#! /usr/bin/python3
# Last edited on 2020-10-06 11:11:59 by jstolfi

from math import floor

def compute_abscissas(yr0,mt0,yr1,mt1,x0,x1):
  """Computes abscissas for the verical lines
  separating presidential mandates. Assumes that abscissa {x0}
  corresponds to year {yr0} and month {mt0}, while abscissa
  {x1} corresponds to year {yr1} and month {mt1}."""
  
  # Convert first and last dates to month counts:
  ym0 = 12*yr0 + (mt0-1);
  ym1 = 12*yr1 + (mt1-1);
  
  cissas = [];
  for yp in (1989, 1993, 1997, 2001, 2005, 2009, 2013, 2017):
    # Convert {yp,'Jan'} to month count:
    ymp = yp*12;
    # Affine map:
    xp = int(floor(x0 + (ymp - ym0)*(x1-x0)/(ym1 - ym0) + 0.5));
    cissas.append(xp)
  return cissas
  
print("05-40", compute_abscissas(1989, 1,2020, 9,61,799))
  
print("07-16", compute_abscissas(1988, 9,2020, 9,61,799))
   
print("09-41", compute_abscissas(1987,10,2020, 6,61,799))