Moon phase factor

I want to make an item that can inform the player what the moon phase of the current world is (in night time). I did the codings but I don't know how to convert the number to the direct moon phase (is it a 0-scale or a 1-scale?)

I have the types of minecraft moon phases:

full moon

waning gibbous

third quarter

waning crescent 

new moon 

waxing crescent

first quarter

waxing gibbous

what moon phase factors do each of the moon phases have?