// Package integrity provides data integrity and hashing utilities
package integrity

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"runtime"
	"sort"
	"strings"
	"sync"
)

// FileSHA256Hex computes SHA256 hash of a single file
func FileSHA256Hex(path string) (string, error) {
	f, err := os.Open(filepath.Clean(path))
	if err != nil {
		return "", err
	}
	defer func() { _ = f.Close() }()

	h := sha256.New()
	if _, err := io.Copy(h, f); err != nil {
		return "", err
	}
	return fmt.Sprintf("%x", h.Sum(nil)), nil
}

// NormalizeSHA256ChecksumHex normalizes a SHA256 checksum string
func NormalizeSHA256ChecksumHex(checksum string) (string, error) {
	checksum = strings.TrimSpace(checksum)
	checksum = strings.TrimPrefix(checksum, "sha256:")
	checksum = strings.TrimPrefix(checksum, "SHA256:")
	checksum = strings.TrimSpace(checksum)
	if checksum == "" {
		return "", nil
	}
	if len(checksum) != 64 {
		return "", fmt.Errorf("expected sha256 hex length 64, got %d", len(checksum))
	}
	if _, err := hex.DecodeString(checksum); err != nil {
		return "", fmt.Errorf("invalid sha256 hex: %w", err)
	}
	return strings.ToLower(checksum), nil
}

// DirOverallSHA256Hex computes overall SHA256 of directory contents
func DirOverallSHA256Hex(root string) (string, error) {
	root = filepath.Clean(root)
	info, err := os.Stat(root)
	if err != nil {
		return "", err
	}
	if !info.IsDir() {
		return "", fmt.Errorf("not a directory")
	}

	var files []string
	err = filepath.WalkDir(root, func(path string, d os.DirEntry, walkErr error) error {
		if walkErr != nil {
			return walkErr
		}
		if d.IsDir() {
			return nil
		}
		rel, err := filepath.Rel(root, path)
		if err != nil {
			return err
		}
		files = append(files, rel)
		return nil
	})
	if err != nil {
		return "", err
	}

	// Deterministic order
	sort.Strings(files)

	// Hash file hashes to avoid holding all bytes
	overall := sha256.New()
	for _, rel := range files {
		p := filepath.Join(root, rel)
		sum, err := FileSHA256Hex(p)
		if err != nil {
			return "", err
		}
		overall.Write([]byte(sum))
	}
	return fmt.Sprintf("%x", overall.Sum(nil)), nil
}

// DirOverallSHA256HexParallel computes directory hash using parallel workers
func DirOverallSHA256HexParallel(root string) (string, error) {
	root = filepath.Clean(root)
	info, err := os.Stat(root)
	if err != nil {
		return "", err
	}
	if !info.IsDir() {
		return "", fmt.Errorf("not a directory")
	}

	// Collect all files with size info
	var files []string
	var totalSize int64
	err = filepath.WalkDir(root, func(path string, d os.DirEntry, walkErr error) error {
		if walkErr != nil {
			return walkErr
		}
		if d.IsDir() {
			return nil
		}
		rel, err := filepath.Rel(root, path)
		if err != nil {
			return err
		}
		files = append(files, rel)

		// Track total size for optimization decisions
		if info, err := d.Info(); err == nil {
			totalSize += info.Size()
		}
		return nil
	})
	if err != nil {
		return "", err
	}

	// Sort for deterministic order
	sort.Strings(files)

	// Parallel hashing with worker pool
	numWorkers := runtime.NumCPU()
	if numWorkers > 8 {
		numWorkers = 8
	}

	type result struct {
		index int
		hash  string
		err   error
	}

	workCh := make(chan int, len(files))
	resultCh := make(chan result, len(files))

	var wg sync.WaitGroup
	for i := 0; i < numWorkers; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			for idx := range workCh {
				rel := files[idx]
				p := filepath.Join(root, rel)
				hash, err := FileSHA256Hex(p)
				resultCh <- result{index: idx, hash: hash, err: err}
			}
		}()
	}

	go func() {
		for i := range files {
			workCh <- i
		}
		close(workCh)
	}()

	go func() {
		wg.Wait()
		close(resultCh)
	}()

	hashes := make([]string, len(files))
	for r := range resultCh {
		if r.err != nil {
			return "", r.err
		}
		hashes[r.index] = r.hash
	}

	// Combine hashes deterministically
	overall := sha256.New()
	for _, h := range hashes {
		overall.Write([]byte(h))
	}
	return fmt.Sprintf("%x", overall.Sum(nil)), nil
}

// EstimateDirSize returns total size of directory contents in bytes
func EstimateDirSize(root string) (int64, error) {
	root = filepath.Clean(root)
	info, err := os.Stat(root)
	if err != nil {
		return 0, err
	}
	if !info.IsDir() {
		return info.Size(), nil
	}

	var totalSize int64
	err = filepath.WalkDir(root, func(path string, d os.DirEntry, walkErr error) error {
		if walkErr != nil {
			return walkErr
		}
		if d.IsDir() {
			return nil
		}
		if info, err := d.Info(); err == nil {
			totalSize += info.Size()
		}
		return nil
	})
	return totalSize, err
}