package sessionread

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"strings"
	"time"

	"git.yeij.top/AskaEth/Cyrene_v2/internal/capability/restrict"
	"git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/history"
	"git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/loop"
)

// Page bounds of the read. They match the REST page (GET /sessions/{id}/messages,
// history.clampLimit) so a tool page and a client page are the same size, with the
// default and the cap applied here rather than rejected as bad arguments.
const (
	DefaultLimit = 50
	MaxLimit     = 200
)

// Sentinels distinguishing the fail-closed refusals from a genuine failure. Each
// case has its own message so the model — and the operator reading the log — can
// tell "not allowed" from "does not exist", which design/22 §8.1 约束 3 requires.
var (
	// ErrNoHistory is returned when no read-only history view was injected.
	ErrNoHistory = errors.New("sessionread: no session-history reader injected")

	// ErrNotOwner is returned when the calling conversation is not the owner's.
	ErrNotOwner = errors.New("sessionread: only the owner conversation may read another session")

	// ErrSessionNotAllowed is returned when the target session id is outside the
	// readable whitelist (Web main session and Web sub sessions only).
	ErrSessionNotAllowed = errors.New("sessionread: target session is not readable (only web / web:<uuid>)")

	// ErrSessionNotFound is returned when the target session does not exist. It is
	// deliberately an error, never empty material: "read nothing" must not be
	// mistakable for "they said nothing".
	ErrSessionNotFound = errors.New("sessionread: target session does not exist")
)

// HistoryReader is the read-only session-history view this package needs. It is
// deliberately narrower than history.HistoryStore: it declares no append, no
// delete and no read-cursor call, so from inside this package a message cannot be
// written to any session and the target session's read_at cannot be advanced
// (design/22 §8.1 约束 1).
type HistoryReader interface {
	// List returns up to limit messages of sessionID, newest first, bounded to the
	// messages strictly older than before when before > 0 (design/16 §3.1).
	List(ctx context.Context, sessionID string, before int64, limit int, platform string) ([]history.Message, error)
	// ListSessions returns every session's metadata (id, title, role).
	ListSessions(ctx context.Context) ([]history.SessionMeta, error)
}

// PersonaNamer resolves role ids to display names. The package keeps this a narrow
// interface on purpose: importing internal/persona/registry here would couple the
// runtime to the persona loader, so the composition root supplies an adapter.
type PersonaNamer interface {
	// PersonaName returns the display name of role id. ok is false for a role this
	// runtime does not know, in which case the raw id is shown instead of dropping
	// the attribution.
	PersonaName(id string) (string, bool)
	// DefaultPersonaID returns the global default role id, or "" when none is set.
	// An unset role id on a message or session means "the default role".
	DefaultPersonaID() string
}

// noPersonaNames is the fallback namer: it knows no role names, so the material
// shows raw ids rather than inventing a display name.
type noPersonaNames struct{}

// PersonaName implements PersonaNamer.
func (noPersonaNames) PersonaName(string) (string, bool) { return "", false }

// DefaultPersonaID implements PersonaNamer.
func (noPersonaNames) DefaultPersonaID() string { return "" }

// Request is one session-read request after argument decoding.
type Request struct {
	// SessionID is the target session ("web" or "web:<uuid>").
	SessionID string
	// Limit caps the page; 0 or less means DefaultLimit, above MaxLimit is clamped.
	Limit int
	// Before is the millisecond cursor: only messages strictly older than it are
	// returned. 0 or less means the newest page.
	Before int64
}

// Reader performs the fail-closed authorization and the read, then renders the
// material. It holds no per-call state and is safe for concurrent use.
type Reader struct {
	history  HistoryReader
	personas PersonaNamer
	loc      *time.Location
	log      *slog.Logger
}

// Read authorizes the call, reads one page of the target session and returns the
// formatted external material.
//
// The checks run in this order deliberately: the caller's environment first (a
// non-owner conversation learns nothing about any session), the id whitelist
// second (a session outside web/web:<uuid> is refused even if it exists), and only
// then the existence lookup.
func (r *Reader) Read(ctx context.Context, req Request) (string, error) {
	if r.history == nil {
		return "", ErrNoHistory
	}
	env := loop.SessionEnvFrom(ctx)
	if err := r.authorize(env, req.SessionID); err != nil {
		return "", err
	}
	meta, err := r.sessionMeta(ctx, req.SessionID)
	if err != nil {
		return "", err
	}
	limit := clampLimit(req.Limit)
	msgs, err := r.history.List(ctx, req.SessionID, req.Before, limit, "")
	if err != nil {
		return "", fmt.Errorf("sessionread: list %s: %w", req.SessionID, err)
	}
	m := material{
		sessionID: req.SessionID,
		title:     meta.Title,
		personaID: meta.PersonaID,
		before:    req.Before,
		msgs:      msgs,
		hasOlder:  len(msgs) == limit,
	}
	// Who read what, and how much — the provenance of design/22 §8.1 约束 4 — is
	// logged here; the message bodies are not. The audit record itself stays with the
	// loop's gate/OnAudit, which this tool neither writes to nor bypasses.
	r.log.Info("external session material read",
		"actor", string(env.ActorEnv), "target", req.SessionID, "count", len(msgs),
		"limit", limit, "before", req.Before, "has_older", m.hasOlder)
	return r.render(m), nil
}

// authorize enforces the fail-closed whitelist: the calling conversation must be
// the owner's, and the target must be the Web main session or a Web sub session.
// Default deny, never default allow (design/22 §8.1 约束 3).
func (r *Reader) authorize(env loop.SessionEnv, sessionID string) error {
	if env.ActorEnv != restrict.ActorOwner {
		r.log.Warn("external session read refused", "target", sessionID, "env", string(env.ActorEnv))
		return ErrNotOwner
	}
	if !ReadableSessionID(sessionID) {
		r.log.Warn("external session read refused", "target", sessionID, "reason", "session not readable")
		return ErrSessionNotAllowed
	}
	return nil
}

// ReadableSessionID reports whether id is in the readable set: the Web main session
// or a Web sub session. It is the fail-closed whitelist of design/22 §8.1 约束 3 —
// IM group and stranger sessions live outside it and are exactly the ones that must
// never leak.
func ReadableSessionID(id string) bool {
	if id == history.MainSessionID {
		return true
	}
	return strings.HasPrefix(id, history.MainSessionID+":") && len(id) > len(history.MainSessionID)+1
}

// sessionMeta looks the session up in the session list, so a session that does not
// exist is a hard error rather than empty material. The list read is a single
// parameter-free query the store already serves for the session list page.
func (r *Reader) sessionMeta(ctx context.Context, sessionID string) (history.SessionMeta, error) {
	sessions, err := r.history.ListSessions(ctx)
	if err != nil {
		return history.SessionMeta{}, fmt.Errorf("sessionread: list sessions: %w", err)
	}
	for _, meta := range sessions {
		if meta.ID == sessionID {
			return meta, nil
		}
	}
	return history.SessionMeta{}, fmt.Errorf("sessionread: session %q: %w", sessionID, ErrSessionNotFound)
}

// clampLimit bounds a requested page size to [DefaultLimit, MaxLimit], mirroring
// history.clampLimit so a tool page and a client page are the same size. A
// non-positive request (an absent argument, a non-numeric one, or a negative one)
// means DefaultLimit; a request above the cap is clamped, not rejected.
func clampLimit(limit int) int {
	if limit < 1 {
		return DefaultLimit
	}
	if limit > MaxLimit {
		return MaxLimit
	}
	return limit
}