How can I read data faster?

Question

Hmmm... its kinda challenging to find a method for reading/writing data faster enough to get ACCEPTED in this problem ( https://www.spoj.pl/problems/INTEST/ ) using F#.

My code ( http://paste.ubuntu.com/548748/ ) gets TLE...

Any ideas how to speed up data reading?

Answer 1

This version of mine passes the time limit (but is still terribly slow ~14 seconds):

open System
open System.IO

// need to change standard buffer, not to add an additional one
let stream = new StreamReader(Console.OpenStandardInput(4096))

let stdin = Seq.unfold (fun s -> if s = null then None else Some (s,stream.ReadLine())) <| stream.ReadLine()

let inline s2i (s : string) = Array.fold (fun a d -> a*10u + (uint32 d - uint32 '0') ) 0u <| s.ToCharArray()

let calc = 
    let fl = Seq.head stdin
    let [|_;ks|] = fl.Split(' ')
    let k = uint32 ks
    Seq.fold (fun a s -> if (s2i s) % k = 0u then a+1 else a) 0 <| Seq.skip 1 stdin

printf "%A" calc

Though the bottle-neck of this version is actually string -> uint32 conversion (standard uint32 cast from string is even slower) the reading itself takes around 2 sec (vs 6 sec of total time) on my sample input (~100M file) - still not a great result. Once s2i is rewritten in imperative style, the total run-time can be reduced to 10 sec on spoj:

let inline s2i (s : string) =
    let mutable a = 0u
    for i in 0..s.Length-1 do a <- a*10u + uint32 (s.Chars(i)) - uint32 '0'
    a

Answer 2

I don't actually know, but I would guess that reading a since character at a time is bad, and you should read e.g. 4k into a buffer at a time and then process the buffer.

Answer 3

let buf =
    let raw = System.Console.OpenStandardInput()
    let bytebuf = new System.IO.BufferedStream(raw)
    new System.IO.StreamReader(bytebuf)

buf.Read()     // retrieves a single character as an int from the buffer
buf.ReadLine() // retrieves a whole line from the buffer