package discovery

import (
	"os"
	"strings"
)

// PrefixTree narrows a Tree to the subtree below Prefix; paths passed in and
// reported back are relative to that subtree.
type PrefixTree struct {
	Tree   Tree
	Prefix string
}

func (p *PrefixTree) Walk(fn func(rel string, isDir bool) error) error {
	prefix := strings.TrimSuffix(p.Prefix, "/") + "/"
	return p.Tree.Walk(func(rel string, isDir bool) error {
		if rel == p.Prefix || strings.HasPrefix(rel, prefix) {
			sub := strings.TrimPrefix(rel, prefix)
			if sub == "" {
				return nil
			}
			return fn(sub, isDir)
		}
		return nil
	})
}

func (p *PrefixTree) ReadFile(rel string) ([]byte, error) {
	return p.Tree.ReadFile(p.Prefix + "/" + rel)
}

// FilesUnder delegates to the underlying tree with the prefix reapplied.
func (p *PrefixTree) FilesUnder(prefix string) ([]string, error) {
	lister, ok := p.Tree.(interface {
		FilesUnder(prefix string) ([]string, error)
	})
	if !ok {
		return nil, os.ErrNotExist
	}
	full := prefix
	if p.Prefix != "" {
		full = p.Prefix + "/" + strings.TrimSuffix(prefix, "/")
	}
	files, err := lister.FilesUnder(full)
	if err != nil {
		return nil, err
	}
	trim := p.Prefix + "/"
	out := make([]string, 0, len(files))
	for _, f := range files {
		out = append(out, strings.TrimPrefix(f, trim))
	}
	return out, nil
}

// MaterializeFile delegates to the underlying tree with the prefix reapplied.
func (p *PrefixTree) MaterializeFile(rel, dst string) error {
	mt, ok := p.Tree.(interface {
		MaterializeFile(rel, dst string) error
	})
	if !ok {
		return os.ErrNotExist
	}
	return mt.MaterializeFile(p.Prefix+"/"+rel, dst)
}