package httpapi

import (
	"context"
	"encoding/json"
	"errors"
	"log/slog"
	"net/http"
	"strconv"
	"time"

	"textmachine/platform/internal/auth"
	"textmachine/platform/internal/pgstore"
	"textmachine/platform/internal/pricing"
	"textmachine/platform/internal/runner"
	"textmachine/platform/internal/runs"
)

// Library is the read side of the contract surface: everything the handlers below need, as an
// interface, so this package keeps knowing nothing about SQL.
type Library interface {
	ListBooks(ctx context.Context, userID string, limit int, cursor string) (pgstore.Library, error)
	GetBook(ctx context.Context, userID, bookID string) (pgstore.Book, *pgstore.Run, error)
	ReadUsage(ctx context.Context, userID string) (pgstore.Usage, error)
}

// Runs is the write side: the run lifecycle, as the HTTP layer needs to see it.
type Runs interface {
	Bounds(ctx context.Context, userID, bookID string) (pricing.Bounds, error)
	Start(ctx context.Context, in runs.StartRequest) (pgstore.Run, error)
}

// contractRoutes registers the /v0 surface.
//
// Every path is written with the version prefix in the PATTERN and registered on the same mux as the
// ops endpoints: a nested mux behind StripPrefix hands the inner handler a copy of the request and
// the pattern never comes back, which would put raw paths carrying book ids into the access log.
func contractRoutes(mux *http.ServeMux, d Deps, guard func(int64, http.Handler) http.Handler) {
	h := &v0{lib: d.Library, runs: d.Runs, log: d.Log}
	// The READS need a read model and nothing else. An instance with no engine binary is a read
	// replica, not a broken one, and mounting nothing unless it could ALSO start runs made it answer
	// 404 to a library it was holding — while its own boot line said it was serving one.
	if d.Library != nil {
		mux.Handle("GET "+APIPrefix+"/books", guard(DefaultMaxBody, http.HandlerFunc(h.listBooks)))
		mux.Handle("GET "+APIPrefix+"/books/{bookId}", guard(DefaultMaxBody, http.HandlerFunc(h.getBook)))
		mux.Handle("GET "+APIPrefix+"/usage", guard(DefaultMaxBody, http.HandlerFunc(h.usage)))
	}
	// The run surface needs the run lifecycle. Where it is absent the paths stay a guarded 404 rather
	// than a handler that answers 500 on every call.
	if d.Runs != nil {
		mux.Handle("GET "+APIPrefix+"/books/{bookId}/run-options", guard(DefaultMaxBody, http.HandlerFunc(h.runOptions)))
		mux.Handle("POST "+APIPrefix+"/books/{bookId}/runs", guard(DefaultMaxBody, http.HandlerFunc(h.startRun)))
	}
}

type v0 struct {
	lib  Library
	runs Runs
	log  *slog.Logger
}

// The wire shapes below are the contract's, field for field (openapi 0.2.0). They are written out
// rather than generated because a generated struct would still need the projection written by hand,
// and a second copy of the field names is what makes a divergence invisible.

type wireProgress struct {
	Draft      wireCounter `json:"draft"`
	Edit       wireCounter `json:"edit"`
	ETASeconds *int        `json:"eta_seconds"`
}

type wireCounter struct {
	Done  int `json:"done"`
	Total int `json:"total"`
}

type wireBook struct {
	ID             string       `json:"id"`
	Title          string       `json:"title"`
	SourceLang     string       `json:"source_lang"`
	TargetLang     string       `json:"target_lang"`
	Genre          string       `json:"genre"`
	ChapterCount   int          `json:"chapter_count"`
	CharacterCount int64        `json:"character_count"`
	AddedAt        time.Time    `json:"added_at"`
	Status         string       `json:"status"`
	Progress       wireProgress `json:"progress"`
	NoteCount      int          `json:"note_count"`
}

type wireLibrary struct {
	Revision   int64      `json:"revision"`
	NextCursor *string    `json:"next_cursor"`
	Books      []wireBook `json:"books"`
}

type wireRun struct {
	ID              string     `json:"id"`
	Revision        int64      `json:"revision"`
	Status          string     `json:"status"`
	VerifyBank      bool       `json:"verify_bank"`
	CeilingChapters int        `json:"ceiling_chapters"`
	PausedReason    *string    `json:"paused_reason"`
	StartedAt       time.Time  `json:"started_at"`
	FinishedAt      *time.Time `json:"finished_at"`
}

type wireBookDetail struct {
	Revision int64    `json:"revision"`
	Book     wireBook `json:"book"`
	Run      *wireRun `json:"run"`
}

type wireCeilingBounds struct {
	MinChapters     int `json:"min_chapters"`
	MaxChapters     int `json:"max_chapters"`
	DefaultChapters int `json:"default_chapters"`
}

type wireRunOptions struct {
	Ceiling wireCeilingBounds `json:"ceiling"`
}

type wireRunRequest struct {
	// Pointers because both fields are REQUIRED and "absent" has to be told from "false" and from
	// "zero": a run started without a declared ceiling would spend past the limit the user is
	// entitled to set beforehand (contract §RunRequest).
	VerifyBank      *bool `json:"verify_bank"`
	CeilingChapters *int  `json:"ceiling_chapters"`
}

type wireUsage struct {
	State            string  `json:"state"`
	RemainingPercent int     `json:"remaining_percent"`
	PausedReason     *string `json:"paused_reason"`
}

func (h *v0) listBooks(w http.ResponseWriter, r *http.Request) {
	user, ok := principal(w, r)
	if !ok {
		return
	}
	limit, ok := pageLimit(w, r)
	if !ok {
		return
	}
	lib, err := h.lib.ListBooks(r.Context(), user, limit, r.URL.Query().Get("cursor"))
	if err != nil {
		h.fail(w, r, err)
		return
	}
	out := wireLibrary{Revision: lib.Revision, Books: make([]wireBook, 0, len(lib.Books))}
	if lib.NextCursor != "" {
		out.NextCursor = &lib.NextCursor
	}
	for _, b := range lib.Books {
		out.Books = append(out.Books, projectBook(b))
	}
	writeJSON(w, r, http.StatusOK, out, h.log)
}

func (h *v0) getBook(w http.ResponseWriter, r *http.Request) {
	user, ok := principal(w, r)
	if !ok {
		return
	}
	book, run, err := h.lib.GetBook(r.Context(), user, r.PathValue("bookId"))
	if err != nil {
		h.fail(w, r, err)
		return
	}
	// ONE counter per book, and it is the book's (contract §Revision: "every book-scoped read and the
	// id of every stream frame of that book's run carry the same number"). Reading it off the run row
	// made the card lag: a unit_done bumps the book and the chapter and not the run, so a client that
	// had applied frame id=2 got revision 0 back and, obeying the contract, dropped the read.
	out := wireBookDetail{Revision: book.Revision, Book: projectBook(book)}
	if run != nil {
		wr := projectRun(*run)
		wr.Revision = book.Revision
		out.Run = &wr
	}
	writeJSON(w, r, http.StatusOK, out, h.log)
}

func (h *v0) runOptions(w http.ResponseWriter, r *http.Request) {
	user, ok := principal(w, r)
	if !ok {
		return
	}
	b, err := h.runs.Bounds(r.Context(), user, r.PathValue("bookId"))
	if err != nil {
		h.fail(w, r, err)
		return
	}
	writeJSON(w, r, http.StatusOK, wireRunOptions{Ceiling: wireCeilingBounds{
		MinChapters:     b.Min,
		MaxChapters:     b.Max,
		DefaultChapters: b.Default,
	}}, h.log)
}

func (h *v0) startRun(w http.ResponseWriter, r *http.Request) {
	user, ok := principal(w, r)
	if !ok {
		return
	}
	// Unknown properties are IGNORED, not refused: RunRequest does not declare
	// additionalProperties: false, and inside 0.x a minor bump is where optional fields appear — a
	// server that rejected the whole request would break a client generated against 0.3.0 for a field
	// it was free to ignore.
	var req wireRunRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		WriteProblem(w, http.StatusBadRequest, "Request could not be read", "")
		return
	}
	if req.VerifyBank == nil || req.CeilingChapters == nil {
		WriteProblem(w, http.StatusBadRequest, "Request is incomplete", "")
		return
	}
	// The schema's own minimum (RunRequest.ceiling_chapters, minimum: 1) belongs HERE and not in the
	// service: a request that violates the schema is malformed, and answering it with 409 would tell
	// the client the bounds had moved — so it would re-read run-options and retry, forever, a request
	// that can never succeed.
	if *req.CeilingChapters < 1 {
		WriteProblem(w, http.StatusBadRequest, "Request could not be read", "")
		return
	}
	run, err := h.runs.Start(r.Context(), runs.StartRequest{
		UserID:          user,
		BookID:          r.PathValue("bookId"),
		VerifyBank:      *req.VerifyBank,
		CeilingChapters: *req.CeilingChapters,
	})
	if err != nil {
		h.fail(w, r, err)
		return
	}
	writeJSON(w, r, http.StatusAccepted, projectRun(run), h.log)
}

func (h *v0) usage(w http.ResponseWriter, r *http.Request) {
	user, ok := principal(w, r)
	if !ok {
		return
	}
	u, err := h.lib.ReadUsage(r.Context(), user)
	if err != nil {
		h.fail(w, r, err)
		return
	}
	out := wireUsage{State: usageState(u.RemainingPercent, u.Spendable), RemainingPercent: u.RemainingPercent}
	if u.PausedReason != "" {
		out.PausedReason = &u.PausedReason
	}
	writeJSON(w, r, http.StatusOK, out, h.log)
}

// lowCredit is the share at which the interface warns. The threshold belongs to the platform and
// never travels: a client that computed it from the percentage would carry a second copy of the
// policy, and the two would diverge on the day it changes (contract §Usage).
const lowCredit = 10

// spendable, not the percentage, decides "exhausted". The share is floored, so a large grant with a
// small remainder rounds to 0% — and the account screen then said "nothing left" while the run dialog
// offered a 300-chapter scale that started successfully. The two screens now answer from the same
// fact.
func usageState(percent int, spendable bool) string {
	switch {
	case !spendable:
		return "exhausted"
	case percent <= lowCredit:
		return "low"
	default:
		return "ok"
	}
}

func projectBook(b pgstore.Book) wireBook {
	return wireBook{
		ID: b.ID, Title: b.Title, SourceLang: b.SourceLang, TargetLang: b.TargetLang,
		Genre: b.Genre, ChapterCount: b.ChapterCount, CharacterCount: b.CharacterCount,
		AddedAt: b.AddedAt, Status: b.Status, NoteCount: b.NoteCount,
		Progress: wireProgress{
			Draft:      wireCounter{Done: b.Progress.DraftDone, Total: b.Progress.DraftTotal},
			Edit:       wireCounter{Done: b.Progress.EditDone, Total: b.Progress.EditTotal},
			ETASeconds: b.Progress.ETASeconds,
		},
	}
}

func projectRun(r pgstore.Run) wireRun {
	out := wireRun{
		ID: r.ID, Revision: r.Revision, Status: r.Status, VerifyBank: r.VerifyBank,
		CeilingChapters: r.CeilingChapters, StartedAt: r.StartedAt, FinishedAt: r.FinishedAt,
	}
	if r.PausedReason != "" {
		out.PausedReason = &r.PausedReason
	}
	return out
}

// principal reads the caller established by the middleware. Absent means the guard did not run,
// which is a wiring defect rather than an unauthenticated request — so it is a 500, and it is
// logged: answering 401 would hide a route mounted without its guard.
func principal(w http.ResponseWriter, r *http.Request) (string, bool) {
	p, ok := auth.FromContext(r.Context())
	if !ok || p.UserID == "" {
		WriteProblem(w, http.StatusInternalServerError, "Internal error", "")
		return "", false
	}
	return p.UserID, true
}

func pageLimit(w http.ResponseWriter, r *http.Request) (int, bool) {
	raw := r.URL.Query().Get("limit")
	if raw == "" {
		return 0, true // the collection's own default
	}
	n, err := strconv.Atoi(raw)
	if err != nil || n < 1 {
		WriteProblem(w, http.StatusBadRequest, "Request could not be read", "")
		return 0, false
	}
	return n, true
}

// fail maps a domain error onto the contract's status codes.
//
// Neither title nor detail ever carries engine or database text (contract §Problem): what a client
// has to put on a screen is a product phrase, and an error string written for an operator becomes
// the sentence a reader gets.
func (h *v0) fail(w http.ResponseWriter, r *http.Request, err error) {
	switch {
	case errors.Is(err, pgstore.ErrNoBook), errors.Is(err, pgstore.ErrNoAccount):
		WriteProblem(w, http.StatusNotFound, "Object not found", "")
	case errors.Is(err, pgstore.ErrBadCursor):
		WriteProblem(w, http.StatusBadRequest, "Request could not be read", "")
	case errors.Is(err, pgstore.ErrRunInFlight):
		WriteProblem(w, http.StatusConflict, "This book is already being translated", "")
	case errors.Is(err, runs.ErrCeilingOutOfBounds), errors.Is(err, pgstore.ErrInsufficientCredit):
		// 409 and not 400: the request was legal when the bounds were read, and a hold taken for
		// another book between that read and this call is what moved them (contract §startRun).
		WriteProblem(w, http.StatusConflict, "The chosen limit no longer fits", "")
	case errors.Is(err, runner.ErrCeilingNotWired), errors.Is(err, runs.ErrRunnerIncomplete):
		// A DEPLOYMENT that cannot start runs: it has no way to tell the engine its ceiling (row 145)
		// or no way to record how a unit ended. ⚠ 503 is NOT among the statuses the contract
		// enumerates for this operation; it is used because every alternative lies — the request is
		// valid, the object exists, and the state is not in conflict. Raised as a question to the
		// contract's owner rather than resolved by editing the spec.
		h.log.ErrorContext(r.Context(), "run refused: this deployment cannot start runs", "err", err)
		WriteProblem(w, http.StatusServiceUnavailable, "Translation cannot be started right now", "")
	default:
		h.log.ErrorContext(r.Context(), "request failed", "err", err)
		WriteProblem(w, http.StatusInternalServerError, "Internal error", "")
	}
}

func writeJSON(w http.ResponseWriter, r *http.Request, status int, body any, log *slog.Logger) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	if err := json.NewEncoder(w).Encode(body); err != nil {
		log.DebugContext(r.Context(), "response body not delivered", "err", err) // the client went away
	}
}