As part of excersise to better understand F# which I am currently learning , I wrote function to split given string into n-grams.
1) I would like to receive feedback about my function : can this be written simpler or in more efficient way?

2) My overall goal is to write function that returns string similarity (on 0.0 .. 1.0 scale) based on n-gram similarity; Does this approach works well for short strings comparisons , or can this method reliably be used to compare large strings (like articles for example).

3) I am aware of the fact that n-gram comparisons ignore context of two strings. What method would you suggest to accomplish my goal?

//s:string - target string to split into n-grams
//n:int - n-gram size to split string into
let ngram_split (s:string, n:int) =
    let ngram_count = s.Length - (s.Length % n)
    let ngram_list = List.init ngram_count (fun i ->
        if( i + n >= s.Length ) then
        s.Substring(i,s.Length - i) + String.init ((i + n) - s.Length)
            (fun i -> "#")
        else
            s.Substring(i,n)
    )
    let ngram_array_unique = ngram_list
                            |> Seq.ofList
                            |> Seq.distinct
                            |> Array.ofSeq

//produce tuples of ngrams (ngram string,how much occurrences in original string)

    Seq.init ngram_array_unique.Length (fun i -> (ngram_array_unique.[i],
        ngram_list |> List.filter(fun item -> item = ngram_array_unique.[i])
                                   |> List.length)
                                        )