// Package books owns a book's INTAKE: receiving the file the user uploads, putting it where the
// engine will look for it, and turning it into a chapter tree.
//
// The shape mirrors the run lifecycle deliberately (package runs): the request only gets the book as
// far as "the bytes are here", and everything after that is a step some later sweep can finish. A
// parse is one $0 call of the engine that takes seconds on a large book, and holding a request open
// for it — or losing the book when the process doing it is restarted — are the two failures this
// split exists to avoid.
package books

import (
	"context"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"log/slog"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"
	"unicode"
	"unicode/utf8"

	"textmachine/platform/internal/ingest"
	"textmachine/platform/internal/pgstore"
)

// Manifester is the engine's $0 producer of a chapter tree: `tmctl manifest`. An interface so the
// intake's decisions can be pinned without an engine binary.
type Manifester interface {
	Manifest(ctx context.Context, binary, workdir string) (ingest.Manifest, error)
}

// Reader materializes what a client reads — the chapter tree and the pairs — once the engine has cut
// the book. Nil where this deployment serves no reading surface.
//
// RefreshCut takes the manifest the caller has already read, so an intake does not pay for a second
// re-chunk of the same source to learn the same cut. A parse that never reaches it, or reaches it and
// fails, has still recorded the DEBT — the materializer's own pass is what answers that.
type Reader interface {
	// Claim takes the debt this intake has just recorded, so the materializer's own sweep does not
	// read the same book at the same time. `ok` false means somebody else already holds it.
	Claim(ctx context.Context, b pgstore.OwedBook) (pgstore.OwedBook, bool, error)
	RefreshCut(ctx context.Context, b pgstore.OwedBook, cut ingest.Manifest) error
}

// Enqueuer hands a book to the queue inside the caller's transaction.
type Enqueuer interface {
	EnqueueParse(ctx context.Context, tx pgstore.Tx, bookID string) error
}

// Config is what an operator chooses about intake.
type Config struct {
	// BooksDir is the root the platform creates book directories under. Absolute, and NOT the place
	// an operator keeps hand-made books: everything below it is written and, on a rejected intake,
	// removed by this service.
	BooksDir string
	// EngineBinary is the versioned tmctl path used to parse. Intake is not mounted without it: a
	// deployment that cannot parse would take uploads it can only reject.
	EngineBinary string
	// BookTemplate is the operator's starting `book.yaml`, from which each new book's own is rendered
	// once (form Б, D39.130 — see render.go). Empty means this deployment provisions books by hand,
	// which is what every deployment did before the form was ratified: an unprovisioned book then
	// WAITS rather than being rejected.
	BookTemplate string
	// Pairs is what this deployment declares it can translate, from its configuration — the AVAILABLE
	// half of it. EMPTY refuses every upload: "declares nothing" is not "declares this pair", and the
	// boot refuses to mount an intake with an empty list at all, so this is the second half of one
	// rule. Which pairs exist is DATA (a prompt pack the operator deploys), never a list in this
	// package.
	Pairs []Pair
}

// Service is the intake.
type Service struct {
	Store  *pgstore.Store
	Engine Manifester
	Reader Reader
	Queue  Enqueuer
	Cfg    Config
	Log    *slog.Logger
	// Now is injectable so the graces below are testable without sleeping.
	Now func() time.Time
	// writeBudget shortens what a terminal write gets. Unexported and zero by default: only this
	// package's own tests set it, to reach the case where the cut outlives that budget.
	writeBudget time.Duration
}

func (s *Service) now() time.Time {
	if s.Now != nil {
		return s.Now()
	}
	return time.Now()
}

func (s *Service) log() *slog.Logger {
	if s.Log == nil {
		return slog.New(slog.DiscardHandler)
	}
	return s.Log
}

// ErrBadIntake is a request this route cannot make a book out of. It is the contract's 400.
var ErrBadIntake = errors.New("books: the intake form is not usable")

// ErrUnsupportedPair is a book in a direction this deployment cannot translate.
//
// It is refused AT INTAKE and that is the whole point of the value: a well-formed language code is
// not the same as a supported one, and until 0.3.0 such a book was accepted, written to disk, cut
// into chapters, walked through the money screen — and then died at the start of a run on the
// engine's own configuration check, with no reason that reached the user (companion §2.1). No money
// burned; a user's time did.
var ErrUnsupportedPair = fmt.Errorf("%w: this deployment cannot translate that pair", ErrBadIntake)

// ErrMalformedLanguage is a language code that is not one. Told apart from the rest of ErrBadIntake
// so the refusal can NAME the field: the canon asks the intake's 400 to say which part was wrong,
// and "the request could not be read" is the answer 0.2.3 gave to six different conditions.
var ErrMalformedLanguage = fmt.Errorf("%w: the languages must be codes", ErrBadIntake)

// Pair is one direction this deployment declares it can run.
type Pair struct{ Source, Target string }

// SourceName is the file name the intake writes a book's source under, extension aside. The name is
// FIXED and predictable because the engine finds the source through `source_file:` in the book's own
// config, which somebody else writes (see the provisioning seam in parse.go).
const SourceName = "source"

// MaxTitle bounds a book's display name. The library lists it, and the client-supplied name is
// otherwise the one unbounded string on that screen.
//
// Exported because the rename door enforces the same bound (httpapi.patchedTitle) and it is one
// column: two copies of the number is how the intake and the patch come to disagree about what fits.
const MaxTitle = 200

// langCode is the contract's LangCode: a code, never a name (§LangCode). Validated here as well as
// in the read model because this is where a value from a browser enters.
var langCode = regexp.MustCompile(`^[a-z]{2,3}(-[A-Za-z0-9]{2,8})*$`)

// Intake is one accepted call of POST /books.
type Intake struct {
	UserID string
	// Title is what the person named the book, and the EMPTY STRING is a value rather than an
	// absence: it means "name it from the file" (canon §BookIntake.title).
	Title      string
	SourceLang string
	TargetLang string
	// Filename is the name the client gave the part. It is used for two things and trusted for
	// neither: a title for the library and the extension that tells the engine which reader to use.
	Filename string
	// File is the body of the upload, already bounded by the route's own limit. It is streamed to
	// disk and never held in memory.
	File io.Reader
}

// Accept receives one book.
//
// The order — row, then bytes — is what makes `uploading` a state anything can observe: a second tab
// listing the library while a 60 MB file is still on the wire sees the book with its languages and
// without its size. It also makes an abandoned upload FINDABLE, which is the half that matters
// operationally: the row is the only record that a directory under BooksDir belongs to anyone.
func (s *Service) Accept(ctx context.Context, in Intake) (pgstore.Book, error) {
	if !langCode.MatchString(in.SourceLang) {
		return pgstore.Book{}, fmt.Errorf("%w: source", ErrMalformedLanguage)
	}
	if !langCode.MatchString(in.TargetLang) {
		return pgstore.Book{}, fmt.Errorf("%w: target", ErrMalformedLanguage)
	}
	if !s.canTranslate(in.SourceLang, in.TargetLang) {
		return pgstore.Book{}, ErrUnsupportedPair
	}
	if s.Cfg.BooksDir == "" {
		return pgstore.Book{}, errors.New("books: no books directory is configured")
	}
	id := pgstore.NewBookID()
	dir := filepath.Join(s.Cfg.BooksDir, id)
	if err := os.MkdirAll(dir, 0o750); err != nil {
		return pgstore.Book{}, fmt.Errorf("books: create book directory: %w", err)
	}
	// The first upload is what marks this storage as ours — see storageIsThere. Written here and
	// nowhere else, and deliberately NOT at boot: a marker the boot recreates says "the storage is
	// here" about a directory the boot itself just made.
	if err := s.markStorage(); err != nil {
		s.removeDir(dir)
		return pgstore.Book{}, err
	}
	// The created row is not kept: what the caller gets back is the row as it stands AFTER the file
	// landed, and between the two the status has moved from `uploading` to `parsing`.
	_, err := s.Store.CreateUpload(ctx, id, pgstore.NewUpload{
		OwnerID:    in.UserID,
		Title:      in.title(),
		SourceLang: in.SourceLang,
		TargetLang: in.TargetLang,
		Workdir:    dir,
		Now:        s.now(),
	})
	if err != nil {
		s.removeDir(dir)
		return pgstore.Book{}, err
	}
	// `streamRunes` and not `characters`: for an EPUB or a UTF-16 source the number is not one — the
	// full account is on `counter`, and the wire says which of the two it is carrying through
	// `character_count_exact`.
	streamRunes, err := s.receive(filepath.Join(dir, SourceName+extensionOf(in.Filename)), in.File)
	if err != nil {
		// The upload did not finish, so there is nothing to parse and nothing to keep. The row is
		// DELETED rather than rejected: `rejected` means the file could not be parsed (contract
		// §BookStatus), and there is no delete handle in the contract for the user to clear a row an
		// abandoned upload would otherwise leave in their library forever.
		//
		// On its own context: the ordinary cause of getting here is the client going away, and the
		// request's context is already cancelled by then.
		c, cancel := s.writeCtx(ctx)
		defer cancel()
		s.abandon(c, id, dir)
		return pgstore.Book{}, err
	}
	// On a context that outlives the request: every byte is in, and a client that hung up while
	// waiting for the 201 must not cost the upload it already finished. The ROW is what makes the
	// book findable, so it is written even when nobody is left to read the answer. Bounded on its own
	// (writeCtx): detached is not the same as unlimited, and a hung statement here would hold the
	// goroutine of a request that is already over.
	start, cancelStart := s.writeCtx(ctx)
	defer cancelStart()
	// The job is enqueued here ONLY where no cut runs in this request. Where one does, a job that
	// exists while the cut is going races it for the parse claim — and both outcomes cost the book:
	// the worker winning takes the fail-fast away, and the worker losing spends the job on nothing,
	// leaving a released claim no job comes back for. cutNow enqueues instead, on the one path that
	// needs it (ReleaseParseClaim).
	//
	// The price, named: a process that dies between this row and the cut leaves the book `parsing`
	// with no job, and the backstop sweep takes it after claimGrace rather than at once.
	enqueue := s.enqueue
	if s.Engine != nil {
		enqueue = nil
	}
	book, err := s.Store.StartParsing(start, id, streamRunes, enqueue)
	if err != nil {
		// The row is gone or unreachable, and the directory holds a file nothing points at — which is
		// the one thing the row-first order exists to prevent, so it is undone here too. The ordinary
		// cause is the sweep having abandoned this upload while it was still arriving.
		c, cancel := s.writeCtx(ctx)
		defer cancel()
		s.abandon(c, id, dir)
		return pgstore.Book{}, err
	}
	// The cut runs synchronously: `tmctl manifest` needs no provider call and no key, so "there is no
	// book in this file" and the book's size are both knowable before the upload is over (backlog row
	// 285). Only the engine's own verdict about the SOURCE refuses; a deployment fault falls through
	// to the asynchronous path this route always had — see cutNow.
	if cut := s.cutNow(ctx, book); cut.err != nil {
		c, cancel := s.writeCtx(ctx)
		defer cancel()
		// Nothing was accepted, so nothing is left behind: no row, no file (row 285 closes row 254).
		s.discard(c, id, dir, cut.claimedAt)
		return pgstore.Book{}, cut.err
	}
	// The cut moved the book past `parsing`, so the response carries the row as it stands now.
	//
	// On its OWN budget: `start` was opened before the bytes were received and the cut runs inside
	// this call, so by now that context can be spent — and a re-read on a dead context answers with
	// the pre-cut row, which is the one thing this line exists to avoid.
	read, cancelRead := s.writeCtx(ctx)
	defer cancelRead()
	fresh, err := s.Store.ReadBook(read, id)
	if err == nil {
		book = fresh
	} else {
		// The book is parsed and only this read failed: a stale 201 beats failing an upload that
		// succeeded.
		s.log().WarnContext(ctx, "the parsed book could not be re-read; the response carries the row as it stood before the cut", "err", err)
	}
	// No book id: an INFO line must not identify a user's library (ENGINEERING_STANDARDS
	// §Наблюдаемость, the same rule that keeps raw paths out of the access log — PD-3). The request
	// id the handler carries is what ties this line to the response the user got.
	s.log().InfoContext(ctx, "book accepted", "source_stream_runes", streamRunes, "status", book.Status)
	return book, nil
}

// cutResult is the intake cut's verdict together with the claim it was taken under, which is what
// lets the caller delete a row that is still `parsing`.
type cutResult struct {
	err       error
	claimedAt time.Time
}

// cutNow runs the intake's own parse and returns the refusal to answer the uploader with, if any.
//
// An empty result covers both "the book is cut" and "nothing could be established": the second is not
// a refusal, and the queue finishes the book as before. A deployment fault must never be answered to
// a user as a verdict about their file.
//
// Bounded by its own budget, which keeps a large book from holding the request open: past it the
// upload is accepted `parsing`. The claim is taken here so this and the queue cannot run the engine
// over one project directory at once (see Parse).
func (s *Service) cutNow(ctx context.Context, book pgstore.Book) cutResult {
	if s.Engine == nil {
		return cutResult{}
	}
	c, cancel := context.WithTimeout(context.WithoutCancel(ctx), CutBudget)
	defer cancel()
	now := s.now()
	claim, err := s.Store.ClaimParse(c, book.ID, now, now.Add(-claimGrace))
	if err != nil {
		return cutResult{} // already claimed, or unreachable: not an answer about the file
	}
	switch err := s.parseClaimed(c, claim, true); {
	case errors.Is(err, ErrBadIntake):
		return cutResult{err: err, claimedAt: claim.At}
	case err != nil: // errNotConclusive, or a context this pass could not finish inside
		// Nothing was established, so the claim goes back at once: the queue job is already enqueued
		// and a standing claim would make it do nothing (ReleaseParseClaim).
		s.log().WarnContext(ctx, "the intake's own cut was not conclusive; the queue takes the book", "err", err)
		w, wcancel := s.writeCtx(ctx)
		defer wcancel()
		if err := s.Store.ReleaseParseClaim(w, book.ID, claim.At, s.enqueue); err != nil {
			s.log().ErrorContext(ctx, "the intake's parse claim could not be given back; the backstop sweep takes the book", "err", err)
		}
	}
	return cutResult{}
}

// canTranslate judges an upload against what this deployment declared.
//
// An empty list refuses everything rather than accepting it: "declared nothing" and "declares this
// pair" are not the same answer, and the permissive reading made a deployment whose available half is
// empty accept books it can only fail later. The boot refuses that configuration outright, so this is
// the second half of one rule rather than a policy of its own.
//
// Compared case-INSENSITIVELY: a language tag is case-insensitive by BCP 47, and `zh-Hans` declared
// against `zh-hans` uploaded is the same pair.
func (s *Service) canTranslate(source, target string) bool {
	for _, p := range s.Cfg.Pairs {
		if strings.EqualFold(p.Source, source) && strings.EqualFold(p.Target, target) {
			return true
		}
	}
	return false
}

// receive streams the upload to disk and counts the RUNES OF THE STREAM that went past — see
// `counter` for what that number is and is not.
//
// Streamed, never buffered: the route's limit is tens of megabytes and reading that into memory
// would make one upload per concurrent request the platform's memory profile. The file is created
// with O_EXCL so a book directory can never be written twice by two requests that somehow minted the
// same id.
func (s *Service) receive(path string, body io.Reader) (int64, error) {
	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o640)
	if err != nil {
		return 0, fmt.Errorf("books: create source file: %w", err)
	}
	c := &counter{w: f}
	if _, err := io.Copy(c, body); err != nil {
		f.Close()
		// The error is returned as it came: the handler tells a body that outgrew the route's limit
		// from a client that went away, and both are the caller's to name (http.MaxBytesError).
		return 0, err
	}
	if err := f.Close(); err != nil {
		return 0, fmt.Errorf("books: close source file: %w", err)
	}
	return c.runes, nil
}

// counter writes through and counts the RUNES OF THE BYTE STREAM IT IS WRITING — every byte that is
// not a UTF-8 continuation byte, which needs no state across the chunk boundaries io.Copy hands it.
//
// ⚠ NAME WHAT THIS IS, BECAUSE THE FIELD IT FEEDS IS CALLED `character_count` AND THE SCREEN CALLS IT
// «Знаков» (unified backlog row 282). What the number means depends entirely on what was uploaded,
// and only the first of these three is the count the name promises:
//
//   - **A UTF-8 text source** — it IS the character count, exactly.
//   - **An EPUB** — the source is a ZIP archive, and this counts the non-continuation bytes of
//     COMPRESSED DATA. The figure is not a character count, not an approximation of one, and not
//     even the same order of magnitude reliably: it is a property of the container. The intake
//     accepts `.epub` (extensionOf below, and the engine dispatches an EPUB reader by it), so this
//     is a live case and not a hypothetical.
//   - **A text source in GB18030 or UTF-16** — the engine accepts both and decodes them itself; here
//     the figure is an approximation, and for UTF-16 a poor one.
//
// This is the FALLBACK, not the answer: the engine's manifest carries `source_chars` (runes of the
// ingested text, container and encoding dealt with) and the read model prefers it once a cut has been
// read. `character_count_exact` on the wire says which of the two a response carries.
//
// The counting stays byte-shaped deliberately: decoding here would make this a second reader of the
// source, disagreeing with the engine's on exactly the shapes above.
//
// Pinned by TestTheIntakeCounterCountsTheWriteStreamAndNotCharacters.
type counter struct {
	w io.Writer
	// runes is deliberately not called `characters`: for two of the three source shapes above it is
	// not one.
	runes int64
}

func (c *counter) Write(p []byte) (int, error) {
	n, err := c.w.Write(p)
	for _, b := range p[:n] {
		if b&0xC0 != 0x80 {
			c.runes++
		}
	}
	return n, err
}

func (s *Service) enqueue(ctx context.Context, tx pgstore.Tx, bookID string) error {
	if s.Queue == nil {
		return nil // no queue configured: the sweep picks the book up on its next pass
	}
	return s.Queue.EnqueueParse(ctx, tx, bookID)
}

// abandon undoes an upload that did not finish. Both halves are best effort and both are logged:
// what must not happen is a silent leak of either the row or the directory.
func (s *Service) abandon(ctx context.Context, id, dir string) {
	s.removeIntake(ctx, id, dir, func(c context.Context) error { return s.Store.DeleteUpload(c, id) })
}

// discard removes a book the intake's own cut refused: same halves as abandon, different stage.
func (s *Service) discard(ctx context.Context, id, dir string, claimedAt time.Time) {
	s.removeIntake(ctx, id, dir, func(c context.Context) error {
		return s.Store.DeleteRefusedIntake(c, id, claimedAt)
	})
}

func (s *Service) removeIntake(ctx context.Context, id, dir string, deleteRow func(context.Context) error) {
	// The ROW is deleted first here, and unlike the reject path that order is forced: the directory
	// may only go once it is certain nobody owns it. The sweep decides from a snapshot, and a request
	// that finished in the meantime has moved the book to `parsing` — DeleteUpload's status guard then
	// refuses, and removing the directory anyway would delete the source of a live book under it.
	//
	// The crash window that leaves is a directory with no row, and it is named rather than closed:
	// nothing walks BooksDir looking for orphans (register row PD-175, where the retention sweep that
	// would is filed).
	if err := deleteRow(ctx); err != nil {
		if errors.Is(err, pgstore.ErrNoBook) {
			// The ordinary race: another instance's sweep got there first, or the upload finished. Not
			// an error — an ERROR line on a routine race is noise that teaches operators to skim.
			s.log().InfoContext(ctx, "the abandoned upload was already gone; its directory is left alone")
			return
		}
		s.log().ErrorContext(ctx, "an abandoned upload was not removed; its directory is left alone", "err", err)
		return
	}
	s.removeDir(dir)
}

// removeDir deletes a directory this service created, and NOTHING else.
//
// The guard is not ceremony: a book registered by the dev CLI carries a workdir the operator chose —
// their own project directory, with their own source and their own project database — and no path
// here may ever remove one. Everything under BooksDir was created by this service and holds nothing
// the platform did not put there.
func (s *Service) removeDir(dir string) {
	if !s.owns(dir) {
		s.log().Error("refusing to remove a directory this service did not create", "dir", dir)
		return
	}
	if err := os.RemoveAll(dir); err != nil {
		s.log().Error("book directory could not be removed", "err", err)
	}
}

// StorageMarker is the file that says "this is the storage this platform has been writing books
// into". Exported so an operator provisioning a volume by hand can put one there.
const StorageMarker = ".tmplatform-books"

const storageMarkerText = `This directory holds book sources written by tmplatformd.
Its presence is what tells the intake that the storage is mounted: without it a book whose directory
is missing is treated as "the storage is gone" and WAITS, instead of being rejected with its source
deleted. Do not remove it while books live here.
`

// markStorage writes the marker if it is not there. Idempotent and race-free by O_EXCL: two uploads
// arriving together are both correct, and the loser's EEXIST is the state it wanted.
func (s *Service) markStorage() error {
	f, err := os.OpenFile(filepath.Join(s.Cfg.BooksDir, StorageMarker),
		os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o640)
	if errors.Is(err, fs.ErrExist) {
		return nil
	}
	if err != nil {
		return fmt.Errorf("books: mark the storage: %w", err)
	}
	defer f.Close()
	if _, err := f.WriteString(storageMarkerText); err != nil {
		return fmt.Errorf("books: mark the storage: %w", err)
	}
	return nil
}

// storageIsThere reports whether the books storage is the one this platform wrote into.
//
// ⚠ It asks about the MARKER and not about the directory, and the difference is the whole point. A
// volume mounted AT BooksDir leaves an empty mountpoint behind when it is unmounted, so the directory
// still exists and `Stat` still succeeds — which is how the first version of this guard (PD-192) let
// an unmount reject every book in intake, with their sources deleted, exactly as if each book's own
// directory had been removed. The boot's MkdirAll made it worse by recreating the root after a
// restart. The marker is written by the FIRST upload and by nothing else, so neither an unmount nor a
// fresh MkdirAll can forge it (re-check of the dofix, FP5-10).
//
// A host that has never taken an upload has no marker either, and answers "not there" — which is the
// safe direction: it has no books to reject.
func (s *Service) storageIsThere() bool {
	if s.Cfg.BooksDir == "" {
		return false
	}
	_, err := os.Stat(filepath.Join(s.Cfg.BooksDir, StorageMarker))
	return err == nil
}

func (s *Service) owns(dir string) bool {
	if s.Cfg.BooksDir == "" || dir == "" {
		return false
	}
	rel, err := filepath.Rel(s.Cfg.BooksDir, dir)
	if err != nil {
		return false
	}
	return rel != "." && rel != ".." && !strings.HasPrefix(rel, ".."+string(filepath.Separator))
}

// title is the name the book joins the library under: the one the person typed, or — when they
// typed none — the name of the file they uploaded.
//
// The empty string is the DECLARED way to ask for the second (canon §BookIntake.title), which is why
// it is not an absence: a value present means the person named the book themselves, and no later
// parse overwrites it.
func (in Intake) title() string {
	if t := strings.TrimSpace(in.Title); t != "" {
		// ⚠ THE SAME CLEANING THE DERIVED BRANCH GETS, and it was missing here — the defect the
		// exported MaxTitle above exists to prevent, one field over. A title the user TYPED went
		// through length bounding alone, so a control character reached Postgres, which cannot hold
		// U+0000 in a `text` column: measured on a live database as
		// `ERROR: invalid byte sequence for encoding "UTF8": 0x00`, while U+2028 landed and the book
		// lived with a line separator in its name. The rename door refuses both (httpapi, the PATCH
		// handler); a deployment where one writer of a column is stricter than the other is the class
		// this file's own comment calls "how the intake and the patch come to disagree".
		//
		// Cleaned rather than refused HERE, unlike at the rename door, and the difference is the
		// contract's: `BookIntake.title` is one field of a multipart upload whose body has already
		// been received, and failing the whole upload over a stray character would throw away tens of
		// megabytes the user has just sent. The rename door has nothing to throw away, so it refuses
		// and says which member was wrong.
		if t = withoutControls(t); t == "" {
			// Nothing legible was left, so this is the same as having named nothing at all.
			return titleFrom(in.Filename)
		}
		return boundedTitle(t)
	}
	return titleFrom(in.Filename)
}

// withoutControls drops what may not appear in a display name: Unicode Cc (C0 and C1 — NUL, the
// newline, the escape) and the two line separators U+2028/U+2029.
//
// ⚠ AND DELIBERATELY NOT THE FORMAT CATEGORY (Cf). U+200E/U+200F and ZWJ/ZWNJ are ordinary content in
// Hebrew, Arabic, Devanagari and Persian, and dropping "everything invisible" would quietly mangle a
// legitimate title in a language pair this repository does not contain yet — the generality invariant,
// not a hypothetical. The engine's own inbound fence draws the line in the same place.
func withoutControls(s string) string {
	return strings.TrimSpace(strings.Map(func(r rune) rune {
		if unicode.IsControl(r) || r == '\u2028' || r == '\u2029' {
			return -1
		}
		return r
	}, s))
}

// titleFrom is the book's name derived from the file's own.
func titleFrom(filename string) string {
	name := filepath.Base(filepath.FromSlash(filename))
	if i := strings.LastIndexByte(name, '.'); i > 0 {
		name = name[:i]
	}
	name = strings.Map(func(r rune) rune {
		if unicode.IsControl(r) {
			return -1
		}
		return r
	}, name)
	name = strings.TrimSpace(name)
	if name == "." || name == ".." || name == string(filepath.Separator) {
		return ""
	}
	return boundedTitle(name)
}

// boundedTitle is the one bound on a title, wherever it came from: the library lists it, and it is
// otherwise the one unbounded string on that screen.
func boundedTitle(name string) string {
	if utf8.RuneCountInString(name) > MaxTitle {
		return string([]rune(name)[:MaxTitle])
	}
	return name
}

// extensionOf is the one thing about the FORMAT the platform is allowed to know: the engine
// dispatches its reader by extension (`.epub` → the epub reader, anything else → plain text,
// backend/internal/chunk/ingest.go), so the extension has to survive intake or an EPUB is read as
// text.
//
// It is not a format allowlist, and that is deliberate: which formats exist is the engine's
// question, and a platform that refused an extension the engine had just learned would be a second
// place to teach. What it does refuse is a name that is not an extension — the value goes into a
// path, so anything but lowercase alphanumerics is dropped and the source becomes plain text.
func extensionOf(filename string) string {
	ext := strings.ToLower(filepath.Ext(filepath.Base(filepath.FromSlash(filename))))
	if len(ext) < 2 || len(ext) > 9 {
		return ".txt"
	}
	for _, r := range ext[1:] {
		if (r < 'a' || r > 'z') && (r < '0' || r > '9') {
			return ".txt"
		}
	}
	return ext
}