// Spectral decomposition of the memory graph.
//
// Computes eigenvalues and eigenvectors of the normalized graph Laplacian.
// The eigenvectors provide natural coordinates for each node — connected
// nodes land nearby, communities form clusters, bridges sit between clusters.
//
// The eigenvalue spectrum reveals:
//   - Number of connected components (count of zero eigenvalues)
//   - Number of natural communities (eigenvalues near zero, before the gap)
//   - How well-connected the graph is (Fiedler value = second eigenvalue)
//
// The eigenvectors provide:
//   - Spectral coordinates for each node (the embedding)
//   - Community membership (sign/magnitude of Fiedler vector)
//   - Natural projections (select which eigenvectors to include)

use serde::{Deserialize, Serialize};
use std::collections::{HashMap, HashSet};
use std::path::PathBuf;

/// Per-node spectral embedding, serializable to disk.
#[derive(Serialize, Deserialize)]
pub struct SpectralEmbedding {
    /// Number of dimensions (eigenvectors)
    pub dims: usize,
    /// Eigenvalues for each dimension
    pub eigenvalues: Vec<f64>,
    /// Node key → coordinate vector
    pub coords: HashMap<String, Vec<f64>>,
}

pub fn embedding_path() -> PathBuf {
    crate::store::memory_dir().join("spectral-embedding.json")
}

/// Load embedding from disk.
pub fn load_embedding() -> Result<SpectralEmbedding, String> {
    let path = embedding_path();
    let data = std::fs::read_to_string(&path)
        .map_err(|e| format!("read {}: {}", path.display(), e))?;
    serde_json::from_str(&data)
        .map_err(|e| format!("parse embedding: {}", e))
}

/// Find nearest neighbors to weighted seed nodes, using link weights.
///
/// Each seed has a weight (from query term weighting). For candidates
/// directly linked to a seed, the spectral distance is scaled by
/// 1/link_strength — strong links make effective distance shorter.
/// Seed weight scales the contribution: high-weight seeds pull harder.
///
/// Returns (key, effective_distance) sorted by distance ascending.
pub fn nearest_to_seeds_weighted(
    emb: &SpectralEmbedding,
    seeds: &[(&str, f64)],           // (key, seed_weight)
    graph: Option<&crate::graph::Graph>,
    k: usize,
) -> Vec<(String, f64)> {
    let seed_set: HashSet<&str> = seeds.iter().map(|(s, _)| *s).collect();

    let seed_data: Vec<(&str, &Vec<f64>, f64)> = seeds.iter()
        .filter_map(|(s, w)| {
            emb.coords.get(*s)
                .filter(|c| c.iter().any(|&v| v.abs() > 1e-12)) // skip degenerate seeds
                .map(|c| (*s, c, *w))
        })
        .collect();
    if seed_data.is_empty() {
        return vec![];
    }

    // Build seed→neighbor link strength lookup
    let link_strengths: HashMap<(&str, &str), f32> = if let Some(g) = graph {
        let mut map = HashMap::new();
        for &(seed_key, _) in seeds {
            for (neighbor, strength) in g.neighbors(seed_key) {
                map.insert((seed_key, neighbor.as_str()), strength);
            }
        }
        map
    } else {
        HashMap::new()
    };

    let dim_weights = eigenvalue_weights(&emb.eigenvalues);

    let mut distances: Vec<(String, f64)> = emb.coords.iter()
        .filter(|(k, coords)| {
            !seed_set.contains(k.as_str())
            && coords.iter().any(|&v| v.abs() > 1e-12) // skip degenerate zero-coord nodes
        })
        .map(|(candidate_key, coords)| {
            let min_dist = seed_data.iter()
                .map(|(seed_key, sc, seed_weight)| {
                    let raw_dist = weighted_distance(coords, sc, &dim_weights);

                    // Scale by link strength if directly connected
                    let link_scale = link_strengths
                        .get(&(*seed_key, candidate_key.as_str()))
                        .map(|&s| 1.0 / (1.0 + s as f64))  // strong link → smaller distance
                        .unwrap_or(1.0);

                    raw_dist * link_scale / seed_weight
                })
                .fold(f64::MAX, f64::min);
            (candidate_key.clone(), min_dist)
        })
        .collect();

    distances.sort_by(|a, b| a.1.total_cmp(&b.1));
    distances.truncate(k);
    distances
}

/// Weighted euclidean distance in spectral space.
/// Dimensions weighted by 1/eigenvalue — coarser structure matters more.
fn weighted_distance(a: &[f64], b: &[f64], weights: &[f64]) -> f64 {
    a.iter()
        .zip(b.iter())
        .zip(weights.iter())
        .map(|((&x, &y), &w)| w * (x - y) * (x - y))
        .sum::<f64>()
        .sqrt()
}

/// Compute eigenvalue-inverse weights for distance calculations.
fn eigenvalue_weights(eigenvalues: &[f64]) -> Vec<f64> {
    eigenvalues.iter()
        .map(|&ev| if ev > 1e-8 { 1.0 / ev } else { 0.0 })
        .collect()
}

/// Compute cluster centers (centroids) in spectral space.
pub fn cluster_centers(
    emb: &SpectralEmbedding,
    communities: &HashMap<String, u32>,
) -> HashMap<u32, Vec<f64>> {
    let mut sums: HashMap<u32, (Vec<f64>, usize)> = HashMap::new();

    for (key, coords) in &emb.coords {
        if let Some(&comm) = communities.get(key) {
            let entry = sums.entry(comm)
                .or_insert_with(|| (vec![0.0; emb.dims], 0));
            for (i, &c) in coords.iter().enumerate() {
                entry.0[i] += c;
            }
            entry.1 += 1;
        }
    }

    sums.into_iter()
        .map(|(comm, (sum, count))| {
            let center: Vec<f64> = sum.iter()
                .map(|s| s / count as f64)
                .collect();
            (comm, center)
        })
        .collect()
}

/// Per-node analysis of spectral position relative to communities.
pub struct SpectralPosition {
    pub key: String,
    pub community: u32,
    /// Distance to own community center
    pub dist_to_center: f64,
    /// Distance to nearest OTHER community center
    pub dist_to_nearest: f64,
    /// Which community is nearest (other than own)
    pub nearest_community: u32,
    /// dist_to_center / median_dist_in_community (>1 = outlier)
    pub outlier_score: f64,
    /// dist_to_center / dist_to_nearest (>1 = between clusters, potential bridge)
    pub bridge_score: f64,
}

/// Analyze spectral positions for all nodes.
///
/// Returns positions sorted by outlier_score descending (most displaced first).
pub fn analyze_positions(
    emb: &SpectralEmbedding,
    communities: &HashMap<String, u32>,
) -> Vec<SpectralPosition> {
    let centers = cluster_centers(emb, communities);
    let weights = eigenvalue_weights(&emb.eigenvalues);

    // Compute distances to own community center
    let mut by_community: HashMap<u32, Vec<f64>> = HashMap::new();
    let mut node_dists: Vec<(String, u32, f64)> = Vec::new();

    for (key, coords) in &emb.coords {
        if let Some(&comm) = communities.get(key)
            && let Some(center) = centers.get(&comm) {
                let dist = weighted_distance(coords, center, &weights);
                by_community.entry(comm).or_default().push(dist);
                node_dists.push((key.clone(), comm, dist));
            }
    }

    // Median distance per community for outlier scoring
    let medians: HashMap<u32, f64> = by_community.into_iter()
        .map(|(comm, mut dists)| {
            dists.sort_by(|a, b| a.total_cmp(b));
            let median = if dists.is_empty() {
                1.0
            } else if dists.len() % 2 == 0 {
                (dists[dists.len() / 2 - 1] + dists[dists.len() / 2]) / 2.0
            } else {
                dists[dists.len() / 2]
            };
            (comm, median.max(1e-6))
        })
        .collect();

    let mut positions: Vec<SpectralPosition> = node_dists.into_iter()
        .map(|(key, comm, dist_to_center)| {
            let coords = &emb.coords[&key];

            let (nearest_community, dist_to_nearest) = centers.iter()
                .filter(|&(&c, _)| c != comm)
                .map(|(&c, center)| (c, weighted_distance(coords, center, &weights)))
                .min_by(|a, b| a.1.total_cmp(&b.1))
                .unwrap_or((comm, f64::MAX));

            let median = medians.get(&comm).copied().unwrap_or(1.0);
            let outlier_score = dist_to_center / median;
            let bridge_score = if dist_to_nearest > 1e-8 {
                dist_to_center / dist_to_nearest
            } else {
                0.0
            };

            SpectralPosition {
                key, community: comm,
                dist_to_center, dist_to_nearest, nearest_community,
                outlier_score, bridge_score,
            }
        })
        .collect();

    positions.sort_by(|a, b| b.outlier_score.total_cmp(&a.outlier_score));
    positions
}

/// Classify a spectral position: well-integrated, outlier, bridge, or orphan.
pub fn classify_position(pos: &SpectralPosition) -> &'static str {
    if pos.bridge_score > 0.7 {
        "bridge"      // between two communities
    } else if pos.outlier_score > 2.0 {
        "outlier"     // far from own community center
    } else if pos.outlier_score < 0.5 {
        "core"        // close to community center
    } else {
        "peripheral"  // normal community member
    }
}