// Package gitsource clones external git sources in-process via go-git and
// exposes their commit trees for discovery — no git subprocess at runtime.
package gitsource

import (
	"fmt"
	"io"
	"os"
	"sort"
	"strings"

	"github.com/go-git/go-billy/v5/memfs"
	"github.com/go-git/go-git/v5"
	"github.com/go-git/go-git/v5/plumbing"
	"github.com/go-git/go-git/v5/plumbing/filemode"
	"github.com/go-git/go-git/v5/plumbing/object"
	"github.com/go-git/go-git/v5/plumbing/transport/http"
	"github.com/go-git/go-git/v5/storage/memory"

	"github.com/m3tam3re/agent-lib/internal/discovery"
)

// Env names for optional basic auth on HTTPS sources (deploy/personal token).
const (
	TokenEnv = "AGENT_LIB_GIT_TOKEN"
	UserEnv  = "AGENT_LIB_GIT_USER"
)

// Repo is a cloned source pinned to one resolved revision.
type Repo struct {
	Repository *git.Repository
	Rev        string
	Ref        string
}

// Clone fetches url — at ref (branch or tag), otherwise the default branch —
// fully in-process and pins the resolved revision.
func Clone(url, ref string) (*Repo, error) {
	repo, err := git.Clone(memory.NewStorage(), memfs.New(), cloneOptions(url, ref))
	if err != nil && ref != "" {
		repo, err = git.Clone(memory.NewStorage(), memfs.New(), tagOptions(url, ref))
	}
	if err != nil {
		return nil, fmt.Errorf("cloning %s: %w", url, err)
	}
	return resolveHead(repo, ref)
}

func cloneOptions(url, ref string) *git.CloneOptions {
	opts := &git.CloneOptions{URL: url}
	if ref != "" {
		opts.ReferenceName = plumbing.NewBranchReferenceName(ref)
	}
	applyAuth(url, opts)
	return opts
}

func tagOptions(url, ref string) *git.CloneOptions {
	opts := &git.CloneOptions{URL: url, ReferenceName: plumbing.NewTagReferenceName(ref)}
	applyAuth(url, opts)
	return opts
}

func applyAuth(url string, opts *git.CloneOptions) {
	if !strings.HasPrefix(url, "https://") {
		return
	}
	token := os.Getenv(TokenEnv)
	if token == "" {
		return
	}
	user := os.Getenv(UserEnv)
	if user == "" {
		user = "agent-lib"
	}
	opts.Auth = &http.BasicAuth{Username: user, Password: token}
}

func resolveHead(repo *git.Repository, requestedRef string) (*Repo, error) {
	head, err := repo.Head()
	if err != nil {
		return nil, fmt.Errorf("resolving HEAD: %w", err)
	}
	r := &Repo{Repository: repo, Rev: head.Hash().String()}
	switch {
	case head.Name().IsBranch() || head.Name().IsTag():
		r.Ref = head.Name().Short()
	case requestedRef != "":
		r.Ref = requestedRef
	default:
		r.Ref = head.Name().String()
	}
	return r, nil
}

// Tree returns the commit tree of the pinned revision as a discovery.Tree.
func (r *Repo) Tree() (discovery.Tree, error) {
	commit, err := r.Repository.CommitObject(plumbing.NewHash(r.Rev))
	if err != nil {
		return nil, fmt.Errorf("resolving commit %s: %w", r.Rev, err)
	}
	tree, err := commit.Tree()
	if err != nil {
		return nil, fmt.Errorf("reading tree of %s: %w", r.Rev, err)
	}
	return &GitTree{tree: tree}, nil
}

// GitTree adapts a git commit tree to the discovery.Tree interface.
type GitTree struct {
	tree *object.Tree
}

func (g *GitTree) Walk(fn func(rel string, isDir bool) error) error {
	walker := object.NewTreeWalker(g.tree, true, nil)
	defer walker.Close()
	for {
		name, entry, err := walker.Next()
		if err == io.EOF {
			return nil
		}
		if err != nil {
			return err
		}
		if name == "." {
			continue
		}
		if err := fn(name, entry.Mode == filemode.Dir); err != nil {
			return err
		}
	}
}

func (g *GitTree) ReadFile(rel string) ([]byte, error) {
	f, err := g.tree.File(rel)
	if err != nil {
		return nil, err
	}
	r, err := f.Blob.Reader()
	if err != nil {
		return nil, err
	}
	defer r.Close()
	return io.ReadAll(r)
}

// FilesUnder lists every file (recursively) below prefix, sorted. A prefix of
// "" lists the whole tree.
func (g *GitTree) FilesUnder(prefix string) ([]string, error) {
	var out []string
	walker := object.NewTreeWalker(g.tree, true, nil)
	defer walker.Close()
	for {
		name, entry, err := walker.Next()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, err
		}
		if name == "." || entry.Mode == filemode.Dir {
			continue
		}
		if prefix == "" || strings.HasPrefix(name, strings.TrimSuffix(prefix, "/")+"/") {
			out = append(out, name)
		}
	}
	sort.Strings(out)
	return out, nil
}

// MaterializeFile writes the blob at rel to dst, preserving the executable
// bit from the tree entry.
func (g *GitTree) MaterializeFile(rel, dst string) error {
	data, err := g.ReadFile(rel)
	if err != nil {
		return fmt.Errorf("reading %s: %w", rel, err)
	}
	mode := os.FileMode(0o644)
	entry, err := g.tree.FindEntry(rel)
	if err == nil && entry.Mode == filemode.Executable {
		mode = 0o755
	}
	return os.WriteFile(dst, data, mode)
}