Perlin function usually output valid cosine-like values, ranged as [-1, 1]. But this library is returning values outside that range from time to time. Test case:

package perlin

import (
"testing"
)

func TestNoise2D(t *testing.T) {
x := Noise2D(29, 5, 2007521014155829838, 1., 2., 3)
if x < 0 {
t.Errorf("value below -1")
}
}