// SPDX-License-Identifier: GPL-3.0-or-later

// Package apply is the executor: it carries out one file's plan.Chain,
// actually moving, copying, renaming, trashing or permanently deleting real
// files. See docs/design.md §7.2 for the mechanism each action follows and
// §7.4's last paragraph for the execution-time conflict re-check.
package apply

import (
	"context"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"time"

	"git.labunix.xyz/krino/internal/plan"
	"git.labunix.xyz/krino/internal/scan"
	"git.labunix.xyz/krino/internal/trash"
)

// StepResult is what happened to one step, in the order the executor ran
// them.
type StepResult struct {
	Step    plan.Step
	Status  string    // "ok" | "failed" | "skipped"
	Detail  string    // the failure, or why it was skipped
	Dst     string    // where the file actually ended up (conflict names can change at execution time)
	Size    int64     // of the file at Dst afterwards
	ModTime time.Time // of the file at Dst afterwards
	Entry   string    // trash entry name, for Trash steps; "" otherwise
	// DisplacedEntry is the trash entry name of the file this step
	// displaced; "" when none. Entry and DisplacedEntry describe two
	// different files: the one being acted on (Entry, only for a Trash-kind
	// step), and the one that was in this step's way and had to be trashed
	// first (DisplacedEntry, only when Displaces was set). Spec §7.4's
	// overwrite policy and §9's "displace" action both depend on this name
	// being recoverable — it is chosen inside trash.Put, so nothing
	// downstream of Chain could otherwise re-derive it for undo.
	DisplacedEntry string
	Made           []string // directories this step created, outermost first
}

// Chain runs one file's steps in order and stops at the first failure,
// marking the rest skipped. It never touches a file whose size or mtime no
// longer matches what the plan recorded. It is ChainLogged with no done.
func Chain(c plan.Chain) []StepResult {
	results, _ := ChainLogged(context.Background(), c, nil, nil)
	return results
}

// ChainLogged is Chain, calling done with each step's result as soon as
// that step has run or been skipped, before the next one starts - so a
// caller that logs from done never has a completed step missing from the
// log when the process dies mid-chain. An error from done stops the chain
// at once and is returned with the results so far.
//
// A move or rename that had to take a free name at apply time, because its
// planned destination was taken since planning, stops the chain too: every
// later step was planned against the name the file did not get, and must
// not act on whatever is at that path.
//
// Once ctx is cancelled (an interrupt), the step already under way finishes
// and every later step is skipped as "interrupted": an interrupt stops after
// the current step, not after the file's whole chain (spec §11).
func ChainLogged(ctx context.Context, c plan.Chain, done func(i int, sr StepResult) error, displaced func(i int, step plan.Step, entry string) error) ([]StepResult, error) {
	results := make([]StepResult, len(c.Steps))
	stopWhy := ""

	for i, step := range c.Steps {
		if stopWhy == "" && ctx.Err() != nil {
			stopWhy = "interrupted"
		}
		switch {
		case stopWhy != "":
			results[i] = StepResult{Step: step, Status: "skipped", Detail: stopWhy}
		case step.Skip != "":
			// Planning already decided this step will not run; it must not
			// be attempted, so no pre-step check, no directory creation, no
			// touching the file (spec: a step already marked Skip is
			// reported, not attempted).
			results[i] = StepResult{Step: step, Status: "skipped", Detail: step.Skip}
		default:
			if err := checkUnchanged(step.Src, c.File); err != nil {
				results[i] = StepResult{Step: step, Status: "failed", Detail: err.Error()}
				stopWhy = "an earlier step in this chain failed"
				break
			}
			res := runStep(step, func(entry string) error {
				if displaced == nil {
					return nil
				}
				return displaced(i, step, entry)
			})
			results[i] = res
			switch {
			case res.Status == "failed":
				stopWhy = "an earlier step in this chain failed"
			case (step.Kind == plan.Move || step.Kind == plan.Rename) && res.Dst != step.Dst:
				stopWhy = fmt.Sprintf("an earlier step put the file at %s, not the planned %s", res.Dst, step.Dst)
			}
		}
		if done != nil {
			if err := done(i, results[i]); err != nil {
				return results[:i+1], err
			}
		}
	}

	return results, nil
}

// checkUnchanged is the guard that matters most: before every step, the
// source is Lstat'd and compared against what the plan recorded for the
// whole file. A file rewritten or replaced between planning and applying
// must never be acted on (spec §15.1): its size and mtime must match, it
// must still be a regular file - not a symlink put in its place - and, at
// its planned path, it must be the same inode. The inode is not compared
// once an earlier step has moved the file: a move across filesystems
// copies it to a new inode, and the chain is still following its own file.
func checkUnchanged(src string, f scan.File) error {
	fi, err := os.Lstat(src)
	if err != nil {
		return fmt.Errorf("changed since plan: %w", err)
	}
	if !fi.Mode().IsRegular() {
		return errors.New("changed since plan: no longer a regular file")
	}
	if fi.Size() != f.Size || !fi.ModTime().Equal(f.ModTime) {
		return errors.New("changed since plan")
	}
	if src == f.Path && f.Ino != 0 {
		if now := scan.NewFile(src, f.Rel, fi); now.Dev != f.Dev || now.Ino != f.Ino {
			return errors.New("changed since plan: another file is in its place")
		}
	}
	return nil
}

// runStep dispatches one already-checked, non-skipped step to the code that
// actually carries it out. reportDisplace is called the moment a displaced
// file has reached the Trash, before the step that needed its name begins;
// a step whose displace cannot be reported does not go on to use the name.
func runStep(step plan.Step, reportDisplace func(entry string) error) StepResult {
	switch step.Kind {
	case plan.Copy, plan.Move, plan.Rename:
		return runFileStep(step, reportDisplace)
	case plan.Trash:
		return runTrashStep(step)
	case plan.DeletePermanent:
		return runDeleteStep(step)
	}
	panic(fmt.Sprintf("apply: unknown plan.Kind %d", int(step.Kind)))
}

// runFileStep carries out copy, move and rename. It re-checks the planned
// destination against the filesystem as it is now (spec §7.4): if something
// with Displaces set claims the file to trash first, that happens before
// anything else, and if the displace fails nothing further is attempted for
// this file. Otherwise, if the planned Dst now exists, the step moves to the
// next free stem_N.ext and records the real name in Dst rather than
// overwriting a file the plan never accounted for. Missing destination
// directories are created and recorded in Made, outermost first, whether or
// not the step that needed them goes on to succeed.
//
// The displace is reported through reportDisplace as soon as trash.Put
// returns, not when this step finishes: the user's file is in the Trash
// from that moment, durably, and for a copy or a cross-device move the rest
// of the step is the whole data transfer. A process killed in that window
// used to leave the file in the Trash with nothing in the log to say so.
func runFileStep(step plan.Step, reportDisplace func(entry string) error) StepResult {
	dst := step.Dst
	var displacedEntry string

	if step.Displaces != "" {
		// Re-checked at apply time: only a regular file may be trashed to
		// make room, never a directory or link put there since.
		if fi, err := os.Lstat(step.Displaces); err != nil || !fi.Mode().IsRegular() {
			return StepResult{Step: step, Status: "failed", Detail: "the file to replace is gone or no longer a regular file"}
		}
		// overwrite policy: the file already at dst must be trashed before
		// this step's own destination name is used, so no free-name search
		// applies here — the whole point of displacing was to clear this
		// exact name. The entry name is captured regardless of what happens
		// next in this step: spec §9 logs "displace" as its own action with
		// its own line, independent of whether the move/copy/rename that
		// needed the name then goes on to succeed, so every return below
		// (failure included) carries it once trashing has succeeded.
		entry, err := trash.Put(step.Displaces)
		if err != nil {
			return StepResult{Step: step, Status: "failed", Detail: "displacing the existing file: " + err.Error()}
		}
		displacedEntry = entry
		if reportDisplace != nil {
			if err := reportDisplace(entry); err != nil {
				// The file is already in the Trash and cannot be recorded.
				// Using the name now would compound an unlogged destructive
				// act with a second one.
				return StepResult{Step: step, Status: "failed", DisplacedEntry: entry,
					Detail: "the file it replaces went to the Trash but could not be logged, so undo cannot see it: " + err.Error()}
			}
		}
	} else if _, err := os.Lstat(dst); err == nil {
		free, err := nextFreeName(dst)
		if err != nil {
			return StepResult{Step: step, Status: "failed", Detail: err.Error()}
		}
		dst = free
	} else if !os.IsNotExist(err) {
		return StepResult{Step: step, Status: "failed", Detail: err.Error()}
	}

	made, err := MkdirAllTracked(filepath.Dir(dst))
	if err != nil {
		return StepResult{Step: step, Status: "failed", Detail: err.Error(), Made: made, DisplacedEntry: displacedEntry}
	}

	switch step.Kind {
	case plan.Copy:
		err = copyFile(step.Src, dst)
	case plan.Move:
		err = moveFile(step.Src, dst)
	case plan.Rename:
		err = renameFile(step.Src, dst)
	}
	if err != nil {
		return StepResult{Step: step, Status: "failed", Detail: err.Error(), Made: made, DisplacedEntry: displacedEntry}
	}

	fi, err := os.Stat(dst)
	if err != nil {
		return StepResult{Step: step, Status: "failed", Detail: err.Error(), Made: made, DisplacedEntry: displacedEntry}
	}
	return StepResult{Step: step, Status: "ok", Dst: dst, Size: fi.Size(), ModTime: fi.ModTime(), Made: made, DisplacedEntry: displacedEntry}
}

// runTrashStep carries out (delete): the file goes to the freedesktop.org
// Trash via trash.Put. A file on a different filesystem from the Trash is
// not trashed at all; the spec requires the failure to name the two ways
// forward, since otherwise the user has no path out of it.
//
// Dst, Size and ModTime describe the file at trash.Dir()/files/<entry>,
// even though plan.Step.Dst is always "" for a delete (there is nothing to
// compute or conflict-check at plan time). That is a deliberate reading of
// §9 rather than an oversight forced by the empty plan.Step.Dst: those
// journal columns describe the file at Dst after the step, and after a
// trash step the file genuinely lives there, so recording it is more
// useful than an empty column and stays greppable. It also cannot confuse
// undo: the refusal condition for reversing a trash step is "the entry is
// gone, or Src now exists" — it reads Entry and Src, never Dst.
func runTrashStep(step plan.Step) StepResult {
	entry, err := trash.Put(step.Src)
	if err != nil {
		detail := err.Error()
		if errors.Is(err, trash.ErrOtherFilesystem) {
			detail += "; use (delete permanent) or a move instead"
		}
		return StepResult{Step: step, Status: "failed", Detail: detail}
	}

	dst := filepath.Join(trash.Dir(), "files", entry)
	var size int64
	var modTime time.Time
	if fi, err := os.Stat(dst); err == nil {
		size, modTime = fi.Size(), fi.ModTime()
	}
	return StepResult{Step: step, Status: "ok", Dst: dst, Size: size, ModTime: modTime, Entry: entry}
}

// runDeleteStep carries out (delete permanent): a plain unlink, with no
// Trash and no way back.
func runDeleteStep(step plan.Step) StepResult {
	if err := os.Remove(step.Src); err != nil {
		return StepResult{Step: step, Status: "failed", Detail: err.Error()}
	}
	return StepResult{Step: step, Status: "ok"}
}