I was uncertain whether you should write "a hash" or "an hash".
Searching on the web I found 3.000 results with "an hash" and 300.000 with "a hash", so for sure most people say "a hash".
However "an hash" still sounds better to me, especially when "hash" is used alone (as opposed to "hash table", "hash function" etc.).
That's because I'm used to pronounce "hash" without stressing the h, so its initial sound is similar to that of "hour" (which I'm sure has to be used with "an").
I know that at least some people say it stressing the h, so for them it's for sure correct to use "a", however I wonder if there are different (official) pronunciations and thus both "a hash" and "an hash" are acceptable in written text.
I'm not a native english speaker and I never stayed for long in an english-speaking country, so it's entirely possible that I made up my pronunciation of "hash".