package startup

import (
	"context"
	"crypto/rand"
	"crypto/subtle"
	"encoding/hex"
	"errors"
	"sync"
	"time"
)

// ClockBootstrap only repairs an uninitialized clock; it is not a public
// replacement for the authenticated time settings API.
type ClockBootstrap struct {
	mu          sync.Mutex
	now         func() time.Time
	apply       func(context.Context, time.Time) error
	token       string
	used        bool
	lastAttempt time.Time
}

func NewClockBootstrap(now func() time.Time, apply func(context.Context, time.Time) error) (*ClockBootstrap, error) {
	var token [32]byte
	if _, err := rand.Read(token[:]); err != nil {
		return nil, err
	}
	return &ClockBootstrap{now: now, apply: apply, token: hex.EncodeToString(token[:])}, nil
}
func (b *ClockBootstrap) Token() string {
	b.mu.Lock()
	defer b.mu.Unlock()
	if b.used || b.now().Year() >= 2000 {
		return ""
	}
	return b.token
}
func (b *ClockBootstrap) Sync(ctx context.Context, token string, target time.Time) error {
	b.mu.Lock()
	defer b.mu.Unlock()
	if b.used || b.now().Year() >= 2000 {
		return errors.New("clock bootstrap is no longer available")
	}
	if subtle.ConstantTimeCompare([]byte(token), []byte(b.token)) != 1 {
		return errors.New("invalid clock bootstrap token")
	}
	if target.Year() < 2020 || target.Year() > 2100 {
		return errors.New("PC clock is outside the supported range")
	}
	if !b.lastAttempt.IsZero() && time.Since(b.lastAttempt) < 3*time.Second {
		return errors.New("clock bootstrap retry is too soon")
	}
	b.lastAttempt = time.Now()
	if err := b.apply(ctx, target); err != nil {
		return err
	}
	if b.now().Year() < 2000 {
		return errors.New("device clock has not been updated")
	}
	b.used = true
	return nil
}