package furgit

import (
	"bufio"
	"bytes"
	"fmt"
	"os"
	"path"
	"path/filepath"
	"slices"
	"strings"
)

func (repo *Repository) resolveLooseRef(refname string) (Ref, error) {
	data, err := os.ReadFile(repo.repoPath(refname))
	if err != nil {
		if os.IsNotExist(err) {
			return Ref{}, ErrNotFound
		}
		return Ref{}, err
	}
	line := strings.TrimSpace(string(data))

	if strings.HasPrefix(line, "ref: ") {
		target := strings.TrimSpace(line[5:])
		if target == "" {
			return Ref{Name: refname, Kind: RefKindInvalid}, ErrInvalidRef
		}
		return Ref{
			Name: refname,
			Kind: RefKindSymbolic,
			Ref:  target,
		}, nil
	}

	id, err := repo.ParseHash(line)
	if err != nil {
		return Ref{Name: refname, Kind: RefKindInvalid}, err
	}
	return Ref{
		Name: refname,
		Kind: RefKindDetached,
		Hash: id,
	}, nil
}

func (repo *Repository) resolvePackedRef(refname string) (Ref, error) {
	// According to git-pack-refs(1), symbolic refs are never
	// stored in packed-refs, so we only need to look for detached
	// refs here.

	path := repo.repoPath("packed-refs")
	f, err := os.Open(path)
	if err != nil {
		if os.IsNotExist(err) {
			return Ref{}, ErrNotFound
		}
		return Ref{}, err
	}
	defer func() { _ = f.Close() }()

	want := []byte(refname)
	scanner := bufio.NewScanner(f)

	for scanner.Scan() {
		line := scanner.Bytes()

		if len(line) == 0 || line[0] == '#' || line[0] == '^' {
			continue
		}

		sp := bytes.IndexByte(line, ' ')
		if sp != repo.hashAlgo.size()*2 {
			continue
		}

		name := line[sp+1:]

		if !bytes.Equal(name, want) {
			continue
		}

		hex := string(line[:sp])
		id, err := repo.ParseHash(hex)
		if err != nil {
			return Ref{Name: refname, Kind: RefKindInvalid}, err
		}

		ref := Ref{
			Name: refname,
			Kind: RefKindDetached,
			Hash: id,
		}

		if scanner.Scan() {
			next := scanner.Bytes()
			if len(next) > 0 && next[0] == '^' {
				peeledHex := strings.TrimPrefix(string(next), "^")
				peeledHex = strings.TrimSpace(peeledHex)

				peeledID, err := repo.ParseHash(peeledHex)
				if err != nil {
					return Ref{Name: refname, Kind: RefKindInvalid}, err
				}
				ref.Peeled = peeledID
			}
		}

		if scanErr := scanner.Err(); scanErr != nil {
			return Ref{Name: refname, Kind: RefKindInvalid}, scanErr
		}

		return ref, nil
	}

	if scanErr := scanner.Err(); scanErr != nil {
		return Ref{Name: refname, Kind: RefKindInvalid}, scanErr
	}
	return Ref{}, ErrNotFound
}

// RefKind represents the kind of HEAD reference.
type RefKind int

const (
	// The HEAD reference is invalid.
	RefKindInvalid RefKind = iota
	// The HEAD reference points to a detached commit hash.
	RefKindDetached
	// The HEAD reference points to a symbolic ref.
	RefKindSymbolic
)

// Ref represents a reference.
type Ref struct {
	// Name is the fully qualified ref name (e.g., refs/heads/main).
	// It may be empty for detached hashes that were not looked up
	// by name (e.g., ResolveRef on a raw hash).
	Name string
	// Kind is the kind of the reference.
	Kind RefKind
	// When Kind is RefKindSymbolic, Ref is the fully qualified ref name.
	// Otherwise the value is undefined.
	Ref string
	// When Kind is RefKindDetached, Hash is the commit hash.
	// Otherwise the value is undefined.
	Hash Hash
	// When Kind is RefKindDetached, and the ref supposedly points to an
	// annotated tag, Peeled is the peeled hash, i.e., the hash of the
	// object that the tag points to.
	Peeled Hash
}

type refParseRule struct {
	fmtStr string
	prefix string
	suffix string
}

func parseRule(rule string) refParseRule {
	prefix, suffix, _ := strings.Cut(rule, "%s")
	return refParseRule{
		fmtStr: rule,
		prefix: prefix,
		suffix: suffix,
	}
}

var refRevParseRules = []refParseRule{
	parseRule("%s"),
	parseRule("refs/%s"),
	parseRule("refs/tags/%s"),
	parseRule("refs/heads/%s"),
	parseRule("refs/remotes/%s"),
	parseRule("refs/remotes/%s/HEAD"),
}

func (rule refParseRule) match(name string) (string, bool) {
	if rule.suffix != "" {
		if !strings.HasSuffix(name, rule.suffix) {
			return "", false
		}
		name = strings.TrimSuffix(name, rule.suffix)
	}

	var short string
	n, err := fmt.Sscanf(name, rule.prefix+"%s", &short)
	if err != nil || n != 1 {
		return "", false
	}
	if fmt.Sprintf(rule.prefix+"%s", short) != name {
		return "", false
	}
	return short, true
}

func (rule refParseRule) render(short string) string {
	return rule.prefix + short + rule.suffix
}

// Short returns the shortest unambiguous shorthand for the ref name,
// following the rev-parse rules used by Git. The provided list of refs
// is used to test for ambiguity.
//
// When strict is true, all other rules must fail to resolve to an
// existing ref; otherwise only rules prior to the matched rule must
// fail.
func (ref *Ref) Short(all []Ref, strict bool) string {
	if ref == nil {
		return ""
	}
	name := ref.Name
	if name == "" {
		return ""
	}

	names := make(map[string]struct{}, len(all))
	for _, r := range all {
		if r.Name == "" {
			continue
		}
		names[r.Name] = struct{}{}
	}

	for i := len(refRevParseRules) - 1; i > 0; i-- {
		short, ok := refRevParseRules[i].match(name)
		if !ok {
			continue
		}

		rulesToFail := i
		if strict {
			rulesToFail = len(refRevParseRules)
		}

		ambiguous := false
		for j := 0; j < rulesToFail; j++ {
			if j == i {
				continue
			}
			full := refRevParseRules[j].render(short)
			if _, found := names[full]; found {
				ambiguous = true
				break
			}
		}

		if !ambiguous {
			return short
		}
	}

	return name
}

// ResolveRef reads the given fully qualified ref (such as "HEAD" or "refs/heads/main")
// and interprets its contents as either a symbolic ref ("ref: refs/..."), a detached
// hash, or invalid.
// If path is empty, it defaults to "HEAD".
// (While typically only HEAD may be a symbolic reference, others may be as well.)
func (repo *Repository) ResolveRef(path string) (Ref, error) {
	if path == "" {
		path = "HEAD"
	}

	if !strings.HasPrefix(path, "refs/") && !slices.Contains([]string{
		"HEAD", "ORIG_HEAD", "FETCH_HEAD", "MERGE_HEAD",
		"CHERRY_PICK_HEAD", "REVERT_HEAD", "REBASE_HEAD", "BISECT_HEAD",
	}, path) {
		id, err := repo.ParseHash(path)
		if err == nil {
			return Ref{
				Name: path,
				Kind: RefKindDetached,
				Hash: id,
			}, nil
		}

		// For now let's keep this to prevent e.g., random users from
		// specifying something crazy like objects/... or ./config.
		// There may be other legal pseudo-refs in the future,
		// but it's probably the best to stay cautious for now.
		return Ref{Name: path, Kind: RefKindInvalid}, ErrInvalidRef
	}

	loose, err := repo.resolveLooseRef(path)
	if err == nil {
		return loose, nil
	}
	if err != ErrNotFound {
		return Ref{Name: path, Kind: RefKindInvalid}, err
	}

	packed, err := repo.resolvePackedRef(path)
	if err == nil {
		return packed, nil
	}
	if err != ErrNotFound {
		return Ref{Name: path, Kind: RefKindInvalid}, err
	}

	return Ref{Name: path, Kind: RefKindInvalid}, ErrNotFound
}

// ResolveRefFully resolves a ref by recursively following
// symbolic references until it reaches a detached ref.
// Symbolic cycles are detected and reported.
// Annotated tags are not peeled.
func (repo *Repository) ResolveRefFully(path string) (Ref, error) {
	seen := make(map[string]struct{})
	return repo.resolveRefFully(path, seen)
}

func (repo *Repository) resolveRefFully(path string, seen map[string]struct{}) (Ref, error) {
	if _, found := seen[path]; found {
		return Ref{}, fmt.Errorf("symbolic ref cycle involving %q", path)
	}
	seen[path] = struct{}{}

	ref, err := repo.ResolveRef(path)
	if err != nil {
		return Ref{}, err
	}

	switch ref.Kind {
	case RefKindDetached:
		return ref, nil

	case RefKindSymbolic:
		if ref.Ref == "" {
			return Ref{}, ErrInvalidRef
		}
		return repo.resolveRefFully(ref.Ref, seen)

	default:
		return Ref{}, ErrInvalidRef
	}
}

// ListRefs lists refs similarly to git-show-ref.
//
// The pattern must be empty or begin with "refs/". An empty pattern is
// treated as "refs/*".
//
// Loose refs are resolved using filesystem globbing relative to the
// repository root, then packed refs are read while skipping any names
// that already appeared as loose refs. Packed refs are filtered
// similarly.
func (repo *Repository) ListRefs(pattern string) ([]Ref, error) {
	if pattern == "" {
		pattern = "refs/*"
	}
	if !strings.HasPrefix(pattern, "refs/") {
		return nil, ErrInvalidRef
	}
	if filepath.IsAbs(pattern) {
		return nil, ErrInvalidRef
	}

	var out []Ref
	seen := make(map[string]struct{})

	globPattern := filepath.Join(repo.rootPath, filepath.FromSlash(pattern))
	matches, err := filepath.Glob(globPattern)
	if err != nil {
		return nil, err
	}
	for _, match := range matches {
		info, statErr := os.Stat(match)
		if statErr != nil {
			return nil, statErr
		}
		if info.IsDir() {
			continue
		}

		rel, relErr := filepath.Rel(repo.rootPath, match)
		if relErr != nil {
			return nil, relErr
		}
		name := filepath.ToSlash(rel)
		if !strings.HasPrefix(name, "refs/") {
			continue
		}

		ref, resolveErr := repo.resolveLooseRef(name)
		if resolveErr != nil {
			if resolveErr == ErrNotFound || os.IsNotExist(resolveErr) {
				continue
			}
			return nil, resolveErr
		}

		seen[name] = struct{}{}
		out = append(out, ref)
	}

	packedPath := repo.repoPath("packed-refs")
	f, err := os.Open(packedPath)
	if err != nil {
		if os.IsNotExist(err) {
			return out, nil
		}
		return nil, err
	}
	defer func() { _ = f.Close() }()

	scanner := bufio.NewScanner(f)
	lastIdx := -1
	for scanner.Scan() {
		line := scanner.Bytes()
		if len(line) == 0 || line[0] == '#' {
			continue
		}

		if line[0] == '^' {
			if lastIdx < 0 {
				continue
			}
			peeledHex := strings.TrimPrefix(string(line), "^")
			peeledHex = strings.TrimSpace(peeledHex)
			peeled, parseErr := repo.ParseHash(peeledHex)
			if parseErr != nil {
				return nil, parseErr
			}
			out[lastIdx].Peeled = peeled
			continue
		}

		sp := bytes.IndexByte(line, ' ')
		if sp != repo.hashAlgo.size()*2 {
			lastIdx = -1
			continue
		}

		name := string(line[sp+1:])
		if !strings.HasPrefix(name, "refs/") {
			lastIdx = -1
			continue
		}
		if _, ok := seen[name]; ok {
			lastIdx = -1
			continue
		}

		match, matchErr := path.Match(pattern, name)
		if matchErr != nil {
			return nil, matchErr
		}
		if !match {
			lastIdx = -1
			continue
		}

		hash, parseErr := repo.ParseHash(string(line[:sp]))
		if parseErr != nil {
			return nil, parseErr
		}
		out = append(out, Ref{
			Name: name,
			Kind: RefKindDetached,
			Hash: hash,
		})
		lastIdx = len(out) - 1
	}
	if scanErr := scanner.Err(); scanErr != nil {
		return nil, scanErr
	}

	return out, nil
}