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

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

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

/// 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>,
}

/// KV Router configuration parameters
#[derive(Debug, Clone, Copy, Serialize, Deserialize, Validate)]
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,

    /// Enable durable KV events using NATS JetStream instead of the default event plane.
    /// When false (default), the router uses the event-plane subscriber and requires
    /// workers to have local_indexer enabled for gap recovery.
    /// When true, uses JetStream for durability and multi-replica consistency.
    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 expected_output_tokens.
    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,
}

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,
        }
    }
}

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,
    ) -> 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());
        }

        if self.router_assume_kv_reuse {
            // Compute actual block hashes and sequence hashes
            let block_hashes = compute_block_hash_for_seq(tokens, block_size, None);
            Some(compute_seq_hash_for_block(&block_hashes))
        } else {
            // Generate random hashes (no KV reuse assumed)
            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
    }
}