// SPDX-FileCopyrightText: Copyright (c) 2024-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

use std::fmt;
use std::str::FromStr;

use derive_builder::Builder;
use rand::Rng;
use serde::{Deserialize, Serialize};
use validator::{Validate, ValidationError};

use crate::protocols::{compute_block_hash_for_seq, compute_seq_hash_for_block};

#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum RouterQueuePolicy {
    #[default]
    Fcfs,
    Wspt,
}

impl fmt::Display for RouterQueuePolicy {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Fcfs => f.write_str("fcfs"),
            Self::Wspt => f.write_str("wspt"),
        }
    }
}

impl FromStr for RouterQueuePolicy {
    type Err = String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "fcfs" => Ok(Self::Fcfs),
            "wspt" => Ok(Self::Wspt),
            _ => Err(format!(
                "unknown queue policy: {s:?}, expected 'fcfs' or 'wspt'"
            )),
        }
    }
}

/// Override configuration for router settings that can be specified per-request
#[derive(Debug, Clone, Default, Builder, Serialize, Deserialize, Validate)]
pub struct RouterConfigOverride {
    #[builder(default)]
    pub overlap_score_weight: Option<f64>,

    #[builder(default)]
    #[validate(range(min = 0.0))]
    pub router_temperature: Option<f64>,

    #[builder(default)]
    pub assume_kv_reuse: Option<bool>,
}

/// KV Router configuration parameters
#[derive(Debug, Clone, Serialize, Deserialize, Validate)]
#[validate(schema(function = "validate_kv_router_config"))]
pub struct KvRouterConfig {
    #[validate(range(min = 0.0))]
    pub overlap_score_weight: f64,

    #[validate(range(min = 0.0))]
    pub router_temperature: f64,

    pub use_kv_events: bool,

    /// **Deprecated:** Enable durable KV events using NATS JetStream instead of the default event plane.
    /// This option will be removed in a future release. The event-plane subscriber
    /// (local_indexer mode) is now the recommended path.
    pub durable_kv_events: bool,

    pub router_replica_sync: bool,

    /// Whether to track active blocks in the router (default: true)
    pub router_track_active_blocks: bool,

    /// Whether to track output blocks during generation (default: false)
    /// When enabled, the router adds placeholder blocks as tokens are generated
    /// and applies fractional decay based on progress toward agent_hints.osl.
    pub router_track_output_blocks: bool,

    /// Whether to assume KV cache reuse when tracking active blocks (default: true).
    /// When true, computes actual block hashes for sequence tracking.
    /// When false, generates random hashes (assuming no KV cache reuse).
    pub router_assume_kv_reuse: bool,

    /// Threshold for triggering snapshots. If None, no snapshots will be performed.
    #[validate(range(min = 1))]
    pub router_snapshot_threshold: Option<u32>,

    /// Whether to reset the router state on startup (default: false)
    pub router_reset_states: bool,

    /// TTL for blocks in seconds (only used when use_kv_events is false, default: 120.0)
    #[validate(range(min = 0.0))]
    pub router_ttl_secs: f64,

    /// Maximum tree size before pruning (only used when use_kv_events is false, default: 2^20 = 1048576)
    #[validate(range(min = 1))]
    pub router_max_tree_size: usize,

    /// Target size ratio after pruning (only used when use_kv_events is false, default: 0.8)
    #[validate(range(min = 0.0, max = 1.0))]
    pub router_prune_target_ratio: f64,

    /// Queue threshold fraction for prefill token capacity.
    /// When set, requests are queued if all workers exceed this fraction of max_num_batched_tokens.
    /// If None, queueing is disabled and all requests go directly to ready.
    /// Default: 2.0. Must be > 0.
    #[validate(range(min = 0.0))]
    pub router_queue_threshold: Option<f64>,

    /// Number of event processing threads for the KV indexer.
    /// When > 1, uses ConcurrentRadixTree with a thread pool instead of the
    /// single-threaded RadixTree. Default: 4.
    #[validate(range(min = 1))]
    pub router_event_threads: u32,

    /// Enable cache control (PIN with TTL) via the worker's cache_control service mesh endpoint.
    /// When true, the router creates a cache_control client and honors nvext.cache_control on
    /// requests, firing a pin_prefix call (with TTL) to the worker after generation completes.
    /// When false (default), cache_control is ignored and no cache_control client is created.
    pub router_enable_cache_control: bool,

    /// Skip blocking for workers at init time (default: false).
    /// When true, the router starts immediately without waiting for discovery-based
    /// workers and workers are provided externally per-request (e.g., EPP).
    pub skip_initial_worker_wait: bool,
    /// Scheduling policy for the router queue.
    /// "fcfs" (default): first-come first-served with priority bumps — optimizes tail TTFT.
    /// "wspt": weighted shortest processing time (Smith's rule) — optimizes average TTFT.
    pub router_queue_policy: RouterQueuePolicy,

    /// Component name of a standalone KV indexer to use for overlap scoring.
    /// When set, the router creates a `Remote` indexer that queries the standalone
    /// indexer via the request plane instead of maintaining a local radix tree.
    /// The standalone indexer handles its own event subscription and discovery.
    #[serde(default)]
    pub remote_indexer_component: Option<String>,
}

impl Default for KvRouterConfig {
    fn default() -> Self {
        Self {
            overlap_score_weight: 1.0,
            router_temperature: 0.0,
            use_kv_events: true,
            durable_kv_events: false, // default to NATS Core (local indexer mode)
            router_replica_sync: false,
            router_track_active_blocks: true,
            router_track_output_blocks: false,
            router_assume_kv_reuse: true,
            router_snapshot_threshold: Some(1000000),
            router_reset_states: false,
            router_ttl_secs: 120.0,
            router_max_tree_size: 2usize.pow(20), // 2^20 = 1048576, matches PruneConfig::default()
            router_prune_target_ratio: 0.8,
            router_queue_threshold: Some(2.0),
            router_event_threads: 4,
            router_enable_cache_control: false,
            skip_initial_worker_wait: false,
            router_queue_policy: RouterQueuePolicy::default(),
            remote_indexer_component: None,
        }
    }
}

fn validate_kv_router_config(config: &KvRouterConfig) -> Result<(), ValidationError> {
    if config.durable_kv_events {
        tracing::warn!(
            "--durable-kv-events is deprecated and will be removed in a future release. \
             The event-plane subscriber (local_indexer mode) is now the recommended path."
        );
    }
    if config.durable_kv_events && !config.use_kv_events {
        return Err(ValidationError::new(
            "durable_kv_events requires use_kv_events=true",
        ));
    }
    if config.router_track_output_blocks && !config.router_track_active_blocks {
        return Err(ValidationError::new(
            "router_track_output_blocks requires router_track_active_blocks=true",
        ));
    }
    Ok(())
}

impl KvRouterConfig {
    /// Compute sequence hashes for active block tracking based on configuration.
    ///
    /// Returns:
    /// - `None` if `router_track_active_blocks` is false
    /// - Random hashes if `router_track_active_blocks` is true but `router_assume_kv_reuse` is false
    /// - Actual sequence hashes if both are true
    pub fn compute_seq_hashes_for_tracking(
        &self,
        tokens: &[u32],
        block_size: u32,
        config_override: Option<&RouterConfigOverride>,
        lora_name: Option<&str>,
    ) -> Option<Vec<u64>> {
        if !self.router_track_active_blocks {
            return None;
        }

        let num_blocks = tokens.len() / block_size as usize;
        if num_blocks == 0 {
            return Some(Vec::new());
        }

        let assume_kv_reuse = config_override
            .and_then(|cfg| cfg.assume_kv_reuse)
            .unwrap_or(self.router_assume_kv_reuse);

        if assume_kv_reuse {
            let block_hashes = compute_block_hash_for_seq(tokens, block_size, None, lora_name);
            Some(compute_seq_hash_for_block(&block_hashes))
        } else {
            let mut rng = rand::rng();
            Some((0..num_blocks).map(|_| rng.random::<u64>()).collect())
        }
    }

    /// Check if KV event subscription should be started.
    ///
    /// Returns false if:
    /// - KV events are disabled (`use_kv_events=false`)
    /// - Overlap scoring is disabled (`overlap_score_weight=0`)
    ///
    /// When false, the router skips starting the KV event subscription entirely,
    /// avoiding the need to query workers for their local indexer state.
    pub fn should_subscribe_to_kv_events(&self) -> bool {
        self.use_kv_events && self.overlap_score_weight > 0.0
    }
}
