package fs

import (
	"context"
	"fmt"
	"os"
)

// FileSpec describes a single synthetic file exposed by a Tree.
type FileSpec struct {
	Name    string
	Mode    os.FileMode
	Read    func() ([]byte, error)
	Write   func([]byte) error                                                                  // nil = read-only
	Wait    func(ctx context.Context, base string) (content []byte, nextBase string, err error) // nil = not waitable
	Size    func() int64                                                                        // optional; if nil, len(Read())
	OneShot bool                                                                                // true = yields data once per open
	Async   bool                                                                                // true = writes dispatched asynchronously
}

// NewSynth creates a Tree backed by a table of FileSpecs (synthetic files).
func NewSynth(specs []FileSpec) *Tree {
	index := make(map[string]int, len(specs))
	for i, s := range specs {
		index[s.Name] = i
	}

	lookup := func(name string) (*FileSpec, bool) {
		i, ok := index[name]
		if !ok {
			return nil, false
		}
		return &specs[i], true
	}

	return NewTree(nil, 0,
		WithLister(func(_ []string) ([]os.DirEntry, error) {
			entries := make([]os.DirEntry, len(specs))
			for i, s := range specs {
				entries[i] = FileEntry(s.Name, s.Mode)
			}
			return entries, nil
		}),
		WithOpener(func(_ []string, name string) (File, error) {
			spec, ok := lookup(name)
			if !ok {
				return nil, fmt.Errorf("%s: not found", name)
			}
			writeFn := func([]byte) error { return fmt.Errorf("%s: read-only", name) }
			if spec.Write != nil {
				writeFn = spec.Write
			}
			waitFn := func(context.Context, string) ([]byte, string, error) {
				return nil, "", fmt.Errorf("%s: not a wait file", name)
			}
			if spec.Wait != nil {
				waitFn = spec.Wait
			}
			return &FileConfig{
				StatFn: func() (os.FileInfo, error) {
					var size int64
					if spec.Size != nil {
						size = spec.Size()
					} else if spec.Wait != nil {
						size = 4096
					} else if data, err := spec.Read(); err == nil {
						size = int64(len(data))
					}
					return &SyntheticFileInfo{Name_: name, Mode_: spec.Mode, Size_: size}, nil
				},
				ReadFn:         spec.Read,
				WriteFn:        writeFn,
				BlockingReadFn: waitFn,
				OneShot_:       spec.OneShot || spec.Wait != nil,
				IsBlocking_:    spec.Wait != nil,
				Async_:         spec.Async,
			}, nil
		}),
	)
}