// Package runs owns the life of a translation run: admitting one, spawning its transient systemd
// unit, tailing what it writes, and deciding what happened to it afterwards.
//
// The shape follows from D39.106: the engine is NOT a child of this service, so no component here
// waits on a process. Admission (the queue) only hands out permission to start; everything after
// that is a RECONCILER that reads the world — Postgres, the book's journal, the exit marker — and
// moves the read model to match. That is what makes a run survive a deploy: the process that
// started it and the process that finishes it are usually not the same process.
package runs

import (
	"context"
	"errors"
	"fmt"
	"io/fs"
	"log/slog"
	"os"
	"sync"
	"time"

	"textmachine/platform/internal/books"
	"textmachine/platform/internal/ingest"
	"textmachine/platform/internal/money"
	"textmachine/platform/internal/pgstore"
	"textmachine/platform/internal/pricing"
	"textmachine/platform/internal/runner"
)

// UnitRunner is the transient-unit half of the seam. An interface so the reconciler's decisions can
// be pinned without systemd, and so a test that DOES want systemd is obviously a different test.
type UnitRunner interface {
	Start(ctx context.Context, s runner.Spec) error
	Stop(ctx context.Context, unit string) error
	Alive(ctx context.Context, unit string) (bool, error)
}

// EngineStatus is the ratified repair channel: `tmctl status --json` against a book.
type EngineStatus interface {
	Status(ctx context.Context, binary, workdir string) (ingest.StatusReport, error)
}

// BankApplier is the correction channel: `tmctl bank-apply` against a book (D39.156 pack 2в). An
// interface for the same reason EngineStatus is one: the door's decisions are pinned without an
// engine binary, and a test that wants the real verb is visibly a different test.
type BankApplier interface {
	BankApply(ctx context.Context, binary, workdir, decisionsPath string, preview bool) (runner.BankApplyOutcome, error)
}

// Config is everything the runner needs that an operator chooses.
type Config struct {
	// StateDir is where exit markers live. Platform state, deliberately NOT the book's directory:
	// the engine owns that one and the platform does not write into it (D39.110).
	StateDir string
	// BooksDir is the root the intake creates book directories under — the same path that package
	// holds, and READ ONLY: this package never writes under it. It is here for one question, asked
	// at the door when a book's directory is missing (sourceThere): is this book's own directory
	// gone, or the whole volume? Empty answers "the storage is not there", which refuses the start
	// as a deployment fault — the safe direction on an instance that was never configured to hold
	// books, since it cannot tell the two apart and must not blame a book for it.
	BooksDir string
	// EngineBinary is the VERSIONED path of tmctl (unified backlog row 139). An attempt is pinned to
	// the path it was started with, so a deploy of the engine mid-run cannot change what a resumed
	// attempt executes.
	EngineBinary string
	// MarkerArgv is the command systemd runs as ExecStopPost to write the exit marker.
	MarkerArgv []string
	// Ceiling is how the run's ceiling reaches the engine, as the argv template of the landed flag
	// (row 145). Empty means the configured override rendered nothing — whitespace, say, since an
	// unset variable takes the default — and then starting a run is refused rather than started under
	// the book's own limit.
	Ceiling runner.CeilingTemplate
	// KeysFile is the deployment's provider-key file, passed to `translate` as `--keys-file`
	// (row 211). The path is an engine argument: keys are never put into the unit's environment and
	// never pass through this process. Empty passes no flag.
	KeysFile string
	// MemoryMax and TasksMax bound ONE run's cgroup — the answer to PD-13.
	MemoryMax string
	TasksMax  int
	// AllowEngineVersionChange lets a RESUME run a different engine build than the one its run
	// started with. Off by default and it has to be turned on deliberately (unified backlog row 139):
	// the engine is deployed more often than a translation finishes, so the quiet behaviour would be
	// for a resumed run to continue under a program nobody chose for it.
	AllowEngineVersionChange bool
	// RunBudget is what ONE run may cost a pass of the sweep; zero takes the default. It bounds the
	// starvation the pass itself cannot: the list is ordered the same way every time, so a handful of
	// runs whose engine hangs used to mean the tail was never reached (register row PD-169).
	RunBudget time.Duration
	// MaxAttempts is how many attempts one run may have before this platform stops restarting it;
	// zero takes the default. It is the ONLY bound on the restart loop, and until it existed there
	// was none: an attempt that ends with no marker is restarted by the reconciler, so on a
	// deployment whose marker machinery is broken every restart ends the same way, and the loop is
	// money-NEUTRAL per turn — the old hold comes back whole and an equal one is taken — which is
	// exactly why no balance ever stops it (unified backlog row 398). Each turn still writes an
	// attempt row and three ledger rows, which is the rubbish D39.240 forbids leaving behind.
	//
	// ⚠ IT IS COUNTED IN `run_attempts.attempt_no`, NEVER IN A COUNTER INSIDE ONE ATTEMPT, and that
	// is the whole of why it works. `reconcile_failures` lives on the attempt and a restart opens a
	// NEW attempt row, so a cap keyed on it starts from zero every turn: the defect repeated, wearing
	// the look of a cure.
	//
	// ⚠ THE PRICE OF COUNTING attempt_no, named because it is what makes this a choice: a user's own
	// resume increments it too, so a run resumed more often than the cap allows loses its AUTOMATIC
	// restart — their resume itself still works, the cap binds the reconciler alone. At the shipped
	// default that population is empty, because a healthy run's count is a handful (the admission, a
	// stop and its resume, a bank stop and its resume) and the one pause that recurs daily is not
	// resumable at all (ErrCeilingReached). ⛔ THE CONDITION UNDER WHICH THAT STOPS HOLDING is
	// therefore exactly this: if resumes ever become frequent enough for a WORKING run to approach
	// the cap, the count has to move to automatic restarts only — which needs a column this
	// deliberately does not add.
	MaxAttempts int
	// ResyncEvery is how often a live run is reconciled from `tmctl status --json`. It is a SLOW
	// poll on purpose: every call re-ingests and re-chunks the source (~1.4-1.5 s of CPU on a 23 MB
	// book, unified backlog row 100). Since the emitter landed (D39.131) it is the REPAIR channel and
	// not the source of progress — see maybeResync for which runs still reach it.
	ResyncEvery time.Duration
}

// Service is the run lifecycle.
type Service struct {
	Store   *pgstore.Store
	Runner  UnitRunner
	Engine  EngineStatus
	Bank    BankApplier
	Pricing pricing.Model
	Queue   Enqueuer
	Cfg     Config
	Log     *slog.Logger
	// Now is injectable so the reconciler's clocks are testable without sleeping.
	Now func() time.Time

	// resynced remembers when each run was last reconciled from status. In memory on purpose: it is
	// a rate limit, not a fact — losing it on restart costs one extra status call.
	resynced map[string]time.Time
	// parkedSaid rate-limits the one line a PARKED attempt produces, keyed by attempt. The state it
	// reports can last a run's whole life and is not written down anywhere, so neither extreme is
	// right: said every pass it is the line an operator filters (this file's own rule, deferItem),
	// said once it is invisible to anyone who starts watching afterwards — and unlike a deferral,
	// nothing else carries the fact. Said at the crossing and then no more often than the repair
	// channel speaks.
	parkedSaid map[int64]time.Time
	// books serializes one book's admissions, resumes and corrections against each other — the
	// per-book rule of the correction door (see bank.go, lockBook). In memory on purpose: the
	// sections it guards live inside one process's calls.
	booksMu sync.Mutex
	books   map[string]*bookLock
}

// Enqueuer hands a run to the queue.
//
// Two methods because the two callers need different atomicity, not different queues: an ADMISSION
// writes the run, its hold and its queue entry in one transaction (a job for a run that does not
// exist, or a hold with no job, are both ways to lose money or work), while a RESUME re-opens a run
// that already exists and only then asks for a worker. A lost entry costs a sweep interval either
// way — the reconciler is the backstop for both.
type Enqueuer interface {
	EnqueueRun(ctx context.Context, tx pgstore.Tx, runID string) error
	EnqueueRunNow(ctx context.Context, runID string) error
}

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
}

// ErrCeilingOutOfBounds is a requested ceiling the account or the book cannot carry. It is the
// contract's 409 on a run start: the bounds are read by GET run-options and may move between that
// read and this call, because a hold taken for another book lowers what is left.
var ErrCeilingOutOfBounds = errors.New("runs: the requested ceiling is outside the bounds")

// ErrRePassUnavailable — a re-pass was asked for on a book that has nothing to re-pass: the bank
// has not moved since the last run, or its move touched no already-paid unit. The remedy is the
// options read, which is what says whether a re-pass exists to buy (P10 §3.2).
var ErrRePassUnavailable = errors.New("runs: the book has nothing to re-pass")

// ErrRunnerIncomplete is a DEPLOYMENT that cannot start runs — the exit-marker command is missing,
// so a unit that ended would have no way to say so and its hold would stay reserved forever.
//
// It is a refusal of the run and NOT of the service: an instance whose runner is misconfigured still
// serves every read, and taking the library down with it would turn a partial outage into a total
// one during exactly the operation that causes it, a binary upgrade.
var ErrRunnerIncomplete = errors.New("runs: this deployment cannot record how a run ends")

// ErrBookNotReady is a run asked for on a book whose intake has not finished — one still arriving,
// still being cut, or rejected. The contract's 409: the book exists and the client can see it, and
// what cannot happen yet is a translation of it.
var ErrBookNotReady = errors.New("runs: the book is not ready to be translated")

// ErrSourceGone is a book whose project directory is not on this deployment any more.
//
// ⚠ IT IS A KIND OF ErrBookNotReady rather than a code of its own, and the wrapping is what a client
// reads: the contract's `book_not_ready` already means "the book is there and a translation of it
// cannot happen", which is exactly this, and the second-level cause is where the difference belongs —
// waiting clears the ordinary one and never clears this one. What it must not become is silence
// after the money moved: a run over a directory that is not there spawns, fails, respawns and holds
// the account's credit until a person comes (PD-162).
var ErrSourceGone = fmt.Errorf("%w: its source directory is not on this deployment", ErrBookNotReady)

// ErrStorageUnavailable is the book STORAGE ROOT missing under a book whose directory is missing
// with it — an unmounted volume, or an instance pointed at a path that is not there yet.
//
// ⛔ TOLD APART FROM ErrSourceGone BECAUSE THE FAULT IS THE HOST'S, and reading the host's fault as
// the book's is a defect this repository has already paid for once: in the intake it rejected every
// book on the volume with a reason that blames the user's file and has no way back (PD-192). Here the
// price would be smaller and the same in kind — a user told their book is unusable while the truth is
// that an operator has a volume to mount — so it travels as the deployment's own 503, beside the
// refusals of a runner that was never wired.
var ErrStorageUnavailable = errors.New("runs: the book storage is not mounted on this deployment")

// ErrNotStoppable is a stop asked for on a run that is already over.
var ErrNotStoppable = errors.New("runs: the run is not live")

// ErrNotResumable is a resume that would not move the run: a run that is already going, or one that
// finished. A bank stop is NOT among them — signing is one act over the whole bank and `resume`
// lifts that stop with the decisions as they stand (D39.144).
var ErrNotResumable = errors.New("runs: the run cannot be continued")

// ErrStopRequested is a resume of a run that is GOING and has already been asked to stop.
//
// It is the cut in the idempotent answer, and the cut is what keeps D39.240's hard stop hard: every
// other live run answers this call with the run, because the run is continuing and that is what the
// caller asked for — but a run under a stop is not continuing, and a 202 saying it is would read as
// the stop having been undone. The remedy is the same call once the stop has landed.
//
// Its own sentinel rather than a bare ErrNotResumable because the client is told WHY: waiting works
// here, which is the opposite of what `ceiling_reached` means.
var ErrStopRequested = fmt.Errorf("%w: it has been asked to stop", ErrNotResumable)

// ErrCeilingReached is a resume of a run that stopped at a LIMIT — the chapters it bought, or the
// credit behind them. It is ErrNotResumable with the one cause the contract enumerates, because the
// remedy differs and is the thing a client has to be told: a limit travels with the START of a run,
// so a NEW run with a larger one continues the work and this call never will.
var ErrCeilingReached = fmt.Errorf("%w: it stopped at a limit", ErrNotResumable)

// ErrCreditUnavailable is a resume of a run that has room LEFT in its limit, over an account that
// cannot cover the rest of it. Told apart from ErrCeilingReached because the remedies are opposite:
// this one clears when the account is topped up and the SAME run then continues, while that one is
// finished with and needs a new run. Answering either in place of the other sends the user to do the
// wrong thing (canon §resumeRun).
var ErrCreditUnavailable = fmt.Errorf("%w: the account cannot cover the rest of it", ErrNotResumable)

// CreditHeldError is ErrCeilingOutOfBounds with the book that caused it: another book of the account
// has a run holding the credit, and naming it is what lets a client offer the user somewhere to go.
type CreditHeldError struct{ BookID string }

func (e *CreditHeldError) Error() string {
	return "runs: the account's credit is held by another book"
}

func (e *CreditHeldError) Is(target error) bool { return target == ErrCeilingOutOfBounds }

// ErrNotPriced is a book the engine has not priced, so no order over it can be honest.
//
// ⛔ A REFUSAL AND NOT A FALLBACK, and this is the line the per-chapter constant used to stand on.
// $0.03 a chapter was measured 4.47× low (D39.179 §1) and it made the last chapters of every book
// unbuyable at any balance (PD-440); replacing it with «guess again, more carefully» would keep the
// shape of the defect. There is now one source for what a book costs — the engine's own projection,
// published in `manifest --json` — and when that source has not spoken this platform says so.
//
// It clears by itself: the book's reading surface is owed the moment a manifest can be read, and the
// materializer's drain writes the projection with the tree. An operator can force it with
// `tmplatformctl book refresh --book <id>`.
var ErrNotPriced = errors.New("runs: this book has no price projection, so an order over it cannot be quoted")

// Options is the whole order form for one book: what is left, what the balance covers, and the quote
// for the default order — the whole book (D39.196 §1).
type Options struct {
	pricing.Options
	// BlockedBy is the book whose hold is holding this one's order down, or "".
	BlockedBy string
	// Refusal is what the door would answer a start on this book right now, from the book's own
	// facts — nil when nothing about the book stands in the way. ADVISORY by construction: it is read
	// outside the book lock, so a run can begin or end between this read and the click, and the start
	// remains the only authority on whether a start may happen (canon §RunOptions).
	//
	// ⚠ What it is FOR is the systematic case rather than the one-second race: a run that has been
	// going for minutes was invisible to this form, so the form said `covers_all` over a book the
	// door would refuse and the user learned it by clicking (PD-455). Racing the door is not what
	// this closes and not what a client should expect of it.
	Refusal error
	// Structure is where the chapter cut came from, verbatim from the engine. ChapterOrders says
	// whether an order may be phrased in CHAPTERS against it at all — see ingest.ChapterOrdersOffered,
	// which trusts `detected` alone.
	Structure     string
	ChapterOrders bool
	// SourceChars is what is left to translate, in characters — the unit an order takes when chapter
	// orders are not offered.
	SourceChars int64
}

// Order computes the order form for a book: what it costs, what the balance covers, and what the
// buyer is told before the click.
func (s *Service) Order(ctx context.Context, userID, bookID string) (Options, error) {
	// ⚠ OWNERSHIP IS THIS QUERY'S OWN, not a separate check before it. `ReadBookForOrder` selects
	// `where b.id = $1 and b.owner_id = $2` and answers ErrNoBook for a stranger's book exactly as
	// for a missing one — telling them apart is what would let anyone enumerate other people's
	// libraries. A guard call ahead of it read the same row through a lateral join to say the same
	// thing, on the path a client polls.
	priced, err := s.Store.ReadBookForOrder(ctx, userID, bookID)
	if err != nil {
		return Options{}, err
	}
	acct, err := s.Store.ReadAccount(ctx, userID)
	if err != nil {
		return Options{}, err
	}
	if !priced.Priced {
		return Options{}, ErrNotPriced
	}
	// The door's own read of the book, and a SECOND read beside the one above rather than more
	// columns on it. The two answer differently shaped questions — one row of facts about the book
	// against one row per chapter — which is the reason PricedBook gives for being separate, and it
	// holds in this direction too: the form asks the door's question with the door's own query, so
	// there is one definition of "may a run start here" and not a copy of it phrased for the form.
	facts, err := s.Store.ReadBookForRun(ctx, userID, bookID)
	if err != nil {
		return Options{}, err
	}
	held, heldAmount, err := s.Store.CreditHeldBy(ctx, userID, bookID)
	if err != nil {
		return Options{}, err
	}
	book := priceBook(priced)
	// Balance AS IT IS. A hold is a debit when it is taken, so the balance already excludes the holds
	// open against it and subtracting Reserved a second time would halve the scale (D39.115 §2a).
	out := Options{
		Options:       s.Pricing.Order(book, acct.Balance),
		Structure:     priced.Structure,
		ChapterOrders: ingest.Manifest{Structure: priced.Structure}.ChapterOrdersOffered(),
		Refusal:       startable(facts),
	}
	for _, c := range book.Remaining {
		out.SourceChars += c.SourceChars
	}
	// `blocked` answers "why is the order SMALLER than this account could afford" (canon §RunOptions),
	// so a hold that costs this book nothing must not name another book the user would then go and
	// stop for nothing.
	if held != "" && s.Pricing.Affordable(book, acct.Balance+heldAmount) > out.AffordableChapters {
		out.BlockedBy = held
	}
	return out, nil
}

// priceBook maps the store's rows onto the arithmetic's own shape. It is a mapping and not a second
// derivation: every figure in it was written by the materializer from one manifest.
func priceBook(p pgstore.PricedBook) pricing.Book {
	book := pricing.Book{StepMax: p.StepMax, BookOnce: p.BookOnce,
		Remaining: make([]pricing.Chapter, 0, len(p.Remaining))}
	for _, c := range p.Remaining {
		book.Remaining = append(book.Remaining, pricing.Chapter{
			ID: c.ID, Number: c.Number, Units: c.Units, Expected: c.Expected, SourceChars: c.SourceChars,
		})
	}
	return book
}

// ErrBalanceCannotCarry is an order the account's money does not reach.
//
// ⛔ ITS OWN ERROR BECAUSE ITS REMEDY IS ITS OWN, and folding it into ErrCeilingOutOfBounds sent the
// buyer to do the wrong thing. That error means «the options moved between the read and the call»,
// and the wire word for it, `bounds_moved`, promises a client that re-reading and retrying will
// work. Here nothing moved and retrying is futile: what is missing is money, and the remedy is to
// top up. This is the ONE refusal the whole order form exists to make honest — it is the answer to
// «хватает или нет» arriving after the click — and it was travelling under somebody else's word.
var ErrBalanceCannotCarry = errors.New("runs: the account's balance does not carry this order")

// ErrChapterOrdersUnavailable is an order phrased in CHAPTERS over a book whose chapter cut is not
// trustworthy enough to sell against — see ingest.ChapterOrdersOffered, which trusts `detected`
// alone. The remedy is in the options read: such a book takes an order in CHARACTERS, or the whole
// book, and the client is told the structure word so it can say why.
var ErrChapterOrdersUnavailable = errors.New("runs: this book's chapter boundaries cannot be ordered against")

// StartRequest is one accepted call of POST /books/{id}/runs.
//
// THE THREE KINDS OF ORDER (D39.196 §1, unified backlog row 279), and nothing chooses between them
// by guessing: the whole book (both members nil, the default — «не тронута ни одна ручка ⇒ заказ =
// вся книга»), a number of CHAPTERS, or a number of CHARACTERS. A re-pass is a fourth thing and buys
// no volume at all.
type StartRequest struct {
	UserID     string
	BookID     string
	VerifyBank bool
	// Chapters is how many of the REMAINING chapters to buy; nil is the whole book. It is refused on a
	// book whose cut is not trusted, because there «chapter 12» does not name what a buyer thinks.
	Chapters *int
	// Characters is how much SOURCE TEXT to buy, in runes. It resolves to the shortest prefix of units
	// that reaches it — units being the granularity the engine ships and stops at — and it is the only
	// partial order a book with one chapter can carry.
	Characters *int64
	// RePass buys the re-pass instead of volume (P10, D39.165 §3): legal only when the options
	// announced work (the bank moved and touched paid units), it admits a run whose hold is the
	// materialized projection and whose bar walks the book. Chapters and Characters are ignored.
	RePass bool
}

// Start admits a run.
//
// The order is re-judged here and not trusted from the client: the options read is a READ, the
// balance moves under it — another book's hold lowers what is left — and the number that decides how
// much money is reserved cannot be one the caller chose unilaterally.
func (s *Service) Start(ctx context.Context, in StartRequest) (pgstore.Run, error) {
	// Refused BEFORE the money moves: a run the engine cannot be given a ceiling for would spend
	// under the book's own limit instead of the account's, and that is not a difference to discover
	// after the hold (see runner.CeilingTemplate).
	if err := s.runnable(); err != nil {
		return pgstore.Run{}, err
	}
	// The book's own serialization (bank.go): a run admitted while the correction door is mid-verb
	// on this book would spawn a `translate` straight into the verb's flock — a wasted attempt. The
	// door's own budget bounds each queued call, and the caller's context bounds THIS wait.
	unlock, err := s.lockBook(ctx, in.BookID)
	if err != nil {
		return pgstore.Run{}, err
	}
	defer unlock()
	book, err := s.Store.ReadBookForRun(ctx, in.UserID, in.BookID)
	if err != nil {
		return pgstore.Run{}, err
	}
	// The book's own half of admission, AUTHORITATIVE here because it is read under the lock. The
	// order form asks the same predicate before the click, where the answer is advisory (Order).
	if err := startable(book); err != nil {
		return pgstore.Run{}, err
	}
	// And the half only the disk can answer, asked BEFORE the money: a run over a book whose
	// directory is not there can never move, and until it was asked here the platform took the hold
	// first and discovered the absence afterwards, in a place with no way back (PD-162).
	if err := s.sourceThere(book.Workdir); err != nil {
		return pgstore.Run{}, err
	}
	acct, err := s.Store.ReadAccount(ctx, in.UserID)
	if err != nil {
		return pgstore.Run{}, err
	}
	priced, err := s.Store.ReadBookForOrder(ctx, in.UserID, in.BookID)
	if err != nil {
		return pgstore.Run{}, err
	}
	if !priced.Priced {
		// No projection, no sale. See ErrNotPriced: the alternative is the constant that made the last
		// chapters of every book unbuyable, and a quieter version of it would be worse, not better.
		return pgstore.Run{}, ErrNotPriced
	}
	pb := priceBook(priced)
	// The re-pass consents, decided HERE — under the book lock, once — and stored on the run row so
	// every spawn of every attempt renders the same argv (P10 §3.1).
	//
	// ⛔ `--resnapshot` NOW RIDES EVERY CONTINUATION, not only a run over an edited bank, and that is
	// PD-422 closed rather than a widening. The auto-bank grows by MINING during an ordinary run;
	// mining moves the ENRICHED memory version; the enriched version is folded into the EDIT wave's
	// snapshot alone (backend/internal/pipeline/snapshot.go, snapshotIDForWave). So the SECOND
	// purchase of a mining book meets its own already-pinned edit jobs under a moved snapshot and the
	// drift guard stops the engine — `exit 1`, which this platform can only report as `failed`, after
	// the hold was taken and with nothing translated. The condition used to be the correction door's
	// flag alone, which no amount of mining ever sets.
	//
	// ⚠ Passing it where nothing moved costs NOTHING: the engine re-pins only where a snapshot
	// actually differs (stagerun.go compares per job), and the guard is per JOB, so it never concerns
	// a chapter this run does not touch. And the volume ceiling admits NEW book before re-made book
	// (volume.go, the unitFresh-then-unitRework pass), so a continuation spends its grant on
	// undelivered chapters rather than on re-making the beginning of the book.
	resnapshot := book.BankMoved || book.HasPriorRun
	if in.RePass {
		// The re-pass purchase (P10 §3.2): «a re-pass costs up to your hold». What it would actually
		// cost is unknowable at admission (errata 28.08-к) — the engine's status is blind to a
		// correction until the next translate folds the bank in — so the hold is the honest ceiling of
		// the work bought: the WHOLE book's projection, of which untouched units come back at $0 and
		// the difference is released on settlement. The consent cap equals the hold: funded by
		// construction, and never the bare blanket form.
		if !book.BankMoved {
			return pgstore.Run{}, ErrRePassUnavailable
		}
		hold, bondFunded := s.Pricing.Hold(priced.Expected-priced.BookOnce, priced.StepMax, priced.BookOnce, acct.Balance)
		if hold > acct.Balance {
			return pgstore.Run{}, fmt.Errorf("%w: a re-pass of this book reserves %s", ErrBalanceCannotCarry, hold.USD())
		}
		offset, err := journalSize(book.Workdir)
		if err != nil {
			return pgstore.Run{}, err
		}
		started, err := s.Store.StartRun(ctx, pgstore.StartRunInput{
			UserID:          in.UserID,
			BookID:          in.BookID,
			OrderedChapters: 0,
			Ceiling:         hold,
			Now:             s.now(),
			Resnapshot:      true,
			AcceptRebill:    hold,
			// ⚠ RECORDED HERE TOO, and its absence was a lie about a whole class of runs: a re-pass
			// holds the WHOLE book's projection, so it funds the book-level pass more often than any
			// ordinary purchase — and the field that exists so the platform does not stay silent about
			// an unfunded pass was reporting every re-pass as unfunded.
			BondFunded: bondFunded,
		}, offset, s.enqueue)
		if err != nil {
			return pgstore.Run{}, err
		}
		s.log().InfoContext(ctx, "re-pass admitted", "run", started.ID)
		return started.Run, nil
	}
	quote, order, err := s.resolveOrder(ctx, in, priced, pb, acct.Balance)
	if err != nil {
		return pgstore.Run{}, err
	}
	if quote.Chapters == 0 {
		return pgstore.Run{}, fmt.Errorf("%w: there is nothing left of this book to buy", ErrCeilingOutOfBounds)
	}
	if quote.Hold > acct.Balance {
		// WHY the order does not fit decides what the client can offer next: a balance that moved is
		// waited out or topped up, while another book's hold is somewhere the user can go and act.
		// ⚠ THE TEST IS «WOULD THIS ORDER HAVE FIT WITHOUT THAT HOLD», and the first edition of it was
		// a TAUTOLOGY: it compared this order's hold against a re-quote of the SAME order plus the
		// held amount, which is true whenever the amount is positive — so every refusal named another
		// book, including the ones that had nothing to do with it. The honest question compares
		// against the BALANCE the account would have, and it is the same question `Order` answers when
		// it decides whether to fill `blocked` at all.
		if held, amount, err := s.Store.CreditHeldBy(ctx, in.UserID, in.BookID); err == nil && held != "" &&
			quote.Hold <= acct.Balance+amount {
			return pgstore.Run{}, &CreditHeldError{BookID: held}
		}
		return pgstore.Run{}, fmt.Errorf("%w: it reserves %s", ErrBalanceCannotCarry, quote.Hold.USD())
	}
	offset, err := journalSize(book.Workdir)
	if err != nil {
		return pgstore.Run{}, err
	}
	started, err := s.Store.StartRun(ctx, pgstore.StartRunInput{
		UserID:     in.UserID,
		BookID:     in.BookID,
		VerifyBank: in.VerifyBank,
		// How many chapters the order SPANS. For an order phrased in chapters that is what was bought,
		// and it is what the bar counts and what the wire publishes. For one phrased in CHARACTERS it
		// is only stored: the span overstates what was bought — a single unit of a four-unit chapter
		// spans `1` — so the wire publishes `ordered_units` instead and nulls this
		// (pgstore.runOrderedChapters). ⚠ It is never 0 for either shape (`max(chapters, 1)` in
		// QuoteUnits), and that is load-bearing: 0 is the RE-PASS's mark. Pinned by
		// runs.TestACharacterOrdersVolumeIsPublishedInTheUnitItWasSoldIn and its chapter-shaped twin.
		// The ORDER itself lives on the book, in units.
		OrderedChapters: quote.Chapters,
		Ceiling:         quote.Hold,
		Now:             s.now(),
		Resnapshot:      resnapshot,
		AcceptRebill:    rebillConsent(resnapshot, quote.Hold),
		// ⚠ RECORDED FROM THE QUOTE THAT WAS ACTUALLY ACTED ON, not from the one the form showed. The
		// form is a read and the balance moves under it, so a buyer shown «funded» can be sold a run
		// that is not — and the passes degrade rather than halt, which makes the difference invisible
		// in the book. The run carries what it really got.
		BondFunded: quote.BondFunded,
		// ⚠ SET FOR EVERY ORDER PHRASED IN CHARACTERS — including one that happens to land on a chapter
		// boundary, see pricing.Quote.UnitShaped. It is what switches the run's bar to units: counted
		// in chapters, an order that stops inside one reads `0/N` for its whole life, which is the
		// state this very file refuses a book for at admission (PD-405). See migration 00033.
		OrderedUnits: unitShapedOrder(quote),
		Order:        &order,
	}, offset, s.enqueue)
	if err != nil {
		return pgstore.Run{}, err
	}
	// Neither the book id nor the money: an INFO line names the run and nothing that identifies a
	// user's library or the size of their wallet (D39.84, PD-99).
	s.log().InfoContext(ctx, "run admitted", "run", started.ID, "ordered_chapters", quote.Chapters)
	return started.Run, nil
}

// rebillConsent is the sum a re-pinning run may spend on re-payment: its own hold, and never the
// blanket form.
//
// FUNDED BY CONSTRUCTION, which is the property the adversarial pass K7 bought: a cap larger than
// the hold lets a run burn a budget it does not have on re-billing, and a cap of «yes» lets a
// projection grown past what the buyer saw be paid silently. Zero when nothing is being re-pinned.
func rebillConsent(resnapshot bool, hold money.MicroUSD) money.MicroUSD {
	if !resnapshot {
		return 0
	}
	return hold
}

// resolveOrder turns the three kinds of order into ONE quote and the BOUNDARY the book row stores.
//
// ⚠ The boundary is an IDENTITY and never a count: a book cut again re-numbers its chapters and
// re-mints every unit id, so a stored count would silently become an order for different text
// (migration 00033, ratified 05.09). An order for the WHOLE BOOK has no boundary at all, which is
// the only honest way to say «all of it, however it is cut».
func (s *Service) resolveOrder(ctx context.Context, in StartRequest, priced pgstore.PricedBook,
	pb pricing.Book, balance money.MicroUSD) (pricing.Quote, pgstore.BookOrder, error) {
	switch {
	case in.Chapters != nil:
		if !(ingest.Manifest{Structure: priced.Structure}).ChapterOrdersOffered() {
			return pricing.Quote{}, pgstore.BookOrder{}, fmt.Errorf("%w: its structure is %q",
				ErrChapterOrdersUnavailable, priced.Structure)
		}
		if *in.Chapters < 1 {
			return pricing.Quote{}, pgstore.BookOrder{}, fmt.Errorf("%w: %d chapters",
				ErrCeilingOutOfBounds, *in.Chapters)
		}
		q := s.Pricing.Quote(pb, balance, *in.Chapters)
		if q.ThroughChapterID == "" {
			// The order reached the end of the book, so it IS the whole book — recorded as such rather
			// than as a boundary, for the reason above.
			return q, pgstore.BookOrder{}, nil
		}
		return q, pgstore.BookOrder{
			ThroughChapterID:     q.ThroughChapterID,
			ThroughChapterNumber: q.ThroughChapter,
		}, nil
	case in.Characters != nil:
		rows, err := s.Store.RemainingUnits(ctx, in.UserID, in.BookID)
		if err != nil {
			return pricing.Quote{}, pgstore.BookOrder{}, err
		}
		units := make([]pricing.Unit, len(rows))
		for i, u := range rows {
			units[i] = pricing.Unit{ID: u.ID, Expected: u.Expected, SourceChars: u.SourceChars}
		}
		n, ok := pricing.UnitsFor(units, *in.Characters)
		if !ok || n < 1 {
			return pricing.Quote{}, pgstore.BookOrder{}, fmt.Errorf("%w: %d characters buys no unit of this book",
				ErrCeilingOutOfBounds, *in.Characters)
		}
		if n >= len(units) {
			return s.Pricing.Quote(pb, balance, 0), pgstore.BookOrder{}, nil
		}
		// ⛔ PRICED FROM THE UNITS THEMSELVES, not from the chapters they fall in. Quoting a unit
		// order at the price of every chapter it touches is what made this order meaningless exactly
		// where it is the only one available: a book with no chapter structure is ONE chapter, so a
		// thousand characters of it reserved the whole book. The chapter span below goes into the ROW
		// and no further: nothing publishes it any more (pgstore.runOrderedChapters nulls the view for
		// a unit-shaped run, because the span overstates what was bought), and the bar counts units
		// because `UnitShaped` is set two lines down. What the stored span is still FOR is the re-pass
		// discriminator — `ceiling_chapters = 0` is the re-pass's own mark, and a zero written here
		// would make this order look like one to three internal readers.
		q := s.Pricing.QuoteUnits(pb, balance, units, n, chaptersSpanning(pb, n))
		q.UnitShaped = true
		return q, pgstore.BookOrder{ThroughUnitID: units[n-1].ID}, nil
	default:
		// Neither handle touched: the whole book, and no boundary to store.
		return s.Pricing.Quote(pb, balance, 0), pgstore.BookOrder{}, nil
	}
}

// chaptersSpanning is how many remaining chapters a prefix of `units` units reaches into — at least
// one, because a bar with a denominator of zero is not a bar.
func chaptersSpanning(b pricing.Book, units int) int {
	seen, n := 0, 0
	for _, c := range b.Remaining {
		n++
		seen += c.Units
		if seen >= units {
			break
		}
	}
	return max(n, 1)
}

// readyToTranslate is the half of the book vocabulary a run may start from: everything a finished
// intake can leave behind, plus every state a previous run can leave a parsed book in.
//
// Written as an allowlist and not as "not uploading, not parsing, not rejected": a status added
// later is one this function has never thought about, and refusing it is the safe direction — the
// user gets a 409 they can report, rather than an engine started against a book in a state nobody
// designed for.
func readyToTranslate(status string) bool {
	switch status {
	case "not_started", "translating", "awaiting_bank", "ready", "paused", "stopped", "failed":
		return true
	}
	return false
}

// startable is what THIS BOOK cannot do, beside runnable's what this DEPLOYMENT cannot do: the whole
// rule a start is judged by before money or disk is looked at, and the one definition of it.
//
// ⚠ IT HAS TWO BINDINGS OF DIFFERENT AUTHORITY, and that asymmetry is the design rather than a
// shortcut. `Start` binds it under the book lock, where its answer decides whether the run happens;
// `Order` binds it on the path a client POLLS, where the same answer is advisory and can be stale
// before the user clicks — the canon says as much in its own words, «the only authority on whether
// THIS start may happen; every read below can be stale» (§RunOptions). What one definition buys is
// that the two cannot drift apart: while the live run was known to the door alone, the form answered
// `covers_all` over a book the door was about to refuse, and the click met a refusal the form had
// the facts to predict (PD-455). A rule changed here changes both.
func startable(book pgstore.BookRunContext) error {
	if !readyToTranslate(book.Status) {
		// A book whose intake has not finished has no chapter tree to translate and, while it is
		// `uploading`, half a file on disk. Refused here rather than discovered by the engine inside a
		// transient unit, where the only trace would be a marker reading "exit-code 1" — and after the
		// account's money had already been held for it.
		return fmt.Errorf("%w: it is %s", ErrBookNotReady, book.Status)
	}
	if book.ChapterCount > 0 && !book.HasTree {
		// The book DECLARES chapters and its tree has not been materialised: the intake committed
		// `not_started` and the materialisation that follows it — in a separate transaction, outside
		// the intake's — failed or has not run. Refused HERE, before the hold, because there is no
		// honest bar for such a run and no way out of it either (PD-405). Every counter the screen
		// shows is a count over `chapters`, so the run would read 0/total for its entire life while
		// spending; and the tree's own debt is FROZEN while it runs, because the sweep that pays that
		// debt skips a book with a live run. The window is the intake's normal one — measured at
		// 0.018 s on the healthy path — and unbounded exactly when the materialisation broke, which is
		// the only shape worth refusing.
		//
		// `book_not_ready` is the word for it, not a new one: the canon's own gloss is "still
		// arriving, still being cut, or was rejected", and a book that owes its tree is still being
		// cut. The remedy is to wait for the intake, which is what the status says.
		//
		// ⚠ ONE population does not clear by waiting, and the operator's handle for it exists: a book
		// whose reading-surface debt was WRITTEN OFF after its attempts (AbandonReadModelDebt) owes
		// nothing, so no sweep will build its tree and this refusal stands until somebody asks again —
		// `tmplatformctl book refresh --book <id>`, which is exactly what that command is for, and the
		// books behind it are listed by `tmplatformctl books --abandoned`. Named here because this
		// guard is what turns a book that merely showed a frozen bar into one that cannot be started:
		// the refusal is the honest half, and being able to find the remedy is the other.
		return fmt.Errorf("%w: its chapters are still being materialised", ErrBookNotReady)
	}
	if book.HasLiveRun {
		return pgstore.ErrRunInFlight
	}
	return nil
}

// sourceThere is the DISK half of admission: the book's project directory, asked for before any
// money moves.
//
// ⛔ IT IS THE DOOR'S ALONE AND DELIBERATELY NOT THE FORM'S, unlike startable above. The form is
// polled, and a stat of a hung mount would take the poll down with it; the door is one call the user
// is already waiting on. The price of the asymmetry is named rather than hidden: the form stays
// silent about a vanished directory and says `covers_all` over it — what it no longer does is stay
// silent while the money is TAKEN, which is the half PD-162 is about.
//
// ⚠ There is no "the journal has not been written yet" case to tell apart here, and looking for one
// sends a reader to the wrong file: the directory is created ON INTAKE, before the row that points
// at it (books.Accept, MkdirAll ahead of the insert), so a row always names a directory that existed.
// The first-run exception belongs to the JOURNAL inside it — journalSize maps ENOENT to a zero
// offset, and that is the line this check stands beside rather than duplicates.
func (s *Service) sourceThere(workdir string) error {
	if st, err := os.Stat(workdir); err == nil {
		if st.IsDir() {
			return nil
		}
		// Something is at the path and it is not a project directory. The platform never wrote that,
		// so it is not this book's own end to announce — it is an anomaly an operator is owed.
		return fmt.Errorf("%w: the book's directory is not a directory", ErrStorageUnavailable)
	} else if !errors.Is(err, fs.ErrNotExist) {
		// A directory that is there and cannot be read — a permission, an I/O error — is the host's
		// business exactly as an absent volume is, and the run is refused before the money either way.
		//
		// ⚠ The PATH is dropped and only the operation and the errno are kept. `os.Stat` answers a
		// *fs.PathError, whose text carries the book's own directory, and this error reaches an ERROR
		// log through the handler: a user's book has no business being in one (PD-139, PD-99). The run
		// id the log line already carries says which book to an operator who is allowed to ask.
		var pe *fs.PathError
		if errors.As(err, &pe) {
			return fmt.Errorf("%w: %s: %w", ErrStorageUnavailable, pe.Op, pe.Err)
		}
		return fmt.Errorf("%w: %w", ErrStorageUnavailable, err)
	}
	// WHICH absence this is decides who is at fault, and the two are identical from one book: an
	// unmounted volume leaves an empty mountpoint behind, so every book under it reads as "gone".
	// Blaming the book there is the defect PD-192 cost the intake — a host-wide fault, answered as
	// this book's own end — so the question is put to the storage marker the first upload wrote, not
	// to the root's existence, and the two answers travel under different words: a deployment's 503
	// and the book's own 409.
	//
	// ⛔ AND THE MARKER IS ONLY EVIDENCE ABOUT THE ROOT IT LIVES IN. A book whose directory is NOT
	// under the intake's root — `tmplatformctl book add --workdir` puts one anywhere, and so does an
	// instance with no `BooksDir` at all — sits on a volume this platform never wrote and has no
	// sentinel on. Reading the intake's healthy marker as evidence about THAT volume would announce a
	// vanished mount as the book's own end, which is PD-192 again with a longer path. Where there is
	// nothing to ask, the answer is the deployment's: this platform does not tell anyone their book is
	// finished on the strength of a directory it never created.
	if !books.Owns(s.Cfg.BooksDir, workdir) || !books.StorageIsThere(s.Cfg.BooksDir) {
		return ErrStorageUnavailable
	}
	return ErrSourceGone
}

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

// runnable is what this DEPLOYMENT cannot do, as opposed to what this run cannot do. Both refusals
// are of the run and not of the service: an instance whose runner is misconfigured still serves
// every read, and taking the library down with it would turn a partial outage into a total one
// during exactly the operation that causes it, a binary upgrade.
func (s *Service) runnable() error {
	if len(s.Cfg.MarkerArgv) == 0 {
		return ErrRunnerIncomplete
	}
	if len(s.Cfg.Ceiling) == 0 {
		return runner.ErrCeilingNotWired
	}
	return nil
}

// ceilingFor renders the engine's ceiling argument for an amount.
func (s *Service) ceilingFor(amount money.MicroUSD) ([]string, error) {
	return s.Cfg.Ceiling.Args(amount)
}

// unitShapedOrder is the run's own volume when its order was phrased in CHARACTERS, and nil for every
// other shape. Nil is the ordinary case and leaves the bar counted in chapters exactly as before.
//
// ⚠ The condition is the PHRASING (`q.UnitShaped`), not where the order landed — see
// pricing.Quote.UnitShaped for why, and for the measurement that killed the older wording.
func unitShapedOrder(q pricing.Quote) *int {
	if !q.UnitShaped || q.Units <= 0 {
		return nil
	}
	n := q.Units
	return &n
}