function applySocketMessage(dashboard, payload, session) {
  if (!dashboard) return { dashboard, session };

  switch (payload.type) {
    case 'session.ready':
      return {
        session: payload.session || session,
        dashboard: {
          ...dashboard,
          funds: {
            ...dashboard.funds,
            recent_quotes: payload.live?.recent_quotes || dashboard.funds.recent_quotes,
          },
          strategy: payload.live?.recent_lifecycle_rows ? {
            ...dashboard.strategy,
            strategy_state: {
              ...dashboard.strategy.strategy_state,
              recent_lifecycle_rows: payload.live.recent_lifecycle_rows,
              maker_competitiveness:
                payload.live.maker_competitiveness
                || dashboard.strategy.strategy_state.maker_competitiveness,
              quote_lifecycle_statistics: {
                ...(dashboard.strategy.strategy_state.quote_lifecycle_statistics || {}),
                live:
                  payload.live.quote_lifecycle_statistics
                  || dashboard.strategy.strategy_state.quote_lifecycle_statistics?.live
                  || null,
              },
            },
          } : dashboard.strategy,
          status_bar: {
            ...dashboard.status_bar,
            ...(payload.live?.status_bar || {}),
          },
        },
      };
    case 'quotes.recent':
      return {
        session,
        dashboard: {
          ...dashboard,
          funds: {
            ...dashboard.funds,
            recent_quotes: payload.recent_quotes,
          },
        },
      };
    case 'quote_lifecycle.updated':
      return {
        session,
        dashboard: {
          ...dashboard,
          strategy: {
            ...dashboard.strategy,
            strategy_state: {
              ...dashboard.strategy.strategy_state,
              recent_lifecycle_rows:
                payload.recent_lifecycle_rows
                || dashboard.strategy.strategy_state.recent_lifecycle_rows,
              maker_competitiveness:
                payload.maker_competitiveness
                || dashboard.strategy.strategy_state.maker_competitiveness,
              quote_lifecycle_statistics: {
                ...(dashboard.strategy.strategy_state.quote_lifecycle_statistics || {}),
                live:
                  payload.quote_lifecycle_statistics
                  || dashboard.strategy.strategy_state.quote_lifecycle_statistics?.live
                  || null,
              },
            },
          },
        },
      };
    case 'status_bar.updated':
      return {
        session,
        dashboard: {
          ...dashboard,
          status_bar: {
            ...dashboard.status_bar,
            ...payload.status_bar,
          },
          funds: {
            ...dashboard.funds,
            profitability: {
              ...dashboard.funds.profitability,
              recent_submission_count:
                payload.status_bar.recent_submission_count
                ?? dashboard.funds.profitability.recent_submission_count,
              last_submission_at:
                payload.status_bar.last_submission_at
                || dashboard.funds.profitability.last_submission_at,
            },
          },
        },
      };
    case 'environment_status.updated': {
      const existing = dashboard.system?.environment_status || { recent_changes: [] };
      return {
        session,
        dashboard: {
          ...dashboard,
          system: {
            ...dashboard.system,
            environment_status: {
              ...existing,
              current: payload.environment_status?.payload || existing.current || null,
              recent_changes: [
                payload.environment_status,
                ...(existing.recent_changes || []),
              ].filter(Boolean).slice(0, 20),
              latest_durable_change_at:
                payload.environment_status?.payload?.changed_at
                || payload.environment_status?.observed_at
                || existing.latest_durable_change_at
                || null,
            },
          },
        },
      };
    }
    default:
      return { dashboard, session };
  }
}

export const initialDashboardState = {
  session: null,
  dashboard: null,
  page: null,
  notice: null,
  error: null,
  websocketState: 'connecting',
  lastControlResult: null,
};

export function dashboardReducer(state, action) {
  switch (action.type) {
    case 'session.loaded':
      return {
        ...state,
        session: action.session,
      };
    case 'bootstrap.loaded':
      return {
        ...state,
        dashboard: action.dashboard,
        page: state.page || action.dashboard.default_page || 'funds',
      };
    case 'submissionLedger.loaded':
      return {
        ...state,
        dashboard: {
          ...state.dashboard,
          funds: {
            ...state.dashboard.funds,
            submission_ledger: action.submissionLedger,
          },
        },
      };
    case 'page.changed':
      return {
        ...state,
        page: action.page,
      };
    case 'notice.changed':
      return {
        ...state,
        notice: action.notice,
      };
    case 'error.changed':
      return {
        ...state,
        error: action.error,
      };
    case 'control.result':
      return {
        ...state,
        lastControlResult: action.result,
      };
    case 'websocket.state.changed':
      return {
        ...state,
        websocketState: action.websocketState,
      };
    case 'socket.message.received': {
      const next = applySocketMessage(state.dashboard, action.payload, state.session);
      return {
        ...state,
        session: next.session,
        dashboard: next.dashboard,
      };
    }
    default:
      return state;
  }
}