package common

import (
	"math"
	"sort"
	"sync"

	"gonum.org/v1/gonum/mat"
)

// Hit is one TopK match returned by MemStore.
type Hit struct {
	Index int
	ID    string
	Score float64 // cosine similarity in [−1, 1]
}

// MemStore is an in-memory product store with exact-cosine TopK retrieval.
// It is the single source of truth for in-run dedup (replacing Python's ES
// KNN over the current run's products).
//
// Vectors are stored as one contiguous float64 row-major matrix (matDense) so
// the per-query dot-product pass — the hot path for document-level structure
// dedup and dataset-level cross-document dedup — is a single level-2 BLAS
// matrix-vector product (mat.VecDense.MulVec) instead of N separate scalar
// loops. gonum only supports float64, so each incoming float32 vector is
// widened when written. All vectors in one store are assumed to share the same
// dimension (embeddings from one model); addLocked pads/truncates a stray
// vector to cols defensively. Precomputed L2 norms complete the cosine.
type MemStore struct {
	mu       sync.RWMutex
	items    []Product
	cols     int        // uniform embedding dimension; 0 while empty
	matData  []float64  // row-major vectors, len == cols*len(items)
	matDense *mat.Dense // view over matData, rebuilt when the row count changes
	norms    []float64
	byID     map[string]int
}

// NewMemStore constructs an empty MemStore.
func NewMemStore() *MemStore {
	return &MemStore{byID: make(map[string]int)}
}

// KeepAction is the decision returned by a DedupCallback during DedupeAdd.
type KeepAction int

const (
	// KeepAdd inserts the incoming product as a new entry.
	KeepAdd KeepAction = iota
	// KeepDrop discards the incoming product (it is a duplicate).
	KeepDrop
	// KeepMerge overwrites the best existing product with the incoming one
	// (identity preserved via the existing id).
	KeepMerge
)

// DedupCallback decides, given the best existing match (score is the cosine
// similarity in [−1, 1]; existing is the zero Product when no candidate
// qualifies), whether to add, drop, or merge the incoming product. When it
// returns KeepMerge, the returned Product replaces the existing entry — its
// ID is forced to the existing entry's ID so merges preserve identity
// (mirrors Python's _struct_rebuild_doc_storage_doc preserve_id=True).
type DedupCallback func(existing Product, score float64) (KeepAction, Product, error)

// Add appends a product and its vector to the store.
func (m *MemStore) Add(p Product) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.addLocked(p)
}

// DedupeAdd atomically checks the store for a near-duplicate of row and acts on
// the DedupCallback's decision. The callback is invoked OUTSIDE the store lock
// so it may perform slow work (e.g. an LLM merge judgment) without blocking
// concurrent callers. The check-then-act is race-free: the best candidate is
// selected under a read lock, the callback runs unlocked, then the add/merge is
// applied under a write lock using the previously resolved index.
func (m *MemStore) DedupeAdd(row Product, threshold float64, cb DedupCallback) (KeepAction, error) {
	m.mu.RLock()
	bestIdx, bestScore := m.bestMatchLocked(row.Vector, threshold)
	var best Product
	if bestIdx >= 0 && bestIdx < len(m.items) {
		best = m.items[bestIdx]
	}
	m.mu.RUnlock()

	action, replacement, err := cb(best, bestScore)
	if err != nil {
		return 0, err
	}

	m.mu.Lock()
	defer m.mu.Unlock()
	// Re-resolve the candidate by ID under the write lock. The unlocked
	// callback may have allowed a concurrent Delete to delete or reindex the
	// slice, so the index captured under the read lock (bestIdx) is stale and
	// unsafe to index — it could point at a different product or be
	// out-of-range (M11).
	resolvedIdx := -1
	if best.ID != "" {
		if idx, ok := m.byID[best.ID]; ok && idx < len(m.items) && m.items[idx].ID == best.ID {
			resolvedIdx = idx
		}
	}
	switch action {
	case KeepDrop:
		return KeepDrop, nil
	case KeepMerge:
		if resolvedIdx < 0 {
			m.addLocked(row)
			return KeepAdd, nil
		}
		// Merges preserve the existing entry's identity (Python preserve_id).
		replacement.ID = m.items[resolvedIdx].ID
		m.items[resolvedIdx] = replacement
		m.replaceMatLocked(resolvedIdx, replacement.Vector)
		m.norms[resolvedIdx] = l2Norm(replacement.Vector)
		return KeepMerge, nil
	default:
		m.addLocked(row)
		return KeepAdd, nil
	}
}

func (m *MemStore) addLocked(p Product) {
	m.items = append(m.items, p)
	m.appendMatLocked(p.Vector)
	m.norms = append(m.norms, l2Norm(p.Vector))
	if p.ID != "" {
		m.byID[p.ID] = len(m.items) - 1
	}
}

// Upsert replaces an existing product by ID, or appends if absent.
func (m *MemStore) Upsert(p Product) {
	m.mu.Lock()
	defer m.mu.Unlock()
	if idx, ok := m.byID[p.ID]; ok {
		m.items[idx] = p
		m.replaceMatLocked(idx, p.Vector)
		m.norms[idx] = l2Norm(p.Vector)
		return
	}
	m.addLocked(p)
}

// Delete removes a product by ID.
func (m *MemStore) Delete(id string) {
	m.mu.Lock()
	defer m.mu.Unlock()
	idx, ok := m.byID[id]
	if !ok {
		return
	}
	m.items = append(m.items[:idx], m.items[idx+1:]...)
	m.removeMatLocked(idx)
	m.norms = append(m.norms[:idx], m.norms[idx+1:]...)
	delete(m.byID, id)
	m.reindexLocked()
}

func (m *MemStore) reindexLocked() {
	m.byID = make(map[string]int, len(m.items))
	for i, it := range m.items {
		if it.ID != "" {
			m.byID[it.ID] = i
		}
	}
}

// Len returns the number of stored products.
func (m *MemStore) Len() int {
	m.mu.RLock()
	defer m.mu.RUnlock()
	return len(m.items)
}

// TopK returns up to k products whose cosine similarity to vec is >= threshold,
// sorted by descending similarity. A zero threshold returns the top-k by score
// regardless of absolute value.
func (m *MemStore) TopK(vec []float32, k int, threshold float64) []Hit {
	m.mu.RLock()
	defer m.mu.RUnlock()
	if m.matDense == nil {
		return nil
	}
	dots := m.matDotLocked(vec)
	qn := l2Norm(vec)
	if qn == 0 {
		qn = 1
	}
	type cand struct {
		idx   int
		score float64
	}
	var cands []cand
	for i := range m.items {
		vn := m.norms[i]
		if vn == 0 {
			vn = 1
		}
		score := dots[i] / (qn * vn)
		if threshold <= 0 || score >= threshold {
			cands = append(cands, cand{i, score})
		}
	}
	sort.Slice(cands, func(a, b int) bool { return cands[a].score > cands[b].score })
	if k > 0 && len(cands) > k {
		cands = cands[:k]
	}
	hits := make([]Hit, 0, len(cands))
	for _, c := range cands {
		hits = append(hits, Hit{Index: c.idx, ID: m.items[c.idx].ID, Score: c.score})
	}
	return hits
}

// Snapshot returns a shallow copy of the stored products.
func (m *MemStore) Snapshot() []Product {
	m.mu.RLock()
	defer m.mu.RUnlock()
	out := make([]Product, len(m.items))
	copy(out, m.items)
	return out
}

// MergeSourceChunkIDs unions the given chunk ids into the source_chunk_ids
// list of the entry identified by id. Used by structure's dedup path to fold
// a dropped duplicate's provenance into the surviving canonical entity
// (mirrors Python's _struct_merge_graph_entities). No-op when id is absent.
func (m *MemStore) MergeSourceChunkIDs(id string, chunkIDs []string) {
	if id == "" || len(chunkIDs) == 0 {
		return
	}
	m.mu.Lock()
	defer m.mu.Unlock()
	idx, ok := m.byID[id]
	if !ok {
		return
	}
	p := m.items[idx]
	if p.Meta == nil {
		p.Meta = map[string]any{}
	}
	existing, _ := p.Meta["source_chunk_ids"].([]string)
	seen := map[string]bool{}
	for _, s := range existing {
		seen[s] = true
	}
	for _, s := range chunkIDs {
		if !seen[s] {
			existing = append(existing, s)
			seen[s] = true
		}
	}
	p.Meta["source_chunk_ids"] = existing
	m.items[idx] = p
}

// bestMatchLocked returns the index and cosine similarity of the stored vector
// most similar to q among those meeting threshold (threshold<=0 keeps every
// candidate). The dot-product pass is a single BLAS gemv over the row-major
// matrix. Caller must hold at least a read lock.
func (m *MemStore) bestMatchLocked(q []float32, threshold float64) (int, float64) {
	if m.matDense == nil {
		return -1, 0
	}
	dots := m.matDotLocked(q)
	qn := l2Norm(q)
	if qn == 0 {
		qn = 1
	}
	bestIdx, bestScore := -1, 0.0
	for i := range m.items {
		vn := m.norms[i]
		if vn == 0 {
			vn = 1
		}
		score := dots[i] / (qn * vn)
		if threshold <= 0 || score >= threshold {
			if bestIdx == -1 || score > bestScore {
				bestIdx, bestScore = i, score
			}
		}
	}
	return bestIdx, bestScore
}

// matDotLocked returns matDense * q — the raw dot products of q against every
// stored vector — computed in one level-2 BLAS operation. Caller must hold at
// least a read lock.
func (m *MemStore) matDotLocked(q []float32) []float64 {
	qv := make([]float64, m.cols)
	for i := 0; i < m.cols && i < len(q); i++ {
		qv[i] = float64(q[i])
	}
	var out mat.VecDense
	out.MulVec(m.matDense, mat.NewVecDense(m.cols, qv))
	return out.RawVector().Data
}

// appendMatLocked widens vec to cols and appends it as a new matrix row. Caller
// must hold a write lock.
func (m *MemStore) appendMatLocked(v []float32) {
	if m.cols == 0 {
		m.cols = len(v)
	}
	m.matData = append(m.matData, float32ToF64(v, m.cols)...)
	// gonum panics on non-positive dimensions, so skip the view until a real
	// dimension is known (e.g. a product arrived with an empty vector). The
	// TopK/bestMatch paths treat a nil matDense as "no usable vectors".
	if m.cols > 0 {
		m.matDense = mat.NewDense(len(m.items), m.cols, m.matData)
	} else {
		m.matDense = nil
	}
}

// replaceMatLocked overwrites matrix row idx with vec (widened to cols, missing
// trailing dims zero-filled, extra dims truncated). Caller must hold a write
// lock.
func (m *MemStore) replaceMatLocked(idx int, v []float32) {
	if m.cols == 0 {
		m.cols = len(v)
		if m.cols > 0 {
			m.matDense = mat.NewDense(len(m.items), m.cols, m.matData)
		}
	}
	base := idx * m.cols
	for j := 0; j < m.cols; j++ {
		if j < len(v) {
			m.matData[base+j] = float64(v[j])
		} else {
			m.matData[base+j] = 0
		}
	}
}

// removeMatLocked removes matrix row idx and rebuilds the view. Caller must
// hold a write lock.
func (m *MemStore) removeMatLocked(idx int) {
	copy(m.matData[idx*m.cols:], m.matData[(idx+1)*m.cols:])
	m.matData = m.matData[:len(m.matData)-m.cols]
	if len(m.matData) == 0 {
		m.cols = 0
		m.matDense = nil
		return
	}
	if m.cols > 0 {
		m.matDense = mat.NewDense(len(m.items), m.cols, m.matData)
	} else {
		m.matDense = nil
	}
}

// float32ToF64 widens v to a row of cols float64 values, zero-filling missing
// trailing dims and truncating excess ones.
func float32ToF64(v []float32, cols int) []float64 {
	out := make([]float64, cols)
	for i := 0; i < cols && i < len(v); i++ {
		out[i] = float64(v[i])
	}
	return out
}

func l2Norm(v []float32) float64 {
	var s float64
	for _, x := range v {
		s += float64(x) * float64(x)
	}
	return math.Sqrt(s)
}