lcg_test.py

# test of cc65's rand.s LCG

import math
import random

seed = 1

def srand(x):
	global seed
	seed = x & 0xFFFF

def rand():
	global seed
	seed *= 0x01010101
	seed += 0x31415927
	seed &= 0xFFFFFFFF
	return (seed >> 8) & 0x7FFF

def randfunc(a, b):
	return a + (rand() % b)

def test_rand(width, iters, rfunc):
	count = [0] * width
	run_lengths = [[] for i in range(width+1)]
	last = width
	run = 0
	for i in range(0,iters):
		v = rfunc(0, width)
		count[v] += 1
		if v == last:
			run += 1
		else:
			run_lengths[last].append(run)
			run = 1
			last = v
	print("rand() %% %d statistics:" % (width))
	for i in range(0,width):
		# Perhaps, due to the way run length is tailed, the following should be done?
		# Even then, there is a significant difference.
		#run_lengths[i] = map(lambda x: math.log(float(x)), run_lengths[i])
		avg = float(sum(run_lengths[i]))/float(len(run_lengths[i]))
		run_length_sd = math.sqrt(sum(map((lambda x: (avg-float(x))**2), run_lengths[i]))/float(len(run_lengths[i])))
		print("%d happened %10d times, average run %.3f run length SD %10.3f" % (i,count[i],avg,run_length_sd))

test_rand(2,100000,randfunc)
test_rand(8,100000,randfunc)

test_rand(2,100000,random.randrange)
test_rand(8,100000,random.randrange)