Complex sentence: The first element seems to be the genitive case of the name of the people "Haddingjar" or of the male name "Haddingi" and in Flateyjarbók a man "Haddingr" is mentioned as the king of Hallingdal.
Split into simpler sentences: The first element seems to be the genitive case of the name of the people "Haddingjar" or of the male name "Haddingi". In Flateyjarbók, a man named "Haddingr" is mentioned as the king of Hallingdal.