package workspace

import (
	"fmt"
	"os"
	"path"
	"sync"
	"syscall"
)

// isWorkspace returns whether a given string is a valid path pointing to a
// Bazel workspace directory.
func isWorkspace(dir string) bool {
	w := path.Join(dir, "WORKSPACE")
	if _, err := os.Stat(w); err == nil {
		return true
	}
	return false
}

// getPathFSID returns an opaque filesystem identifier for a given path.
func getPathFSID(dir string) (uint64, error) {
	st, err := os.Stat(dir)
	if err != nil {
		// No need to wrap err, as stat errors are already quite explicit
		// (they also include the stat'd path).
		return 0, err
	}
	switch x := st.Sys().(type) {
	case *syscall.Stat_t:
		return x.Dev, nil
	default:
		return 0, fmt.Errorf("unsupported operating system (got stat type %+v)", st.Sys())
	}
}

// lookForWorkspace recurses up a directory until it finds a WORKSPACE, the
// root, or crosses a filesystem boundary.
func lookForWorkspace(dir string) (string, error) {
	fsid, err := getPathFSID(dir)
	if err != nil {
		return "", fmt.Errorf("could not get initial FSID: %w", err)
	}
	for {
		if dir == "." || dir == "/" || dir == "" {
			return "", fmt.Errorf("got up to root before finding workspace")
		}

		fsid2, err := getPathFSID(dir)
		if err != nil {
			return "", fmt.Errorf("could not get parent FWID: %w", err)
		}
		if fsid2 != fsid {
			return "", fmt.Errorf("crossed filesystem boundaries before finding workspace")
		}

		if isWorkspace(dir) {
			return dir, nil
		}
		dir = path.Dir(dir)
	}
}

// Get returns the workspace directory from which a given
// command line tool is running. This handles the following
// cases:
//
// 1. The command line tool was invoked via `bazel run`.
// 2. The command line tool was started in the workspace directory or a
//    subdirectory.
//
// If the workspace directory path cannot be inferred based on the above
// assumptions, an error is returned.
func Get() (string, error) {
	workspaceOnce.Do(func() {
		workspace, workspaceErr = get()
	})
	return workspace, workspaceErr
}

var (
	workspace     string
	workspaceErr  error
	workspaceOnce sync.Once
)

func get() (string, error) {
	if p := os.Getenv("BUILD_WORKSPACE_DIRECTORY"); p != "" && isWorkspace(p) {
		return p, nil
	}

	wd, err := os.Getwd()
	if err != nil {
		return "", err
	}
	p, err := lookForWorkspace(wd)
	if err != nil {
		return "", fmt.Errorf("not invoked from `bazel run` and could not find workspace root by traversing upwards: %w", err)
	}
	return p, nil
}