## Evalivate a lisp like expression.
##  - Only single digit numbers are allowed.
##  - No space is allowed.
##
## (+(*23)(/9(-85))) is equivalent of
## (+ (* 2 3) (/ 9 (-8 5))) = 9

tests = {
  "(+(*23)(/9(-85)))" : 9,
  "(-(*33)(+5(-43)))" : 3,
  "+(/6(+1(-65)))(-9(*42))" : 4,
  "-(*(+2(*1(-54)))3)(+1(/88))" : 7
}

def main()
  for expr in tests
    r = eval(expr, 0); assert(r[1] != -1)
    val = r[0]
    assert(val == tests[expr])
  end
  print("ALL TESTS PASSED")
end

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

## Return [value, index], if error index will be -1.
def eval(expr, ind)
  c = expr[ind]; ind += 1

  if c == '('
    r = eval(expr, ind)
    val = r[0]
    ind = r[1]
    if ind == -1 then return [null, -1] end
    if ind == expr.length then
      print("Invalid expression.")
      return [null, -1]
    end
    assert(ind < expr.length)
    c = expr[ind]; ind += 1
    if c != ')'
      print("Expected ')'"); return [null, -1]
    end
    return [val, ind]
  
  else if c == '+'
    r = binary_op(expr, ind)
    if not r[0] then return [null, -1] end
    return [r[1] + r[2], r[3]]
  
  else if c == '-'
    r = binary_op(expr, ind)
    if not r[0] then return [null, -1] end
    return [r[1] - r[2], r[3]]
  
  else if c == '*'
    r = binary_op(expr, ind)
    if not r[0] then return [null, -1] end
    return [r[1] * r[2], r[3]]
  
  else if c == '/'
    r = binary_op(expr, ind)
    if not r[0] then return [null, -1] end
    return [r[1] / r[2], r[3]]
    
  else if isnum(c)
    val = ord(c) - ord('0')
    assert(0 <= val and val < 10)
    return [val, ind]

  else
    print("Uexpected token:", c)
    return [null, -1]
    
  end ## switch(c)
  
end

## return [success, v1, v2, index]
def binary_op(expr, ind)
  r = eval(expr, ind)
  v1 = r[0]; ind = r[1]
  if ind == -1 then return [false, null, null, -1] end
    
  r = eval(expr, ind)
  v2 = r[0]; ind = r[1]
  if ind == -1 then return [false, null, null, -1] end
  
  return [true, v1, v2, ind]
end


## Return true if c in numeric.
def isnum(c)
  k = ord(c) - ord('0')
  ## TODO: k in 0..10
  return (0 <= k and k < 10)
end

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

## if __name__ == '__main__' ;)
  main()