## Tail call optimization implementation.

from lang import clock

## Reference:
## !!CON2019. TAIL CALL OPTIMIZATION (The musical)
## https://www.youtube.com/watch?v=-PX0BV9hGZY

N = 10000

def woo(n)
	if n == 0 then return '' end
	return woo(n-1) + '!'
end

def woo_toc(n, acc)
	if n == 0 then return acc end
	return woo_toc(n-1, acc + '!')
end

s = clock()
print(woo_toc(N, '')) 
print('elapsed:', clock() - s)

## ----------------------------------------------------------------------------

def fac_toc(n, acc)
  if n == 0 then return acc end
  return fac_toc(n-1, acc*n)
end

def fac(n)
  if n == 0 then return 1 end
  return fac(n-1) * n
end


## Limiting to 50 because fac(i) will
## cause a stack overflow
for i in 0..50
  fi = fac(i);
  ft = fac_toc(i, 1)
  if i < 24
    assert(fi == ft)
  else ## For huge values it'll differ by a tiny value.
    assert((fi - ft)/ft < 0.00000000001)
  end
end

## This will run without any problem but we can't
## assert the return value since it's WAY too large for
## a 64 bit value (100000! = 2.824229407 E+456573).
start = clock()
fac_toc(100000, 1)
## expected 0.006s average (on my pc, spec on readme).
print('fac_toc(100000, 1): elapsed:', clock() - start, 's')
assert(true)

def g(n)
  if n == 0 then return 1 end
  return f(n -1)
end

def f(n)
  if n == 0 then return 0 end
  return g(n-1)
end

for i in 0..100
  assert(i%2 == f(i))
end

## The below function caused a bug after compile time stack size calculation
## bug fixed. Added here to check for regression.
def h()
  f() ## <-- Not tail called here (anymore) because it's not returned.
end

# If we got here, that means all test were passed.
print('All TESTS PASSED')