Comment by Turskarama

There's a lot that can be, take this for instance (line 3336):

  if (c == '\033')
      term->print_state = 1;
  else if (c == (unsigned char)'\233')
      term->print_state = 2;
  else if (c == '[' && term->print_state == 1)
      term->print_state = 2;
  else if (c == '4' && term->print_state == 2)
      term->print_state = 3;
  else if (c == 'i' && term->print_state == 3)
      term->print_state = 4;
  else
      term->print_state = 0;

This could be turned into a pure function that takes c and print_state as input, and returns a new print_state which the outer function assigns. That's 12 lines turned into 1.

  term->print_state = newState(c, term->print_state);

(I am not a C developer so the sytax could be wrong). Just because the outer function is impure doesn't mean it can't in turn call pure functions.