29 lines
1.1 KiB
OCaml
29 lines
1.1 KiB
OCaml
(* pre-bake & return an inner-loop function to bin & assemble a character frequency map *)
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let get_fproc (m: (char, int) Hashtbl.t) :(char -> unit) =
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(fun (c:char) -> try
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Hashtbl.replace m c ( (Hashtbl.find m c) + 1)
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with Not_found -> Hashtbl.add m c 1)
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(* pre-bake and return an inner-loop function to do the actual entropy calculation *)
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let get_calc (slen:int) :(float -> float) =
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let slen_float = float_of_int slen in
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let log_2 = log 2.0 in
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(fun v -> let pt = v /. slen_float in
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pt *. ((log pt) /. log_2) )
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(* main function, given a string argument it:
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builds a (mutable) frequency map (initial alphabet size of 255, but it's auto-expanding),
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extracts the relative probability values into a list,
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folds-in the basic entropy calculation and returns the result. *)
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let shannon (s:string) :float =
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let freq_hash = Hashtbl.create 255 in
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String.iter (get_fproc freq_hash) s;
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let relative_probs = Hashtbl.fold (fun k v b -> (float v)::b) freq_hash [] in
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let calc = get_calc (String.length s) in
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-1.0 *. List.fold_left (fun b x -> b +. calc x) 0.0 relative_probs
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