///|
/// Compute 2 raised to the power of x.
///
/// # Examples
///
/// ```moonbit nocheck
/// assert_eq(exp2(-1), 0.5);
/// assert_eq(exp2(-2), 0.25);
/// assert_eq(exp2(-3), 0.125);
/// assert_eq(exp2(-4), 0.0625);
/// assert_eq(exp2(0), 1);
/// assert_eq(exp2(1), 2);
/// assert_eq(exp2(2), 4);
/// assert_eq(exp2(3), 8);
/// ```
///
/// # Accuracy
///
/// 1 ulp (unit in the last place)
///
/// # Special Cases
///
/// 1. exp2(+Inf) = +Inf
/// 2. exp2(-Inf) = 0
///.3. exp2(NaN) = NaN
pub fn exp2(x : Double) -> Double {
  pow(2.0, x)
}

///|
test "exp2" {
  fn assert_exp2_ulp(input, expect) raise {
    assert_ulp(expect, exp2(input), EXP2_MAX_ULP)
  }

  assert_exp2_ulp(-1, 0.5)
  assert_exp2_ulp(-2, 0.25)
  assert_exp2_ulp(-3, 0.125)
  assert_exp2_ulp(-4, 0.0625)
  assert_exp2_ulp(-5, 0.03125)
  assert_exp2_ulp(-6, 0.015625)
  assert_exp2_ulp(-7, 0.0078125)
  assert_exp2_ulp(-8, 0.00390625)
  assert_exp2_ulp(-9, 0.001953125)
  assert_exp2_ulp(1, 2)
  assert_exp2_ulp(2, 4)
  assert_exp2_ulp(3, 8)
  assert_exp2_ulp(4, 16)
  assert_exp2_ulp(5, 32)
  assert_exp2_ulp(6, 64)
  assert_exp2_ulp(7, 128)
  assert_exp2_ulp(8, 256)
  assert_exp2_ulp(9, 512)
  assert_exp2_ulp(10, 1024)
  assert_exp2_ulp(100, 1267650600228229400000000000000)
  assert_exp2_ulp(10000, @double.infinity)
  assert_exp2_ulp(2.5, 5.656854249492381)
  assert_exp2_ulp(3.4, 10.556063286183154)
  assert_exp2_ulp(5.3, 39.396621227037315)
  assert_exp2_ulp(6.2, 73.51669471981025)
  assert_exp2_ulp(7.1, 137.1870032046455)
  assert_exp2_ulp(8.9, 477.7128916668455)
  assert_exp2_ulp(9.8, 891.4437768152316)
  assert_exp2_ulp(10.7, 1663.4929077375696)
  assert_exp2_ulp(101.6, 3842798030625539400000000000000)
  assert_exp2_ulp(1.542, 2.9119790979981732)
  assert_exp2_ulp(2.846, 7.190041012888239)
  assert_exp2_ulp(7.881, 235.73137848449025)
  assert_exp2_ulp(3.772, 13.661083430259916)
  assert_exp2_ulp(-1.542, 0.3434090583574056)
  assert_exp2_ulp(-2.846, 0.13908126507310425)
  assert_exp2_ulp(-7.881, 0.004242116626258961)
  assert_exp2_ulp(-3.772, 0.07320063632617563)
  assert_exp2_ulp(-1, 0.5)
  assert_exp2_ulp(0, 1)
  assert_exp2_ulp(-0, 1)
  assert_exp2_ulp(@double.not_a_number, @double.not_a_number)
  assert_exp2_ulp(@double.infinity, @double.infinity)
  assert_exp2_ulp(@double.neg_infinity, 0)
}