// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package tokens stores personal access tokens for programmatic writes
// to the public API. Tokens live in tokens.toml next to users.toml; the
// raw token is shown exactly once, at creation time, and only its
// SHA-256 digest is stored.
package tokens

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"slices"
	"strings"
	"sync"
	"time"

	"sourcedock.dev/petrbalvin/interpres/v2"
	"sourcedock.dev/petrbalvin/volumen/internal/tomlfile"
)

// TokenPrefix marks volumen API tokens.
const TokenPrefix = "vol_"

// ValidScopes are the recognised API token scopes. There is no read
// scope: every read endpoint is public, so a token can only widen
// access to the write and delete operations.
var ValidScopes = []string{"write", "delete"}

// ErrNoValidScope is returned when a token was requested with an
// explicit scope list that names no recognised scope. Treating it as
// "unrestricted" would hand out more access than the caller asked for.
var ErrNoValidScope = errors.New("no valid scope in the requested list")

// Token is one stored access token (digest only, never the raw value).
type Token struct {
	Name      string
	TokenHash string
	Created   string
	LastUsed  string
	Scopes    []string // nil means unrestricted
}

// HasScope reports whether the token grants scope.
func (t *Token) HasScope(scope string) bool {
	if t.Scopes == nil {
		return true
	}
	return slices.Contains(t.Scopes, scope)
}

// HashToken returns the SHA-256 hex digest of a raw token.
func HashToken(raw string) string {
	sum := sha256.Sum256([]byte(raw))
	return hex.EncodeToString(sum[:])
}

// GenerateToken mints a new raw token with the volumen prefix.
// crypto/rand.Text returns 128 bits of randomness in a URL-safe alphabet,
// and panics on a system failure rather than returning a weak value.
func GenerateToken() string {
	return TokenPrefix + rand.Text()
}

// Store is the file-backed store of API access tokens, with an mtime
// snapshot cache so that authentication does not re-read the file on
// every request.
type Store struct {
	path string

	mu        sync.Mutex
	cached    []Token
	snapshot  fileSnapshot
	haveCache bool

	lock sync.Mutex
}

type fileSnapshot struct {
	present bool
	mtime   int64
	size    int64
}

// New opens the token store at path.
func New(path string) *Store {
	return &Store{path: path}
}

// All returns every stored token record.
func (s *Store) All() []Token {
	s.mu.Lock()
	defer s.mu.Unlock()
	tokens, err := s.loadLocked()
	if err != nil {
		slog.Error("tokens: cannot read the token file", "path", s.path, "error", err)
		return nil
	}
	return slices.Clone(tokens)
}

// loadLocked returns the cached records, rebuilding them when the file
// changed. A missing file is not an error; an unreadable or unparsable
// one is. The caller must hold s.mu.
func (s *Store) loadLocked() ([]Token, error) {
	snapshot := s.buildSnapshot()
	if s.haveCache && snapshot == s.snapshot {
		return s.cached, nil
	}
	tokens, err := s.readFile()
	if err != nil {
		return nil, err
	}
	s.snapshot = snapshot
	s.cached = tokens
	s.haveCache = true
	return tokens, nil
}

func (s *Store) buildSnapshot() fileSnapshot {
	info, err := os.Stat(s.path)
	if err != nil {
		return fileSnapshot{}
	}
	return fileSnapshot{present: true, mtime: info.ModTime().UnixNano(), size: info.Size()}
}

func (s *Store) readFile() ([]Token, error) {
	raw, err := os.ReadFile(s.path)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return nil, nil
		}
		return nil, fmt.Errorf("read %s: %w", s.path, err)
	}
	data, err := interpres.ParseMap(raw)
	if err != nil {
		return nil, fmt.Errorf("parse %s: %w", s.path, err)
	}
	var out []Token
	for _, entry := range tomlfile.Tables(data["tokens"]) {
		name := tomlfile.String(entry["name"])
		hash := tomlfile.String(entry["token_hash"])
		if name == "" || hash == "" {
			continue
		}
		token := Token{
			Name:      name,
			TokenHash: hash,
			Created:   tomlfile.String(entry["created"]),
			LastUsed:  tomlfile.String(entry["last_used"]),
		}
		if scopes := tomlfile.Strings(entry["scopes"]); len(scopes) > 0 {
			token.Scopes = scopes
		}
		out = append(out, token)
	}
	return out, nil
}

// Create mints a token. It returns nil and an empty raw value when the
// name is taken or empty, when the scope list names no recognised
// scope, or when the file cannot be written. An empty scope list creates
// an unrestricted token, which only a caller that asks for one gets.
func (s *Store) Create(name string, scopes []string) (*Token, string, error) {
	name = strings.TrimSpace(name)
	if name == "" {
		return nil, "", errors.New("token name must not be empty")
	}
	if len(scopes) > 0 {
		valid := make([]string, 0, len(scopes))
		for _, scope := range scopes {
			if slices.Contains(ValidScopes, scope) && !slices.Contains(valid, scope) {
				valid = append(valid, scope)
			}
		}
		if len(valid) == 0 {
			return nil, "", ErrNoValidScope
		}
		scopes = valid
	}
	s.lock.Lock()
	defer s.lock.Unlock()
	existing, err := s.reload()
	if err != nil {
		return nil, "", err
	}
	for _, token := range existing {
		if token.Name == name {
			return nil, "", fmt.Errorf("a token named %q already exists", name)
		}
	}
	raw := GenerateToken()
	record := Token{
		Name:      name,
		TokenHash: HashToken(raw),
		Created:   nowISO(),
		Scopes:    scopes,
	}
	if err := s.persist(append(existing, record)); err != nil {
		return nil, "", err
	}
	return &record, raw, nil
}

// Authenticate returns the matching record for a presented raw token.
func (s *Store) Authenticate(raw string) *Token {
	if !strings.HasPrefix(raw, TokenPrefix) {
		return nil
	}
	digest := HashToken(raw)
	all := s.All()
	for i := range all {
		if hmac.Equal([]byte(all[i].TokenHash), []byte(digest)) {
			return &all[i]
		}
	}
	return nil
}

// Touch refreshes last_used, at most once per UTC day per token.
func (s *Store) Touch(name string) {
	today := nowISO()[:10]
	// Fast path under the cache lock: when the cached records already
	// carry today for this token, no write and no reload are needed, so
	// authentication does not serialise on the file.
	s.mu.Lock()
	if cached, err := s.loadLocked(); err == nil {
		for i := range cached {
			if cached[i].Name == name && strings.HasPrefix(cached[i].LastUsed, today) {
				s.mu.Unlock()
				return
			}
		}
	}
	s.mu.Unlock()

	s.lock.Lock()
	defer s.lock.Unlock()
	tokens, err := s.reload()
	if err != nil {
		slog.Warn("tokens: cannot refresh last_used", "path", s.path, "error", err)
		return
	}
	changed := false
	for i := range tokens {
		if tokens[i].Name == name && !strings.HasPrefix(tokens[i].LastUsed, today) {
			tokens[i].LastUsed = nowISO()
			changed = true
		}
	}
	if changed {
		if err := s.persist(tokens); err != nil {
			slog.Warn("tokens: cannot persist last_used", "path", s.path, "error", err)
		}
	}
}

// Revoke removes a token by name; false when nothing was removed or the
// file cannot be written.
func (s *Store) Revoke(name string) bool {
	s.lock.Lock()
	defer s.lock.Unlock()
	tokens, err := s.reload()
	if err != nil {
		slog.Error("tokens: refusing to revoke, the token file is unreadable",
			"path", s.path, "error", err)
		return false
	}
	remaining := make([]Token, 0, len(tokens))
	for _, token := range tokens {
		if token.Name != name {
			remaining = append(remaining, token)
		}
	}
	if len(remaining) == len(tokens) {
		return false
	}
	return s.persist(remaining) == nil
}

// reload re-reads the file, refusing to carry on when it cannot be
// parsed: writing back a list derived from an unreadable file would
// destroy the tokens it contains. The caller must hold s.lock.
func (s *Store) reload() ([]Token, error) {
	s.Invalidate()
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.loadLocked()
}

// Health reports why the token file cannot be read, or nil when it is
// fine or absent.
func (s *Store) Health() error {
	s.mu.Lock()
	defer s.mu.Unlock()
	_, err := s.loadLocked()
	return err
}

// Invalidate drops the cached records so the next read re-reads the
// file.
func (s *Store) Invalidate() {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.cached = nil
	s.snapshot = fileSnapshot{}
	s.haveCache = false
}

func (s *Store) persist(tokens []Token) error {
	entries := make([]map[string]any, 0, len(tokens))
	for _, token := range tokens {
		entry := map[string]any{
			"name":       token.Name,
			"token_hash": token.TokenHash,
			"created":    token.Created,
		}
		if token.LastUsed != "" {
			entry["last_used"] = token.LastUsed
		}
		if len(token.Scopes) > 0 {
			scopes := make([]string, len(token.Scopes))
			for i, scope := range token.Scopes {
				scopes[i] = scope
			}
			entry["scopes"] = scopes
		}
		entries = append(entries, entry)
	}
	if err := tomlfile.Write(s.path, "tokens", entries); err != nil {
		slog.Error("tokens: cannot persist", "path", s.path, "error", err)
		return err
	}
	s.Invalidate()
	return nil
}

func nowISO() string {
	return time.Now().UTC().Format("2006-01-02T15:04:05-07:00")
}