// Package config holds Basking's settings file: where to listen, where notes live, the password
// hash and the API tokens. The notes themselves are not in here.
package config

import (
	"crypto/pbkdf2"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"
	"sync"
	"time"
)

type Token struct {
	Name    string    `json:"name"`
	Hash    string    `json:"hash"`
	Created time.Time `json:"created"`
}

type Config struct {
	Listen  string `json:"listen"`
	DataDir string `json:"data_dir"`

	// PasswordHash is the one password that opens the dashboard.
	PasswordHash string `json:"password_hash,omitempty"`
	// Secret signs session cookies, so a restart does not sign you out. It is made on first run.
	Secret string  `json:"secret,omitempty"`
	Tokens []Token `json:"tokens,omitempty"`

	// CookieSecure and TrustProxy are for running behind an HTTPS reverse proxy.
	CookieSecure bool `json:"cookie_secure,omitempty"`
	TrustProxy   bool `json:"trust_proxy,omitempty"`
}

var tokenName = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9 ._-]{0,39}$`)

// Defaults fills in what is missing.
func (c *Config) Defaults() {
	if c.Listen == "" {
		c.Listen = "127.0.0.1:8490"
	}
	if c.DataDir == "" {
		c.DataDir = "data"
	}
	if c.Secret == "" {
		b := make([]byte, 32)
		rand.Read(b)
		c.Secret = base64.RawURLEncoding.EncodeToString(b)
	}
}

func (c *Config) Validate() error {
	seen := map[string]bool{}
	for _, t := range c.Tokens {
		if !tokenName.MatchString(t.Name) {
			return fmt.Errorf("token name %q must be 1 to 40 letters, numbers, spaces, dots, dashes or underscores", t.Name)
		}
		if seen[strings.ToLower(t.Name)] {
			return fmt.Errorf("there is already a token called %q", t.Name)
		}
		seen[strings.ToLower(t.Name)] = true
	}
	return nil
}

// Holder keeps the live configuration and writes every change to disk.
type Holder struct {
	path string
	mu   sync.RWMutex
	cfg  Config
}

// Load reads the file. A missing file gives the defaults (and is created by the first Update).
func Load(path string) (*Holder, error) {
	h := &Holder{path: path}
	b, err := os.ReadFile(path)
	switch {
	case errors.Is(err, os.ErrNotExist):
	case err != nil:
		return nil, err
	default:
		if err := json.Unmarshal(b, &h.cfg); err != nil {
			return nil, fmt.Errorf("%s: %w", path, err)
		}
	}
	h.cfg.Defaults()
	if err := h.cfg.Validate(); err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	return h, nil
}

func (h *Holder) Path() string { return h.path }

// Exists says whether the file is on disk yet.
func (h *Holder) Exists() bool { _, err := os.Stat(h.path); return err == nil }

// Get returns a private copy; change it freely.
func (h *Holder) Get() Config {
	h.mu.RLock()
	defer h.mu.RUnlock()
	return clone(h.cfg)
}

func clone(c Config) Config {
	b, _ := json.Marshal(c)
	var out Config
	json.Unmarshal(b, &out)
	return out
}

// Update applies fn to a copy, validates it, saves it and only then makes it current.
func (h *Holder) Update(fn func(*Config) error) error {
	h.mu.Lock()
	defer h.mu.Unlock()
	c := clone(h.cfg)
	if err := fn(&c); err != nil {
		return err
	}
	c.Defaults()
	if err := c.Validate(); err != nil {
		return err
	}
	if err := save(h.path, c); err != nil {
		return err
	}
	h.cfg = c
	return nil
}

func save(path string, c Config) error {
	b, err := json.MarshalIndent(c, "", "  ")
	if err != nil {
		return err
	}
	if dir := filepath.Dir(path); dir != "." {
		os.MkdirAll(dir, 0o755)
	}
	tmp := path + ".tmp"
	if err := os.WriteFile(tmp, append(b, '\n'), 0o600); err != nil {
		return err
	}
	return os.Rename(tmp, path)
}

// ---- passwords and tokens ----

const pbkdf2Rounds = 600_000

// HashPassword returns "pbkdf2$rounds$salt$key".
func HashPassword(password string) (string, error) {
	salt := make([]byte, 16)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key, err := pbkdf2.Key(sha256.New, password, salt, pbkdf2Rounds, 32)
	if err != nil {
		return "", err
	}
	return fmt.Sprintf("pbkdf2$%d$%s$%s", pbkdf2Rounds, base64.RawStdEncoding.EncodeToString(salt), base64.RawStdEncoding.EncodeToString(key)), nil
}

// CheckPassword compares in constant time. An unparseable hash never matches.
func CheckPassword(hash, password string) bool {
	parts := strings.Split(hash, "$")
	if len(parts) != 4 || parts[0] != "pbkdf2" {
		return false
	}
	rounds, err := strconv.Atoi(parts[1])
	if err != nil || rounds < 1 || rounds > 10_000_000 {
		return false
	}
	salt, err1 := base64.RawStdEncoding.DecodeString(parts[2])
	want, err2 := base64.RawStdEncoding.DecodeString(parts[3])
	if err1 != nil || err2 != nil {
		return false
	}
	got, err := pbkdf2.Key(sha256.New, password, salt, rounds, len(want))
	return err == nil && subtle.ConstantTimeCompare(got, want) == 1
}

// NewToken makes an API token and the hash to store for it. The token itself is shown once.
func NewToken() (token, hash string, err error) {
	b := make([]byte, 32)
	if _, err = rand.Read(b); err != nil {
		return
	}
	token = "bsk_" + base64.RawURLEncoding.EncodeToString(b)
	return token, HashToken(token), nil
}

func HashToken(token string) string {
	sum := sha256.Sum256([]byte(token))
	return hex.EncodeToString(sum[:])
}

// RandomPassword makes a password for a first run.
func RandomPassword() string {
	b := make([]byte, 12)
	rand.Read(b)
	return base64.RawURLEncoding.EncodeToString(b)
}