// Package git resolves and inspects a repository's authoritative Git remote.
package git

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"

	"gitea.maximumdirect.net/eric/ReleaseKit/internal/process"
)

var (
	ErrInvalidRemote         = errors.New("invalid Git remote")
	ErrMultiplePushURLs      = errors.New("Git remote has multiple push URLs")
	ErrEndpointChanged       = errors.New("Git publication endpoint changed")
	ErrRemoteMainMissing     = errors.New("authoritative remote has no main branch")
	ErrRemoteMainConflict    = errors.New("authoritative main does not identify the candidate commit")
	ErrRemoteTagConflict     = errors.New("version tag already exists on the authoritative remote")
	ErrRemoteStateUnreadable = errors.New("authoritative remote state is unreadable")
)

// Endpoint identifies the single effective publication destination captured
// from a selected remote. ScopedName is a transient remote name used with
// command-scoped configuration; it is never written to the repository.
type Endpoint struct {
	RemoteName string
	PushURL    string
	ScopedName string
	transport  []string
}

// RefState records whether an exact remote ref was found by a successful query.
type RefState uint8

const (
	RefUnreadable RefState = iota
	RefAbsent
	RefPresent
	RefConflict
)

// RemoteRef holds an exact ref's object and optional peeled commit. State is
// RefConflict only when more than one exact result or malformed data is found.
type RemoteRef struct {
	State  RefState
	Object string
	Peeled string
}

// Snapshot is the result of querying refs/heads/main and one exact version tag.
type Snapshot struct {
	Main RemoteRef
	Tag  RemoteRef
}

// Resolve captures the selected remote's effective push URL and establishes a
// command-scoped remote whose read and push URLs resolve to that same endpoint.
func Resolve(ctx context.Context, repositoryDir, remoteName string) (Endpoint, error) {
	if strings.TrimSpace(remoteName) == "" || strings.HasPrefix(remoteName, "-") {
		return Endpoint{}, fmt.Errorf("%w: %q", ErrInvalidRemote, remoteName)
	}
	urls, err := pushURLs(ctx, repositoryDir, remoteName)
	if err != nil {
		return Endpoint{}, err
	}
	if len(urls) != 1 {
		return Endpoint{}, fmt.Errorf("%w: %s resolves to %d distinct destinations", ErrMultiplePushURLs, remoteName, len(urls))
	}
	scopedName, err := newScopedName()
	if err != nil {
		return Endpoint{}, fmt.Errorf("create command-scoped remote name: %w", err)
	}
	endpoint := Endpoint{RemoteName: remoteName, PushURL: urls[0], ScopedName: scopedName}
	endpoint.transport, err = transportConfig(ctx, repositoryDir, remoteName, scopedName)
	if err != nil {
		return Endpoint{}, err
	}
	if err := endpoint.verifyScopedURLs(ctx, repositoryDir); err != nil {
		return Endpoint{}, err
	}
	return endpoint, nil
}

// Recheck confirms both the selected remote's effective push destination and
// the scoped remote's effective read and push URLs still identify this endpoint.
func (endpoint Endpoint) Recheck(ctx context.Context, repositoryDir string) error {
	urls, err := pushURLs(ctx, repositoryDir, endpoint.RemoteName)
	if err != nil {
		return fmt.Errorf("re-resolve remote %q: %w", endpoint.RemoteName, err)
	}
	if len(urls) != 1 || urls[0] != endpoint.PushURL {
		return fmt.Errorf("%w: remote %q now resolves to %q", ErrEndpointChanged, endpoint.RemoteName, strings.Join(urls, ", "))
	}
	return endpoint.verifyScopedURLs(ctx, repositoryDir)
}

// Query reads exact main and version-tag refs. A command failure is unreadable,
// never evidence that a ref is absent.
func (endpoint Endpoint) Query(ctx context.Context, repositoryDir, version string) (Snapshot, error) {
	if !validVersionRef(version) {
		return Snapshot{}, fmt.Errorf("invalid version tag name %q", version)
	}
	mainRef := "refs/heads/main"
	tagRef := "refs/tags/" + version
	peeledRef := tagRef + "^{}"
	output, err := run(ctx, repositoryDir, endpoint.scopedConfig(), "ls-remote", endpoint.ScopedName, mainRef, tagRef, peeledRef)
	if err != nil {
		snapshot := Snapshot{Main: RemoteRef{State: RefUnreadable}, Tag: RemoteRef{State: RefUnreadable}}
		return snapshot, fmt.Errorf("%w: query authoritative refs at %s: %w", ErrRemoteStateUnreadable, endpoint.PushURL, err)
	}
	main := RemoteRef{State: RefAbsent}
	tag := RemoteRef{State: RefAbsent}
	for _, line := range strings.Split(strings.TrimSuffix(output, "\n"), "\n") {
		if line == "" {
			continue
		}
		object, name, ok := strings.Cut(line, "\t")
		if !ok || !validObjectID(object) {
			return Snapshot{Main: RemoteRef{State: RefUnreadable}, Tag: RemoteRef{State: RefUnreadable}}, fmt.Errorf("%w: malformed ls-remote output %q", ErrRemoteStateUnreadable, line)
		}
		switch name {
		case mainRef:
			main = mergeRef(main, object, false)
		case tagRef:
			tag = mergeRef(tag, object, false)
		case peeledRef:
			tag = mergeRef(tag, object, true)
		}
	}
	return Snapshot{Main: main, Tag: tag}, nil
}

// RequireCandidate requires exact main equality and an absent remote version
// tag. It deliberately does not accept a main branch that merely contains the
// candidate as an ancestor.
func (endpoint Endpoint) RequireCandidate(ctx context.Context, repositoryDir, version, commit string) (Snapshot, error) {
	snapshot, err := endpoint.Query(ctx, repositoryDir, version)
	if err != nil {
		return snapshot, err
	}
	if snapshot.Main.State == RefAbsent {
		return snapshot, ErrRemoteMainMissing
	}
	if snapshot.Main.State != RefPresent || snapshot.Main.Object != commit {
		snapshot.Main.State = RefConflict
		return snapshot, fmt.Errorf("%w: candidate %s, remote main %s", ErrRemoteMainConflict, commit, snapshot.Main.Object)
	}
	if snapshot.Tag.State != RefAbsent {
		if snapshot.Tag.State == RefUnreadable {
			return snapshot, ErrRemoteStateUnreadable
		}
		snapshot.Tag.State = RefConflict
		return snapshot, fmt.Errorf("%w: %s", ErrRemoteTagConflict, snapshot.Tag.Object)
	}
	return snapshot, nil
}

func (endpoint Endpoint) verifyScopedURLs(ctx context.Context, repositoryDir string) error {
	readURL, err := run(ctx, repositoryDir, endpoint.scopedConfig(), "ls-remote", "--get-url", endpoint.ScopedName)
	if err != nil {
		return fmt.Errorf("resolve scoped read URL: %w", err)
	}
	resolved := outputLines(readURL)
	if len(resolved) != 1 || resolved[0] != endpoint.PushURL {
		return fmt.Errorf("%w: scoped URL resolves to %q, expected %q", ErrEndpointChanged, resolved, endpoint.PushURL)
	}
	// The scoped remote sets its push URL explicitly to the same captured value.
	// Git applies the same insteadOf rules to both explicit URLs; pushInsteadOf
	// applies only when no push URL exists and therefore cannot change this one.
	return nil
}

func (endpoint Endpoint) scopedConfig() []string {
	config := []string{
		"remote." + endpoint.ScopedName + ".url=" + endpoint.PushURL,
		"remote." + endpoint.ScopedName + ".pushurl=" + endpoint.PushURL,
		"remote." + endpoint.ScopedName + ".mirror=false",
	}
	// These options affect Git transport behavior and are carried into the
	// scoped remote without copying refspecs, mirror mode, or push destinations.
	config = append(config, endpoint.transport...)
	return config
}

// transportConfig evaluates Git's normal configuration, including includes,
// then copies transport settings under the transient remote's name.
func transportConfig(ctx context.Context, repositoryDir, remoteName, scopedName string) ([]string, error) {
	var config []string
	for _, key := range []string{"proxy", "uploadpack", "receivepack", "vcs", "serveroption"} {
		output, err := run(ctx, repositoryDir, nil, "config", "--get-all", "remote."+remoteName+"."+key)
		if err != nil {
			// git config exits 1 when the key is absent; that is expected.
			var exitErr interface{ ExitCode() int }
			if errors.As(err, &exitErr) && exitErr.ExitCode() == 1 {
				continue
			}
			return nil, fmt.Errorf("read transport setting remote.%s.%s: %w", remoteName, key, err)
		}
		for _, value := range outputLines(output) {
			config = append(config, "remote."+scopedName+"."+key+"="+value)
		}
	}
	return config, nil
}

func pushURLs(ctx context.Context, repositoryDir, remoteName string) ([]string, error) {
	output, err := run(ctx, repositoryDir, nil, "remote", "get-url", "--push", "--all", remoteName)
	if err != nil {
		return nil, fmt.Errorf("resolve push URL for remote %q: %w", remoteName, err)
	}
	values := outputLines(output)
	unique := make([]string, 0, len(values))
	seen := make(map[string]struct{}, len(values))
	for _, value := range values {
		if _, ok := seen[value]; ok {
			continue
		}
		seen[value] = struct{}{}
		unique = append(unique, value)
	}
	return unique, nil
}

func run(ctx context.Context, dir string, config []string, args ...string) (string, error) {
	commandArgs := make([]string, 0, len(config)*2+len(args))
	for _, value := range config {
		commandArgs = append(commandArgs, "-c", value)
	}
	commandArgs = append(commandArgs, args...)
	var stdout, stderr strings.Builder
	err := process.Run(ctx, process.Invocation{
		Args:   append([]string{"git"}, commandArgs...),
		Dir:    dir,
		Stdout: &stdout,
		Stderr: &stderr,
	})
	if err != nil {
		message := strings.TrimSpace(stderr.String())
		if message != "" {
			return stdout.String(), fmt.Errorf("%w: %s", err, message)
		}
		return stdout.String(), err
	}
	return stdout.String(), nil
}

func newScopedName() (string, error) {
	var bytes [12]byte
	if _, err := rand.Read(bytes[:]); err != nil {
		return "", err
	}
	return "releasekit-" + hex.EncodeToString(bytes[:]), nil
}

func outputLines(output string) []string {
	output = strings.TrimSuffix(output, "\n")
	if output == "" {
		return nil
	}
	lines := strings.Split(output, "\n")
	for i := range lines {
		lines[i] = strings.TrimSuffix(lines[i], "\r")
	}
	return lines
}

func mergeRef(ref RemoteRef, object string, peeled bool) RemoteRef {
	if ref.State == RefConflict {
		return ref
	}
	if peeled {
		if ref.State == RefAbsent {
			ref.State = RefPresent
			ref.Peeled = object
			return ref
		}
		if ref.Peeled != "" {
			return RemoteRef{State: RefConflict, Object: ref.Object}
		}
		ref.Peeled = object
		return ref
	}
	if ref.State == RefAbsent {
		ref.State, ref.Object = RefPresent, object
		return ref
	}
	if ref.Object != "" {
		return RemoteRef{State: RefConflict, Object: ref.Object}
	}
	ref.Object = object
	return ref
}

func validVersionRef(version string) bool {
	if len(version) < 6 || !strings.HasPrefix(version, "v") {
		return false
	}
	parts := strings.Split(version[1:], ".")
	if len(parts) != 3 {
		return false
	}
	for _, part := range parts {
		if part == "" || (len(part) > 1 && part[0] == '0') {
			return false
		}
		for _, char := range part {
			if char < '0' || char > '9' {
				return false
			}
		}
	}
	return true
}

func validObjectID(object string) bool {
	if len(object) != 40 && len(object) != 64 {
		return false
	}
	for _, char := range object {
		if (char < '0' || char > '9') && (char < 'a' || char > 'f') && (char < 'A' || char > 'F') {
			return false
		}
	}
	return true
}