package backendupdate

import (
	"bytes"
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"

	"yms-daemon/internal/filestore"
	"yms-daemon/internal/hostnginx"
	"yms-daemon/internal/systemd"
	"yms-daemon/internal/transaction"
)

// gatewayOperation 切换宿主 Nginx 后端上游的事务操作。Apply 应用 after 快照
// 并写入回执，Inspect 通过对比当前配置与 before/after 快照判断执行状态。
type gatewayOperation struct {
	// controller 宿主 Nginx 配置的读取与应用控制器。
	controller gatewayController
	// before 切换前的 Nginx 配置快照。
	before hostnginx.Snapshot
	// after 切换后的 Nginx 配置快照。
	after hostnginx.Snapshot
	// receiptPath 切换成功后写入回执摘要的文件路径。
	receiptPath string
}

// Apply 应用 after 快照到宿主 Nginx，并在成功后写入摘要回执文件。
// 回执文件用于 Inspect 区分“已应用”与“应用内容不一致”等状态。
func (o *gatewayOperation) Apply(ctx context.Context) error {
	if err := o.controller.Apply(ctx, o.after); err != nil {
		return err
	}
	return writeImmutableFile(o.receiptPath, []byte(snapshotDigest(o.after)), 0o600)
}

// Inspect 读取当前 Nginx 配置并与 after/before 快照对比，返回该操作是否已应用、
// 未应用或状态未知。对比同时校验端口与配置内容；回执文件的存在与摘要一致用于
// 强化“已应用”判定。
func (o *gatewayOperation) Inspect(context.Context) (transaction.Inspection, error) {
	current, err := o.controller.Read()
	if err != nil {
		return transaction.Inspection{}, err
	}
	receipt, receiptErr := os.ReadFile(o.receiptPath)
	if current.ActivePort == o.after.ActivePort && bytes.Equal(current.Content, o.after.Content) {
		if receiptErr == nil && string(receipt) == snapshotDigest(o.after) {
			return transaction.Inspection{Status: transaction.InspectionApplied, Result: gatewayResult(current)}, nil
		}
		if errors.Is(receiptErr, os.ErrNotExist) {
			return transaction.Inspection{Status: transaction.InspectionNotApplied, Result: gatewayResult(current)}, nil
		}
		if receiptErr != nil {
			return transaction.Inspection{}, fmt.Errorf("read host Nginx switch receipt: %w", receiptErr)
		}
		return transaction.Inspection{Status: transaction.InspectionUnknown, Result: gatewayResult(current)}, nil
	}
	if current.ActivePort == o.before.ActivePort && bytes.Equal(current.Content, o.before.Content) {
		return transaction.Inspection{Status: transaction.InspectionNotApplied, Result: gatewayResult(current)}, nil
	}
	return transaction.Inspection{Status: transaction.InspectionUnknown, Result: gatewayResult(current)}, nil
}

// pathOperation 将某文件系统路径调整到期望状态的事务操作。before 保存操作前
// 的路径状态，desired 保存期望达到的路径状态，Inspect 据此判断执行结果。
type pathOperation struct {
	// path 被操作的路径。
	path string
	// before 操作前的路径状态，用于判断“未应用”。
	before pathState
	// desired 期望达到的路径状态，用于判断“已应用”。
	desired pathState
}

// Apply 将目标路径调整到 desired 状态。
func (o *pathOperation) Apply(context.Context) error {
	return applyPathState(o.path, o.desired)
}

// Inspect 判断路径当前是否已匹配 desired 或仍匹配 before，返回相应的执行状态。
func (o *pathOperation) Inspect(context.Context) (transaction.Inspection, error) {
	desired, err := pathMatches(o.path, o.desired)
	if err != nil {
		return transaction.Inspection{}, err
	}
	if desired {
		return transaction.Inspection{Status: transaction.InspectionApplied, Result: pathResult(o.path, o.desired)}, nil
	}
	before, err := pathMatches(o.path, o.before)
	if err != nil {
		return transaction.Inspection{}, err
	}
	if before {
		return transaction.Inspection{Status: transaction.InspectionNotApplied, Result: pathResult(o.path, o.before)}, nil
	}
	return transaction.Inspection{Status: transaction.InspectionUnknown}, nil
}

// unitStopOperation 停止某个 systemd 单元的事务操作。
type unitStopOperation struct {
	// units systemd 管理器。
	units systemd.Manager
	// name 待停止的单元名。
	name string
}

// Apply 停止指定 systemd 单元。
func (o *unitStopOperation) Apply(ctx context.Context) error {
	return o.units.Stop(ctx, o.name)
}

// Inspect 查询单元状态并判断是否已停止。inactive 与 failed 均视为已停止，因为
// systemd 可能把成功停止的遗留服务标记为 failed（其追踪的 JVM 以 SIGTERM 退出，
// 状态码 143），二者都表示已无活动进程，即本步骤所需结果。
func (o *unitStopOperation) Inspect(ctx context.Context) (transaction.Inspection, error) {
	unit, err := o.units.Inspect(ctx, o.name)
	if err != nil {
		return transaction.Inspection{}, err
	}
	result, err := json.Marshal(unit)
	if err != nil {
		return transaction.Inspection{}, err
	}
	// systemd may leave a successfully stopped legacy service in failed when
	// its tracked JVM exits with SIGTERM (status 143). Both states
	// mean no process is active, which is the required result of this step.
	if unit.ActiveState == "inactive" || unit.ActiveState == "failed" {
		return transaction.Inspection{Status: transaction.InspectionApplied, Result: result}, nil
	}
	if unitRunning(unit) {
		return transaction.Inspection{Status: transaction.InspectionNotApplied, Result: result}, nil
	}
	return transaction.Inspection{Status: transaction.InspectionUnknown, Result: result}, nil
}

// snapshotPath 检查指定路径并把其当前状态快照为 pathState。若路径不存在则返回
// absent 状态；若为符号链接则记录其目标；若为普通文件则复制一份备份并记录其
// 尺寸与摘要身份，便于后续校验与恢复。
func snapshotPath(path string, backupPath string) (pathState, error) {
	info, err := os.Lstat(path)
	if errors.Is(err, os.ErrNotExist) {
		return pathState{Kind: pathKindAbsent}, nil
	}
	if err != nil {
		return pathState{}, fmt.Errorf("inspect compatibility path %s: %w", path, err)
	}
	if info.Mode()&os.ModeSymlink != 0 {
		target, err := os.Readlink(path)
		if err != nil {
			return pathState{}, fmt.Errorf("read compatibility link %s: %w", path, err)
		}
		return pathState{Kind: pathKindSymlink, Target: target}, nil
	}
	if !info.Mode().IsRegular() {
		return pathState{}, fmt.Errorf("compatibility path is neither a regular file nor symbolic link: %s", path)
	}
	identity, err := copyFileSnapshot(path, backupPath, info.Mode().Perm())
	if err != nil {
		return pathState{}, err
	}
	return pathState{Kind: pathKindRegular, BackupPath: backupPath, Identity: identity, Mode: uint32(info.Mode().Perm())}, nil
}

// applyPathState 将目标路径调整到指定状态：absent 则删除，symlink 则原子地替换
// 为指向目标的新链接，regular 则校验备份后原子地恢复为普通文件。父目录必须是
// 直接目录。所有变更完成后都会同步父目录以确保持久化。
func applyPathState(path string, state pathState) error {
	parent := filepath.Dir(path)
	info, err := os.Lstat(parent)
	if err != nil {
		return fmt.Errorf("inspect path state parent %s: %w", parent, err)
	}
	if !info.IsDir() || info.Mode()&os.ModeSymlink != 0 {
		return fmt.Errorf("path state parent is not a direct directory: %s", parent)
	}
	switch state.Kind {
	case pathKindAbsent:
		if err := os.Remove(path); err != nil && !errors.Is(err, os.ErrNotExist) {
			return fmt.Errorf("remove path state %s: %w", path, err)
		}
		return syncDirectory(parent)
	case pathKindSymlink:
		if !filepath.IsAbs(state.Target) {
			return errors.New("path state symbolic link target must be absolute")
		}
		temporary := filepath.Join(parent, ".yms-daemon-link-"+rand.Text())
		if err := os.Symlink(state.Target, temporary); err != nil {
			return fmt.Errorf("create temporary compatibility link: %w", err)
		}
		defer os.Remove(temporary)
		if err := os.Rename(temporary, path); err != nil {
			return fmt.Errorf("replace compatibility link %s: %w", path, err)
		}
		return syncDirectory(parent)
	case pathKindRegular:
		if err := verifyFileIdentity(state.BackupPath, state.Identity); err != nil {
			return fmt.Errorf("verify compatibility file snapshot: %w", err)
		}
		return copyFileAtomic(state.BackupPath, path, os.FileMode(state.Mode))
	default:
		return fmt.Errorf("unsupported path state kind: %q", state.Kind)
	}
}

// pathMatches 判断路径当前是否匹配指定状态：absent 状态要求路径不存在，symlink
// 状态要求为指向相同目标的符号链接，regular 状态要求为权限位与身份摘要都一致的
// 普通文件。
func pathMatches(path string, state pathState) (bool, error) {
	info, err := os.Lstat(path)
	if errors.Is(err, os.ErrNotExist) {
		return state.Kind == pathKindAbsent, nil
	}
	if err != nil {
		return false, err
	}
	switch state.Kind {
	case pathKindAbsent:
		return false, nil
	case pathKindSymlink:
		if info.Mode()&os.ModeSymlink == 0 {
			return false, nil
		}
		target, err := os.Readlink(path)
		return target == state.Target, err
	case pathKindRegular:
		if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || uint32(info.Mode().Perm()) != state.Mode {
			return false, nil
		}
		if err := verifyFileIdentity(path, state.Identity); err != nil {
			return false, nil
		}
		return true, nil
	default:
		return false, fmt.Errorf("unsupported path state kind: %q", state.Kind)
	}
}

// copyFileSnapshot 将源文件复制到目标路径并返回其尺寸与 SHA-256 身份信息。
// 复制采用先写临时文件、同步、再原子重命名的流程，并同步父目录，保证落盘一致。
func copyFileSnapshot(sourcePath string, destinationPath string, mode os.FileMode) (filestore.Identity, error) {
	if err := os.MkdirAll(filepath.Dir(destinationPath), 0o750); err != nil {
		return filestore.Identity{}, err
	}
	source, err := os.Open(sourcePath)
	if err != nil {
		return filestore.Identity{}, err
	}
	defer source.Close()
	temporary, err := os.CreateTemp(filepath.Dir(destinationPath), ".snapshot-*")
	if err != nil {
		return filestore.Identity{}, err
	}
	temporaryPath := temporary.Name()
	defer os.Remove(temporaryPath)
	if err := temporary.Chmod(mode); err != nil {
		_ = temporary.Close()
		return filestore.Identity{}, err
	}
	digest := sha256.New()
	size, copyErr := io.Copy(io.MultiWriter(temporary, digest), source)
	if copyErr != nil {
		_ = temporary.Close()
		return filestore.Identity{}, copyErr
	}
	if err := temporary.Sync(); err != nil {
		_ = temporary.Close()
		return filestore.Identity{}, err
	}
	if err := temporary.Close(); err != nil {
		return filestore.Identity{}, err
	}
	if err := os.Rename(temporaryPath, destinationPath); err != nil {
		return filestore.Identity{}, err
	}
	if err := syncDirectory(filepath.Dir(destinationPath)); err != nil {
		return filestore.Identity{}, err
	}
	return filestore.Identity{Size: size, SHA256: hex.EncodeToString(digest.Sum(nil))}, nil
}

// copyFileAtomic 原子地将源文件复制到目标路径：先写入同目录临时文件，同步并
// 关闭后再重命名，最后同步父目录。用于普通文件路径状态的恢复。
func copyFileAtomic(sourcePath string, destinationPath string, mode os.FileMode) error {
	source, err := os.Open(sourcePath)
	if err != nil {
		return err
	}
	defer source.Close()
	parent := filepath.Dir(destinationPath)
	temporary, err := os.CreateTemp(parent, ".restore-*")
	if err != nil {
		return err
	}
	temporaryPath := temporary.Name()
	defer os.Remove(temporaryPath)
	if err := temporary.Chmod(mode); err != nil {
		_ = temporary.Close()
		return err
	}
	if _, err := io.Copy(temporary, source); err != nil {
		_ = temporary.Close()
		return err
	}
	if err := temporary.Sync(); err != nil {
		_ = temporary.Close()
		return err
	}
	if err := temporary.Close(); err != nil {
		return err
	}
	if err := os.Rename(temporaryPath, destinationPath); err != nil {
		return err
	}
	return syncDirectory(parent)
}

// writeImmutableFile 以排他方式写入不可变文件：若路径已存在则要求其内容与待写
// 内容完全一致，否则报错；若不存在则以指定权限原子地创建并同步。写入失败时会
// 清理残留文件。
func writeImmutableFile(path string, content []byte, mode os.FileMode) error {
	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
		return err
	}
	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_WRONLY, mode)
	if errors.Is(err, os.ErrExist) {
		existing, readErr := os.ReadFile(path)
		if readErr != nil {
			return readErr
		}
		if !bytes.Equal(existing, content) {
			return fmt.Errorf("immutable file already exists with different content: %s", path)
		}
		return nil
	}
	if err != nil {
		return err
	}
	complete := false
	defer func() {
		if !complete {
			_ = os.Remove(path)
		}
	}()
	if _, err := file.Write(content); err != nil {
		_ = file.Close()
		return err
	}
	if err := file.Sync(); err != nil {
		_ = file.Close()
		return err
	}
	if err := file.Close(); err != nil {
		return err
	}
	if err := syncDirectory(filepath.Dir(path)); err != nil {
		return err
	}
	complete = true
	return nil
}

// readGatewaySnapshot 读取持久化的 Nginx 配置快照文件，并校验其中活动端口与
// 期望端口一致，返回快照内容与端口。
func readGatewaySnapshot(path string, port int) (hostnginx.Snapshot, error) {
	content, err := os.ReadFile(path)
	if err != nil {
		return hostnginx.Snapshot{}, err
	}
	actual, err := hostnginx.ActiveBackendPort(content)
	if err != nil {
		return hostnginx.Snapshot{}, err
	}
	if actual != port {
		return hostnginx.Snapshot{}, fmt.Errorf("persisted host Nginx snapshot port mismatch: got %d, want %d", actual, port)
	}
	return hostnginx.Snapshot{Content: content, ActivePort: port}, nil
}

// verifyFileIdentity 校验文件的实际尺寸与 SHA-256 摘要是否与给定身份一致，任何
// 不一致都返回错误。
func verifyFileIdentity(path string, identity filestore.Identity) error {
	if err := identity.Validate(); err != nil {
		return err
	}
	file, err := os.Open(path)
	if err != nil {
		return err
	}
	digest := sha256.New()
	size, copyErr := io.Copy(digest, file)
	closeErr := file.Close()
	if err := errors.Join(copyErr, closeErr); err != nil {
		return err
	}
	if size != identity.Size || !strings.EqualFold(hex.EncodeToString(digest.Sum(nil)), identity.SHA256) {
		return errors.New("file identity mismatch")
	}
	return nil
}

// snapshotDigest 计算 Nginx 快照内容的 SHA-256 摘要（十六进制字符串），用于
// 回执与意图中的一致性标识。
func snapshotDigest(snapshot hostnginx.Snapshot) string {
	digest := sha256.Sum256(snapshot.Content)
	return hex.EncodeToString(digest[:])
}

// gatewayResult 将 Nginx 快照的活动端口与内容摘要序列化为事务检查结果的 JSON。
func gatewayResult(snapshot hostnginx.Snapshot) json.RawMessage {
	result, _ := json.Marshal(struct {
		ActivePort int    `json:"activePort"`
		SHA256     string `json:"sha256"`
	}{snapshot.ActivePort, snapshotDigest(snapshot)})
	return result
}

// pathResult 将路径及其状态序列化为事务检查结果的 JSON。
func pathResult(path string, state pathState) json.RawMessage {
	result, _ := json.Marshal(struct {
		Path  string    `json:"path"`
		State pathState `json:"state"`
	}{path, state})
	return result
}

// syncDirectory 打开目录并执行 fsync，将目录项变更持久化到磁盘。
func syncDirectory(directory string) error {
	file, err := os.Open(directory)
	if err != nil {
		return err
	}
	syncErr := file.Sync()
	closeErr := file.Close()
	return errors.Join(syncErr, closeErr)
}