package execenv

import (
	"archive/tar"
	"compress/gzip"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strings"
	"time"
)

// PythonBin 返回托管解释器。未安装时 ok=false。
func (r *Root) PythonBin() (string, bool) {
	bin := filepath.Join(r.PythonDir(), "current", "bin", "python3")
	st, err := os.Stat(bin)
	if err != nil || st.IsDir() {
		return "", false
	}
	return bin, true
}

// PythonPrefix 是需要在沙箱里重新挂载的前缀（current 的真实目录）。
func (r *Root) PythonPrefix() string {
	dest, err := filepath.EvalSymlinks(filepath.Join(r.PythonDir(), "current"))
	if err != nil {
		return ""
	}
	return dest
}

// LinkPython 把已有解释器收成托管前缀：python/host/bin/python3 -> 绝对路径。
// 这是下载独立 CPython 失败时的退化，仍然全系统只有一份，不是每个工作区一份。
func (r *Root) LinkPython(bin string) error {
	if !filepath.IsAbs(bin) {
		return fmt.Errorf("execenv: python bin must be absolute: %s", bin)
	}
	if _, err := os.Stat(bin); err != nil {
		return err
	}
	host := filepath.Join(r.PythonDir(), "host")
	bindir := filepath.Join(host, "bin")
	if err := os.MkdirAll(bindir, 0o755); err != nil {
		return err
	}
	link := filepath.Join(bindir, "python3")
	_ = os.Remove(link)
	if err := os.Symlink(bin, link); err != nil {
		return err
	}
	if err := os.WriteFile(filepath.Join(r.PythonDir(), "SOURCE"), []byte("link "+bin+"\n"), 0o644); err != nil {
		return err
	}
	return r.switchPython("host")
}

func (r *Root) switchPython(name string) error {
	link := filepath.Join(r.PythonDir(), "current")
	tmp := link + ".tmp"
	_ = os.Remove(tmp)
	if err := os.Symlink(name, tmp); err != nil {
		return err
	}
	return os.Rename(tmp, link)
}

// InstallStandalone 下载 python-build-standalone 的 install_only 包并解到 python/。
// url 为空时向 GitHub latest 要与本机架构匹配的 cpython 3.14 资产。
func (r *Root) InstallStandalone(ctx context.Context, url string) error {
	if url == "" {
		var err error
		url, err = latestStandaloneURL(ctx)
		if err != nil {
			return err
		}
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return err
	}
	client := &http.Client{Timeout: 5 * time.Minute}
	resp, err := client.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode != 200 {
		return fmt.Errorf("execenv: download python: %s", resp.Status)
	}
	dest := filepath.Join(r.PythonDir(), "cpython-standalone")
	if err := os.RemoveAll(dest); err != nil {
		return err
	}
	if err := os.MkdirAll(dest, 0o755); err != nil {
		return err
	}
	if err := extractTarGz(resp.Body, dest); err != nil {
		return err
	}
	bin := filepath.Join(dest, "python", "bin", "python3")
	if _, err := os.Stat(bin); err != nil {
		return fmt.Errorf("execenv: standalone archive has no python/bin/python3")
	}
	// 让 current/bin/python3 成立：current -> cpython-standalone/python。
	inner := filepath.Join("cpython-standalone", "python")
	if err := os.WriteFile(filepath.Join(r.PythonDir(), "SOURCE"), []byte("standalone "+url+"\n"), 0o644); err != nil {
		return err
	}
	return r.switchPython(inner)
}

func latestStandaloneURL(ctx context.Context) (string, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet,
		"https://api.github.com/repos/astral-sh/python-build-standalone/releases/latest", nil)
	if err != nil {
		return "", err
	}
	req.Header.Set("Accept", "application/vnd.github+json")
	req.Header.Set("User-Agent", "agent-system-execenv")
	resp, err := (&http.Client{Timeout: 30 * time.Second}).Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()
	if resp.StatusCode != 200 {
		return "", fmt.Errorf("execenv: github releases: %s", resp.Status)
	}
	var doc struct {
		Assets []struct {
			Name string `json:"name"`
			URL  string `json:"browser_download_url"`
		} `json:"assets"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&doc); err != nil {
		return "", err
	}
	arch := "x86_64"
	if runtime.GOARCH == "arm64" {
		arch = "aarch64"
	}
	needle := arch + "-unknown-linux-gnu-install_only.tar.gz"
	var fallback string
	for _, a := range doc.Assets {
		if !strings.Contains(a.Name, needle) || !strings.HasPrefix(a.Name, "cpython-3.14") {
			continue
		}
		if strings.Contains(a.Name, "freethreaded") {
			fallback = a.URL
			continue
		}
		return a.URL, nil
	}
	if fallback != "" {
		return fallback, nil
	}
	return "", errors.New("execenv: no cpython 3.14 install_only asset")
}

func extractTarGz(rd io.Reader, dest string) error {
	gz, err := gzip.NewReader(rd)
	if err != nil {
		return err
	}
	defer gz.Close()
	tr := tar.NewReader(gz)
	for {
		hdr, err := tr.Next()
		if errors.Is(err, io.EOF) {
			return nil
		}
		if err != nil {
			return err
		}
		name := filepath.Clean(hdr.Name)
		if name == "." || strings.HasPrefix(name, "..") || filepath.IsAbs(name) {
			return fmt.Errorf("execenv: tar path escapes: %s", hdr.Name)
		}
		target := filepath.Join(dest, name)
		if !strings.HasPrefix(target, filepath.Clean(dest)+string(os.PathSeparator)) && target != filepath.Clean(dest) {
			return fmt.Errorf("execenv: tar path escapes: %s", hdr.Name)
		}
		switch hdr.Typeflag {
		case tar.TypeDir:
			if err := os.MkdirAll(target, 0o755); err != nil {
				return err
			}
		case tar.TypeReg:
			if err := os.MkdirAll(filepath.Dir(target), 0o755); err != nil {
				return err
			}
			mode := os.FileMode(hdr.Mode) & 0o777
			if mode == 0 {
				mode = 0o644
			}
			f, err := os.OpenFile(target, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, mode)
			if err != nil {
				return err
			}
			_, cerr := io.Copy(f, tr)
			f.Close()
			if cerr != nil {
				return cerr
			}
		case tar.TypeSymlink:
			if err := os.MkdirAll(filepath.Dir(target), 0o755); err != nil {
				return err
			}
			_ = os.Remove(target)
			if err := os.Symlink(hdr.Linkname, target); err != nil {
				return err
			}
		}
	}
}

// BootstrapVenv 用系统 python3 做一份唯一的托管前缀。解释器仍指向系统 python，
// 但 pip 在这份前缀里，构建器可以下载 wheel。
func (r *Root) BootstrapVenv(python string) error {
	if python == "" {
		python = "/usr/bin/python3"
	}
	dest := filepath.Join(r.PythonDir(), "venv")
	if _, err := os.Stat(filepath.Join(dest, "bin", "python3")); err != nil {
		cmd := exec.Command(python, "-m", "venv", dest)
		out, err := cmd.CombinedOutput()
		if err != nil {
			return fmt.Errorf("execenv: venv: %v: %s", err, out)
		}
	}
	if err := os.WriteFile(filepath.Join(r.PythonDir(), "SOURCE"), []byte("venv "+python+"\n"), 0o644); err != nil {
		return err
	}
	return r.switchPython("venv")
}