// Command deps makes a module's dependencies available to the catalogue's check, or says why it cannot
// (novox/hq issue 385).
//
// A module's tests run only when its dependencies can be fetched. The build seat clones what a check
// needs with the forge credential and forgets that credential before the check runs, so a module pinning
// an SDK commit the seat's module cache does not hold cannot fetch it from the forge. The SDK is cloned
// beside the check all the same (MESH_CHECK_BESIDE, mesh-controller's besideRefs): every requirement of the
// mesh's own forge is taken from the clone of its repository there, at the commit or the tag its version
// names, through a go.mod of its own (-modfile) — the module's own files are not changed, and no forge
// credential is added.
//
// Usage: deps -beside <dir> -work <dir> <module dir>. It prints the go.mod to build the module with, or
// nothing when the module fetches what it needs; when it cannot, it prints why and exits 1, and the check
// fails: a module whose tests could not run is a failure, never a note.
package main

import (
	"archive/tar"
	"bytes"
	"encoding/json"
	"errors"
	"flag"
	"fmt"
	"io"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"strings"
)

// forge is the prefix of the Go modules of the mesh's own forge: <forge><repository>[/<directory>].
const forge = "git.novox.be/novox/"

func main() {
	beside := flag.String("beside", "", "where the repositories beside the check are cloned (MESH_CHECK_BESIDE)")
	work := flag.String("work", os.TempDir(), "where the go.mod and the sources taken from beside are written")
	flag.Parse()
	if flag.NArg() != 1 {
		fmt.Println("usage: deps -beside <dir> -work <dir> <module dir>")
		os.Exit(2)
	}
	modfile, err := resolve(flag.Arg(0), *beside, *work)
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	if modfile != "" {
		fmt.Println(modfile)
	}
}

// resolve is the go.mod the module at dir builds with: "" when its own fetches everything, else one
// beside it in work that takes every requirement of the mesh's forge from its clone beside the check.
func resolve(dir, beside, work string) (string, error) {
	out, err := goIn(dir, "mod", "download")
	if err == nil {
		return "", nil
	}
	fetching := lastLine(out)
	required, err := forgeRequirements(dir)
	if err != nil {
		return "", err
	}
	if len(required) == 0 {
		return "", fmt.Errorf("its dependencies cannot be fetched: %s", fetching)
	}
	if beside == "" {
		return "", fmt.Errorf("its dependencies cannot be fetched (%s), and nothing was cloned beside the check to "+
			"take %s from", fetching, repositories(required))
	}
	into, err := os.MkdirTemp(work, filepath.Base(dir)+"-")
	if err != nil {
		return "", err
	}
	modfile := filepath.Join(into, "go.mod")
	if err := copyFile(filepath.Join(dir, "go.mod"), modfile); err != nil {
		return "", err
	}
	if err := copyFile(filepath.Join(dir, "go.sum"), filepath.Join(into, "go.sum")); err != nil &&
		!errors.Is(err, os.ErrNotExist) {
		return "", err
	}
	for _, r := range required {
		source, err := export(beside, r, into)
		if err != nil {
			return "", fmt.Errorf("its dependencies cannot be fetched (%s), and %s@%s cannot be taken from the "+
				"clone beside the check: %w", fetching, r.Path, r.Version, err)
		}
		if out, err := goIn(dir, "mod", "edit", "-replace", r.Path+"@"+r.Version+"="+source, modfile); err != nil {
			return "", fmt.Errorf("%s@%s cannot be taken from the clone beside the check: %s", r.Path, r.Version,
				lastLine(out))
		}
	}
	if out, err := goIn(dir, "mod", "download", "-modfile="+modfile); err != nil {
		return "", fmt.Errorf("its dependencies cannot be fetched even with %s taken from the clone beside the "+
			"check: %s", repositories(required), lastLine(out))
	}
	return modfile, nil
}

// requirement is one module the module requires.
type requirement struct {
	Path    string
	Version string
}

// forgeRequirements is every requirement of the module at dir that comes from the mesh's own forge.
func forgeRequirements(dir string) ([]requirement, error) {
	out, err := goIn(dir, "mod", "edit", "-json")
	if err != nil {
		return nil, fmt.Errorf("its go.mod cannot be read: %s", lastLine(out))
	}
	var mod struct{ Require []requirement }
	if err := json.Unmarshal(out, &mod); err != nil {
		return nil, fmt.Errorf("its go.mod cannot be read: %w", err)
	}
	var on []requirement
	for _, r := range mod.Require {
		if strings.HasPrefix(r.Path, forge) {
			on = append(on, r)
		}
	}
	return on, nil
}

// repositoryOf splits a module path of the forge into its repository and the directory in it.
func repositoryOf(path string) (string, string) {
	repo, sub, _ := strings.Cut(strings.TrimPrefix(path, forge), "/")
	return repo, sub
}

func repositories(rs []requirement) string {
	var names []string
	for _, r := range rs {
		repo, _ := repositoryOf(r.Path)
		names = append(names, repo)
	}
	return strings.Join(names, ", ")
}

// pseudo is a pseudo-version's commit: v0.1.14-0.20261012082113-9747f3e1b1ac → 9747f3e1b1ac.
var pseudo = regexp.MustCompile(`[-.]\d{14}-([0-9a-f]{12})$`)

// refOf is the ref a version names in its repository: a pseudo-version's commit, else the tag a module in
// a directory is released under (<directory>/<version>).
func refOf(sub, version string) string {
	version = strings.TrimSuffix(version, "+incompatible")
	if m := pseudo.FindStringSubmatch(version); m != nil {
		return m[1]
	}
	if sub != "" {
		return sub + "/" + version
	}
	return version
}

// export writes the module r names, at its version, from its repository's clone beside the check into
// into, and answers its directory.
func export(beside string, r requirement, into string) (string, error) {
	repo, sub := repositoryOf(r.Path)
	clone := filepath.Join(beside, repo)
	if _, err := os.Stat(filepath.Join(clone, ".git")); err != nil {
		return "", fmt.Errorf("no clone of %s is beside the check", repo)
	}
	ref := refOf(sub, r.Version)
	commit, err := exec.Command("git", "-C", clone, "rev-parse", "--verify", "--quiet", ref+"^{commit}").Output()
	if err != nil {
		return "", fmt.Errorf("the clone of %s holds no %s (a commit or tag not pushed to the forge?)", repo, ref)
	}
	at := strings.TrimSpace(string(commit))
	args := []string{"-C", clone, "archive", "--format=tar", at}
	if sub != "" {
		args = append(args, "--", sub)
	}
	var stderr bytes.Buffer
	archive := exec.Command("git", args...)
	archive.Stderr = &stderr
	raw, err := archive.Output()
	if err != nil {
		return "", fmt.Errorf("git archive of %s at %s: %v %s", repo, ref, err, strings.TrimSpace(stderr.String()))
	}
	root := filepath.Join(into, repo+"@"+at[:12])
	if err := untar(raw, root); err != nil {
		return "", err
	}
	source := filepath.Join(root, filepath.FromSlash(sub))
	if _, err := os.Stat(filepath.Join(source, "go.mod")); err != nil {
		return "", fmt.Errorf("%s at %s has no go.mod in %q", repo, ref, sub)
	}
	return source, nil
}

// untar writes a tar's directories and regular files under root.
func untar(raw []byte, root string) error {
	r := tar.NewReader(bytes.NewReader(raw))
	for {
		h, err := r.Next()
		if err == io.EOF {
			return nil
		}
		if err != nil {
			return err
		}
		name := filepath.Clean(filepath.FromSlash(h.Name))
		if name == "." || strings.HasPrefix(name, "..") || filepath.IsAbs(name) {
			continue
		}
		path := filepath.Join(root, name)
		switch h.Typeflag {
		case tar.TypeDir:
			if err := os.MkdirAll(path, 0o755); err != nil {
				return err
			}
		case tar.TypeReg:
			if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
				return err
			}
			body, err := io.ReadAll(r)
			if err != nil {
				return err
			}
			if err := os.WriteFile(path, body, 0o644); err != nil {
				return err
			}
		}
	}
}

func copyFile(from, to string) error {
	raw, err := os.ReadFile(from)
	if err != nil {
		return err
	}
	return os.WriteFile(to, raw, 0o644)
}

// goIn runs the go command in dir.
func goIn(dir string, args ...string) ([]byte, error) {
	cmd := exec.Command("go", args...)
	cmd.Dir = dir
	var stderr bytes.Buffer
	cmd.Stderr = &stderr
	out, err := cmd.Output()
	if err != nil {
		return append(out, stderr.Bytes()...), err
	}
	return out, nil
}

// lastLine is the last line a command said, which is where go says what failed.
func lastLine(out []byte) string {
	lines := strings.Split(strings.TrimSpace(string(out)), "\n")
	return strings.TrimSpace(lines[len(lines)-1])
}