Farsi and Dari are two dialects of the same language, mutually intelligible in written format, but very different when spoken. ... Tajik too has diverged from Farsi in both script and vocabulary and they are often mutually intelligible when spoken but not when written.
By way of Early New Persian, Dari Persian, like Iranian Persian and Tajik, is a continuation of Middle Persian, the official religious and literary language of the Sassanian Empire (224–651 CE), itself a continuation of Old Persian, the language of the Achaemenids (550–330 BC).
They must have thought and prepared themselves about this eventuality.This move of KSA is going to hurt Russian but if it continued,it would break back of Iran and would bankrupt US shale gas producers.