// Package scratchpad manages the project-scoped markdown files described
// in SPEC §3. Files live under
// $XDG_DATA_HOME/patterm/projects/<project-key>/scratchpads/. Last-write-
// wins with a revision token (SPEC §14).
package scratchpad

import (
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
)

// Store is the per-project scratchpad directory.
type Store struct {
	dir string
}

// Open returns a Store rooted at the SPEC §3 path for projectKey.
func Open(projectKey string) (*Store, error) {
	base, err := DataDir()
	if err != nil {
		return nil, err
	}
	dir := filepath.Join(base, "projects", projectKey, "scratchpads")
	if err := os.MkdirAll(dir, 0o700); err != nil {
		return nil, fmt.Errorf("scratchpad: mkdir %s: %w", dir, err)
	}
	return &Store{dir: dir}, nil
}

// DataDir resolves $XDG_DATA_HOME/patterm with the conventional fallback.
func DataDir() (string, error) {
	if h := os.Getenv("XDG_DATA_HOME"); h != "" {
		return filepath.Join(h, "patterm"), nil
	}
	home, err := os.UserHomeDir()
	if err != nil {
		return "", err
	}
	return filepath.Join(home, ".local", "share", "patterm"), nil
}

// Entry is what List returns; SPEC §7 `scratchpad_list` shape.
type Entry struct {
	Name       string
	Size       int64
	ModifiedAt string // RFC3339; kept as string so MCP serialization is trivial later
}

func (s *Store) Dir() string { return s.dir }

func (s *Store) List() ([]Entry, error) {
	entries, err := os.ReadDir(s.dir)
	if err != nil {
		return nil, err
	}
	out := make([]Entry, 0, len(entries))
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		info, err := e.Info()
		if err != nil {
			continue
		}
		out = append(out, Entry{
			Name:       e.Name(),
			Size:       info.Size(),
			ModifiedAt: info.ModTime().UTC().Format("2006-01-02T15:04:05Z"),
		})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
	return out, nil
}

func (s *Store) Read(name string) (content string, revision string, err error) {
	p, err := s.safePath(name)
	if err != nil {
		return "", "", err
	}
	b, err := os.ReadFile(p)
	if err != nil {
		return "", "", err
	}
	return string(b), revisionOf(b), nil
}

// RevisionMismatchError is returned by Write when expectedRevision
// doesn't match the on-disk revision. The MCP scratchpad_write tool
// surfaces CurrentRevision as `current_revision` in the response so
// the caller can re-read and merge (SPEC §7 / §14).
type RevisionMismatchError struct {
	CurrentRevision string
}

func (e *RevisionMismatchError) Error() string {
	return "scratchpad: revision mismatch (current=" + e.CurrentRevision + ")"
}

// Write replaces the file's contents. expectedRevision, if non-empty,
// must match the current revision or the write is rejected with a
// *RevisionMismatchError (SPEC §14 last-write-wins-with-token).
func (s *Store) Write(name, content, expectedRevision string) (string, error) {
	p, err := s.safePath(name)
	if err != nil {
		return "", err
	}
	if expectedRevision != "" {
		if cur, err := os.ReadFile(p); err == nil {
			curRev := revisionOf(cur)
			if curRev != expectedRevision {
				return "", &RevisionMismatchError{CurrentRevision: curRev}
			}
		}
	}
	if err := os.WriteFile(p, []byte(content), 0o600); err != nil {
		return "", err
	}
	return revisionOf([]byte(content)), nil
}

func (s *Store) Append(name, content string) error {
	p, err := s.safePath(name)
	if err != nil {
		return err
	}
	f, err := os.OpenFile(p, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0o600)
	if err != nil {
		return err
	}
	defer f.Close()
	_, err = f.WriteString(content)
	return err
}

func (s *Store) safePath(name string) (string, error) {
	if name == "" || strings.ContainsAny(name, "/\\") || name == "." || name == ".." {
		return "", errors.New("scratchpad: invalid name")
	}
	return filepath.Join(s.dir, name), nil
}

func revisionOf(b []byte) string {
	sum := sha256.Sum256(b)
	return hex.EncodeToString(sum[:6])
}