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

// Package scan walks a directory tree and reports the files krino will
// consider sorting, and why the rest were skipped.
package scan

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"time"

	"krino/internal/ignore"
)

// File is a regular file found by Walk.
type File struct {
	Path    string // absolute, cleaned
	Rel     string // slash-separated, relative to the root
	Name    string // base name
	Size    int64
	ModTime time.Time
	Mode    fs.FileMode
}

// Reason is why an entry was not returned as a File.
type Reason int

const (
	Ignored    Reason = iota // matched an ignore pattern
	Busy                     // a sibling NAME<busy-suffix> exists
	TooNew                   // modified less than MinAge ago
	Symlink                  // a symbolic link (never followed)
	NotRegular               // fifo, socket, device
	Unreadable               // a directory that could not be read
	TooBig                   // larger than MaxSize
)

// String names a Reason the way it should read in a report.
func (r Reason) String() string {
	switch r {
	case Ignored:
		return "ignored"
	case Busy:
		return "busy"
	case TooNew:
		return "too new"
	case Symlink:
		return "symlink"
	case NotRegular:
		return "not a regular file"
	case Unreadable:
		return "unreadable"
	case TooBig:
		return "too big"
	default:
		return fmt.Sprintf("Reason(%d)", int(r))
	}
}

// Skipped is one entry Walk did not return as a File, and why.
type Skipped struct {
	Rel    string
	Reason Reason
}

// Options controls how Walk traverses a directory.
type Options struct {
	Recursive bool
	MaxDepth  int             // 0: unlimited; 1: the root's own entries only
	Ignore    *ignore.Matcher // nil: nothing ignored
	Exclude   []string        // absolute directories never entered
	Busy      []string        // suffixes, e.g. ".part"
	MinAge    time.Duration
	MaxSize   int64 // bytes; 0: no limit
	Now       time.Time
}

// Result is everything Walk found under a root.
type Result struct {
	Files   []File    // sorted by Rel
	Skipped []Skipped // sorted by Rel
}

// Walk lists the files under root that krino will consider, and reports why
// the rest were skipped. root must exist, be a directory, and be readable;
// an unreadable subdirectory found during the walk is reported as
// Unreadable and does not abort the scan.
func Walk(root string, opt Options) (*Result, error) {
	root, err := filepath.Abs(root)
	if err != nil {
		return nil, err
	}
	info, err := os.Stat(root)
	if err != nil {
		return nil, err
	}
	if !info.IsDir() {
		return nil, fmt.Errorf("%s is not a directory", root)
	}
	entries, err := os.ReadDir(root)
	if err != nil {
		return nil, err
	}

	exclude := make(map[string]bool, len(opt.Exclude))
	for _, e := range opt.Exclude {
		exclude[filepath.Clean(e)] = true
	}

	w := &walker{root: root, opt: opt, exclude: exclude}
	if err := w.walk(root, "", 1, entries); err != nil {
		return nil, err
	}

	sort.Slice(w.result.Files, func(i, j int) bool { return w.result.Files[i].Rel < w.result.Files[j].Rel })
	sort.Slice(w.result.Skipped, func(i, j int) bool { return w.result.Skipped[i].Rel < w.result.Skipped[j].Rel })
	return &w.result, nil
}

// walker accumulates the Result across recursive calls.
type walker struct {
	opt     Options
	root    string
	exclude map[string]bool
	result  Result
}

// walk applies the skip checks to entries, the already-read contents of dir
// (at relDir relative to the root, dir's own entries at depth). A
// subdirectory it recurses into is read here, right before recursing, so a
// ReadDir failure on it can be reported as Unreadable and skipped without
// aborting the rest of the walk; only a failure reading dir itself (passed
// in by the caller) would need to propagate, and only Walk's own read of
// the root works that way.
func (w *walker) walk(dir, relDir string, depth int, entries []os.DirEntry) error {
	// The set of names in this directory, built once, for the busy check.
	names := make(map[string]bool, len(entries))
	for _, e := range entries {
		names[e.Name()] = true
	}

	for _, e := range entries {
		name := e.Name()
		rel := name
		if relDir != "" {
			rel = relDir + "/" + name
		}
		path := filepath.Join(dir, name)

		// A symlink is never followed, whatever it points to; DirEntry's
		// Type is Lstat-like and does not resolve it.
		if e.Type()&fs.ModeSymlink != 0 {
			w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: Symlink})
			continue
		}

		if e.IsDir() {
			// Directories themselves are not reported, with two
			// exceptions: the symlink case above, and an ignored
			// directory (C2) — reported once for the directory itself,
			// not for each file inside it, since pruning it without
			// descending is the whole point; without this, its contents
			// would appear in no count and in no -v listing at all.
			if !w.opt.Recursive {
				continue
			}
			if w.opt.Ignore != nil && w.opt.Ignore.Match(rel, true) {
				w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: Ignored})
				continue
			}
			if w.exclude[filepath.Clean(path)] {
				continue
			}
			if w.opt.MaxDepth > 0 && depth+1 > w.opt.MaxDepth {
				continue
			}
			subEntries, err := os.ReadDir(path)
			if err != nil {
				w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: Unreadable})
				continue
			}
			if err := w.walk(path, rel, depth+1, subEntries); err != nil {
				return err
			}
			continue
		}

		info, err := e.Info()
		if err != nil {
			if errors.Is(err, fs.ErrNotExist) {
				continue // vanished mid-walk (a download finishing, say): silently skipped
			}
			w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: Unreadable})
			continue
		}
		if !info.Mode().IsRegular() {
			w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: NotRegular})
			continue
		}
		if w.opt.Ignore != nil && w.opt.Ignore.Match(rel, false) {
			w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: Ignored})
			continue
		}
		if busy := w.isBusy(name, names); busy {
			w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: Busy})
			continue
		}
		if w.opt.Now.Sub(info.ModTime()) < w.opt.MinAge {
			w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: TooNew})
			continue
		}
		if w.opt.MaxSize > 0 && info.Size() > w.opt.MaxSize {
			w.result.Skipped = append(w.result.Skipped, Skipped{Rel: rel, Reason: TooBig})
			continue
		}
		w.result.Files = append(w.result.Files, File{
			Path:    path,
			Rel:     rel,
			Name:    name,
			Size:    info.Size(),
			ModTime: info.ModTime(),
			Mode:    info.Mode(),
		})
	}
	return nil
}

// isBusy reports whether name+suffix, for any configured Busy suffix, is
// among names — the sibling of an in-progress download.
func (w *walker) isBusy(name string, names map[string]bool) bool {
	for _, suffix := range w.opt.Busy {
		if names[name+suffix] {
			return true
		}
	}
	return false
}