// Copyright (C) 2026 Dcntrlized
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published
// by the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.

// Package licenser applies AGPL headers to a pull request's added files and
// pushes them back as its own commit on top of the head — to the base
// repository for same-repo PRs and to the contributor's fork for cross-repo PRs
// (Gitea's allow_maintainer_edit, on by default). The contributor's commit is
// never rewritten, and a branch that advances while the bot works is re-read and
// re-processed instead of being abandoned.
package licenser

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"
	"time"
)

// Options describes one pull-request head to fix.
type Options struct {
	// RemoteURL is the credential-bearing clone/push URL of the HEAD repository
	// (the fork, or the base repository for same-repo PRs).
	RemoteURL string
	// Ref is the head branch name; SHA is the head commit the PR was read at.
	// A later attempt works from the branch's current tip instead.
	Ref string
	SHA string
	// Files are repository-relative paths of changed files to check.
	Files []string
	// AddlicenseBin is the addlicense executable.
	AddlicenseBin string
	// Holder is the copyright holder passed to addlicense -c.
	Holder string
	// BotName/BotEmail are the committer identity of the amended commit.
	BotName  string
	BotEmail string
	// WorkRoot is the parent directory for the temporary worktree.
	WorkRoot string
	// DryRun stops before committing/pushing.
	DryRun bool
	// Logger receives progress; may be nil.
	Logger *slog.Logger
}

// Result reports what happened.
type Result struct {
	// Changed lists files that received a header.
	Changed []string
	// NewSHA is the bot's commit on top of the pull request head, empty when
	// nothing changed.
	NewSHA string
	// Pushed reports whether that commit reached the remote.
	Pushed bool
	// Blocked reports a push refused by the server (typically
	// allow_maintainer_edit disabled on the PR, or a protected branch).
	Blocked bool
	// BlockedReason carries the redacted server message for Blocked.
	BlockedReason string
}

// ErrRemoteMoved means the remote head kept advancing for every attempt: each
// rejection makes the bot drop its local work, re-read the branch, re-apply the
// headers and push again, and this error is only returned once that loop is
// exhausted. Nothing is ever force-pushed or rewritten, so the caller can simply
// try again later.
var ErrRemoteMoved = errors.New("remote branch kept moving; will retry later")

// maxPushAttempts bounds the re-read/re-apply/push loop. A normal race is
// resolved on the second attempt; the bound only stops a pathological
// push storm from spinning forever.
const maxPushAttempts = 5

const cloneDepth = 50

// commitSubject/commitBody describe the bot's own commit. It is deliberately a
// new commit on top of the pull request head — never an amend — so the
// contributor's work and message stay byte-for-byte intact and the bot's change
// can be reverted or dropped with a single `git revert` / `git reset`.
const commitSubject = "chore(license): add AGPL-3.0 headers"

// EnsureHeaders runs addlicense over opts.Files in a fresh worktree of the PR
// head, records the result in a separate commit and pushes it on top of the
// head branch (fast-forward, no force).
func EnsureHeaders(ctx context.Context, opts Options) (*Result, error) {
	log := opts.Logger
	if log == nil {
		log = slog.Default()
	}
	if opts.RemoteURL == "" {
		return nil, errors.New("RemoteURL is required")
	}
	if opts.Ref == "" || opts.SHA == "" {
		return nil, errors.New("Ref and SHA are required")
	}

	var last *Result
	for attempt := 1; attempt <= maxPushAttempts; attempt++ {
		res, moved, err := ensureHeadersOnce(ctx, opts, log, attempt == 1)
		if err != nil {
			return res, err
		}
		if !moved {
			return res, nil
		}
		// The branch advanced under us: drop the local work (the temp clone is
		// discarded), re-read the branch, re-run the check and try again.
		last = res
		log.Warn("branch advanced while working; re-reading it and re-applying the headers",
			"ref", opts.Ref, "attempt", attempt, "of", maxPushAttempts)
	}
	if last != nil {
		last.NewSHA = "" // nothing reached the remote
	}
	return last, fmt.Errorf("%w: %s moved on every one of %d attempts", ErrRemoteMoved, opts.Ref, maxPushAttempts)
}

// ensureHeadersOnce performs one clone/apply/commit/push round. first selects
// the starting point: the head commit the pull request was read at on the first
// attempt, the branch's current tip afterwards. moved reports a rejected
// non-fast-forward push, which the caller answers by starting over.
func ensureHeadersOnce(ctx context.Context, opts Options, log *slog.Logger, first bool) (res *Result, moved bool, err error) {
	dir, err := os.MkdirTemp(opts.WorkRoot, "license-bot-*")
	if err != nil {
		return nil, false, fmt.Errorf("create worktree: %w", err)
	}
	defer os.RemoveAll(dir)

	if _, err := git(ctx, dir, "clone", "--quiet", "--depth", strconv.Itoa(cloneDepth), "--single-branch", "--branch", opts.Ref, opts.RemoteURL, dir); err != nil {
		return nil, false, fmt.Errorf("clone %s: %w", Redact(opts.RemoteURL), err)
	}

	target := opts.SHA
	if !first {
		// Re-read the branch: the previous attempt lost a race, so the work must
		// be redone on top of whatever is there now.
		if out, err := git(ctx, dir, "rev-parse", "origin/"+opts.Ref); err == nil {
			if tip := strings.TrimSpace(out); tip != "" {
				target = tip
			}
		}
	}
	if !commitExists(ctx, dir, target) {
		if _, err := git(ctx, dir, "fetch", "--quiet", "--unshallow", "origin"); err != nil {
			log.Warn("unshallow failed, using fetched history", "err", err)
		}
	}
	if !commitExists(ctx, dir, target) {
		return nil, false, fmt.Errorf("head commit %s not present in %s", target, Redact(opts.RemoteURL))
	}
	if _, err := git(ctx, dir, "checkout", "--quiet", target); err != nil {
		return nil, false, fmt.Errorf("checkout %s: %w", target, err)
	}

	tpl := filepath.Join(dir, ".license-header.tpl")
	if _, err := os.Stat(tpl); err != nil {
		return nil, false, fmt.Errorf(".license-header.tpl missing at %s: %w", target, err)
	}
	patterns := ReadIgnorePatterns(filepath.Join(dir, ".licenseignore"))

	res = &Result{}
	year := strconv.Itoa(time.Now().Year())
	for _, f := range opts.Files {
		if f == "" {
			continue
		}
		abs := filepath.Join(dir, filepath.FromSlash(f))
		if st, err := os.Stat(abs); err != nil || st.IsDir() {
			continue // removed again or not a regular file
		}
		args := []string{"-v", "-f", ".license-header.tpl", "-c", opts.Holder, "-y", year}
		for _, p := range patterns {
			args = append(args, "-ignore", p)
		}
		args = append(args, f)
		out, err := run(ctx, dir, opts.AddlicenseBin, args...)
		if err != nil {
			return nil, false, fmt.Errorf("addlicense %s: %w: %s", f, err, strings.TrimSpace(out))
		}
	}

	porcelain, err := git(ctx, dir, "status", "--porcelain")
	if err != nil {
		return nil, false, fmt.Errorf("git status: %w", err)
	}
	res.Changed = ChangedFiles(porcelain)
	if len(res.Changed) == 0 {
		// Everything this pull request added is already licensed, so there is
		// nothing to publish — including on a retry after a concurrent push
		// fixed the files itself.
		return res, false, nil
	}
	log.Info("adding license headers", "files", len(res.Changed), "dry_run", opts.DryRun)
	if opts.DryRun {
		return res, false, nil
	}

	if _, err := git(ctx, dir, "add", "-A"); err != nil {
		return nil, false, fmt.Errorf("git add: %w", err)
	}
	body := "Added by the Dcntrlized license bot to the files this pull request\n" +
		"introduced without a license header. Only the files this pull request adds\n" +
		"are touched, and the header text comes from .license-header.tpl.\n\n" +
		"This commit is separate from the contributor's work, so it can be reverted\n" +
		"or dropped freely.\n\nFiles:\n"
	for _, f := range res.Changed {
		body += "- " + f + "\n"
	}
	if _, err := git(ctx, dir, "-c", "user.name="+opts.BotName, "-c", "user.email="+opts.BotEmail,
		"commit", "--quiet", "-m", commitSubject, "-m", body); err != nil {
		return nil, false, fmt.Errorf("git commit: %w", err)
	}
	newSHA, err := git(ctx, dir, "rev-parse", "HEAD")
	if err != nil {
		return nil, false, fmt.Errorf("rev-parse: %w", err)
	}
	res.NewSHA = strings.TrimSpace(newSHA)

	// Fast-forward push only: no --force, no --force-with-lease. If anyone
	// pushed in the meantime the push is rejected and the caller starts over
	// from the new tip rather than rewriting their history.
	out, err := git(ctx, dir, "push", "--quiet", opts.RemoteURL, "HEAD:refs/heads/"+opts.Ref)
	if err != nil {
		msg := strings.TrimSpace(out)
		if isPushRefused(msg) {
			res.Blocked = true
			res.BlockedReason = Redact(msg)
			return res, false, nil
		}
		if isRemoteMoved(msg) {
			res.NewSHA = ""
			return res, true, nil
		}
		return res, false, fmt.Errorf("git push %s: %w: %s", Redact(opts.RemoteURL), err, Redact(msg))
	}
	res.Pushed = true
	return res, false, nil
}

func commitExists(ctx context.Context, dir, sha string) bool {
	_, err := git(ctx, dir, "cat-file", "-e", sha+"^{commit}")
	return err == nil
}

func git(ctx context.Context, dir string, args ...string) (string, error) {
	return runEnv(ctx, dir, []string{"GIT_TERMINAL_PROMPT=0"}, "git", args...)
}

func run(ctx context.Context, dir, bin string, args ...string) (string, error) {
	return runEnv(ctx, dir, nil, bin, args...)
}

// runEnv runs a command and, on failure, returns an error carrying the
// redacted combined output so diagnostics never leak the bot token.
func runEnv(ctx context.Context, dir string, extraEnv []string, bin string, args ...string) (string, error) {
	cmd := exec.CommandContext(ctx, bin, args...)
	cmd.Dir = dir
	cmd.Env = append(os.Environ(), extraEnv...)
	var buf strings.Builder
	cmd.Stdout = &buf
	cmd.Stderr = &buf
	err := cmd.Run()
	out := buf.String()
	if ctx.Err() != nil {
		return out, ctx.Err()
	}
	if err != nil {
		if msg := strings.TrimSpace(Redact(out)); msg != "" {
			return out, fmt.Errorf("%w: %s", err, msg)
		}
	}
	return out, err
}

var (
	// A refusal the bot cannot fix by retrying: permissions, disabled
	// maintainer edits, protected branch rules.
	rePushDenied = regexp.MustCompile(`(?i)(403|forbidden|denied|not allowed|not permitted|permission|protected)`)
	// The remote advanced under us: a plain non-fast-forward rejection.
	reRemoteMoved = regexp.MustCompile(`(?i)(non-fast-forward|fetch first|stale info|\[remote rejected\]|behind)`)
)

func isPushRefused(msg string) bool { return rePushDenied.MatchString(msg) }
func isRemoteMoved(msg string) bool { return reRemoteMoved.MatchString(msg) }

// ChangedFiles extracts paths from `git status --porcelain` output.
func ChangedFiles(porcelain string) []string {
	var out []string
	for _, line := range strings.Split(porcelain, "\n") {
		if len(line) < 4 {
			continue
		}
		p := line[3:]
		if strings.HasPrefix(p, "\"") {
			if uq, err := strconv.Unquote(p); err == nil {
				p = uq
			}
		}
		if p = strings.TrimSpace(p); p != "" {
			out = append(out, p)
		}
	}
	return out
}

// ReadIgnorePatterns returns the non-empty, non-comment lines of a
// .licenseignore file. A missing file yields no patterns.
func ReadIgnorePatterns(path string) []string {
	b, err := os.ReadFile(path)
	if err != nil {
		return nil
	}
	return ParseIgnore(string(b))
}

// ParseIgnore splits .licenseignore content into addlicense -ignore patterns.
// addlicense's patterns are doublestar globs, not prefixes, so lines are used
// verbatim.
func ParseIgnore(body string) []string {
	var out []string
	for _, line := range strings.Split(body, "\n") {
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		out = append(out, line)
	}
	return out
}

// Redact removes credentials embedded in a URL or message so logs and PR
// comments never leak the bot token.
func Redact(s string) string {
	// scheme://user:pass@host -> scheme://host
	re := regexp.MustCompile(`([a-zA-Z][a-zA-Z0-9+.-]*://)[^/@\s]*@`)
	return re.ReplaceAllString(s, "$1")
}