How to implement the Salsa20 hash function?
You are right about the interpretation of the power 10: it's a tenfold iteration. So we apply the function 10 times, starting with $x$, feeding the output as input for the next step. So C-like (I write x for the vector of 16 words $x_0,\ldots,x_{15}$):

y=x; 
for (i=0; i&lt; 10; i++){ 
    y = doubleround(y)
}; 
return y  

The inverse of little-endian is indeed the function that sends a word (32 bits) back to the sequence of 4 bytes in a little endian way, so the least significant byte goes first, and the most significant byte goes last. So it maps $w$ to w &amp; 0xff, (w &gt;&gt; 8) &amp; 0xff, (w &gt;&gt; 16) &amp; 0xff, (w &gt;&gt; 24) &amp; 0xff

Finally, the iteration gives you a vector 16 words, which you add componentwise in the same order with the original input vector. Then you convert them to bytes again, in that same order, and every word in the little-endian fashion described above. (We could also have converted them to bytes first (using the inverse of litleenian) and then done 64 byte additions in order, but this is less efficient, as we then do 64 byte additions instead of 16 word additions).