Pad/internal/browser/index.go

package browser

import (
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"
)

// DirectoryIndex holds a cached index of a directory's contents.
// Per design: index stores raw metadata; sorting is pre-computed via position maps.
type DirectoryIndex struct {
	Path       string           `json:"path"`
	Mtime      time.Time        `json:"mtime"`
	EntryCount int              `json:"entry_count"`
	Entries    []Entry          `json:"entries"`
	SortOrders map[string][]int `json:"sort_orders,omitempty"` // key: "name_asc", "name_desc", etc.
}

// getCachePath returns the path to the cached index file for a directory.
func getCachePath(dirPath string) string {
	// Use .pad/indices/ relative to the directory being indexed
	indicesDir := filepath.Join(dirPath, ".pad", "indices")
	// Simple hash: use the directory path itself (URL-encoded style)
	encoded := strings.NewReplacer("/", "_", "\\", "_").Replace(dirPath)
	return filepath.Join(indicesDir, fmt.Sprintf("browser_%s.json", encoded))
}

// buildIndex reads the directory and produces an index with pre-computed position maps.
// Sorting happens at index build time, not during frame rendering.
func buildIndex(dirPath string) (*DirectoryIndex, error) {
	// Try to load from cache first
	if cached, ok := loadCachedIndex(dirPath); ok {
		return cached, nil
	}

	// 1. Read directory entries
	entries, err := os.ReadDir(dirPath)
	if err != nil {
		return nil, fmt.Errorf("browser: read dir %s: %w", dirPath, err)
	}

	// 2. Convert to Entry structs (unsorted)
	var browserEntries []Entry
	for _, e := range entries {
		// Skip hidden directories like .pad
		if e.Name() == ".pad" || e.Name() == ".git" {
			continue
		}
		info, err := e.Info()
		if err != nil {
			continue
		}
		browserEntries = append(browserEntries, Entry{
			Path:    filepath.Join(dirPath, e.Name()),
			Name:    e.Name(),
			Size:    info.Size(),
			ModTime: info.ModTime(),
			IsDir:   info.IsDir(),
		})
	}

	// 3. Build position maps for all sort modes
	sortOrders := make(map[string][]int)
	for mode := SortMode(0); mode < SortMode(modeCount()); mode++ {
		key := sortModeKey(mode)
		if key != "" {
			sortOrders[key] = buildPositionMap(browserEntries, mode)
		}
	}

	// 4. Build index (unsorted entries with pre-computed position maps)
	dirInfo, err := os.Stat(dirPath)
	if err != nil {
		return nil, fmt.Errorf("browser: stat dir %s: %w", dirPath, err)
	}

	idx := &DirectoryIndex{
		Path:       dirPath,
		Mtime:      dirInfo.ModTime(),
		EntryCount: len(browserEntries),
		Entries:    browserEntries,
		SortOrders: sortOrders,
	}

	// 5. Write to cache
	cacheIndex(idx)

	return idx, nil
}

// buildPositionMap creates a sorted index → raw index mapping.
// The returned slice maps sorted positions to raw entry indices.
// Uses sort.SliceStable for O(n log n) performance on large directories.
func buildPositionMap(entries []Entry, mode SortMode) []int {
	n := len(entries)
	indices := make([]int, n)
	for i := range indices {
		indices[i] = i
	}

	// Sort indices based on entry comparison
	cmp := comparator(mode)
	sort.SliceStable(indices, func(i, j int) bool {
		a, b := entries[indices[i]], entries[indices[j]]
		return cmp(a, b) < 0
	})

	return indices
}

// sortModeKey converts a SortMode to its JSON key string.
func sortModeKey(mode SortMode) string {
	switch mode {
	case SortModeNameAsc:
		return "name_asc"
	case SortModeNameDesc:
		return "name_desc"
	case SortModeDateAsc:
		return "date_asc"
	case SortModeDateDesc:
		return "date_desc"
	default:
		return ""
	}
}

// modeCount returns the total number of sort modes (4).
func modeCount() int {
	return int(SortModeDateDesc) + 1
}

// loadCachedIndex attempts to load a cached index from disk.
// Returns the index and true if valid, or nil and false if not.
// Cache validation uses both mtime and entry count to handle
// filesystems where mtime doesn't update immediately.
func loadCachedIndex(dirPath string) (*DirectoryIndex, bool) {
	cachePath := getCachePath(dirPath)

	data, err := os.ReadFile(cachePath)
	if err != nil {
		return nil, false
	}

	var idx DirectoryIndex
	if err := json.Unmarshal(data, &idx); err != nil {
		return nil, false
	}

	// Check if directory mtime has changed since cache was written
	dirInfo, err := os.Stat(dirPath)
	if err != nil {
		return nil, false
	}

	// Also check entry count as a secondary invalidation signal.
	// Directory mtime may not update immediately on some filesystems
	// (e.g., tmpfs), but the entry count will always reflect changes.
	actualEntries, err := os.ReadDir(dirPath)
	if err != nil {
		return nil, false
	}
	// Count actual entries (excluding .pad and .git)
	actualCount := 0
	for _, e := range actualEntries {
		if e.Name() != ".pad" && e.Name() != ".git" {
			actualCount++
		}
	}

	// Cache is valid only if BOTH mtime and entry count match
	if idx.Mtime.Equal(dirInfo.ModTime()) && actualCount == idx.EntryCount {
		return &idx, true
	}

	return nil, false
}

// cacheIndex writes the directory index to the cache file.
func cacheIndex(idx *DirectoryIndex) {
	cachePath := getCachePath(idx.Path)

	// Ensure directory exists
	indicesDir := filepath.Dir(cachePath)
	if err := os.MkdirAll(indicesDir, 0755); err != nil {
		// Non-fatal; continue without caching
		return
	}

	data, err := json.Marshal(idx)
	if err != nil {
		return
	}

	// Write to temp file first, then rename for atomicity
	tmpPath := cachePath + ".tmp"
	if err := os.WriteFile(tmpPath, data, 0644); err != nil {
		return
	}
	os.Rename(tmpPath, cachePath)
}