package api

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"time"

	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
)

// ContentHash returns the lowercase hex sha256 of b. It is the note's
// content_hash handle: brain_write / brain_update return it, brain_get
// recomputes it from the file on disk, and brain_update stamps the prior
// note's hash into the new note's `supersedes` frontmatter.
func ContentHash(b []byte) string {
	sum := sha256.Sum256(b)
	return hex.EncodeToString(sum[:])
}

// resolveWithin maps a brainDir-relative path to an absolute path and
// guarantees it does not escape brainDir. Returns the cleaned relPath
// (forward-slashed) and the absolute path.
func resolveWithin(brainDir, relPath string) (rel, abs string, err error) {
	clean := filepath.Clean("/" + filepath.ToSlash(relPath))
	rel = strings.TrimPrefix(clean, "/")
	abs = filepath.Join(brainDir, filepath.FromSlash(rel))
	check, err := filepath.Rel(brainDir, abs)
	if err != nil || check == ".." || strings.HasPrefix(check, ".."+string(filepath.Separator)) {
		return "", "", fmt.Errorf("path %q escapes brain dir", relPath)
	}
	return rel, abs, nil
}

// UpdateNoteOptions identifies the note to supersede and supplies its new
// body. Path takes precedence; otherwise the target is resolved from
// Wing/Hall/Slug via brain.NotePath.
type UpdateNoteOptions struct {
	Path    string // brainDir-relative path; takes precedence over wing/hall/slug
	Wing    string
	Hall    string
	Slug    string
	Content string // new full body (whole-note replace)
	Reason  string // optional; stamped as supersede_reason
}

// UpdateNote supersedes an existing note in place. It replaces the body
// with opts.Content, preserves the existing frontmatter (created_at,
// wing, hall, and any custom fields), and stamps updated_at, supersedes
// (the prior content hash), and supersede_reason (when given).
//
// It never creates: if the target does not exist, it returns an error so
// the caller can fall back to brain_write. Returns the note's relPath,
// the new content hash, and the prior content hash.
//
// Embeddings are NOT refreshed here. The rewritten file's mtime advances,
// which the mtime-driven vectorstore.Sync ticker uses to re-embed it on
// its next pass — the same out-of-band mechanism brain_write relies on.
func UpdateNote(brainDir string, opts UpdateNoteOptions) (relPath, contentHash, priorHash string, err error) {
	if opts.Content == "" {
		return "", "", "", fmt.Errorf("content is required")
	}

	var rel string
	if opts.Path != "" {
		rel = opts.Path
	} else {
		full, perr := brain.NotePath(brainDir, opts.Wing, opts.Hall, opts.Slug)
		if perr != nil {
			return "", "", "", perr
		}
		rel, _ = filepath.Rel(brainDir, full)
		rel = filepath.ToSlash(rel)
	}

	rel, abs, err := resolveWithin(brainDir, rel)
	if err != nil {
		return "", "", "", err
	}

	prior, err := os.ReadFile(abs)
	if err != nil {
		if os.IsNotExist(err) {
			return "", "", "", fmt.Errorf("note %q does not exist: use brain_write to create", rel)
		}
		return "", "", "", fmt.Errorf("read target: %w", err)
	}
	priorHash = ContentHash(prior)

	fm, _ := parseFrontmatter(string(prior))
	fm.set("updated_at", time.Now().UTC().Format(time.RFC3339))
	fm.set("supersedes", priorHash)
	if opts.Reason != "" {
		fm.set("supersede_reason", opts.Reason)
	}

	out := []byte(fm.render() + opts.Content)
	if err := os.WriteFile(abs, out, 0o644); err != nil {
		return "", "", "", fmt.Errorf("write: %w", err)
	}
	return rel, ContentHash(out), priorHash, nil
}

// ReadNote reads the note at the brainDir-relative relPath and returns
// its parsed frontmatter, body, and content hash. It is the read-after-
// write primitive behind brain_get: the hash it returns equals the hash
// brain_write / brain_update returned for the same bytes.
func ReadNote(brainDir, relPath string) (fm map[string]string, body, contentHash string, err error) {
	_, abs, err := resolveWithin(brainDir, relPath)
	if err != nil {
		return nil, "", "", err
	}
	raw, err := os.ReadFile(abs)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, "", "", fmt.Errorf("note %q does not exist", relPath)
		}
		return nil, "", "", fmt.Errorf("read note: %w", err)
	}
	parsed, body := parseFrontmatter(string(raw))
	fm = make(map[string]string, len(parsed.lines))
	for _, l := range parsed.lines {
		if l.key != "" {
			fm[l.key] = l.value
		}
	}
	return fm, body, ContentHash(raw), nil
}