// Package vectord owns the vector-search surface — HNSW indexes
// keyed by string IDs with optional opaque JSON metadata. The
// underlying library is github.com/coder/hnsw (pure Go, no cgo).
//
// G1 scope: in-memory only. Persistence to storaged + rehydrate
// across restart is the next piece — keeping it out of this layer
// makes the index API easier to test and keeps the storaged
// dependency optional for downstream tooling.
package vectord

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"math"
	"sync"

	"github.com/coder/hnsw"
)

// Distance names accepted by IndexParams.Distance.
const (
	DistanceCosine    = "cosine"
	DistanceEuclidean = "euclidean"
)

// Default HNSW parameters — match coder/hnsw's NewGraph defaults
// which are tuned for OpenAI-shaped embeddings (1536-d, but the
// hyperparameters generalize).
const (
	DefaultM        = 16
	DefaultEfSearch = 20
)

// smallIndexRebuildThreshold guards against coder/hnsw v0.6.1's
// degenerate-state nil-deref (graph.go:95 layerNode.search) which
// fires when the graph transitions through low-len states with a
// stale entry pointer. Below this threshold, Add and BatchAdd
// rebuild the entire graph from scratch — fresh graph + one
// variadic Add never exercises the buggy incremental path.
//
// Why 32: HNSW's value is sub-linear search at large N; at N<32 a
// rebuild's O(n) cost (snapshot ids + bulk Add) is negligible
// (~µs at 768-d). The boundary is intentionally above the small
// playbook-corpus regime (where multitier_100k surfaced the bug)
// but well below realistic working-set indexes.
//
// The recover() guard in BatchAdd remains as belt-and-suspenders
// for any incremental-path edge cases past the threshold.
const smallIndexRebuildThreshold = 32

// IndexParams describes one vector index. Once an Index is built,
// these are fixed — changing M / dimension / distance requires a
// rebuild.
type IndexParams struct {
	Name      string `json:"name"`
	Dimension int    `json:"dimension"`
	M         int    `json:"m"`
	EfSearch  int    `json:"ef_search"`
	Distance  string `json:"distance"`
}

// Result is one search hit. Distance semantics depend on the
// configured distance function — for cosine it's `1 - cos(a,b)`
// where smaller = closer; for euclidean it's the L2 norm of
// `a - b`. Either way, smaller = closer and the result list is
// sorted ascending.
type Result struct {
	ID       string          `json:"id"`
	Distance float32         `json:"distance"`
	Metadata json.RawMessage `json:"metadata,omitempty"`
}

// Index wraps a coder/hnsw graph plus side maps of opaque JSON
// metadata and raw vectors per ID. Concurrency: read-heavy via
// Search (read-lock); Add and Delete take the write lock.
//
// Why we keep vectors in a side map (i.vectors) in addition to the
// graph: coder/hnsw v0.6.1 has a known bug where the graph
// transitions through degenerate states after Delete cycles, and
// later operations (Add / Lookup) can panic with nil-deref. The
// side map is independent of graph state, so the rebuild path can
// always reconstruct a clean graph even if the current one is
// corrupted. Memory cost is ~2x for vectors (also held in graph),
// which is acceptable for the safety it buys. Verified necessary
// 2026-05-01 multitier_100k where the bug fired at len=40.
type Index struct {
	params IndexParams
	g      *hnsw.Graph[string]
	meta   map[string]json.RawMessage
	// vectors is the panic-safe source of truth — every successful
	// Add stores the vector here, every Delete removes it, and
	// rebuildGraphLocked reads from this map (not i.g.Lookup) so
	// it tolerates a corrupted graph. Map keys are also the
	// canonical ID set (replaces the prior i.ids map).
	vectors map[string][]float32
	mu      sync.RWMutex
}

// Errors surfaced to HTTP handlers. Sentinel-based so the wire
// layer can map to status codes via errors.Is.
var (
	ErrDimensionMismatch = errors.New("vectord: vector dimension mismatch")
	ErrUnknownDistance   = errors.New("vectord: unknown distance function")
	ErrInvalidParams     = errors.New("vectord: invalid index params")
)

// NewIndex builds a fresh index from validated params.
func NewIndex(p IndexParams) (*Index, error) {
	if p.Name == "" {
		return nil, fmt.Errorf("%w: empty name", ErrInvalidParams)
	}
	if p.Dimension <= 0 {
		return nil, fmt.Errorf("%w: dimension must be > 0 (got %d)", ErrInvalidParams, p.Dimension)
	}
	if p.M <= 0 {
		p.M = DefaultM
	}
	if p.EfSearch <= 0 {
		p.EfSearch = DefaultEfSearch
	}
	if p.Distance == "" {
		p.Distance = DistanceCosine
	}
	dist, err := distanceFn(p.Distance)
	if err != nil {
		return nil, err
	}

	g := hnsw.NewGraph[string]()
	g.M = p.M
	g.EfSearch = p.EfSearch
	g.Distance = dist
	// Ml stays at the library default (0.25); exposing it as a knob
	// is a G2 concern when we have real tuning data.

	return &Index{
		params:  p,
		g:       g,
		meta:    make(map[string]json.RawMessage),
		vectors: make(map[string][]float32),
	}, nil
}

// distanceFn maps the string name to the underlying function.
// Easier to unit-test than calling out to coder/hnsw's registry.
func distanceFn(name string) (hnsw.DistanceFunc, error) {
	switch name {
	case DistanceCosine, "":
		return hnsw.CosineDistance, nil
	case DistanceEuclidean:
		return hnsw.EuclideanDistance, nil
	}
	return nil, fmt.Errorf("%w: %q (want cosine or euclidean)", ErrUnknownDistance, name)
}

// Params returns a copy of the immutable index params.
func (i *Index) Params() IndexParams { return i.params }

// Len returns the number of vectors currently in the index.
//
// Reads from i.vectors (the panic-safe source of truth) rather
// than i.g.Len() — the latter can drift past Len during a corrupted
// graph state. i.vectors only changes on successful Add/Delete.
func (i *Index) Len() int {
	i.mu.RLock()
	defer i.mu.RUnlock()
	return len(i.vectors)
}

// IDs returns a snapshot of every ID currently stored in the index.
// Allocated under the read lock so callers receive a stable copy and
// can iterate without holding the lock. Used by the merge endpoint
// (OPEN #1: periodic fresh→main index merge — drains the fresh
// corpus into the main one when it crosses the operational ceiling).
//
// Source of truth: the i.vectors keyset. The meta map stays sparse
// (only items with explicit metadata appear there, per the K-B1
// nil-vs-{} distinction); using meta as the ID set would silently
// miss items added with nil metadata.
func (i *Index) IDs() []string {
	i.mu.RLock()
	defer i.mu.RUnlock()
	out := make([]string, 0, len(i.vectors))
	for id := range i.vectors {
		out = append(out, id)
	}
	return out
}

// Add inserts a vector with optional metadata, with replace
// semantics for the vector: if id already exists, the prior
// vector is removed first. Dim must match the index dim or
// ErrDimensionMismatch is returned.
//
// Metadata semantics (post-scrum K-B1): nil meta is "leave
// existing alone" (upsert-style); to clear metadata, pass an
// empty `{}` or Delete+Add. This avoids silent metadata loss
// when the JSON `metadata` field is omitted on re-add.
//
// Validates that all vector components are finite (post-scrum
// O-W3). NaN/Inf in any component poisons HNSW: distance
// comparisons return false for both `<` and `>`, breaking the
// search heap invariants. Zero-norm vectors are also rejected
// under cosine distance — cos(0,x) = NaN.
//
// Note: coder/hnsw's Graph.Add panics on re-adding an existing
// key (internal "node not added" length-invariant check). We
// pre-Delete to make Add idempotent on re-insert.
func (i *Index) Add(id string, vec []float32, meta json.RawMessage) error {
	if id == "" {
		return errors.New("vectord: empty id")
	}
	if len(vec) != i.params.Dimension {
		return fmt.Errorf("%w: index dim=%d, got=%d", ErrDimensionMismatch, i.params.Dimension, len(vec))
	}
	if err := validateVector(vec, i.params.Distance); err != nil {
		return err
	}
	i.mu.Lock()
	defer i.mu.Unlock()
	// Re-add: drop existing graph entry AND side-store entry before
	// the new Add. Without removing from i.vectors, the rebuild path
	// below would see both old and new entries and double-add.
	// safeGraphDelete tolerates a corrupted graph; i.vectors is
	// authoritative regardless.
	if _, exists := i.vectors[id]; exists {
		_ = safeGraphDelete(i.g, id)
		delete(i.vectors, id)
	}
	newNode := hnsw.MakeNode(id, vec)
	postLen := len(i.vectors) + 1
	addOK := false
	if postLen <= smallIndexRebuildThreshold {
		i.rebuildGraphLocked([]hnsw.Node[string]{newNode})
		addOK = true
	} else {
		// Warm path: try incremental Add. If the graph is in a
		// degenerate state from a prior Delete cycle, this panics;
		// we recover and rebuild from the panic-safe i.vectors map.
		addOK = safeGraphAdd(i.g, newNode)
		if !addOK {
			i.rebuildGraphLocked([]hnsw.Node[string]{newNode})
			addOK = true
		}
	}
	if !addOK {
		return errors.New("vectord: hnsw add failed even after rebuild — should never happen")
	}
	// Commit to the side stores after the graph mutation succeeded.
	out := make([]float32, len(vec))
	copy(out, vec)
	i.vectors[id] = out
	if meta != nil {
		// Per scrum K-B1 (Kimi): only OVERWRITE on explicit non-nil.
		// nil = "leave existing meta alone" (upsert). To clear, the
		// caller should send an empty `{}` body or Delete the id.
		i.meta[id] = meta
	}
	return nil
}

// safeGraphAdd wraps coder/hnsw's variadic Graph.Add with a
// recover() so v0.6.1's degenerate-state nil-deref returns false
// instead of crashing the caller. Caller is expected to fall back
// to rebuildGraphLocked on false.
func safeGraphAdd(g *hnsw.Graph[string], nodes ...hnsw.Node[string]) (ok bool) {
	defer func() {
		if r := recover(); r != nil {
			ok = false
		}
	}()
	g.Add(nodes...)
	return true
}

// safeGraphDelete wraps Graph.Delete with recover for the same
// reason — Delete can also touch corrupted layer state.
func safeGraphDelete(g *hnsw.Graph[string], id string) (ok bool) {
	defer func() {
		if r := recover(); r != nil {
			ok = false
		}
	}()
	return g.Delete(id)
}

// rebuildGraphLocked replaces i.g with a fresh graph containing
// the current items (snapshotted from the panic-safe i.vectors
// map) plus the supplied extras, in one bulk Add into a freshly-
// created graph. Caller MUST hold the write lock.
//
// Independence from i.g state is the load-bearing property — even
// if i.g is corrupted from a prior coder/hnsw v0.6.1 panic, this
// rebuild produces a clean graph because i.vectors is maintained
// only on successful Add/Delete.
//
// Caller MUST ensure that any extra IDs already present in
// i.vectors have been removed first (otherwise the bulk Add will
// see duplicate IDs and panic).
func (i *Index) rebuildGraphLocked(extras []hnsw.Node[string]) {
	g := hnsw.NewGraph[string]()
	g.M = i.params.M
	g.EfSearch = i.params.EfSearch
	g.Distance = i.g.Distance

	nodes := make([]hnsw.Node[string], 0, len(i.vectors)+len(extras))
	for id, vec := range i.vectors {
		nodes = append(nodes, hnsw.MakeNode(id, vec))
	}
	nodes = append(nodes, extras...)

	if len(nodes) > 0 {
		g.Add(nodes...)
	}
	i.g = g
}

// ValidateVector is the exported form of validateVector — the HTTP
// handler pre-validates batches before committing, so it needs the
// same predicate Add uses internally. Per scrum O-I3 (G1P).
func ValidateVector(vec []float32, distance string) error {
	return validateVector(vec, distance)
}

// validateVector rejects vectors that would poison the HNSW
// graph or produce NaN distances. Per scrum O-W3 (Opus, G1).
func validateVector(vec []float32, distance string) error {
	var sumSq float64
	for j, v := range vec {
		f := float64(v)
		if math.IsNaN(f) || math.IsInf(f, 0) {
			return fmt.Errorf("vectord: vec[%d] is non-finite (got %v)", j, v)
		}
		sumSq += f * f
	}
	if distance == DistanceCosine && sumSq == 0 {
		return errors.New("vectord: zero-norm vector under cosine distance")
	}
	return nil
}

// BatchItem is one entry in a BatchAdd call. Same per-field
// contract as Add: ID + Vector required, Metadata follows
// upsert-style semantics (nil = leave existing alone).
type BatchItem struct {
	ID       string
	Vector   []float32
	Metadata json.RawMessage
}

// BatchAdd inserts a slice of items under a single write-lock, with
// one variadic call into coder/hnsw's Graph.Add. Net win vs. a loop
// of single Add calls: N→1 lock acquisitions per HTTP batch and one
// variadic library call instead of N.
//
// Contract: items MUST be pre-validated by the caller (id non-empty,
// vector dimension matches, vector finite + non-zero-norm under
// cosine). Pre-validation lives in the HTTP handler so per-item
// error messages stay precise; reproducing it here would force
// position-encoded errors on every consumer.
//
// Intra-batch duplicate IDs: dedup'd internally with last-write-wins
// semantics (matches map-style behavior — second occurrence of an
// ID replaces the first). Without dedup, coder/hnsw's "node not
// added" length-invariant panics on the second occurrence. Caught
// by 2026-04-29 cross-lineage scrum (Opus BLOCK).
func (i *Index) BatchAdd(items []BatchItem) error {
	if len(items) == 0 {
		return nil
	}

	// Intra-batch dedup, last-write-wins. Walk forward, record the
	// LAST index for each ID, then keep only items whose index is
	// the recorded last. Preserves order of last occurrences in the
	// original positions.
	if hasDup := containsDuplicateID(items); hasDup {
		items = dedupBatchLastWins(items)
	}

	i.mu.Lock()
	defer i.mu.Unlock()

	// Pre-pass: drop any existing IDs from BOTH the graph and the
	// side-store map so the rebuild snapshot doesn't double-add and
	// the warm path's variadic Add never sees a re-add. Graph Delete
	// is wrapped in safeGraphDelete because corrupted graphs can also
	// panic on Delete; the side store remains authoritative.
	for _, it := range items {
		if _, exists := i.vectors[it.ID]; exists {
			_ = safeGraphDelete(i.g, it.ID)
			delete(i.vectors, it.ID)
		}
	}

	nodes := make([]hnsw.Node[string], len(items))
	for j, it := range items {
		nodes[j] = hnsw.MakeNode(it.ID, it.Vector)
	}

	// Below threshold: rebuild from scratch unconditionally — fresh
	// graph + one bulk Add never exercises v0.6.1's degenerate-state
	// path. At/above threshold: try warm incremental Add, fall back
	// to rebuild on panic. The rebuild always succeeds because
	// i.vectors is independent of graph state.
	postLen := len(i.vectors) + len(nodes)
	if postLen <= smallIndexRebuildThreshold {
		i.rebuildGraphLocked(nodes)
	} else {
		if !safeGraphAdd(i.g, nodes...) {
			i.rebuildGraphLocked(nodes)
		}
	}

	// Commit to side stores after the graph is in good shape.
	for _, it := range items {
		out := make([]float32, len(it.Vector))
		copy(out, it.Vector)
		i.vectors[it.ID] = out
		if it.Metadata != nil {
			i.meta[it.ID] = it.Metadata
		}
	}
	return nil
}

// containsDuplicateID is a fast pre-check — if no dups, skip the
// dedup allocation. Most batches won't have dups so this is a hot
// path.
func containsDuplicateID(items []BatchItem) bool {
	seen := make(map[string]struct{}, len(items))
	for _, it := range items {
		if _, ok := seen[it.ID]; ok {
			return true
		}
		seen[it.ID] = struct{}{}
	}
	return false
}

// dedupBatchLastWins keeps only the last occurrence of each ID,
// preserving the relative order of those last occurrences. This
// matches map-style "set X to A then to B" semantics: B wins.
func dedupBatchLastWins(items []BatchItem) []BatchItem {
	lastIdx := make(map[string]int, len(items))
	for j, it := range items {
		lastIdx[it.ID] = j
	}
	out := make([]BatchItem, 0, len(lastIdx))
	for j, it := range items {
		if lastIdx[it.ID] == j {
			out = append(out, it)
		}
	}
	return out
}

// Delete removes id from the index. Returns true if present.
//
// The side store i.vectors is the authority on presence; the graph
// Delete is best-effort (can panic on corrupted state, recovered
// via safeGraphDelete). The side store always reflects the
// post-Delete truth so the next rebuild produces a clean graph.
func (i *Index) Delete(id string) bool {
	i.mu.Lock()
	defer i.mu.Unlock()
	_, present := i.vectors[id]
	if !present {
		return false
	}
	delete(i.meta, id)
	delete(i.vectors, id)
	_ = safeGraphDelete(i.g, id)
	return true
}

// Search returns the k nearest neighbors of query, sorted
// ascending by distance.
//
// Note: coder/hnsw's Search returns `[]Node[K]` without distances —
// they're computed internally in the search candidate heap but
// dropped from the public API. We recompute distance from the
// returned vectors. O(k·dim) per search, negligible at typical
// k=10 / dim<2048.
func (i *Index) Search(query []float32, k int) ([]Result, error) {
	if len(query) != i.params.Dimension {
		return nil, fmt.Errorf("%w: index dim=%d, got=%d", ErrDimensionMismatch, i.params.Dimension, len(query))
	}
	if k <= 0 {
		return nil, errors.New("vectord: k must be > 0")
	}
	i.mu.RLock()
	defer i.mu.RUnlock()

	// Per scrum O-I2 (Opus): use the stored distance function
	// directly rather than re-resolving the string name on every
	// search. The graph's Distance is set once at NewIndex.
	dist := i.g.Distance
	hits := i.g.Search(query, k)
	out := make([]Result, len(hits))
	for j, n := range hits {
		out[j] = Result{
			ID:       n.Key,
			Distance: dist(query, n.Value),
			Metadata: i.meta[n.Key],
		}
	}
	return out, nil
}

// IndexEnvelope is the JSON shape persisted alongside the binary
// HNSW graph bytes. params + metadata + format version travel
// together; the graph itself is opaque binary that round-trips
// through hnsw.Graph.Export / Import.
//
// Version history:
//   - v1: original; metadata-only ID inference on decode (loses
//     items added with nil metadata across persistence).
//   - v2: explicit IDs slice closes the v1 gap. Older v1 envelopes
//     still load (backward-compat fallback path in DecodeIndex).
type IndexEnvelope struct {
	Version  int                        `json:"version"`
	Params   IndexParams                `json:"params"`
	Metadata map[string]json.RawMessage `json:"metadata"`
	IDs      []string                   `json:"ids,omitempty"` // v2+ — canonical ID set
}

// envelopeVersion is the version this build EMITS. Decoders accept
// any envelope ≤ this version (with version-specific fallbacks for
// older shapes). Bumping this constant means the format changed
// incompatibly going forward; readers without the version-specific
// branch will return ErrVersionMismatch.
const envelopeVersion = 2

// ErrVersionMismatch is returned by DecodeIndex when the envelope
// claims a version this build doesn't understand.
var ErrVersionMismatch = errors.New("vectord: unknown envelope version")

// Encode writes the index's JSON envelope (params + metadata) and
// the binary HNSW graph bytes through two writers. Two-stream
// shape lets the persistor write each to a distinct storaged key
// without reframing.
//
// envelopeW receives params+metadata as JSON; graphW receives the
// raw output of hnsw.Graph.Export.
func (i *Index) Encode(envelopeW, graphW io.Writer) error {
	i.mu.RLock()
	defer i.mu.RUnlock()

	// v2: serialize the canonical ID set explicitly so DecodeIndex
	// can restore i.vectors without depending on meta-key inference.
	idList := make([]string, 0, len(i.vectors))
	for id := range i.vectors {
		idList = append(idList, id)
	}
	env := IndexEnvelope{
		Version:  envelopeVersion,
		Params:   i.params,
		Metadata: i.meta,
		IDs:      idList,
	}
	if err := json.NewEncoder(envelopeW).Encode(env); err != nil {
		return fmt.Errorf("encode envelope: %w", err)
	}
	if err := i.g.Export(graphW); err != nil {
		return fmt.Errorf("export graph: %w", err)
	}
	return nil
}

// DecodeIndex reconstructs an Index from a previously-Encoded pair
// of streams. The returned Index is independent — closing either
// reader after this call doesn't affect the result.
func DecodeIndex(envelopeR, graphR io.Reader) (*Index, error) {
	var env IndexEnvelope
	if err := json.NewDecoder(envelopeR).Decode(&env); err != nil {
		return nil, fmt.Errorf("decode envelope: %w", err)
	}
	// Accept v1 (legacy: ids inferred from meta keys) and v2 (explicit
	// ids slice). Future versions are unknown — return mismatch
	// rather than half-decoding.
	if env.Version < 1 || env.Version > envelopeVersion {
		return nil, fmt.Errorf("%w: have %d, got %d",
			ErrVersionMismatch, envelopeVersion, env.Version)
	}
	idx, err := NewIndex(env.Params)
	if err != nil {
		return nil, err
	}
	if err := idx.g.Import(graphR); err != nil {
		return nil, fmt.Errorf("import graph: %w", err)
	}
	if env.Metadata != nil {
		idx.meta = env.Metadata
	}
	// Reconstruct i.vectors from the imported graph. Source of IDs:
	// v2 envelope's explicit IDs slice (canonical), or v1 fallback
	// via the meta keys. We then call i.g.Lookup on each ID to
	// recover the vector — Lookup on a freshly Imported graph is
	// safe (no degenerate state from prior Delete cycles).
	var idSource []string
	if env.Version >= 2 && env.IDs != nil {
		idSource = env.IDs
	} else {
		// v1 backward-compat path. Old envelopes don't carry ids
		// explicitly; the metadata keyset is the best signal we have.
		idSource = make([]string, 0, len(idx.meta))
		for id := range idx.meta {
			idSource = append(idSource, id)
		}
	}
	for _, id := range idSource {
		if vec, ok := idx.g.Lookup(id); ok {
			out := make([]float32, len(vec))
			copy(out, vec)
			idx.vectors[id] = out
		}
	}
	return idx, nil
}

// Lookup returns the stored vector + metadata for an id.
//
// Per scrum O-W1 (Opus): the vector is COPIED before return.
// coder/hnsw's Lookup hands back the underlying graph slice;
// caller mutation would corrupt the index without locking.
func (i *Index) Lookup(id string) (vec []float32, meta json.RawMessage, ok bool) {
	i.mu.RLock()
	defer i.mu.RUnlock()
	v, found := i.g.Lookup(id)
	if !found {
		return nil, nil, false
	}
	out := make([]float32, len(v))
	copy(out, v)
	return out, i.meta[id], true
}