#!/usr/bin/env python
# generate.py
# Glenn G. Chappell
# 3 Feb 2012
#
# For CS 321 Spring 2012
# Example of coroutines in Python


# ----------------------------------------------------------------------
# Standard iterative computation of Fibonacci numbers.
#
# Fibonacci computation function is called in a loop.
#
# NOTE: The following two functions are not currently called.
# ----------------------------------------------------------------------


def fibo(n):
   """Return F(n), the nth Fibonacci number.
   
   F(0) = 0. F(1) = 1. For n >= 2, F(n) = F(n-2)+F(n-1).

   """
   prev = 1
   curr = 0
   for i in xrange(n):  # i = 0, 1, 2, ..., n-1
       next = prev + curr
       prev = curr
       curr = next
   return curr


def print_fibos():
    """Print first 20 Fibonacci numbers, each on a separate line."""

    for n in xrange(20):  # n = 0, 1, 2, ..., 19
        print fibo(n)


# ----------------------------------------------------------------------
# The above is inefficient, since computing fibo(19) requires computing
# all previous Fibonacci numbers. These were already computed and
# printed, but not saved.
#
# One way to deal with this would be to print the numbers inside the
# computation loop. But that makes the function less versatile. Another
# would be to save the computed numbers in a data structure. But that
# takes storage, as well as extra code to create the storage, save
# the values, and then check for them when they are requested.
#
# Another way is to use a coroutine, which Python makes available in the
# form of "generators": functions that can "yield" a value. Yielding is
# like returning, except that the function can then be restarted from
# the point just after the yield. Such a function actually returns an
# iterator; the yielded values can be obtained using a standard loop
# construction.
# ----------------------------------------------------------------------


def fibos_gen():
    """Generator. Return Fibonacci numbers 0, 1, 1, 2, 3, 5, ...."""
    prev = 1
    curr = 0
    yield curr
    while True:
        next = prev + curr
        prev = curr
        curr = next
        yield curr


def print_fibos_from_generator():
    """Print first 20 Fibonacci numbers, each on a separate line.

    Same as print_fibos, but uses a generator function.

    """
    count = 0
    for f in fibos_gen():
        print f
        count += 1
        if count >= 20:
            break


# ----------------------------------------------------------------------
# Call function to print Fibonacci numbers.
# ----------------------------------------------------------------------


print "Fibonacci numbers (computed by generator):"
print_fibos_from_generator()

# Lines below are commented out
#print "Fibonacci numbers:"
#print_fibos()