package delivery

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"path"
	"sort"
	"strings"
)

// NamedTreePrefix opens the tree input of a module whose every artifact names its paths, so such a value is never read
// as a git tree id and a reader of a kept Tree can tell the two apart.
const NamedTreePrefix = "named:"

// NamedTree is the tree input of the new-form fingerprint of a module whose every artifact names the paths it is built
// from (novox/hq ADR 0499 clause 2): `named:` and the hex sha256 of the lines "manifest <digest>" and, sorted,
// "path <path> <object>", joined by a newline. manifest is the module's ManifestDigest (the sha256 of its module.json's
// exact bytes, as Module.Manifest and the merge preview carry it); paths are the named paths, each clean and relative to
// the module's directory as the manifest names them (catalogue.Manifest.NamedPaths), with the git object id each has at
// the commit (a blob for a file, a tree for a directory).
//
// **One function computes it** (clause 2): the indexer, its merge preview (buildstep.ModuleTrees) and the build modules
// call this one, and no second computation exists. It refuses no path, a named path with no object (absent at the
// commit), a path that is not clean, and a manifest that is not a digest, naming them: the indexer then gives the
// module none, with that reason (clause 3).
func NamedTree(manifest string, paths map[string]string) (string, error) {
	if _, err := ManifestsKey(manifest); err != nil {
		return "", err
	}
	if len(paths) == 0 {
		return "", fmt.Errorf("delivery: the module names no path, so it has no named tree")
	}
	lines := make([]string, 0, len(paths))
	for p, object := range paths {
		if p == "" || p != path.Clean(p) || strings.HasPrefix(p, "/") || p == ".." || strings.HasPrefix(p, "../") {
			return "", fmt.Errorf("delivery: the named path %q is not a clean path inside the module's directory", p)
		}
		if object == "" {
			return "", fmt.Errorf("delivery: the named path %q has no git object at the commit", p)
		}
		lines = append(lines, "path "+p+" "+object)
	}
	sort.Strings(lines)
	sum := sha256.Sum256([]byte("manifest " + manifest + "\n" + strings.Join(lines, "\n")))
	return NamedTreePrefix + hex.EncodeToString(sum[:]), nil
}