dynamo-mocker 1.4.0

// SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

use ddsketchy::DDSketch;
use rustc_hash::FxHashMap;
use serde::Serialize;
use serde::ser::{SerializeMap, Serializer};
use std::fmt::{Display, Formatter, Result as FmtResult};
use uuid::Uuid;

use crate::common::protocols::OutputSignal;

// 0.1% relative quantile error. The enlarged store covers latency/rate values
// spanning roughly 10^28 within one sign while remaining bounded (~512 KiB for
// the two stores at their maximum size, ~1 MiB for both global sketches).
const DDSKETCH_RELATIVE_ACCURACY: f64 = 0.001;
const DDSKETCH_MAX_BINS: usize = 32_768;

#[derive(Debug, Clone)]
pub struct TraceSimulationReport {
    pub request_counts: TraceRequestCounts,
    pub throughput: TraceThroughputStats,
    pub prefix_cache_reused_ratio: f64,
    pub first_admission_prefix_cache_reused_ratio: f64,
    pub latency: TraceLatencyStats,
    /// SLA-goodput stats. `Some` only when an SLA was supplied to the collector
    /// (via `set_sla_thresholds`); `None` otherwise — goodput is undefined
    /// without an SLA, so the `goodput_*` keys are omitted from the report.
    pub goodput: Option<TraceGoodputStats>,
    /// Per-request records, one per admitted request. Populated by
    /// `TraceCollector::finish`. Intentionally NOT serialized into the summary
    /// JSON (see custom `Serialize` impl below) — consumers that want per-
    /// request granularity should access this field directly and serialize
    /// it themselves (e.g., the `--report-jsonl` CLI path).
    pub per_request: Vec<PerRequestRecord>,
}

#[derive(Debug, Clone)]
pub struct TraceRequestCounts {
    pub num_requests: usize,
    pub completed_requests: usize,
    pub total_input_tokens: usize,
    pub total_output_tokens: usize,
}

#[derive(Debug, Clone)]
pub struct TraceThroughputStats {
    pub duration_ms: f64,
    pub wall_time_ms: f64,
    pub request_throughput_rps: f64,
    pub input_throughput_tok_s: f64,
    pub output_throughput_tok_s: f64,
    pub total_throughput_tok_s: f64,
    /// Provisioned worker-time per role, in **worker-seconds**: the time-integral
    /// of the *provisioned* worker count over the whole simulated run. The
    /// provisioned count is every worker physically holding a GPU (active +
    /// starting-up + draining), so this captures the startup ramp and the
    /// scale-down drain tail, unlike a snapshot of the active/serving count.
    /// Populated on the collector by the runtime: `add_worker_seconds` accrues
    /// the integral each clock advance (agg / disagg), and
    /// `set_static_worker_count` covers the single-worker path; 0.0 otherwise.
    /// Multiply by GPUs-per-worker for GPU-seconds (/3600 for GPU-hours).
    /// Aggregated replay reports through `decode_worker_seconds`, leaving
    /// `prefill_worker_seconds` at 0.0.
    pub prefill_worker_seconds: f64,
    pub decode_worker_seconds: f64,
    /// GPUs per worker per role, derived from the mocker engine parallelism
    /// (`MockEngineArgs::aic_gpus_per_worker` = tensor parallelism × materialized
    /// DP topology); the runtime sets it on the collector. 0 when not set.
    pub prefill_gpus_per_worker: usize,
    pub decode_gpus_per_worker: usize,
    /// GPU-hours = Σ_role `worker_seconds × gpus_per_worker / 3600` — the
    /// deployment's provisioned GPU-time (already including the startup ramp and
    /// drain tail, since `*_worker_seconds` do). Computed in `finish()` straight
    /// from the mocker's own worker parallelism, so it needs no external config.
    pub gpu_hours: f64,
}

/// Goodput: throughput restricted to the requests that satisfy the SLA. Present
/// on the report only when an SLA was supplied to the collector (goodput is
/// undefined without one). A completed request counts as "good" per
/// [`SlaThresholds::is_good`].
#[derive(Debug, Clone)]
pub struct TraceGoodputStats {
    /// Completed requests that satisfied the SLA.
    pub completed_requests: usize,
    /// Good requests per second, over the simulated `duration_s`.
    pub request_throughput_rps: f64,
    /// Output tokens from good requests per second, over `duration_s`.
    pub output_throughput_tok_s: f64,
}

#[derive(Debug, Clone)]
pub struct TraceDistributionStats {
    pub mean_ms: f64,
    pub min_ms: f64,
    pub max_ms: f64,
    pub median_ms: f64,
    pub p75_ms: f64,
    pub p90_ms: f64,
    pub p95_ms: f64,
    pub p99_ms: f64,
    pub std_ms: f64,
}

#[derive(Debug, Clone)]
pub struct TraceLatencyStats {
    pub ttft: TraceDistributionStats,
    pub ttst: TraceDistributionStats,
    pub tpot: TraceDistributionStats,
    pub itl: TraceInterTokenLatencyStats,
    pub e2e: TraceDistributionStats,
    pub output_token_throughput_per_user: TraceDistributionStats,
}

#[derive(Debug, Clone)]
pub struct TraceInterTokenLatencyStats {
    pub distribution: TraceDistributionStats,
    pub max_ms: f64,
}

impl TraceSimulationReport {
    pub fn with_wall_time_ms(mut self, wall_time_ms: f64) -> Self {
        self.throughput.wall_time_ms = wall_time_ms;
        self
    }

    pub fn processed_tokens(&self) -> usize {
        self.request_counts.total_input_tokens + self.request_counts.total_output_tokens
    }

    pub fn processed_tokens_per_s(&self) -> f64 {
        if self.throughput.wall_time_ms <= 0.0 {
            return 0.0;
        }
        self.processed_tokens() as f64 / self.throughput.wall_time_ms * 1000.0
    }

    pub fn processed_output_tokens_per_s(&self) -> f64 {
        if self.throughput.wall_time_ms <= 0.0 {
            return 0.0;
        }
        self.request_counts.total_output_tokens as f64 / self.throughput.wall_time_ms * 1000.0
    }
}

impl Display for TraceSimulationReport {
    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
        writeln!(
            f,
            "  completed_requests: {}",
            self.request_counts.completed_requests
        )?;
        writeln!(
            f,
            "  request_throughput_rps: {:.6}",
            self.throughput.request_throughput_rps
        )?;
        writeln!(
            f,
            "  output_throughput_tok_s: {:.6}",
            self.throughput.output_throughput_tok_s
        )?;
        writeln!(
            f,
            "  total_input_tokens: {}",
            self.request_counts.total_input_tokens
        )?;
        writeln!(
            f,
            "  total_output_tokens: {}",
            self.request_counts.total_output_tokens
        )?;
        writeln!(
            f,
            "  processed_tokens_per_s: {:.6}",
            self.processed_tokens_per_s()
        )?;
        writeln!(
            f,
            "  processed_output_tokens_per_s: {:.6}",
            self.processed_output_tokens_per_s()
        )?;
        writeln!(f, "  mean_ttft_ms: {:.6}", self.latency.ttft.mean_ms)?;
        writeln!(f, "  mean_e2e_latency_ms: {:.6}", self.latency.e2e.mean_ms)?;
        writeln!(
            f,
            "  prefix_cache_reused_ratio: {:.6}",
            self.prefix_cache_reused_ratio
        )?;
        writeln!(
            f,
            "  first_admission_prefix_cache_reused_ratio: {:.6}",
            self.first_admission_prefix_cache_reused_ratio
        )?;
        write!(f, "  wall_time_ms: {:.6}", self.throughput.wall_time_ms)
    }
}

impl Serialize for TraceSimulationReport {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let mut map = serializer.serialize_map(Some(70))?;
        map.serialize_entry("num_requests", &self.request_counts.num_requests)?;
        map.serialize_entry(
            "completed_requests",
            &self.request_counts.completed_requests,
        )?;
        map.serialize_entry(
            "total_input_tokens",
            &self.request_counts.total_input_tokens,
        )?;
        map.serialize_entry(
            "total_output_tokens",
            &self.request_counts.total_output_tokens,
        )?;
        map.serialize_entry("duration_ms", &self.throughput.duration_ms)?;
        map.serialize_entry("wall_time_ms", &self.throughput.wall_time_ms)?;
        map.serialize_entry(
            "request_throughput_rps",
            &self.throughput.request_throughput_rps,
        )?;
        map.serialize_entry(
            "input_throughput_tok_s",
            &self.throughput.input_throughput_tok_s,
        )?;
        map.serialize_entry(
            "output_throughput_tok_s",
            &self.throughput.output_throughput_tok_s,
        )?;
        map.serialize_entry(
            "total_throughput_tok_s",
            &self.throughput.total_throughput_tok_s,
        )?;
        map.serialize_entry(
            "prefill_worker_seconds",
            &self.throughput.prefill_worker_seconds,
        )?;
        map.serialize_entry(
            "decode_worker_seconds",
            &self.throughput.decode_worker_seconds,
        )?;
        map.serialize_entry(
            "prefill_gpus_per_worker",
            &self.throughput.prefill_gpus_per_worker,
        )?;
        map.serialize_entry(
            "decode_gpus_per_worker",
            &self.throughput.decode_gpus_per_worker,
        )?;
        map.serialize_entry("gpu_hours", &self.throughput.gpu_hours)?;
        if let Some(goodput) = &self.goodput {
            map.serialize_entry("goodput_completed_requests", &goodput.completed_requests)?;
            map.serialize_entry(
                "goodput_request_throughput_rps",
                &goodput.request_throughput_rps,
            )?;
            map.serialize_entry(
                "goodput_output_throughput_tok_s",
                &goodput.output_throughput_tok_s,
            )?;
        }
        map.serialize_entry("processed_tokens", &self.processed_tokens())?;
        map.serialize_entry("processed_tokens_per_s", &self.processed_tokens_per_s())?;
        map.serialize_entry(
            "processed_output_tokens_per_s",
            &self.processed_output_tokens_per_s(),
        )?;
        map.serialize_entry("prefix_cache_reused_ratio", &self.prefix_cache_reused_ratio)?;
        map.serialize_entry(
            "first_admission_prefix_cache_reused_ratio",
            &self.first_admission_prefix_cache_reused_ratio,
        )?;
        serialize_distribution(&mut map, "ttft", &self.latency.ttft)?;
        serialize_distribution(&mut map, "ttst", &self.latency.ttst)?;
        serialize_distribution(&mut map, "tpot", &self.latency.tpot)?;
        serialize_distribution(&mut map, "itl", &self.latency.itl.distribution)?;
        map.serialize_entry("max_itl_ms", &self.latency.itl.max_ms)?;
        serialize_distribution(&mut map, "e2e_latency", &self.latency.e2e)?;
        serialize_rate_distribution(
            &mut map,
            "output_token_throughput_per_user",
            &self.latency.output_token_throughput_per_user,
        )?;
        map.end()
    }
}

fn serialize_distribution<S>(
    map: &mut S,
    prefix: &str,
    stats: &TraceDistributionStats,
) -> Result<(), S::Error>
where
    S: SerializeMap,
{
    map.serialize_entry(&format!("mean_{prefix}_ms"), &stats.mean_ms)?;
    map.serialize_entry(&format!("min_{prefix}_ms"), &stats.min_ms)?;
    map.serialize_entry(&format!("max_{prefix}_ms"), &stats.max_ms)?;
    map.serialize_entry(&format!("median_{prefix}_ms"), &stats.median_ms)?;
    map.serialize_entry(&format!("p75_{prefix}_ms"), &stats.p75_ms)?;
    map.serialize_entry(&format!("p90_{prefix}_ms"), &stats.p90_ms)?;
    map.serialize_entry(&format!("p95_{prefix}_ms"), &stats.p95_ms)?;
    map.serialize_entry(&format!("p99_{prefix}_ms"), &stats.p99_ms)?;
    map.serialize_entry(&format!("std_{prefix}_ms"), &stats.std_ms)?;
    Ok(())
}

fn serialize_rate_distribution<S>(
    map: &mut S,
    prefix: &str,
    stats: &TraceDistributionStats,
) -> Result<(), S::Error>
where
    S: SerializeMap,
{
    map.serialize_entry(&format!("mean_{prefix}"), &stats.mean_ms)?;
    map.serialize_entry(&format!("min_{prefix}"), &stats.min_ms)?;
    map.serialize_entry(&format!("max_{prefix}"), &stats.max_ms)?;
    map.serialize_entry(&format!("median_{prefix}"), &stats.median_ms)?;
    map.serialize_entry(&format!("p75_{prefix}"), &stats.p75_ms)?;
    map.serialize_entry(&format!("p90_{prefix}"), &stats.p90_ms)?;
    map.serialize_entry(&format!("p95_{prefix}"), &stats.p95_ms)?;
    map.serialize_entry(&format!("p99_{prefix}"), &stats.p99_ms)?;
    map.serialize_entry(&format!("std_{prefix}"), &stats.std_ms)?;
    Ok(())
}

#[derive(Debug)]
struct TraceRequestStats {
    arrival_time_ms: f64,
    first_admit_ms: Option<f64>,
    terminal_time_ms: Option<f64>,
    terminal_status: Option<ReplayTerminalStatus>,
    token_timeline: TokenTimeline,
    input_length: usize,
    requested_output_length: usize,
    reused_input_tokens: usize,
    first_admission_reused_input_tokens: usize,
    /// Index of the prefill worker that handled this request, if any.
    /// `None` in two situations:
    ///   - Aggregated replay (no separate prefill pool) — meaningless field.
    ///   - Offline disagg with conditional-prefill bypass — request was
    ///     routed directly to a decode worker without going through prefill.
    ///
    /// Downstream tooling derives "was_bypassed" as `prefill_worker_idx is None`
    /// in disagg mode.
    prefill_worker_idx: Option<usize>,
    /// Index of the decode worker that handled this request, if any.
    decode_worker_idx: Option<usize>,
    /// Session / turn metadata copied from the workload driver, when the
    /// trace source carries it (e.g., multi-turn Mooncake). `None` for raw
    /// single-shot request lists.
    session_id: Option<String>,
    turn_index: Option<usize>,
    detail: Option<Box<PerRequestDetail>>,
}

#[derive(Debug)]
enum TokenTimeline {
    Recording(Vec<f64>),
    Finalized(FinalizedTokenTimeline),
}

impl Default for TokenTimeline {
    fn default() -> Self {
        Self::Recording(Vec::new())
    }
}

#[derive(Debug, Clone, Copy)]
struct FinalizedTokenTimeline {
    first_ms: f64,
    second_ms: Option<f64>,
    last_ms: f64,
    len: usize,
}

#[derive(Debug)]
struct StreamingDistribution {
    sketch: DDSketch,
    count: u64,
    mean: f64,
    sum_squared_deviations: f64,
    min: f64,
    max: f64,
}

impl Default for StreamingDistribution {
    fn default() -> Self {
        let sketch = match DDSketch::with_max_bins(DDSKETCH_RELATIVE_ACCURACY, DDSKETCH_MAX_BINS) {
            Ok(sketch) => sketch,
            Err(error) => panic!("invalid built-in DDSketch configuration: {error}"),
        };
        Self {
            sketch,
            count: 0,
            mean: 0.0,
            sum_squared_deviations: 0.0,
            min: f64::INFINITY,
            max: f64::NEG_INFINITY,
        }
    }
}

impl StreamingDistribution {
    fn add(&mut self, value: f64) {
        if !value.is_finite() {
            return;
        }

        self.sketch.add(value);
        self.count += 1;
        let delta = value - self.mean;
        self.mean += delta / self.count as f64;
        let delta_after_mean_update = value - self.mean;
        self.sum_squared_deviations += delta * delta_after_mean_update;
        self.min = self.min.min(value);
        self.max = self.max.max(value);
    }

    fn finish(&self) -> TraceDistributionStats {
        if self.count == 0 {
            return empty_distribution_stats();
        }

        TraceDistributionStats {
            mean_ms: self.mean,
            min_ms: self.min,
            max_ms: self.max,
            median_ms: self.percentile(50.0),
            p75_ms: self.percentile(75.0),
            p90_ms: self.percentile(90.0),
            p95_ms: self.percentile(95.0),
            p99_ms: self.percentile(99.0),
            std_ms: (self.sum_squared_deviations / self.count as f64).sqrt(),
        }
    }

    /// Preserve the report's historical rounded-rank percentile definition;
    /// DDSketch itself uses a floored rank for its `quantile` input.
    fn percentile(&self, percentile: f64) -> f64 {
        let span = self.count.saturating_sub(1);
        let rank = (span as f64 * percentile / 100.0).round() as u64;
        let quantile = if span == 0 || rank >= span {
            1.0
        } else {
            (rank as f64 + 0.5) / span as f64
        };
        match self.sketch.quantile(quantile) {
            Ok(value) => value,
            Err(error) => panic!("invalid built-in DDSketch quantile {quantile}: {error}"),
        }
    }
}

#[derive(Debug, Default)]
struct PerRequestDetail {
    prefill_reused_input_tokens: Option<usize>,
    prefill_admit_ms: Option<f64>,
    source_held_ms: Option<f64>,
    destination_reserved_ms: Option<f64>,
    destination_activated_ms: Option<f64>,
    decode_admit_ms: Option<f64>,
    source_released_ms: Option<f64>,
    decode_reused_input_tokens: Option<usize>,
    prefill_route_overlap_tokens: Option<usize>,
    decode_route_overlap_tokens: Option<usize>,
}

#[derive(Debug, Clone, Copy, Eq, PartialEq, Serialize)]
#[serde(rename_all = "snake_case")]
pub enum ReplayTerminalStatus {
    Completed,
    Rejected,
    Canceled,
    Failed,
}

/// Flat per-request record for `--report-jsonl` emission. One JSON line per
/// request in the JSONL output; consumed by external analysis tools that want
/// per-request granularity (TTFT vs. ISL scatter, worker-residency analysis,
/// bypass classification, etc.).
#[derive(Debug, Clone, Serialize)]
pub struct PerRequestRecord {
    /// Session identifier from the trace, when present. Mirrors AIPerf's
    /// `conversation_id` field for the same purpose: bucket per-request
    /// records by multi-turn session. Placed first in the serialized output
    /// so each JSONL row leads with its session/turn identity, matching
    /// AIPerf's `profile_export.jsonl` layout.
    pub session_id: Option<String>,
    /// Zero-based turn index within `session_id`, when present.
    pub turn_index: Option<usize>,
    pub uuid: String,
    pub arrival_time_ms: f64,
    pub first_admit_ms: Option<f64>,
    pub terminal_time_ms: f64,
    pub first_token_ms: Option<f64>,
    pub last_token_ms: Option<f64>,
    pub ttft_ms: Option<f64>,
    pub ttst_ms: Option<f64>,
    pub e2e_latency_ms: Option<f64>,
    /// Inter-token latency for this request, in milliseconds. Matches
    /// AIPerf's `inter_token_latency` field — one scalar per request.
    pub itl_ms: Option<f64>,
    pub input_length: usize,
    /// Number of output tokens requested by the workload trace.
    pub requested_output_length: usize,
    /// Number of output tokens actually emitted by the mock engine.
    pub output_length: usize,
    pub reused_input_tokens: usize,
    pub prefill_worker_idx: Option<usize>,
    pub decode_worker_idx: Option<usize>,
    pub prefill_admit_ms: Option<f64>,
    pub source_held_ms: Option<f64>,
    pub destination_reserved_ms: Option<f64>,
    pub destination_activated_ms: Option<f64>,
    pub decode_admit_ms: Option<f64>,
    pub source_released_ms: Option<f64>,
    pub decode_reused_input_tokens: Option<usize>,
    pub prefill_route_overlap_tokens: Option<usize>,
    pub decode_route_overlap_tokens: Option<usize>,
    pub terminal_status: ReplayTerminalStatus,
}

#[cfg(test)]
#[derive(Debug, Clone, PartialEq)]
pub(crate) struct TraceRequestStatsSnapshot {
    pub arrival_time_ms: f64,
    pub first_admit_ms: Option<f64>,
    pub first_token_ms: Option<f64>,
    pub last_token_ms: Option<f64>,
    pub input_length: usize,
    pub requested_output_length: usize,
    pub output_length: usize,
    pub reused_input_tokens: usize,
    pub first_admission_reused_input_tokens: usize,
}

/// SLA thresholds used to classify requests for goodput. Mirrors Spica's
/// `SLATarget` shape: set `ttft_ms` + `itl_ms` together, or `e2e_ms` alone.
/// Only the thresholds that are set are checked, so an e2e-only SLA gates on
/// e2e and a ttft+itl SLA gates on both. All-`None` (the default) means "no
/// SLA", which suppresses goodput entirely.
#[derive(Debug, Clone, Copy, Default)]
pub struct SlaThresholds {
    pub ttft_ms: Option<f64>,
    pub itl_ms: Option<f64>,
    pub e2e_ms: Option<f64>,
}

impl SlaThresholds {
    pub(crate) fn is_set(&self) -> bool {
        self.ttft_ms.is_some() || self.itl_ms.is_some() || self.e2e_ms.is_some()
    }

    /// Whether a completed request satisfies the SLA. Each *set* threshold must
    /// hold; unset thresholds are ignored.
    ///
    /// - `ttft_ms`: time-to-first-token ≤ bound.
    /// - `e2e_ms`: end-to-end latency ≤ bound.
    /// - `itl_ms`: the per-request **average inter-token latency** ≤ bound,
    ///   computed the same way as aiperf / genai-perf:
    ///   `avg_itl = (e2e_ms − ttft_ms) / (output_length − 1)`. When
    ///   `output_length ≤ 1` there is no inter-token interval, so the ITL check
    ///   is skipped (treated as satisfied).
    fn is_good(&self, ttft_ms: f64, e2e_ms: f64, output_length: usize) -> bool {
        if let Some(bound) = self.e2e_ms
            && e2e_ms > bound
        {
            return false;
        }
        if let Some(bound) = self.ttft_ms
            && ttft_ms > bound
        {
            return false;
        }
        if let Some(bound) = self.itl_ms
            && output_length > 1
        {
            let avg_itl_ms = (e2e_ms - ttft_ms) / (output_length as f64 - 1.0);
            if avg_itl_ms > bound {
                return false;
            }
        }
        true
    }

    fn is_good_without_tokens(&self, e2e_ms: f64) -> bool {
        self.ttft_ms.is_none()
            && self.itl_ms.is_none()
            && self.e2e_ms.is_some_and(|bound| e2e_ms <= bound)
    }
}

#[derive(Debug, Default)]
pub(crate) struct TraceCollector {
    requests: FxHashMap<Uuid, TraceRequestStats>,
    /// Global per-token distributions are folded in as requests terminate, so
    /// completed requests no longer retain one timestamp per emitted token.
    itl_distribution: StreamingDistribution,
    output_token_throughput_per_user: StreamingDistribution,
    /// Keep completed token timelines until `finish()` instead of folding them
    /// synchronously in `on_terminal`.
    defer_token_timeline_finalization: bool,
    /// When `true`, `finish()` populates `TraceSimulationReport::per_request`.
    /// Default `false` to skip the ~100ms terminal pass + ~30MB allocation
    /// when the caller doesn't need per-request granularity.
    capture_per_request: bool,
    /// SLA thresholds for goodput classification. All-`None` by default, in
    /// which case `finish()` leaves `TraceSimulationReport::goodput` as `None`.
    sla: SlaThresholds,
    /// Accumulated provisioned worker-seconds per role, integrated by the
    /// runtime over the sim clock (see `add_worker_seconds`). Used for the
    /// runtimes that have an event loop (agg / disagg), where the provisioned
    /// count varies with startup / drain / scaling.
    prefill_worker_seconds: f64,
    decode_worker_seconds: f64,
    /// Static provisioned worker counts `(prefill, decode)` for runtimes with a
    /// fixed worker (the single-worker path, which has no event loop to
    /// integrate). When `Some`, `finish()` derives worker-seconds as
    /// `count × duration_s` instead of using the accumulator.
    static_worker_count: Option<(usize, usize)>,
    /// GPUs per worker per role, from the mocker engine parallelism. Used in
    /// `finish()` to turn worker-seconds into gpu_hours.
    prefill_gpus_per_worker: usize,
    decode_gpus_per_worker: usize,
}

impl TraceRequestStats {
    fn first_token_ms(&self) -> Option<f64> {
        match &self.token_timeline {
            TokenTimeline::Recording(times) => times.first().copied(),
            TokenTimeline::Finalized(summary) => Some(summary.first_ms),
        }
    }

    fn last_token_ms(&self) -> Option<f64> {
        match &self.token_timeline {
            TokenTimeline::Recording(times) => times.last().copied(),
            TokenTimeline::Finalized(summary) => Some(summary.last_ms),
        }
    }

    fn actual_output_length(&self) -> usize {
        match &self.token_timeline {
            TokenTimeline::Recording(times) => times.len(),
            TokenTimeline::Finalized(summary) => summary.len,
        }
    }

    fn mean_tpot_ms(&self) -> Option<f64> {
        let num_gaps = self.actual_output_length().saturating_sub(1);
        if num_gaps == 0 {
            return None;
        }

        let first_token_ms = self.first_token_ms()?;
        let last_token_ms = self.last_token_ms()?;
        Some((last_token_ms - first_token_ms).max(0.0) / num_gaps as f64)
    }

    fn ttst_ms(&self) -> Option<f64> {
        let (first_token_ms, second_token_ms) = match &self.token_timeline {
            TokenTimeline::Recording(times) => {
                let [first_token_ms, second_token_ms, ..] = times.as_slice() else {
                    return None;
                };
                (*first_token_ms, *second_token_ms)
            }
            TokenTimeline::Finalized(summary) => (summary.first_ms, summary.second_ms?),
        };
        Some((second_token_ms - first_token_ms).max(0.0))
    }

    fn finalize_token_timeline(
        &mut self,
        include_in_distributions: bool,
        itl_distribution: &mut StreamingDistribution,
        output_token_throughput_per_user: &mut StreamingDistribution,
    ) {
        let TokenTimeline::Recording(times) = &self.token_timeline else {
            return;
        };

        if include_in_distributions {
            for window in times.windows(2) {
                let itl_ms = (window[1] - window[0]).max(0.0);
                itl_distribution.add(itl_ms);
                if itl_ms > 0.0 {
                    output_token_throughput_per_user.add(1000.0 / itl_ms);
                }
            }
        }

        let Some(first_ms) = times.first().copied() else {
            self.token_timeline = TokenTimeline::default();
            return;
        };
        let summary = FinalizedTokenTimeline {
            first_ms,
            second_ms: times.get(1).copied(),
            last_ms: times.last().copied().unwrap_or(first_ms),
            len: times.len(),
        };
        self.token_timeline = TokenTimeline::Finalized(summary);
    }
}

impl TraceCollector {
    /// Defer token-timeline folding until the entire replay has ended.
    pub(crate) fn set_defer_token_timeline_finalization(&mut self, value: bool) {
        self.defer_token_timeline_finalization = value;
    }

    /// Toggle whether `finish()` should build per-request records. Off by
    /// default; the runtimes flip it on when the caller asks for JSONL output.
    pub(crate) fn set_capture_per_request(&mut self, value: bool) {
        self.capture_per_request = value;
    }

    /// Set the SLA thresholds used to classify goodput in `finish()`. With no
    /// SLA set (the default), the report's `goodput` field stays `None`.
    pub(crate) fn set_sla_thresholds(&mut self, sla: SlaThresholds) {
        self.sla = sla;
    }

    /// Add provisioned worker-seconds for the interval just elapsed. The runtime
    /// calls this each time it advances the sim clock, with
    /// `provisioned_count × dt_ms / 1000` per role — the time-integral of the
    /// *provisioned* worker count (active + starting-up + draining), so the
    /// startup ramp and drain tail are included. Agg replay passes `prefill = 0`
    /// and reports through `decode`.
    pub(crate) fn add_worker_seconds(&mut self, prefill: f64, decode: f64) {
        self.prefill_worker_seconds += prefill;
        self.decode_worker_seconds += decode;
    }

    /// Declare a fixed `(prefill, decode)` provisioned worker count for a runtime
    /// with no event loop to integrate (the single-worker path). `finish()` then
    /// reports `count × duration_s` worker-seconds.
    pub(crate) fn set_static_worker_count(&mut self, prefill: usize, decode: usize) {
        self.static_worker_count = Some((prefill, decode));
    }

    pub(crate) fn clear_static_worker_count(&mut self) {
        self.static_worker_count = None;
    }

    /// Set GPUs-per-worker per role (from the mocker engine parallelism). Used
    /// in `finish()` to derive gpu_hours from the worker-seconds.
    pub(crate) fn set_gpus_per_worker(&mut self, prefill: usize, decode: usize) {
        self.prefill_gpus_per_worker = prefill;
        self.decode_gpus_per_worker = decode;
    }

    pub(crate) fn on_arrival(
        &mut self,
        uuid: Uuid,
        arrival_time_ms: f64,
        input_length: usize,
        requested_output_length: usize,
    ) {
        self.requests.insert(
            uuid,
            TraceRequestStats {
                arrival_time_ms,
                first_admit_ms: None,
                terminal_time_ms: None,
                terminal_status: None,
                token_timeline: TokenTimeline::default(),
                input_length,
                requested_output_length,
                reused_input_tokens: 0,
                prefill_worker_idx: None,
                decode_worker_idx: None,
                session_id: None,
                turn_index: None,
                first_admission_reused_input_tokens: 0,
                detail: self
                    .capture_per_request
                    .then(|| Box::new(PerRequestDetail::default())),
            },
        );
    }

    /// Attach session/turn metadata to a request. Called by the disagg/agg
    /// runtimes when the workload driver provides it (multi-turn traces).
    /// Idempotent — set-once semantics, so calling on the same uuid more than
    /// once is a no-op after the first.
    pub(crate) fn on_session_metadata(
        &mut self,
        uuid: Uuid,
        session_id: String,
        turn_index: usize,
    ) {
        if !self.capture_per_request {
            return;
        }
        if let Some(stats) = self.requests.get_mut(&uuid)
            && stats.session_id.is_none()
        {
            stats.session_id = Some(session_id);
            stats.turn_index = Some(turn_index);
        }
    }

    /// Record that `uuid` was dispatched to `worker_idx` on the prefill pool
    /// (offline disagg replay only). Idempotent — subsequent calls are no-ops
    /// once a value is set, so the first dispatch wins. Aggregated replay does
    /// not call this; for those requests `prefill_worker_idx` stays `None`.
    pub(crate) fn on_prefill_assigned(&mut self, uuid: Uuid, worker_idx: usize) {
        if let Some(stats) = self.requests.get_mut(&uuid)
            && stats.prefill_worker_idx.is_none()
        {
            stats.prefill_worker_idx = Some(worker_idx);
        }
    }

    /// Record that `uuid` was dispatched to `worker_idx` on the decode pool
    /// (offline disagg replay), or to the only pool (aggregated replay).
    /// Idempotent.
    pub(crate) fn on_decode_assigned(&mut self, uuid: Uuid, worker_idx: usize) {
        if let Some(stats) = self.requests.get_mut(&uuid)
            && stats.decode_worker_idx.is_none()
        {
            stats.decode_worker_idx = Some(worker_idx);
        }
    }

    pub(crate) fn on_admit(&mut self, uuid: Uuid, admit_time_ms: f64, reused_input_tokens: usize) {
        if let Some(stats) = self.requests.get_mut(&uuid) {
            if stats.first_admit_ms.is_none() {
                stats.first_admission_reused_input_tokens = reused_input_tokens;
                stats.first_admit_ms = Some(admit_time_ms);
            }
            stats.reused_input_tokens = stats.reused_input_tokens.max(reused_input_tokens);
        }
    }

    pub(crate) fn on_prefill_admit(
        &mut self,
        uuid: Uuid,
        admit_time_ms: f64,
        reused_input_tokens: usize,
    ) {
        self.on_admit(uuid, admit_time_ms, reused_input_tokens);
        if let Some(detail) = self.detail_mut(uuid) {
            detail.prefill_admit_ms.get_or_insert(admit_time_ms);
            detail.prefill_reused_input_tokens = Some(
                detail
                    .prefill_reused_input_tokens
                    .unwrap_or_default()
                    .max(reused_input_tokens),
            );
        }
    }

    pub(crate) fn on_decode_admit(
        &mut self,
        uuid: Uuid,
        admit_time_ms: f64,
        reused_input_tokens: usize,
    ) {
        self.on_admit(uuid, admit_time_ms, reused_input_tokens);
        if let Some(detail) = self.detail_mut(uuid) {
            detail.decode_admit_ms.get_or_insert(admit_time_ms);
            detail.decode_reused_input_tokens = Some(
                detail
                    .decode_reused_input_tokens
                    .unwrap_or_default()
                    .max(reused_input_tokens),
            );
        }
    }

    pub(crate) fn on_source_held(&mut self, uuid: Uuid, at_ms: f64) {
        if let Some(detail) = self.detail_mut(uuid) {
            detail.source_held_ms.get_or_insert(at_ms);
        }
    }

    pub(crate) fn on_destination_reserved(&mut self, uuid: Uuid, at_ms: f64) {
        if let Some(detail) = self.detail_mut(uuid) {
            detail.destination_reserved_ms.get_or_insert(at_ms);
        }
    }

    pub(crate) fn on_destination_activated(&mut self, uuid: Uuid, at_ms: f64) {
        if let Some(detail) = self.detail_mut(uuid) {
            detail.destination_activated_ms.get_or_insert(at_ms);
        }
    }

    pub(crate) fn on_source_released(&mut self, uuid: Uuid, at_ms: f64) {
        if let Some(detail) = self.detail_mut(uuid) {
            detail.source_released_ms.get_or_insert(at_ms);
        }
    }

    pub(crate) fn on_prefill_route_overlap(&mut self, uuid: Uuid, tokens: usize) {
        if let Some(detail) = self.detail_mut(uuid) {
            detail.prefill_route_overlap_tokens.get_or_insert(tokens);
        }
    }

    pub(crate) fn on_decode_route_overlap(&mut self, uuid: Uuid, tokens: usize) {
        if let Some(detail) = self.detail_mut(uuid) {
            detail.decode_route_overlap_tokens.get_or_insert(tokens);
        }
    }

    pub(crate) fn on_terminal(
        &mut self,
        uuid: Uuid,
        terminal_time_ms: f64,
        status: ReplayTerminalStatus,
    ) {
        let Self {
            requests,
            itl_distribution,
            output_token_throughput_per_user,
            defer_token_timeline_finalization,
            ..
        } = self;
        if let Some(stats) = requests.get_mut(&uuid)
            && stats.terminal_status.is_none()
        {
            stats.terminal_time_ms = Some(terminal_time_ms);
            stats.terminal_status = Some(status);
            if !*defer_token_timeline_finalization {
                stats.finalize_token_timeline(
                    status == ReplayTerminalStatus::Completed && stats.first_admit_ms.is_some(),
                    itl_distribution,
                    output_token_throughput_per_user,
                );
            }
        }
    }

    fn detail_mut(&mut self, uuid: Uuid) -> Option<&mut PerRequestDetail> {
        if !self.capture_per_request {
            return None;
        }
        self.requests.get_mut(&uuid)?.detail.as_deref_mut()
    }

    pub(crate) fn on_token(&mut self, uuid: Uuid, token_time_ms: f64) {
        if let Some(stats) = self.requests.get_mut(&uuid)
            && let TokenTimeline::Recording(times) = &mut stats.token_timeline
        {
            times.push(token_time_ms);
        }
    }

    /// Move the tokens emitted by one scheduler pass to a shared completion
    /// boundary. Scheduler cores record their rank-local end time while the
    /// pass is formed; attention-DP replay then aligns every rank in the group
    /// to the slowest rank before the pass becomes externally visible.
    pub(crate) fn align_pass_token_times(
        &mut self,
        output_signals: &[OutputSignal],
        completion_time_ms: f64,
    ) {
        let mut emitted_by_request = FxHashMap::default();
        for signal in output_signals {
            if signal.token_id.is_some() {
                *emitted_by_request.entry(signal.uuid).or_insert(0usize) += 1;
            }
        }

        for (uuid, emitted) in emitted_by_request {
            let Some(stats) = self.requests.get_mut(&uuid) else {
                continue;
            };
            let TokenTimeline::Recording(times) = &mut stats.token_timeline else {
                continue;
            };
            let start = times
                .len()
                .checked_sub(emitted)
                .expect("scheduler emitted more output signals than collector tokens");
            times[start..].fill(completion_time_ms);
        }
    }

    /// Return (ttft_ms, mean_itl_ms) for a completed request, if available.
    pub(crate) fn request_latencies(&self, uuid: Uuid) -> Option<(f64, f64)> {
        let stats = self.requests.get(&uuid)?;
        let first_token_ms = stats.first_token_ms()?;
        let ttft_ms = (first_token_ms - stats.arrival_time_ms).max(0.0);
        let mean_itl_ms = stats.mean_tpot_ms().unwrap_or(0.0);
        Some((ttft_ms, mean_itl_ms))
    }

    pub(crate) fn actual_output_length(&self, uuid: Uuid) -> Option<usize> {
        self.requests
            .get(&uuid)
            .map(TraceRequestStats::actual_output_length)
    }

    pub(crate) fn finish(mut self) -> TraceSimulationReport {
        let Self {
            requests,
            itl_distribution,
            output_token_throughput_per_user,
            ..
        } = &mut self;
        for stats in requests.values_mut() {
            stats.finalize_token_timeline(
                stats.terminal_status == Some(ReplayTerminalStatus::Completed)
                    && stats.first_admit_ms.is_some(),
                itl_distribution,
                output_token_throughput_per_user,
            );
        }

        // Build per-request records before we move `self.requests` into the
        // summary aggregation below. Gated on `capture_per_request` — the
        // ~100ms terminal pass + ~30MB allocation only runs when a caller
        // (e.g. CLI `--report-jsonl`) asked for it. The summary report is
        // unaffected either way (custom Serialize impl skips `per_request`).
        let per_request = if self.capture_per_request {
            self.per_request_records()
        } else {
            Vec::new()
        };
        let sla = self.sla;
        let static_worker_count = self.static_worker_count;
        let accumulated_prefill_worker_seconds = self.prefill_worker_seconds;
        let accumulated_decode_worker_seconds = self.decode_worker_seconds;
        let prefill_gpus_per_worker = self.prefill_gpus_per_worker;
        let decode_gpus_per_worker = self.decode_gpus_per_worker;
        let itl_distribution = self.itl_distribution.finish();
        let output_token_throughput_per_user = self.output_token_throughput_per_user.finish();
        let requests = self.requests;
        let request_count = requests.len();
        let mut ttfts = Vec::with_capacity(request_count);
        let mut ttsts = Vec::with_capacity(request_count);
        let mut tpots = Vec::with_capacity(request_count);
        let mut e2e_latencies = Vec::with_capacity(request_count);
        let mut duration_ms = 0.0_f64;
        let mut total_input_tokens = 0usize;
        let mut total_output_tokens = 0usize;
        let mut completed_requests = 0usize;
        let mut total_reused_tokens = 0usize;
        let mut total_first_admission_reused_tokens = 0usize;
        // Goodput: completed requests (and their output tokens) that satisfy the SLA.
        let mut goodput_requests = 0usize;
        let mut goodput_output_tokens = 0usize;

        for stats in requests.values() {
            if stats.first_admit_ms.is_none() {
                continue;
            }
            if stats.terminal_status != Some(ReplayTerminalStatus::Completed) {
                continue;
            }
            let Some(terminal_time_ms) = stats.terminal_time_ms else {
                continue;
            };

            completed_requests += 1;
            total_input_tokens += stats.input_length;
            let output_length = stats.actual_output_length();
            total_output_tokens += output_length;
            total_reused_tokens += stats.reused_input_tokens;
            total_first_admission_reused_tokens += stats.first_admission_reused_input_tokens;
            duration_ms = duration_ms.max(terminal_time_ms);

            let (Some(first_token_ms), Some(last_token_ms)) =
                (stats.first_token_ms(), stats.last_token_ms())
            else {
                let e2e_ms = (terminal_time_ms - stats.arrival_time_ms).max(0.0);
                if sla.is_set() && sla.is_good_without_tokens(e2e_ms) {
                    goodput_requests += 1;
                }
                continue;
            };

            let ttft_ms = (first_token_ms - stats.arrival_time_ms).max(0.0);
            let e2e_ms = (last_token_ms - stats.arrival_time_ms).max(0.0);
            ttfts.push(ttft_ms);
            e2e_latencies.push(e2e_ms);

            // Goodput classification (aiperf avg-ITL; see SlaThresholds::is_good).
            if sla.is_set() && sla.is_good(ttft_ms, e2e_ms, output_length) {
                goodput_requests += 1;
                goodput_output_tokens += output_length;
            }

            if let Some(ttst_ms) = stats.ttst_ms() {
                ttsts.push(ttst_ms);
            }

            if let Some(tpot_ms) = stats.mean_tpot_ms() {
                tpots.push(tpot_ms);
            }
        }

        let duration_s = (duration_ms / 1000.0).max(1e-9);
        // Provisioned worker-seconds: static count × duration for the
        // single-worker path, else the runtime-integrated accumulator.
        let (prefill_worker_seconds, decode_worker_seconds) = match static_worker_count {
            Some((prefill, decode)) => (prefill as f64 * duration_s, decode as f64 * duration_s),
            None => (
                accumulated_prefill_worker_seconds,
                accumulated_decode_worker_seconds,
            ),
        };
        // GPU-hours straight from the mocker's own worker parallelism (no
        // external GPU-count config). 0 when gpus_per_worker was not set.
        let gpu_hours = (prefill_worker_seconds * prefill_gpus_per_worker as f64
            + decode_worker_seconds * decode_gpus_per_worker as f64)
            / 3600.0;
        // Goodput only when an SLA was supplied; otherwise it is undefined.
        let goodput = sla.is_set().then(|| TraceGoodputStats {
            completed_requests: goodput_requests,
            request_throughput_rps: goodput_requests as f64 / duration_s,
            output_throughput_tok_s: goodput_output_tokens as f64 / duration_s,
        });
        TraceSimulationReport {
            request_counts: TraceRequestCounts {
                num_requests: request_count,
                completed_requests,
                total_input_tokens,
                total_output_tokens,
            },
            throughput: TraceThroughputStats {
                duration_ms,
                wall_time_ms: 0.0,
                request_throughput_rps: completed_requests as f64 / duration_s,
                input_throughput_tok_s: total_input_tokens as f64 / duration_s,
                output_throughput_tok_s: total_output_tokens as f64 / duration_s,
                total_throughput_tok_s: (total_input_tokens + total_output_tokens) as f64
                    / duration_s,
                prefill_worker_seconds,
                decode_worker_seconds,
                prefill_gpus_per_worker,
                decode_gpus_per_worker,
                gpu_hours,
            },
            prefix_cache_reused_ratio: if total_input_tokens == 0 {
                0.0
            } else {
                total_reused_tokens as f64 / total_input_tokens as f64
            },
            first_admission_prefix_cache_reused_ratio: if total_input_tokens == 0 {
                0.0
            } else {
                total_first_admission_reused_tokens as f64 / total_input_tokens as f64
            },
            latency: TraceLatencyStats {
                ttft: build_distribution_stats(ttfts),
                ttst: build_distribution_stats(ttsts),
                tpot: build_distribution_stats(tpots),
                itl: TraceInterTokenLatencyStats {
                    max_ms: itl_distribution.max_ms,
                    distribution: itl_distribution,
                },
                e2e: build_distribution_stats(e2e_latencies),
                output_token_throughput_per_user,
            },
            goodput,
            per_request,
        }
    }

    /// Flatten each retained request into a serializable `PerRequestRecord`.
    /// Used by the `--report-jsonl` CLI path to emit one JSON object per
    /// request to the JSONL file, mirroring AIPerf's per-request output shape.
    ///
    /// Only requests with a terminal outcome are emitted. Requests truncated
    /// by a simulation-time cap have no terminal outcome and remain omitted.
    pub fn per_request_records(&self) -> Vec<PerRequestRecord> {
        let mut records = Vec::with_capacity(self.requests.len());
        for (uuid, stats) in &self.requests {
            let Some(detail) = stats.detail.as_deref() else {
                continue;
            };
            let Some(terminal_status) = stats.terminal_status else {
                continue;
            };
            let Some(terminal_time_ms) = stats.terminal_time_ms else {
                continue;
            };
            let first_token_ms = stats.first_token_ms();
            let last_token_ms = stats.last_token_ms();
            records.push(PerRequestRecord {
                session_id: stats.session_id.clone(),
                turn_index: stats.turn_index,
                uuid: uuid.to_string(),
                arrival_time_ms: stats.arrival_time_ms,
                first_admit_ms: stats.first_admit_ms,
                terminal_time_ms,
                first_token_ms,
                last_token_ms,
                ttft_ms: first_token_ms.map(|time| (time - stats.arrival_time_ms).max(0.0)),
                ttst_ms: stats.ttst_ms(),
                e2e_latency_ms: last_token_ms.map(|time| (time - stats.arrival_time_ms).max(0.0)),
                itl_ms: stats.mean_tpot_ms(),
                input_length: stats.input_length,
                requested_output_length: stats.requested_output_length,
                output_length: stats.actual_output_length(),
                reused_input_tokens: detail
                    .prefill_reused_input_tokens
                    .unwrap_or(stats.reused_input_tokens),
                prefill_worker_idx: stats.prefill_worker_idx,
                decode_worker_idx: stats.decode_worker_idx,
                prefill_admit_ms: detail.prefill_admit_ms,
                source_held_ms: detail.source_held_ms,
                destination_reserved_ms: detail.destination_reserved_ms,
                destination_activated_ms: detail.destination_activated_ms,
                decode_admit_ms: detail.decode_admit_ms,
                source_released_ms: detail.source_released_ms,
                decode_reused_input_tokens: detail.decode_reused_input_tokens,
                prefill_route_overlap_tokens: detail.prefill_route_overlap_tokens,
                decode_route_overlap_tokens: detail.decode_route_overlap_tokens,
                terminal_status,
            });
        }
        // Stable ordering: by arrival_time_ms (with uuid as tiebreaker) so the
        // JSONL file is reproducible across runs and matches the order
        // analysis tools usually expect.
        records.sort_by(|a, b| {
            a.arrival_time_ms
                .total_cmp(&b.arrival_time_ms)
                .then_with(|| a.uuid.cmp(&b.uuid))
        });
        records
    }

    #[cfg(test)]
    pub(crate) fn snapshot(&self, uuid: Uuid) -> Option<TraceRequestStatsSnapshot> {
        self.requests
            .get(&uuid)
            .map(|stats| TraceRequestStatsSnapshot {
                arrival_time_ms: stats.arrival_time_ms,
                first_admit_ms: stats.first_admit_ms,
                first_token_ms: stats.first_token_ms(),
                last_token_ms: stats.last_token_ms(),
                input_length: stats.input_length,
                requested_output_length: stats.requested_output_length,
                output_length: stats.actual_output_length(),
                reused_input_tokens: stats.reused_input_tokens,
                first_admission_reused_input_tokens: stats.first_admission_reused_input_tokens,
            })
    }

    #[cfg(test)]
    pub(crate) fn snapshots(&self) -> Vec<TraceRequestStatsSnapshot> {
        self.requests
            .values()
            .map(|stats| TraceRequestStatsSnapshot {
                arrival_time_ms: stats.arrival_time_ms,
                first_admit_ms: stats.first_admit_ms,
                first_token_ms: stats.first_token_ms(),
                last_token_ms: stats.last_token_ms(),
                input_length: stats.input_length,
                requested_output_length: stats.requested_output_length,
                output_length: stats.actual_output_length(),
                reused_input_tokens: stats.reused_input_tokens,
                first_admission_reused_input_tokens: stats.first_admission_reused_input_tokens,
            })
            .collect()
    }

    #[cfg(test)]
    fn retained_token_timestamps(&self) -> usize {
        self.requests
            .values()
            .map(|stats| match &stats.token_timeline {
                TokenTimeline::Recording(times) => times.len(),
                TokenTimeline::Finalized(_) => 0,
            })
            .sum()
    }
}

fn mean(values: &[f64]) -> f64 {
    if values.is_empty() {
        0.0
    } else {
        values.iter().sum::<f64>() / values.len() as f64
    }
}

fn build_distribution_stats(mut values: Vec<f64>) -> TraceDistributionStats {
    if values.is_empty() {
        return empty_distribution_stats();
    }

    let min_ms = values
        .iter()
        .copied()
        .min_by(|left, right| left.total_cmp(right))
        .expect("non-empty values must have a minimum");
    let max_ms = values
        .iter()
        .copied()
        .max_by(|left, right| left.total_cmp(right))
        .expect("non-empty values must have a maximum");

    TraceDistributionStats {
        mean_ms: mean(&values),
        min_ms,
        max_ms,
        median_ms: percentile_in_place(&mut values, 50.0),
        p75_ms: percentile_in_place(&mut values, 75.0),
        p90_ms: percentile_in_place(&mut values, 90.0),
        p95_ms: percentile_in_place(&mut values, 95.0),
        p99_ms: percentile_in_place(&mut values, 99.0),
        std_ms: std_dev(&values),
    }
}

fn empty_distribution_stats() -> TraceDistributionStats {
    TraceDistributionStats {
        mean_ms: 0.0,
        min_ms: 0.0,
        max_ms: 0.0,
        median_ms: 0.0,
        p75_ms: 0.0,
        p90_ms: 0.0,
        p95_ms: 0.0,
        p99_ms: 0.0,
        std_ms: 0.0,
    }
}

fn percentile_in_place(values: &mut [f64], percentile: f64) -> f64 {
    let rank = percentile_rank(values.len(), percentile);
    let (_, selected, _) = values.select_nth_unstable_by(rank, |left, right| left.total_cmp(right));
    *selected
}

fn percentile_rank(len: usize, percentile: f64) -> usize {
    let rank = ((len - 1) as f64 * percentile / 100.0).round() as usize;
    rank.min(len - 1)
}

fn std_dev(values: &[f64]) -> f64 {
    if values.is_empty() {
        return 0.0;
    }

    let mean = mean(values);
    let variance = values
        .iter()
        .map(|value| {
            let centered = value - mean;
            centered * centered
        })
        .sum::<f64>()
        / values.len() as f64;
    variance.sqrt()
}

#[cfg(test)]
mod tests {
    use super::*;

    fn build_distribution_stats_sorted(values: &[f64]) -> TraceDistributionStats {
        if values.is_empty() {
            return TraceDistributionStats {
                mean_ms: 0.0,
                min_ms: 0.0,
                max_ms: 0.0,
                median_ms: 0.0,
                p75_ms: 0.0,
                p90_ms: 0.0,
                p95_ms: 0.0,
                p99_ms: 0.0,
                std_ms: 0.0,
            };
        }

        let mut sorted = values.to_vec();
        sorted.sort_by(|left, right| left.total_cmp(right));
        TraceDistributionStats {
            mean_ms: mean(values),
            min_ms: sorted[0],
            max_ms: *sorted.last().expect("sorted values must be non-empty"),
            median_ms: sorted[percentile_rank(sorted.len(), 50.0)],
            p75_ms: sorted[percentile_rank(sorted.len(), 75.0)],
            p90_ms: sorted[percentile_rank(sorted.len(), 90.0)],
            p95_ms: sorted[percentile_rank(sorted.len(), 95.0)],
            p99_ms: sorted[percentile_rank(sorted.len(), 99.0)],
            std_ms: std_dev(values),
        }
    }

    #[test]
    fn build_distribution_stats_matches_sorted_baseline() {
        let values = vec![
            0.0, 1.0, 1.0, 2.5, 4.0, 4.0, 7.25, 9.5, 15.0, 22.0, 22.0, 100.0,
        ];

        let expected = build_distribution_stats_sorted(&values);
        let actual = build_distribution_stats(values);

        assert_eq!(actual.mean_ms, expected.mean_ms);
        assert_eq!(actual.min_ms, expected.min_ms);
        assert_eq!(actual.max_ms, expected.max_ms);
        assert_eq!(actual.median_ms, expected.median_ms);
        assert_eq!(actual.p75_ms, expected.p75_ms);
        assert_eq!(actual.p90_ms, expected.p90_ms);
        assert_eq!(actual.p95_ms, expected.p95_ms);
        assert_eq!(actual.p99_ms, expected.p99_ms);
        assert_eq!(actual.std_ms, expected.std_ms);
    }

    #[test]
    fn built_in_ddsketch_configuration_and_quantiles_are_valid() {
        let mut distribution = StreamingDistribution::default();
        assert!((distribution.sketch.alpha() - DDSKETCH_RELATIVE_ACCURACY).abs() < f64::EPSILON);
        for percentile in [50.0, 75.0, 90.0, 95.0, 99.0] {
            assert_eq!(distribution.percentile(percentile), 0.0);
        }

        // This is wider than any plausible replay latency or token-rate
        // range, and stays below the configured store's ~10^28 span.
        for value in [1e-9, 1e18] {
            distribution.add(value);
        }
        for (quantile, expected) in [(0.0, 1e-9), (1.0, 1e18)] {
            let actual = distribution.sketch.quantile(quantile).unwrap();
            assert!((actual - expected).abs() <= expected * DDSKETCH_RELATIVE_ACCURACY);
        }
    }

    #[test]
    fn streaming_distribution_preserves_all_zero_samples() {
        let mut distribution = StreamingDistribution::default();
        for _ in 0..128 {
            distribution.add(0.0);
        }

        assert_eq!(distribution.sketch.get_zero_count(), 128);
        let stats = distribution.finish();
        for value in [
            stats.mean_ms,
            stats.min_ms,
            stats.max_ms,
            stats.median_ms,
            stats.p75_ms,
            stats.p90_ms,
            stats.p95_ms,
            stats.p99_ms,
            stats.std_ms,
        ] {
            assert_eq!(value, 0.0);
        }
    }

    #[test]
    fn streaming_percentiles_select_the_historical_rounded_rank() {
        let percentiles = [
            0.0, 1.0, 10.0, 25.0, 49.0, 50.0, 51.0, 75.0, 90.0, 95.0, 99.0, 100.0,
        ];
        for len in [2, 3, 4, 5, 10, 11, 100, 101, 256, 257] {
            let values = (0..len)
                .map(|index| 1_000.0 + index as f64 * 10.0)
                .collect::<Vec<_>>();
            let mut distribution = StreamingDistribution::default();
            for &value in &values {
                distribution.add(value);
            }

            for percentile in percentiles {
                let expected = values[percentile_rank(values.len(), percentile)];
                let actual = distribution.percentile(percentile);
                assert!(
                    (actual - expected).abs() <= expected * DDSKETCH_RELATIVE_ACCURACY,
                    "len={len} percentile={percentile}: expected rank value {expected}, got {actual}"
                );
            }
        }
    }

    #[test]
    fn completed_zero_output_request_counts_without_latency_samples() {
        let mut collector = TraceCollector::default();
        collector.set_static_worker_count(0, 1);
        collector.set_gpus_per_worker(0, 4);
        let uuid = Uuid::from_u128(99);
        collector.on_arrival(uuid, 0.0, 32, 0);
        collector.on_admit(uuid, 5.0, 8);
        collector.on_terminal(uuid, 25.0, ReplayTerminalStatus::Completed);

        let report = collector.finish();

        assert_eq!(report.request_counts.completed_requests, 1);
        assert_eq!(report.request_counts.total_input_tokens, 32);
        assert_eq!(report.request_counts.total_output_tokens, 0);
        assert_eq!(report.throughput.duration_ms, 25.0);
        assert_eq!(report.throughput.decode_worker_seconds, 0.025);
        assert!((report.throughput.gpu_hours - 0.1 / 3600.0).abs() < 1e-12);
        assert_eq!(report.latency.ttft.mean_ms, 0.0);
        assert_eq!(report.latency.e2e.mean_ms, 0.0);
    }

    #[test]
    fn token_before_simulation_cap_does_not_count_as_completion() {
        let mut collector = TraceCollector::default();
        let uuid = Uuid::from_u128(100);
        collector.on_arrival(uuid, 0.0, 32, 4);
        collector.on_admit(uuid, 5.0, 0);
        collector.on_token(uuid, 25.0);

        let report = collector.finish();

        assert_eq!(report.request_counts.completed_requests, 0);
        assert_eq!(report.request_counts.total_input_tokens, 0);
        assert_eq!(report.request_counts.total_output_tokens, 0);
        assert_eq!(report.throughput.duration_ms, 0.0);
    }

    #[test]
    fn zero_output_goodput_requires_e2e_only_sla() {
        let collect = |sla| {
            let mut collector = TraceCollector::default();
            collector.set_sla_thresholds(sla);
            let uuid = Uuid::from_u128(101);
            collector.on_arrival(uuid, 0.0, 32, 0);
            collector.on_admit(uuid, 5.0, 0);
            collector.on_terminal(uuid, 100.0, ReplayTerminalStatus::Completed);
            collector.finish().goodput.unwrap().completed_requests
        };

        assert_eq!(
            collect(SlaThresholds {
                e2e_ms: Some(100.0),
                ..Default::default()
            }),
            1
        );
        assert_eq!(
            collect(SlaThresholds {
                ttft_ms: Some(1_000.0),
                ..Default::default()
            }),
            0
        );
    }

    /// With per-request capture on, a standard disagg-style request lifecycle
    /// (arrival → admit → prefill_assigned → decode_assigned → tokens) yields
    /// exactly one record with all fields populated correctly.
    #[test]
    fn per_request_disagg_record_populates_all_fields() {
        let mut collector = TraceCollector::default();
        collector.set_capture_per_request(true);
        let uuid = Uuid::from_u128(1);
        collector.on_arrival(uuid, 0.0, 100, 4);
        collector.on_prefill_route_overlap(uuid, 64);
        collector.on_prefill_admit(uuid, 5.0, 30);
        collector.on_source_held(uuid, 10.0);
        collector.on_destination_reserved(uuid, 12.0);
        collector.on_destination_activated(uuid, 20.0);
        collector.on_source_released(uuid, 21.0);
        collector.on_decode_route_overlap(uuid, 32);
        collector.on_decode_admit(uuid, 25.0, 40);
        collector.on_prefill_assigned(uuid, 2);
        collector.on_decode_assigned(uuid, 7);
        collector.on_token(uuid, 50.0);
        collector.on_token(uuid, 60.0);
        collector.on_token(uuid, 75.0);
        collector.on_token(uuid, 95.0);
        collector.on_terminal(uuid, 95.0, ReplayTerminalStatus::Completed);

        let report = collector.finish();
        assert_eq!(report.per_request.len(), 1);
        let rec = &report.per_request[0];
        assert_eq!(rec.uuid, uuid.to_string());
        assert_eq!(rec.arrival_time_ms, 0.0);
        assert_eq!(rec.first_admit_ms, Some(5.0));
        assert_eq!(rec.terminal_time_ms, 95.0);
        assert_eq!(rec.first_token_ms, Some(50.0));
        assert_eq!(rec.last_token_ms, Some(95.0));
        assert_eq!(rec.ttft_ms, Some(50.0));
        assert_eq!(rec.ttst_ms, Some(10.0));
        assert_eq!(rec.e2e_latency_ms, Some(95.0));
        // Mean per-token gap across 4 tokens: (10 + 15 + 20) / 3 = 15.0
        assert_eq!(rec.itl_ms, Some(15.0));
        assert_eq!(rec.input_length, 100);
        assert_eq!(rec.output_length, 4);
        assert_eq!(rec.reused_input_tokens, 30);
        assert_eq!(rec.prefill_worker_idx, Some(2));
        assert_eq!(rec.decode_worker_idx, Some(7));
        assert_eq!(rec.prefill_admit_ms, Some(5.0));
        assert_eq!(rec.source_held_ms, Some(10.0));
        assert_eq!(rec.destination_reserved_ms, Some(12.0));
        assert_eq!(rec.destination_activated_ms, Some(20.0));
        assert_eq!(rec.source_released_ms, Some(21.0));
        assert_eq!(rec.decode_admit_ms, Some(25.0));
        assert_eq!(rec.decode_reused_input_tokens, Some(40));
        assert_eq!(rec.prefill_route_overlap_tokens, Some(64));
        assert_eq!(rec.decode_route_overlap_tokens, Some(32));
        assert_eq!(rec.terminal_status, ReplayTerminalStatus::Completed);
    }

    /// A conditional-prefill bypass is reflected by `prefill_worker_idx ==
    /// None` while `decode_worker_idx` is set. This is how downstream tooling
    /// distinguishes bypassed requests from standard disagg flow.
    #[test]
    fn per_request_bypass_leaves_prefill_worker_idx_none() {
        let mut collector = TraceCollector::default();
        collector.set_capture_per_request(true);
        let uuid = Uuid::from_u128(42);
        collector.on_arrival(uuid, 0.0, 100, 2);
        collector.on_admit(uuid, 5.0, 0);
        // No on_prefill_assigned call — request bypassed remote prefill.
        collector.on_decode_assigned(uuid, 1);
        collector.on_token(uuid, 30.0);
        collector.on_token(uuid, 45.0);
        collector.on_terminal(uuid, 45.0, ReplayTerminalStatus::Completed);

        let report = collector.finish();
        assert_eq!(report.per_request.len(), 1);
        let rec = &report.per_request[0];
        assert!(
            rec.prefill_worker_idx.is_none(),
            "bypassed request must have prefill_worker_idx = None"
        );
        assert_eq!(rec.decode_worker_idx, Some(1));
    }

    /// Default: capture is off, so `per_request` is empty and the ~100ms
    /// terminal pass is skipped. The summary report is otherwise identical.
    #[test]
    fn per_request_default_off() {
        let mut collector = TraceCollector::default();
        // Note: NOT calling set_capture_per_request — capture stays false.
        let uuid = Uuid::from_u128(1);
        collector.on_arrival(uuid, 0.0, 100, 2);
        collector.on_admit(uuid, 5.0, 0);
        collector.on_decode_assigned(uuid, 0);
        collector.on_token(uuid, 50.0);
        collector.on_token(uuid, 60.0);
        collector.on_terminal(uuid, 60.0, ReplayTerminalStatus::Completed);

        assert!(collector.requests[&uuid].detail.is_none());

        let report = collector.finish();
        assert!(report.per_request.is_empty());
        // Summary stats still work.
        assert_eq!(report.request_counts.completed_requests, 1);
    }

    /// Register a completed request: arrival, output length (osl), and the
    /// explicit per-output-token timestamps (first → ttft, last → e2e).
    fn add_completed(
        collector: &mut TraceCollector,
        uuid_n: u128,
        arrival_ms: f64,
        output_length: usize,
        token_times_ms: &[f64],
    ) {
        let uuid = Uuid::from_u128(uuid_n);
        collector.on_arrival(uuid, arrival_ms, 100, output_length);
        collector.on_admit(uuid, arrival_ms, 0);
        collector.on_decode_assigned(uuid, 0);
        for &t in token_times_ms {
            collector.on_token(uuid, t);
        }
        let terminal_time_ms = token_times_ms.last().copied().unwrap_or(arrival_ms);
        collector.on_terminal(uuid, terminal_time_ms, ReplayTerminalStatus::Completed);
    }

    /// Goodput classifies a request "good" using aiperf's average ITL,
    /// `avg_itl = (e2e − ttft) / (osl − 1)`, and skips the ITL check when
    /// `osl ≤ 1`.
    #[test]
    fn goodput_classifies_by_aiperf_avg_itl() {
        let mut collector = TraceCollector::default();
        collector.set_sla_thresholds(SlaThresholds {
            ttft_ms: Some(150.0),
            itl_ms: Some(30.0),
            e2e_ms: None,
        });
        // A: ttft=100, e2e=200, osl=3 → avg_itl=(200−100)/2=50 > 30 → BAD.
        add_completed(&mut collector, 1, 0.0, 3, &[100.0, 150.0, 200.0]);
        // B: ttft=100, e2e=140, osl=3 → avg_itl=20 ≤ 30, ttft ok → GOOD.
        add_completed(&mut collector, 2, 0.0, 3, &[100.0, 120.0, 140.0]);
        // C: osl=1 → ITL check skipped; ttft=100 ≤ 150 → GOOD.
        add_completed(&mut collector, 3, 0.0, 1, &[100.0]);

        let goodput = collector
            .finish()
            .goodput
            .expect("SLA set → goodput present");
        assert_eq!(goodput.completed_requests, 2); // B and C
        // duration = max last token = 200ms → 0.2s; good output tokens = 3 (B) + 1 (C) = 4.
        assert!((goodput.output_throughput_tok_s - 4.0 / 0.2).abs() < 1e-6);
        assert!((goodput.request_throughput_rps - 2.0 / 0.2).abs() < 1e-6);
    }

    /// A request straddling the ITL bound flips good↔bad at the boundary.
    #[test]
    fn goodput_itl_boundary_is_inclusive() {
        let sla = SlaThresholds {
            ttft_ms: None,
            itl_ms: Some(50.0),
            e2e_ms: None,
        };
        // avg_itl = (200−100)/(3−1) = 50.0, exactly the bound → good (≤).
        let mut at_bound = TraceCollector::default();
        at_bound.set_sla_thresholds(sla);
        add_completed(&mut at_bound, 1, 0.0, 3, &[100.0, 150.0, 200.0]);
        assert_eq!(at_bound.finish().goodput.unwrap().completed_requests, 1);
        // avg_itl = (201−100)/2 = 50.5 > 50 → bad.
        let mut over = TraceCollector::default();
        over.set_sla_thresholds(sla);
        add_completed(&mut over, 1, 0.0, 3, &[100.0, 150.0, 201.0]);
        assert_eq!(over.finish().goodput.unwrap().completed_requests, 0);
    }

    /// An e2e-only SLA gates on end-to-end latency alone.
    #[test]
    fn goodput_e2e_only_sla() {
        let mut collector = TraceCollector::default();
        collector.set_sla_thresholds(SlaThresholds {
            ttft_ms: None,
            itl_ms: None,
            e2e_ms: Some(150.0),
        });
        add_completed(&mut collector, 1, 0.0, 2, &[100.0, 200.0]); // e2e=200 > 150 → BAD
        add_completed(&mut collector, 2, 0.0, 2, &[60.0, 120.0]); // e2e=120 ≤ 150 → GOOD
        assert_eq!(collector.finish().goodput.unwrap().completed_requests, 1);
    }

    /// No SLA → goodput is omitted entirely.
    #[test]
    fn goodput_absent_without_sla() {
        let mut collector = TraceCollector::default();
        add_completed(&mut collector, 1, 0.0, 2, &[10.0, 20.0]);
        assert!(collector.finish().goodput.is_none());
    }

    /// Worker-seconds: the accumulator (agg/disagg) sums runtime contributions;
    /// the static path (single worker) reports `count × duration_s`.
    #[test]
    fn worker_seconds_accumulated_and_static() {
        let mut accumulated = TraceCollector::default();
        add_completed(&mut accumulated, 1, 0.0, 2, &[10.0, 20.0]);
        accumulated.add_worker_seconds(1.5, 4.0);
        accumulated.add_worker_seconds(0.5, 1.0);
        let report = accumulated.finish();
        assert!((report.throughput.prefill_worker_seconds - 2.0).abs() < 1e-9);
        assert!((report.throughput.decode_worker_seconds - 5.0).abs() < 1e-9);

        let mut static_single = TraceCollector::default();
        static_single.set_static_worker_count(0, 1);
        add_completed(&mut static_single, 1, 0.0, 2, &[100.0, 200.0]); // duration = 0.2s
        let report = static_single.finish();
        assert!(report.throughput.prefill_worker_seconds.abs() < 1e-9);
        assert!((report.throughput.decode_worker_seconds - 0.2).abs() < 1e-9);
    }

    /// gpu_hours derives from worker-seconds x the per-role GPUs/worker that the
    /// runtime records from the mocker's own parallelism.
    #[test]
    fn gpu_hours_from_worker_seconds_and_gpus_per_worker() {
        let mut collector = TraceCollector::default();
        collector.set_gpus_per_worker(2, 4); // prefill 2 GPUs/worker, decode 4
        add_completed(&mut collector, 1, 0.0, 2, &[100.0, 200.0]);
        collector.add_worker_seconds(10.0, 5.0); // prefill_ws=10, decode_ws=5
        let report = collector.finish();
        assert_eq!(report.throughput.prefill_gpus_per_worker, 2);
        assert_eq!(report.throughput.decode_gpus_per_worker, 4);
        // gpu_hours = (10*2 + 5*4) / 3600 = 40 / 3600
        assert!((report.throughput.gpu_hours - 40.0 / 3600.0).abs() < 1e-9);
    }

    /// Records emerge in arrival-time order, so the JSONL file produced from
    /// them is deterministic across runs (important for diff-friendly CI).
    #[test]
    fn per_request_records_are_sorted_by_arrival_time() {
        let mut collector = TraceCollector::default();
        collector.set_capture_per_request(true);
        // Insert out of order on purpose.
        for (uuid_n, arrival) in [(3u128, 30.0), (1, 0.0), (2, 10.0)] {
            let uuid = Uuid::from_u128(uuid_n);
            collector.on_arrival(uuid, arrival, 100, 1);
            collector.on_admit(uuid, arrival + 1.0, 0);
            collector.on_decode_assigned(uuid, 0);
            collector.on_token(uuid, arrival + 5.0);
            collector.on_terminal(uuid, arrival + 5.0, ReplayTerminalStatus::Completed);
        }
        let report = collector.finish();
        let arrivals: Vec<f64> = report
            .per_request
            .iter()
            .map(|r| r.arrival_time_ms)
            .collect();
        assert_eq!(arrivals, vec![0.0, 10.0, 30.0]);
    }

    /// Each record must round-trip cleanly to JSON — this is the format we
    /// emit to `--report-jsonl`. Guards against accidental serde regressions
    /// (e.g., adding a non-serializable field to `PerRequestRecord`).
    #[test]
    fn per_request_record_serializes_to_json_object() {
        let mut collector = TraceCollector::default();
        collector.set_capture_per_request(true);
        let uuid = Uuid::from_u128(123);
        collector.on_arrival(uuid, 0.0, 50, 2);
        collector.on_admit(uuid, 1.0, 10);
        collector.on_prefill_assigned(uuid, 0);
        collector.on_decode_assigned(uuid, 1);
        collector.on_token(uuid, 20.0);
        collector.on_token(uuid, 25.0);
        collector.on_terminal(uuid, 25.0, ReplayTerminalStatus::Completed);

        let report = collector.finish();
        let line = serde_json::to_string(&report.per_request[0])
            .expect("PerRequestRecord must serialize cleanly");
        // Parse it back and spot-check a few keys to confirm shape.
        let parsed: serde_json::Value =
            serde_json::from_str(&line).expect("emitted JSON must parse");
        assert!(parsed.is_object());
        assert_eq!(parsed["uuid"], uuid.to_string());
        assert_eq!(parsed["input_length"], 50);
        assert_eq!(parsed["output_length"], 2);
        assert_eq!(parsed["prefill_worker_idx"], 0);
        assert_eq!(parsed["decode_worker_idx"], 1);
        assert!(parsed["itl_ms"].is_number());
        assert_eq!(parsed["terminal_status"], "completed");
    }

    #[test]
    fn terminal_failures_emit_nullable_latencies_and_unfinished_requests_are_omitted() {
        let mut collector = TraceCollector::default();
        collector.set_capture_per_request(true);
        for (uuid_n, status) in [
            (1, ReplayTerminalStatus::Rejected),
            (2, ReplayTerminalStatus::Canceled),
            (3, ReplayTerminalStatus::Failed),
        ] {
            let uuid = Uuid::from_u128(uuid_n);
            collector.on_arrival(uuid, uuid_n as f64, 64, 2);
            collector.on_terminal(uuid, uuid_n as f64 + 1.0, status);
        }
        collector.on_arrival(Uuid::from_u128(4), 4.0, 64, 2);

        let report = collector.finish();

        assert_eq!(report.per_request.len(), 3);
        assert_eq!(
            report
                .per_request
                .iter()
                .map(|record| record.terminal_status)
                .collect::<Vec<_>>(),
            vec![
                ReplayTerminalStatus::Rejected,
                ReplayTerminalStatus::Canceled,
                ReplayTerminalStatus::Failed,
            ]
        );
        assert!(report.per_request.iter().all(|record| {
            record.first_admit_ms.is_none()
                && record.first_token_ms.is_none()
                && record.last_token_ms.is_none()
                && record.ttft_ms.is_none()
                && record.e2e_latency_ms.is_none()
        }));
    }

    #[test]
    fn first_admission_reuse_ignores_later_readmission_self_reuse() {
        let uuid = Uuid::from_u128(1);
        let mut collector = TraceCollector::default();
        collector.on_arrival(uuid, 0.0, 100, 1);
        collector.on_admit(uuid, 1.0, 0);
        collector.on_admit(uuid, 2.0, 80);
        collector.on_token(uuid, 3.0);
        collector.on_terminal(uuid, 3.0, ReplayTerminalStatus::Completed);

        let report = collector.finish();

        assert_eq!(report.prefix_cache_reused_ratio, 0.8);
        assert_eq!(report.first_admission_prefix_cache_reused_ratio, 0.0);
    }

    #[test]
    fn terminal_request_releases_per_token_timestamps() {
        let uuid = Uuid::from_u128(7);
        let mut collector = TraceCollector::default();
        collector.on_arrival(uuid, 0.0, 128, 100_000);
        collector.on_admit(uuid, 1.0, 0);
        for token_index in 0..100_000 {
            collector.on_token(uuid, token_index as f64 + 10.0);
        }
        assert_eq!(collector.retained_token_timestamps(), 100_000);

        collector.on_terminal(uuid, 100_009.0, ReplayTerminalStatus::Completed);

        assert_eq!(collector.retained_token_timestamps(), 0);
        let snapshot = collector
            .snapshot(uuid)
            .expect("request must remain summarized");
        assert_eq!(snapshot.output_length, 100_000);
        assert_eq!(snapshot.first_token_ms, Some(10.0));
        assert_eq!(snapshot.last_token_ms, Some(100_009.0));
        let report = collector.finish();
        assert_eq!(report.latency.itl.distribution.mean_ms, 1.0);
        assert_eq!(report.latency.itl.distribution.min_ms, 1.0);
        assert_eq!(report.latency.itl.distribution.max_ms, 1.0);
    }

    #[test]
    fn deferred_token_timeline_finalization_folds_at_finish() {
        let uuid = Uuid::from_u128(8);
        let mut collector = TraceCollector::default();
        collector.set_defer_token_timeline_finalization(true);
        collector.on_arrival(uuid, 0.0, 128, 3);
        collector.on_admit(uuid, 1.0, 0);
        collector.on_token(uuid, 10.0);
        collector.on_token(uuid, 12.0);
        collector.on_token(uuid, 15.0);
        collector.on_terminal(uuid, 15.0, ReplayTerminalStatus::Completed);

        assert_eq!(collector.retained_token_timestamps(), 3);

        let report = collector.finish();
        assert_eq!(report.latency.itl.distribution.mean_ms, 2.5);
        assert_eq!(report.latency.itl.distribution.min_ms, 2.0);
        assert_eq!(report.latency.itl.distribution.max_ms, 3.0);
    }
}