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

use rustc_hash::FxHashMap;
use serde::Serialize;
use serde::ser::{SerializeMap, Serializer};
use uuid::Uuid;

#[derive(Debug, Clone)]
pub struct TraceSimulationReport {
    pub request_counts: TraceRequestCounts,
    pub throughput: TraceThroughputStats,
    pub prefix_cache_reused_ratio: f64,
    pub latency: TraceLatencyStats,
}

#[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,
}

#[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
    }
}

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(59))?;
        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("prefix_cache_reused_ratio", &self.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>,
    token_times_ms: Vec<f64>,
    input_length: usize,
    output_length: usize,
    reused_input_tokens: usize,
}

#[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 output_length: usize,
    pub reused_input_tokens: usize,
}

#[derive(Debug, Default)]
pub(crate) struct TraceCollector {
    requests: FxHashMap<Uuid, TraceRequestStats>,
}

impl TraceRequestStats {
    fn first_token_ms(&self) -> Option<f64> {
        self.token_times_ms.first().copied()
    }

    fn last_token_ms(&self) -> Option<f64> {
        self.token_times_ms.last().copied()
    }

    fn mean_tpot_ms(&self) -> Option<f64> {
        let num_gaps = self.token_times_ms.len().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 itls_ms(&self) -> impl Iterator<Item = f64> + '_ {
        self.token_times_ms
            .windows(2)
            .map(|window| (window[1] - window[0]).max(0.0))
    }

    fn ttst_ms(&self) -> Option<f64> {
        let [first_token_ms, second_token_ms, ..] = self.token_times_ms.as_slice() else {
            return None;
        };
        Some((second_token_ms - first_token_ms).max(0.0))
    }
}

impl TraceCollector {
    pub(crate) fn on_arrival(
        &mut self,
        uuid: Uuid,
        arrival_time_ms: f64,
        input_length: usize,
        output_length: usize,
    ) {
        self.requests.insert(
            uuid,
            TraceRequestStats {
                arrival_time_ms,
                first_admit_ms: None,
                token_times_ms: Vec::with_capacity(output_length),
                input_length,
                output_length,
                reused_input_tokens: 0,
            },
        );
    }

    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) {
            stats.first_admit_ms.get_or_insert(admit_time_ms);
            stats.reused_input_tokens = stats.reused_input_tokens.max(reused_input_tokens);
        }
    }

    pub(crate) fn on_token(&mut self, uuid: Uuid, token_time_ms: f64) {
        if let Some(stats) = self.requests.get_mut(&uuid) {
            stats.token_times_ms.push(token_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 finish(self) -> TraceSimulationReport {
        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 itls = Vec::new();
        let mut e2e_latencies = Vec::with_capacity(request_count);
        let mut output_token_throughput_per_user = Vec::new();
        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;

        for stats in requests.values() {
            if stats.first_admit_ms.is_none() {
                continue;
            }
            let Some(first_token_ms) = stats.first_token_ms() else {
                continue;
            };
            let Some(last_token_ms) = stats.last_token_ms() else {
                continue;
            };

            completed_requests += 1;
            total_input_tokens += stats.input_length;
            total_output_tokens += stats.output_length;
            total_reused_tokens += stats.reused_input_tokens;
            duration_ms = duration_ms.max(last_token_ms);

            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);

            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);
                for itl_ms in stats.itls_ms() {
                    if itl_ms > 0.0 {
                        output_token_throughput_per_user.push(1000.0 / itl_ms);
                    }
                    itls.push(itl_ms);
                }
            }
        }

        let duration_s = (duration_ms / 1000.0).max(1e-9);
        let itl_distribution = build_distribution_stats(itls);
        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,
            },
            prefix_cache_reused_ratio: if total_input_tokens == 0 {
                0.0
            } else {
                total_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: build_distribution_stats(
                    output_token_throughput_per_user,
                ),
            },
        }
    }

    #[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,
                output_length: stats.output_length,
                reused_input_tokens: stats.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,
                output_length: stats.output_length,
                reused_input_tokens: stats.reused_input_tokens,
            })
            .collect()
    }
}

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 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 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 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);
    }
}
