/*
Kernel Manager - Download, verify, and manage kernels for Volt hybrid runtime.

Provides kernel lifecycle operations:
  - Download kernels to /var/lib/volt/kernels/
  - Verify SHA-256 checksums
  - List available (local) kernels
  - Default kernel selection (host kernel fallback)
  - Kernel config validation (namespaces, cgroups, Landlock)

Copyright (c) Armored Gates LLC. All rights reserved.
*/
package kernel

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"net/http"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"
	"time"
)

const (
	// DefaultKernelDir is where kernels are stored on disk.
	DefaultKernelDir = "/var/lib/volt/kernels"

	// HostKernelPath is the default host kernel image location.
	HostKernelPath = "/boot/vmlinuz"

	// configGzPath is the compressed kernel config inside /proc.
	configGzPath = "/proc/config.gz"
)

// KernelInfo describes a locally available kernel.
type KernelInfo struct {
	Version   string    // e.g. "6.1.0-42-amd64"
	Path      string    // absolute path to vmlinuz
	Size      int64     // bytes
	SHA256    string    // hex-encoded checksum
	Source    string    // "host", "downloaded", "custom"
	AddedAt   time.Time // when the kernel was registered
	IsDefault bool      // whether this is the active default
}

// RequiredFeature is a kernel config option that must be present.
type RequiredFeature struct {
	Config      string // e.g. "CONFIG_NAMESPACES"
	Description string // human-readable explanation
}

// RequiredFeatures lists kernel config options needed for Volt hybrid mode.
var RequiredFeatures = []RequiredFeature{
	{Config: "CONFIG_NAMESPACES", Description: "Namespace support (PID, NET, MNT, UTS, IPC)"},
	{Config: "CONFIG_PID_NS", Description: "PID namespace isolation"},
	{Config: "CONFIG_NET_NS", Description: "Network namespace isolation"},
	{Config: "CONFIG_USER_NS", Description: "User namespace isolation"},
	{Config: "CONFIG_UTS_NS", Description: "UTS namespace isolation"},
	{Config: "CONFIG_IPC_NS", Description: "IPC namespace isolation"},
	{Config: "CONFIG_CGROUPS", Description: "Control groups support"},
	{Config: "CONFIG_CGROUP_V2", Description: "Cgroups v2 unified hierarchy"},
	{Config: "CONFIG_SECURITY_LANDLOCK", Description: "Landlock LSM filesystem sandboxing"},
	{Config: "CONFIG_SECCOMP", Description: "Seccomp syscall filtering"},
	{Config: "CONFIG_SECCOMP_FILTER", Description: "Seccomp BPF filter programs"},
}

// Manager handles kernel downloads, verification, and selection.
type Manager struct {
	kernelDir string
}

// NewManager creates a new kernel manager rooted at the given directory.
// If kernelDir is empty, DefaultKernelDir is used.
func NewManager(kernelDir string) *Manager {
	if kernelDir == "" {
		kernelDir = DefaultKernelDir
	}
	return &Manager{kernelDir: kernelDir}
}

// Init ensures the kernel directory exists.
func (m *Manager) Init() error {
	return os.MkdirAll(m.kernelDir, 0755)
}

// KernelDir returns the base directory for kernel storage.
func (m *Manager) KernelDir() string {
	return m.kernelDir
}

// ── Download & Verify ────────────────────────────────────────────────────────

// Download fetches a kernel image from url into the kernel directory under the
// given version name. If expectedSHA256 is non-empty the download is verified
// against it; a mismatch causes the file to be removed and an error returned.
func (m *Manager) Download(version, url, expectedSHA256 string) (*KernelInfo, error) {
	if err := m.Init(); err != nil {
		return nil, fmt.Errorf("kernel dir init: %w", err)
	}

	destDir := filepath.Join(m.kernelDir, version)
	if err := os.MkdirAll(destDir, 0755); err != nil {
		return nil, fmt.Errorf("create version dir: %w", err)
	}

	destPath := filepath.Join(destDir, "vmlinuz")

	// Download to temp file first, then rename.
	tmpPath := destPath + ".tmp"
	out, err := os.Create(tmpPath)
	if err != nil {
		return nil, fmt.Errorf("create temp file: %w", err)
	}
	defer func() {
		out.Close()
		os.Remove(tmpPath) // clean up on any failure path
	}()

	client := &http.Client{Timeout: 10 * time.Minute}
	resp, err := client.Get(url)
	if err != nil {
		return nil, fmt.Errorf("download failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("download returned HTTP %d", resp.StatusCode)
	}

	hasher := sha256.New()
	writer := io.MultiWriter(out, hasher)

	if _, err := io.Copy(writer, resp.Body); err != nil {
		return nil, fmt.Errorf("download interrupted: %w", err)
	}

	if err := out.Close(); err != nil {
		return nil, fmt.Errorf("close temp file: %w", err)
	}

	checksum := hex.EncodeToString(hasher.Sum(nil))

	if expectedSHA256 != "" && !strings.EqualFold(checksum, expectedSHA256) {
		os.Remove(tmpPath)
		return nil, fmt.Errorf("checksum mismatch: got %s, expected %s", checksum, expectedSHA256)
	}

	if err := os.Rename(tmpPath, destPath); err != nil {
		return nil, fmt.Errorf("rename to final path: %w", err)
	}

	// Write checksum sidecar.
	checksumPath := filepath.Join(destDir, "sha256")
	os.WriteFile(checksumPath, []byte(checksum+"\n"), 0644)

	fi, _ := os.Stat(destPath)
	return &KernelInfo{
		Version: version,
		Path:    destPath,
		Size:    fi.Size(),
		SHA256:  checksum,
		Source:  "downloaded",
		AddedAt: time.Now(),
	}, nil
}

// VerifyChecksum checks that the kernel at path matches the expected SHA-256
// hex digest. Returns nil on match.
func VerifyChecksum(path, expectedSHA256 string) error {
	f, err := os.Open(path)
	if err != nil {
		return fmt.Errorf("open kernel: %w", err)
	}
	defer f.Close()

	h := sha256.New()
	if _, err := io.Copy(h, f); err != nil {
		return fmt.Errorf("read kernel: %w", err)
	}

	got := hex.EncodeToString(h.Sum(nil))
	if !strings.EqualFold(got, expectedSHA256) {
		return fmt.Errorf("checksum mismatch: got %s, expected %s", got, expectedSHA256)
	}
	return nil
}

// Checksum computes and returns the SHA-256 hex digest of the file at path.
func Checksum(path string) (string, error) {
	f, err := os.Open(path)
	if err != nil {
		return "", fmt.Errorf("open: %w", err)
	}
	defer f.Close()

	h := sha256.New()
	if _, err := io.Copy(h, f); err != nil {
		return "", fmt.Errorf("read: %w", err)
	}
	return hex.EncodeToString(h.Sum(nil)), nil
}

// ── List ─────────────────────────────────────────────────────────────────────

// List returns all locally available kernels sorted by version name.
func (m *Manager) List() ([]KernelInfo, error) {
	entries, err := os.ReadDir(m.kernelDir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, fmt.Errorf("read kernel dir: %w", err)
	}

	var kernels []KernelInfo
	for _, entry := range entries {
		if !entry.IsDir() {
			continue
		}
		version := entry.Name()
		vmlinuz := filepath.Join(m.kernelDir, version, "vmlinuz")
		fi, err := os.Stat(vmlinuz)
		if err != nil {
			continue // not a valid kernel directory
		}

		ki := KernelInfo{
			Version: version,
			Path:    vmlinuz,
			Size:    fi.Size(),
			Source:  "downloaded",
		}

		// Read checksum sidecar if present.
		if data, err := os.ReadFile(filepath.Join(m.kernelDir, version, "sha256")); err == nil {
			ki.SHA256 = strings.TrimSpace(string(data))
		}

		kernels = append(kernels, ki)
	}

	sort.Slice(kernels, func(i, j int) bool {
		return kernels[i].Version < kernels[j].Version
	})

	return kernels, nil
}

// ── Default Kernel Selection ─────────────────────────────────────────────────

// DefaultKernel returns the best kernel to use:
//  1. The host kernel at /boot/vmlinuz-$(uname -r).
//  2. Generic /boot/vmlinuz fallback.
//  3. The latest locally downloaded kernel.
//
// Returns the absolute path to the kernel image.
func (m *Manager) DefaultKernel() (string, error) {
	// Prefer the host kernel matching the running version.
	uname := currentKernelVersion()
	hostPath := "/boot/vmlinuz-" + uname
	if fileExists(hostPath) {
		return hostPath, nil
	}

	// Generic fallback.
	if fileExists(HostKernelPath) {
		return HostKernelPath, nil
	}

	// Check locally downloaded kernels — pick the latest.
	kernels, err := m.List()
	if err == nil && len(kernels) > 0 {
		return kernels[len(kernels)-1].Path, nil
	}

	return "", fmt.Errorf("no kernel found (checked %s, %s, %s)", hostPath, HostKernelPath, m.kernelDir)
}

// ResolveKernel resolves a kernel reference to an absolute path.
// If kernelRef is an absolute path and exists, it is returned directly.
// Otherwise, it is treated as a version name under kernelDir.
// If empty, DefaultKernel() is used.
func (m *Manager) ResolveKernel(kernelRef string) (string, error) {
	if kernelRef == "" {
		return m.DefaultKernel()
	}

	// Absolute path — use directly.
	if filepath.IsAbs(kernelRef) {
		if !fileExists(kernelRef) {
			return "", fmt.Errorf("kernel not found: %s", kernelRef)
		}
		return kernelRef, nil
	}

	// Treat as version name.
	path := filepath.Join(m.kernelDir, kernelRef, "vmlinuz")
	if fileExists(path) {
		return path, nil
	}

	return "", fmt.Errorf("kernel version %q not found in %s", kernelRef, m.kernelDir)
}

// ── Kernel Config Validation ─────────────────────────────────────────────────

// ValidationResult holds the outcome of a kernel config check.
type ValidationResult struct {
	Feature RequiredFeature
	Present bool
	Value   string // "y", "m", or empty
}

// ValidateHostKernel checks the running host kernel's config for required
// features. It reads from /boot/config-$(uname -r) or /proc/config.gz.
func ValidateHostKernel() ([]ValidationResult, error) {
	uname := currentKernelVersion()
	configPath := "/boot/config-" + uname

	configData, err := os.ReadFile(configPath)
	if err != nil {
		// Try /proc/config.gz via zcat
		configData, err = readProcConfigGz()
		if err != nil {
			return nil, fmt.Errorf("cannot read kernel config (tried %s and %s): %w",
				configPath, configGzPath, err)
		}
	}

	return validateConfig(string(configData)), nil
}

// ValidateConfigFile checks a kernel config file at the given path for
// required features.
func ValidateConfigFile(path string) ([]ValidationResult, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("read config file: %w", err)
	}
	return validateConfig(string(data)), nil
}

// validateConfig parses a kernel .config text and checks for required features.
func validateConfig(configText string) []ValidationResult {
	configMap := make(map[string]string)
	for _, line := range strings.Split(configText, "\n") {
		line = strings.TrimSpace(line)
		if line == "" {
			continue
		}
		// Check for "# CONFIG_FOO is not set" pattern.
		if strings.HasPrefix(line, "# ") && strings.HasSuffix(line, " is not set") {
			key := strings.TrimPrefix(line, "# ")
			key = strings.TrimSuffix(key, " is not set")
			configMap[key] = "n"
			continue
		}
		if strings.HasPrefix(line, "#") {
			continue
		}
		parts := strings.SplitN(line, "=", 2)
		if len(parts) == 2 {
			configMap[parts[0]] = parts[1]
		}
	}

	var results []ValidationResult
	for _, feat := range RequiredFeatures {
		val := configMap[feat.Config]
		r := ValidationResult{Feature: feat}
		if val == "y" || val == "m" {
			r.Present = true
			r.Value = val
		}
		results = append(results, r)
	}

	return results
}

// AllFeaturesPresent returns true if every validation result is present.
func AllFeaturesPresent(results []ValidationResult) bool {
	for _, r := range results {
		if !r.Present {
			return false
		}
	}
	return true
}

// MissingFeatures returns only the features that are not present.
func MissingFeatures(results []ValidationResult) []ValidationResult {
	var missing []ValidationResult
	for _, r := range results {
		if !r.Present {
			missing = append(missing, r)
		}
	}
	return missing
}

// ── Helpers ──────────────────────────────────────────────────────────────────

// currentKernelVersion returns the running kernel version string (uname -r).
func currentKernelVersion() string {
	data, err := os.ReadFile("/proc/sys/kernel/osrelease")
	if err == nil {
		return strings.TrimSpace(string(data))
	}
	// Fallback: shell out to uname.
	out, err := exec.Command("uname", "-r").Output()
	if err == nil {
		return strings.TrimSpace(string(out))
	}
	return "unknown"
}

// readProcConfigGz reads kernel config from /proc/config.gz using zcat.
func readProcConfigGz() ([]byte, error) {
	if !fileExists(configGzPath) {
		return nil, fmt.Errorf("%s not found (try: modprobe configs)", configGzPath)
	}
	return exec.Command("zcat", configGzPath).Output()
}

// fileExists returns true if the path exists and is not a directory.
func fileExists(path string) bool {
	fi, err := os.Stat(path)
	if err != nil {
		return false
	}
	return !fi.IsDir()
}