package vectorstore

import (
	"context"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// Embedder produces dense vectors. The embed package's Client satisfies
// this; it's declared locally so vectorstore doesn't depend on embed.
type Embedder interface {
	Embed(ctx context.Context, text string) ([]float32, error)
}

// Store is the subset of PGStore that Sync needs. Lets tests stub it.
type Store interface {
	// KnownPathsWithTime returns every embedded chunk path paired with the
	// row's updated_at. Sync uses the timestamp to detect edits — a file
	// whose mtime is newer than ANY of its chunks' updated_at is re-embedded
	// from scratch (old chunks deleted, fresh chunks upserted).
	KnownPathsWithTime(ctx context.Context) (map[string]time.Time, error)
	Upsert(ctx context.Context, path string, embedding []float32) error
	Delete(ctx context.Context, path string) error
}

// SyncResult tallies what Sync did. Returned for logs / metrics; callers
// generally don't act on the fields directly.
type SyncResult struct {
	Added   int
	Updated int
	Deleted int
	Errors  []error
}

// scanDirs is the set of brainDir subdirectories whose .md files are
// embedded for vector retrieval. wiki/ holds LLM-extracted entity and
// source pages; knowledge/ holds curated hand-written entries.
var scanDirs = []string{"wiki", "knowledge"}

// maxChunkBytes is the per-chunk byte budget passed to ChunkMarkdown.
// Sized to fit comfortably under nomic-embed-text's 2048-token default
// context (~4 chars/token for English markdown → ~8 KB ceiling; we sit
// at 4 KB to leave headroom for unicode, code blocks, and tokenizer
// variance).
const maxChunkBytes = 4000

// Sync brings the embedding store in line with brain/{wiki,knowledge}/
// on disk:
//   - new files (in the tree, not in the store) get embedded + upserted
//   - files whose mtime exceeds the store's updated_at get re-embedded
//   - files no longer on disk get deleted from the store
//
// Designed to be called on a ticker. Best-effort: per-file errors are
// collected into SyncResult.Errors and do not abort the run.
func Sync(ctx context.Context, brainDir string, store Store, embedder Embedder) (SyncResult, error) {
	var res SyncResult
	if store == nil || embedder == nil {
		return res, nil
	}

	known, err := store.KnownPathsWithTime(ctx)
	if err != nil {
		return res, fmt.Errorf("known paths: %w", err)
	}
	// Group known chunks by parent path and remember the EARLIEST
	// updated_at per parent. A file is considered stale if its mtime is
	// after the oldest of its chunk rows — i.e. at least one chunk hasn't
	// been refreshed since the last edit. Also keep the full chunk-path
	// list per parent so we can delete every old chunk before re-embedding
	// (handles "file shrunk → fewer chunks → orphan rows" cleanly).
	type parentState struct {
		minUpdatedAt time.Time
		chunkPaths   []string
	}
	parents := make(map[string]*parentState, len(known))
	for p, t := range known {
		parent := ParentPath(p)
		ps, ok := parents[parent]
		if !ok {
			ps = &parentState{minUpdatedAt: t}
			parents[parent] = ps
		} else if t.Before(ps.minUpdatedAt) {
			ps.minUpdatedAt = t
		}
		ps.chunkPaths = append(ps.chunkPaths, p)
	}
	seenParents := make(map[string]struct{})

	for _, sub := range scanDirs {
		root := filepath.Join(brainDir, sub)
		if _, err := os.Stat(root); os.IsNotExist(err) {
			continue
		}

		err = filepath.WalkDir(root, func(path string, d os.DirEntry, err error) error {
			if err != nil {
				return err
			}
			if d.IsDir() || !strings.HasSuffix(path, ".md") || d.Name() == "_index.md" {
				return nil
			}
			rel, err := filepath.Rel(brainDir, path)
			if err != nil {
				return err
			}
			relSlash := filepath.ToSlash(rel)
			seenParents[relSlash] = struct{}{}

			if ps, ok := parents[relSlash]; ok {
				// File already has chunks in the store. Re-embed only when
				// the file has been edited since the oldest chunk was
				// written. Tolerate clock skew with a sub-second grace.
				info, statErr := d.Info()
				if statErr != nil {
					res.Errors = append(res.Errors, fmt.Errorf("stat %s: %w", relSlash, statErr))
					return nil
				}
				if !info.ModTime().After(ps.minUpdatedAt) {
					return nil
				}
				// Stale: delete old chunks before re-embedding so a shrunk
				// file doesn't leave orphan rows at higher #NNNN indexes.
				for _, oldPath := range ps.chunkPaths {
					if delErr := store.Delete(ctx, oldPath); delErr != nil {
						res.Errors = append(res.Errors, fmt.Errorf("delete %s for re-embed: %w", oldPath, delErr))
						return nil
					}
				}
			}

			content, readErr := os.ReadFile(path)
			if readErr != nil {
				res.Errors = append(res.Errors, fmt.Errorf("read %s: %w", relSlash, readErr))
				return nil
			}
			chunks := NumberChunks(relSlash, ChunkMarkdown(string(content), maxChunkBytes))
			for _, ch := range chunks {
				vec, embErr := embedder.Embed(ctx, ch.Content)
				if embErr != nil {
					res.Errors = append(res.Errors, fmt.Errorf("embed %s: %w", ch.Path, embErr))
					continue
				}
				if upErr := store.Upsert(ctx, ch.Path, vec); upErr != nil {
					res.Errors = append(res.Errors, fmt.Errorf("upsert %s: %w", ch.Path, upErr))
					continue
				}
				res.Added++
			}
			return nil
		})
		if err != nil {
			return res, fmt.Errorf("walk %s: %w", sub, err)
		}
	}

	// Drop chunk rows whose parent file is gone.
	for path := range known {
		if _, ok := seenParents[ParentPath(path)]; ok {
			continue
		}
		if err := store.Delete(ctx, path); err != nil {
			res.Errors = append(res.Errors, fmt.Errorf("delete %s: %w", path, err))
			continue
		}
		res.Deleted++
	}
	return res, nil
}

// StartSync launches Sync on a ticker in a background goroutine. The
// goroutine exits when ctx is cancelled. Failures are logged via slog.
func StartSync(ctx context.Context, brainDir string, store Store, embedder Embedder, interval time.Duration) {
	if interval <= 0 {
		interval = 5 * time.Minute
	}
	go func() {
		t := time.NewTicker(interval)
		defer t.Stop()
		// Run once immediately so first-boot doesn't wait a full tick.
		if r, err := Sync(ctx, brainDir, store, embedder); err != nil {
			slog.Error("embed sync failed", "err", err)
		} else if r.Added+r.Deleted > 0 || len(r.Errors) > 0 {
			slog.Info("embed sync", "added", r.Added, "deleted", r.Deleted, "errors", len(r.Errors))
			for _, e := range r.Errors {
				slog.Warn("embed sync item failed", "err", e)
			}
		}
		for {
			select {
			case <-ctx.Done():
				return
			case <-t.C:
				if r, err := Sync(ctx, brainDir, store, embedder); err != nil {
					slog.Error("embed sync failed", "err", err)
				} else if r.Added+r.Deleted > 0 || len(r.Errors) > 0 {
					slog.Info("embed sync", "added", r.Added, "deleted", r.Deleted, "errors", len(r.Errors))
				}
			}
		}
	}()
}