import swisseph as swe

# Make angle lie between [0, 360)
norm360 = lambda angle: angle % 360

# Makara sankranti means sun enters Makara rashi Capricorn = 10th sign
# longitude = 270° = 30° x 9 previous signs traversed
def func(jd):
    longi = swe.calc_ut(jd, swe.SUN, flags = swe.FLG_SWIEPH | swe.FLG_SIDEREAL)
    l = norm360(longi[0][0]) # degrees
    return l - 270

# Print julian day in readable format
def format_jd(jd):
    yy, mon, dd, hh_mm = swe.revjul(jd)
    hh = hh_mm // 1
    mm = (hh_mm % 1) * 60
    name = swe.get_ayanamsa_name(sidm)
    if name:
        return "%04d-%02d-%02d %02d:%02d\t\t%s" % (yy, mon, dd, hh, mm, name)

# calculates "x", x ∈ [start, stop], such that func(x) == 0
def bisection_search(func, start, stop):
  left = start
  right = stop
  epsilon = 5E-10   # Anything better than this puts the loop below infinite

  while True:
    middle = (left + right) / 2
    midval =  func(middle)
    rtval = func(right)
    if midval * rtval >= 0:
      right = middle
    else:
      left = middle

    if (right - left) <= epsilon: break

  return (right + left) / 2

# We assume that Sankranti occurs between Nov 2022 and Mar 2023
start_jd = swe.julday(2022, 10, 1) # Nov 01
end_jd = swe.julday(2023, 3, 30)   # Mar 30
sankranti = []
for sidm in range(0, 55):
    swe.set_sid_mode(sidm)
    jd_sank = bisection_search(func, start_jd, end_jd)
    string = format_jd(jd_sank)
    if string: sankranti.append(string)

sankranti.sort()
[ print(d) for d in sankranti ]