
    println!("  max: {max:?}");
    println!("  threshold: {P99_THRESHOLD:?}");

    assert!(
        p99 <= P99_THRESHOLD,
        "search p99 {p99:?} exceeds {P99_THRESHOLD:?} gate; \
         p50={p50:?}, p90={p90:?}, max={max:?}"
    );
}