phaseflow/src/app/api/cycle/current/route.ts

// ABOUTME: API route for current cycle phase information.
// ABOUTME: Returns current cycle day, phase, and phase limits.
import { NextResponse } from "next/server";

import { withAuth } from "@/lib/auth-middleware";
import { getCycleDay, getPhase, getPhaseConfig } from "@/lib/cycle";

// Phase boundaries per spec: MENSTRUAL 1-3, FOLLICULAR 4-(cl-16), OVULATION (cl-15)-(cl-14),
// EARLY_LUTEAL (cl-13)-(cl-7), LATE_LUTEAL (cl-6)-cl
function getNextPhaseStart(currentPhase: string, cycleLength: number): number {
  switch (currentPhase) {
    case "MENSTRUAL":
      return 4; // FOLLICULAR starts at 4
    case "FOLLICULAR":
      return cycleLength - 15; // OVULATION starts at (cycleLength - 15)
    case "OVULATION":
      return cycleLength - 13; // EARLY_LUTEAL starts at (cycleLength - 13)
    case "EARLY_LUTEAL":
      return cycleLength - 6; // LATE_LUTEAL starts at (cycleLength - 6)
    case "LATE_LUTEAL":
      return 1; // New cycle starts
    default:
      return 1;
  }
}

/**
 * Calculates the number of days until the next phase begins.
 * For LATE_LUTEAL, calculates days until new cycle starts (MENSTRUAL).
 */
function getDaysUntilNextPhase(cycleDay: number, cycleLength: number): number {
  const currentPhase = getPhase(cycleDay, cycleLength);

  // For LATE_LUTEAL, calculate days until new cycle
  if (currentPhase === "LATE_LUTEAL") {
    return cycleLength - cycleDay + 1;
  }

  const nextPhaseStart = getNextPhaseStart(currentPhase, cycleLength);
  return nextPhaseStart - cycleDay;
}

export const GET = withAuth(async (_request, user) => {
  // Validate user has required cycle data
  if (!user.lastPeriodDate) {
    return NextResponse.json(
      {
        error:
          "User has no lastPeriodDate set. Please log your period start date first.",
      },
      { status: 400 },
    );
  }

  // Calculate current cycle position
  const cycleDay = getCycleDay(
    user.lastPeriodDate,
    user.cycleLength,
    new Date(),
  );
  const phase = getPhase(cycleDay, user.cycleLength);
  const phaseConfig = getPhaseConfig(phase);
  const daysUntilNextPhase = getDaysUntilNextPhase(cycleDay, user.cycleLength);

  return NextResponse.json({
    cycleDay,
    phase,
    phaseConfig,
    daysUntilNextPhase,
    cycleLength: user.cycleLength,
  });
});