// Package lock reads and writes .loom/externals/.locks.
//
// One record per adopted document: where it was fetched from, resolved, and the
// ETag the publisher served with it. The ETag is opaque and is never a hash we
// compute — on gitea it happens to equal the git blob hash and on GitHub it does
// not, so a design that compares a local hash to a remote ETag works on exactly
// one host by coincidence. See .loom/event-log.md.
package lock

import (
	"bufio"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"strings"
)

// Dir is where adopted documents live, relative to the repository root.
const Dir = ".loom/externals"

// File is the lock file, inside Dir.
const File = ".locks"

const header = "# loomctl locks — one record per adopted document.\n" +
	"# path<TAB>url<TAB>etag   The url is resolved: a short form would follow\n" +
	"# whatever the default branch is at the time you ask.\n"

// Record is one adopted document.
type Record struct {
	// Path is relative to Dir, and is for a person to read. The origin is the
	// URL: the path does not round-trip, because it drops the route, the
	// branch, and .loom/published/.
	Path string
	// URL is the resolved origin, branch and all.
	URL string
	// ETag is the publisher's, verbatim, including its quotes.
	ETag string
}

// Set is every lock, keyed by path.
type Set struct {
	root string
	recs map[string]Record
}

func path(root string) string { return filepath.Join(root, Dir, File) }

// Load reads the lock file for the repository at root. A missing file is an
// empty set, not an error: a repository whose externals were fetched by hand has
// no locks, and reporting that is the point.
func Load(root string) (*Set, error) {
	s := &Set{root: root, recs: map[string]Record{}}
	f, err := os.Open(path(root))
	if errors.Is(err, fs.ErrNotExist) {
		return s, nil
	}
	if err != nil {
		return nil, err
	}
	defer f.Close()

	sc := bufio.NewScanner(f)
	for n := 1; sc.Scan(); n++ {
		line := sc.Text()
		if strings.TrimSpace(line) == "" || strings.HasPrefix(line, "#") {
			continue
		}
		parts := strings.Split(line, "\t")
		if len(parts) != 3 {
			return nil, fmt.Errorf("%s:%d: want 3 tab-separated fields, got %d", path(root), n, len(parts))
		}
		s.recs[parts[0]] = Record{Path: parts[0], URL: parts[1], ETag: parts[2]}
	}
	return s, sc.Err()
}

// Get returns the record for a path, and whether there was one.
func (s *Set) Get(p string) (Record, bool) { r, ok := s.recs[p]; return r, ok }

// Put adds or replaces a record.
func (s *Set) Put(r Record) { s.recs[r.Path] = r }

// All returns every record, ordered by path so the file diffs cleanly.
func (s *Set) All() []Record {
	out := make([]Record, 0, len(s.recs))
	for _, r := range s.recs {
		out = append(out, r)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Path < out[j].Path })
	return out
}

// Save writes the lock file, replacing it atomically so an interrupted write
// cannot leave a repository holding half a lock.
func (s *Set) Save() error {
	p := path(s.root)
	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
		return err
	}
	var b strings.Builder
	b.WriteString(header)
	for _, r := range s.All() {
		fmt.Fprintf(&b, "%s\t%s\t%s\n", r.Path, r.URL, r.ETag)
	}
	tmp, err := os.CreateTemp(filepath.Dir(p), ".locks-*")
	if err != nil {
		return err
	}
	if _, err := tmp.WriteString(b.String()); err != nil {
		tmp.Close()
		os.Remove(tmp.Name())
		return err
	}
	if err := tmp.Close(); err != nil {
		os.Remove(tmp.Name())
		return err
	}
	return os.Rename(tmp.Name(), p)
}