package skillpack

import (
	"context"
	"fmt"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"strings"
)

// Source is where a pack's files come from. Fetch retrieves the tree at ref and
// returns it together with the source's own resolved ref (a git commit SHA, or
// the content digest for a plain directory) — provenance a host can show and
// pin against. ref semantics are source-specific and may be empty ("the
// default": a dir's current contents, a repo's default branch).
type Source interface {
	Fetch(ctx context.Context, ref string) (Tree, string, error)
	// Kind is a short stable tag ("dir", "git") for persistence + display.
	Kind() string
	// String is a human-readable identifier (path or URL[/subpath]).
	String() string
}

// DirSource reads a pack from a local directory. ref is ignored (a directory
// has no versions); the resolved ref is the content digest. Useful for
// first-party/builtin packs shipped on disk and for tests.
type DirSource struct {
	Path string
}

func (d DirSource) Kind() string   { return "dir" }
func (d DirSource) String() string { return d.Path }

func (d DirSource) Fetch(_ context.Context, _ string) (Tree, string, error) {
	info, err := os.Stat(d.Path)
	if err != nil {
		return nil, "", fmt.Errorf("skillpack: dir source %q: %w", d.Path, err)
	}
	if !info.IsDir() {
		return nil, "", fmt.Errorf("skillpack: dir source %q is not a directory", d.Path)
	}
	t, err := readTree(os.DirFS(d.Path))
	if err != nil {
		return nil, "", err
	}
	return t, t.Digest(), nil
}

// GitSource fetches a pack from a git repository, optionally from a Subpath
// within it (for repos that publish several packs). ref is any git commit-ish
// (branch, tag, or SHA); empty means the default branch. The resolved ref is
// the checked-out commit SHA.
//
// Fetch clones into a temp dir, reads the subpath tree into memory, and removes
// the clone before returning — the returned Tree is self-contained, so there is
// no clone lifetime to manage and nothing left on disk. Git runs via the system
// `git`; GitRunner is overridable for tests.
type GitSource struct {
	URL     string
	Subpath string
	// GitRunner runs a git command in dir and returns combined output. Nil uses
	// the system git.
	GitRunner func(ctx context.Context, dir string, args ...string) ([]byte, error)
}

func (g GitSource) Kind() string { return "git" }

func (g GitSource) String() string {
	if g.Subpath != "" {
		return g.URL + "//" + g.Subpath
	}
	return g.URL
}

func (g GitSource) run(ctx context.Context, dir string, args ...string) ([]byte, error) {
	if g.GitRunner != nil {
		return g.GitRunner(ctx, dir, args...)
	}
	cmd := exec.CommandContext(ctx, "git", args...)
	cmd.Dir = dir
	out, err := cmd.CombinedOutput()
	if err != nil {
		return out, fmt.Errorf("skillpack: git %s: %w: %s", strings.Join(args, " "), err, strings.TrimSpace(string(out)))
	}
	return out, nil
}

func (g GitSource) Fetch(ctx context.Context, ref string) (Tree, string, error) {
	// Argument-injection guard: a URL or ref beginning with "-" would be parsed
	// by git as an option (e.g. --upload-pack=…), not a value. Reject it rather
	// than rely solely on the "--" separator, which checkout does not honor for
	// a rev. Hosts should also allow-list sources, but this is defense-in-depth
	// for a library.
	if strings.HasPrefix(g.URL, "-") {
		return nil, "", fmt.Errorf("skillpack: git url must not start with '-': %q", g.URL)
	}
	if strings.HasPrefix(ref, "-") {
		return nil, "", fmt.Errorf("skillpack: git ref must not start with '-': %q", ref)
	}

	tmp, err := os.MkdirTemp("", "skillpack-git-*")
	if err != nil {
		return nil, "", err
	}
	defer os.RemoveAll(tmp)

	// --filter=blob:none: a blobless partial clone gets the ref graph cheaply
	// and fetches only the blobs the checkout needs — much less than the full
	// history, while still supporting an arbitrary commit-ish ref. "--" ends
	// option parsing before the URL.
	if _, err := g.run(ctx, "", "clone", "--quiet", "--filter=blob:none", "--", g.URL, tmp); err != nil {
		return nil, "", err
	}
	if ref != "" {
		if _, err := g.run(ctx, tmp, "checkout", "--quiet", "--detach", ref); err != nil {
			return nil, "", err
		}
	}
	shaOut, err := g.run(ctx, tmp, "rev-parse", "HEAD")
	if err != nil {
		return nil, "", err
	}
	sha := strings.TrimSpace(string(shaOut))

	root := tmp
	if g.Subpath != "" {
		clean := path.Clean("/" + g.Subpath) // normalize, strip leading ../
		root = filepath.Join(tmp, filepath.FromSlash(strings.TrimPrefix(clean, "/")))
		if !within(tmp, root) {
			return nil, "", fmt.Errorf("skillpack: subpath %q escapes the repo", g.Subpath)
		}
		if info, err := os.Stat(root); err != nil || !info.IsDir() {
			return nil, "", fmt.Errorf("skillpack: subpath %q not found in %s", g.Subpath, g.URL)
		}
	}
	t, err := readTree(os.DirFS(root))
	if err != nil {
		return nil, "", err
	}
	// Drop a nested .git if the subpath was the repo root.
	for p := range t {
		if p == ".git" || strings.HasPrefix(p, ".git/") {
			delete(t, p)
		}
	}
	return t, sha, nil
}