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Contents
open Base
type palindrome_products = {
value : int;
factors : (int * int) list;
} [@@deriving show, eq]
let invalid_input_error min max =
Error (Printf.sprintf "invalid input: min is %d and max is %d" min max)
let is_palindrome n =
let n = Int.to_string n in String.(rev n = n)
let to_palindrome_products (xs : (int * (int * int)) list): palindrome_products =
let value = fst (List.hd_exn xs) in
let factors = List.map ~f:snd xs in
{value; factors}
let seq stride = Sequence.range ~start:`inclusive ~stop:`inclusive ~stride
let smallest ~min ~max =
if min > max
then invalid_input_error min max
else
let open Sequence.Monad_infix in
let seq = seq 1 in
let products =
seq min max >>= fun x ->
seq x max >>= fun y ->
Sequence.singleton (x * y, (x, y))
|> Sequence.filter ~f:(fun (n, _) -> is_palindrome n) in
products
|> Sequence.to_list
|> List.sort ~compare:(fun (x,_) (y,_) -> Int.compare x y)
|> List.group ~break:(fun (x, _) (y, _) -> x <> y)
|> List.hd
|> Option.map ~f:to_palindrome_products
|> Result.of_option ~error:(Printf.sprintf "no palindrome with factors in the range %d to %d" min max)
let largest ~min ~max =
if min > max
then invalid_input_error min max
else
let open Sequence.Monad_infix in
let seq = seq (-1) in
let products =
seq max min >>= fun x ->
seq x min >>= fun y ->
Sequence.singleton (x * y, (y, x))
|> Sequence.filter ~f:(fun (n, _) -> is_palindrome n) in
products
|> Sequence.to_list
|> List.sort ~compare:(fun (x,_) (y,_) -> Int.compare y x)
|> List.group ~break:(fun (x, _) (y, _) -> x <> y)
|> List.hd
|> Option.map ~f:to_palindrome_products
|> Result.of_option ~error:(Printf.sprintf "no palindrome with factors in the range %d to %d" min max)
Version data entries
42 entries across 42 versions & 1 rubygems