package buildstep

import (
	"fmt"
	"os"
	"path"
	"path/filepath"
	"strings"
)

// narrowKept are the files of a module's directory a narrowed build keeps beside its named paths: its manifest and
// its lock files, each an input of the module's key (ModuleTree's Manifest and Locks). package.json and go.mod are
// not kept: neither is in the named tree, so a build that needs one names it among its artifact's paths, which puts
// it in the key (novox/hq ADR 0499 clause 4).
var narrowKept = append([]string{"module.json"}, lockFiles...)

// Narrow leaves in the fetched tree only what a build of an artifact that names its paths may see (novox/hq ADR 0499
// clause 4): r.Dir's named paths, its manifest and lock files, and .git; everything else is removed, so a file outside
// them cannot change what is built without changing its key. A named path absent at the tree is an error naming it,
// and nothing is removed then. With no paths in r it changes nothing. Both build modules' fetch steps call it.
func Narrow(tree string, r FetchRequest) error {
	if len(r.Paths) == 0 {
		return nil
	}
	keep := map[string]bool{".git": true}
	for _, p := range r.Paths {
		rel := path.Join(r.Dir, p)
		if strings.HasPrefix(rel, "../") || rel == ".." || path.IsAbs(p) {
			return fmt.Errorf("the named path %q leaves the module's directory", p)
		}
		if _, err := os.Lstat(filepath.Join(tree, filepath.FromSlash(rel))); err != nil {
			return fmt.Errorf("the named path %q is not in the fetched tree: %w", p, err)
		}
		keep[rel] = true
	}
	for _, f := range narrowKept {
		keep[path.Join(r.Dir, f)] = true
	}
	return prune(tree, "", keep)
}

// prune removes from dir (rel within the tree) every entry that is neither kept nor on the way to one.
func prune(root, rel string, keep map[string]bool) error {
	entries, err := os.ReadDir(filepath.Join(root, filepath.FromSlash(rel)))
	if err != nil {
		return err
	}
	for _, e := range entries {
		p := path.Join(rel, e.Name())
		if keep[p] {
			continue
		}
		ancestor := false
		for k := range keep {
			if strings.HasPrefix(k, p+"/") {
				ancestor = true
				break
			}
		}
		if ancestor && e.IsDir() {
			if err := prune(root, p, keep); err != nil {
				return err
			}
			continue
		}
		if err := os.RemoveAll(filepath.Join(root, filepath.FromSlash(p))); err != nil {
			return err
		}
	}
	return nil
}