// SPDX-License-Identifier: GPL-3.0-or-later

// Package assurance appends Python-schema-compatible hash-chain records for
// local forensic artifacts. Artifact hashing streams from disk, and previous-
// record lookup reads only a bounded tail of the chain.
package assurance

import (
	"bufio"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"time"
)

const (
	Schema       = "enodia.hash_chain.v1"
	ChainName    = "hash-chain.jsonl"
	maxTailBytes = 1 << 20
)

type Record struct {
	Schema    string  `json:"schema"`
	Time      float64 `json:"time"`
	Kind      string  `json:"kind"`
	Path      string  `json:"path"`
	Size      int64   `json:"size"`
	SHA256    string  `json:"sha256"`
	PrevHash  string  `json:"prev_hash"`
	ChainHash string  `json:"chain_hash"`
}

type Chain struct {
	Path string
	mu   sync.Mutex
}

// New preflights the private append-only chain file before service readiness.
func New(directory string) (*Chain, error) {
	if directory == "" {
		return nil, fmt.Errorf("assurance directory is required")
	}
	if err := os.MkdirAll(directory, 0o750); err != nil {
		return nil, fmt.Errorf("create assurance directory: %w", err)
	}
	path := filepath.Join(directory, ChainName)
	file, err := os.OpenFile(path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o600)
	if err != nil {
		return nil, fmt.Errorf("preflight assurance chain: %w", err)
	}
	if err := file.Close(); err != nil {
		return nil, fmt.Errorf("close assurance chain: %w", err)
	}
	if err := os.Chmod(path, 0o600); err != nil {
		return nil, fmt.Errorf("protect assurance chain: %w", err)
	}
	return &Chain{Path: path}, nil
}

// Append hashes artifact and durably appends its chained evidence record.
func (c *Chain) Append(kind, artifact string, when time.Time) (Record, error) {
	c.mu.Lock()
	defer c.mu.Unlock()
	digest, size, err := fileDigest(artifact)
	if err != nil {
		// Python records unavailable artifacts with empty evidence rather than
		// dropping the chain entry.
		digest, size = "", 0
	}
	previous, err := lastHash(c.Path)
	if err != nil {
		return Record{}, err
	}
	record := Record{
		Schema: Schema, Time: float64(when.UnixNano()) / 1e9, Kind: kind,
		Path: artifact, Size: size, SHA256: digest, PrevHash: previous,
	}
	payload, err := canonicalPayload(record)
	if err != nil {
		return Record{}, err
	}
	sum := sha256.Sum256(append(payload, []byte(previous)...))
	record.ChainHash = hex.EncodeToString(sum[:])
	raw, err := json.Marshal(record)
	if err != nil {
		return Record{}, fmt.Errorf("encode assurance record: %w", err)
	}
	file, err := os.OpenFile(c.Path, os.O_APPEND|os.O_WRONLY, 0o600)
	if err != nil {
		return Record{}, fmt.Errorf("open assurance chain: %w", err)
	}
	defer file.Close()
	if _, err := file.Write(append(raw, '\n')); err != nil {
		return Record{}, fmt.Errorf("append assurance chain: %w", err)
	}
	if err := file.Sync(); err != nil {
		return Record{}, fmt.Errorf("sync assurance chain: %w", err)
	}
	return record, nil
}

func fileDigest(path string) (string, int64, error) {
	file, err := os.Open(path)
	if err != nil {
		return "", 0, err
	}
	defer file.Close()
	hash := sha256.New()
	size, err := io.Copy(hash, file)
	if err != nil {
		return "", 0, err
	}
	return hex.EncodeToString(hash.Sum(nil)), size, nil
}

func lastHash(path string) (string, error) {
	file, err := os.Open(path)
	if err != nil {
		return "", fmt.Errorf("open assurance tail: %w", err)
	}
	defer file.Close()
	info, err := file.Stat()
	if err != nil {
		return "", fmt.Errorf("stat assurance chain: %w", err)
	}
	offset := info.Size() - maxTailBytes
	if offset < 0 {
		offset = 0
	}
	if _, err := file.Seek(offset, io.SeekStart); err != nil {
		return "", fmt.Errorf("seek assurance tail: %w", err)
	}
	raw, err := io.ReadAll(io.LimitReader(file, maxTailBytes))
	if err != nil {
		return "", fmt.Errorf("read assurance tail: %w", err)
	}
	lines := strings.Split(strings.TrimSpace(string(raw)), "\n")
	for index := len(lines) - 1; index >= 0; index-- {
		var record Record
		if json.Unmarshal([]byte(lines[index]), &record) == nil && record.ChainHash != "" {
			return record.ChainHash, nil
		}
	}
	return "", nil
}

func canonicalPayload(record Record) ([]byte, error) {
	// encoding/json sorts string map keys, matching Python's sort_keys=True.
	return json.Marshal(map[string]any{
		"schema": record.Schema, "time": record.Time, "kind": record.Kind,
		"path": record.Path, "size": record.Size, "sha256": record.SHA256,
		"prev_hash": record.PrevHash,
	})
}

// ReadAll decodes a chain for verification and read-only consumers.
func ReadAll(path string) ([]Record, error) {
	file, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer file.Close()
	var records []Record
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		var record Record
		if err := json.Unmarshal(scanner.Bytes(), &record); err != nil {
			return nil, err
		}
		records = append(records, record)
	}
	return records, scanner.Err()
}