package main

import (
	"encoding/json"
	"errors"
	"io"
	"net/http"
	"strconv"
	"strings"
)

const (
	sessionSnapshotItems = 24
	sessionSnapshotBytes = 8 << 20
)

type sessionSnapshot struct {
	SessionID     string            `json:"session_id"`
	Messages      []json.RawMessage `json:"messages"`
	TotalMessages int               `json:"total_messages"`
}

// boundSessionSnapshot caps only the continuity fallback response. Native
// WebUI requests remain untouched, including when an older backend ignores
// its optional `limit` query parameter.
func boundSessionSnapshot(response *http.Response) error {
	request := response.Request
	if request == nil || response.StatusCode != http.StatusOK ||
		request.URL.Query().Get("hermes_fallback") != "1" ||
		!strings.HasPrefix(request.URL.Path, "/api/sessions/") ||
		!strings.HasSuffix(request.URL.Path, "/messages") {
		return nil
	}
	body, err := io.ReadAll(io.LimitReader(response.Body, sessionSnapshotBytes+1))
	if err != nil {
		return err
	}
	_ = response.Body.Close()
	if len(body) > sessionSnapshotBytes {
		return errors.New("session snapshot exceeds safe response limit")
	}
	var payload sessionSnapshot
	if err := json.Unmarshal(body, &payload); err != nil || payload.Messages == nil {
		return errors.New("session snapshot is malformed")
	}
	total := len(payload.Messages)
	if payload.TotalMessages > total {
		total = payload.TotalMessages
	}
	if len(payload.Messages) > sessionSnapshotItems {
		payload.Messages = payload.Messages[len(payload.Messages)-sessionSnapshotItems:]
	}
	payload.TotalMessages = total
	body, err = json.Marshal(payload)
	if err != nil {
		return err
	}
	response.Body = io.NopCloser(strings.NewReader(string(body)))
	response.ContentLength = int64(len(body))
	response.Header.Set("Content-Length", strconv.Itoa(len(body)))
	response.Header.Set("Content-Type", "application/json; charset=utf-8")
	response.Header.Set("Cache-Control", "no-store")
	response.Header.Del("ETag")
	return nil
}