haskell prelude

[François Sarradin]

David,

Even with BigInteger, you cannot have an infinite stream ;)

Nice implem though ^^

About note 1, you not only need tuples but also destructuration.

françois-
 Le 23 juin 2012 03:22, "David Conrad" <drconrad at gmail.com> a écrit :

>
>
> On Fri, 22 Jun 2012 22:37:41 +0200, Fran?ois Sarradin <fsarradin at gmail.com
> > wrote:
>
>>
>> Here is a copy of source code I have written once to produce fib numbers
>> as
>> an infinite stream. It uses an Iterator (as Brian has proposed).
>>
>>    @Test
>>    public void shouldGetFibonacciNumbersAsInfiniteStream() {
>>          Iterator<Integer> fibIterator = new AbstractIterator<Integer>() {
>>            int a = 1;
>>            int b = 0;
>>
>>            @Override
>>            protected Integer computeNext() {
>>                Integer result = b;
>>                b = a + b;
>>                a = result;
>>                return result;
>>            }
>>        };
>>
>>        assertThat(limit(fibIterator, 8)).containsOnly(0, 1, 1, 2, 3, 5,
>> 8, 13);
>>    }
>>
>>
> François,
>
> Not really an infinite sequence unless you use BigInteger, is it? :)
>
> import java.util.Iterator;
> import java.math.BigInteger;
> import static java.math.BigInteger.ONE;
> import static java.math.BigInteger.ZERO;
>
> public class Fibo {
>     private Fibo() {
>         throw new AssertionError("no instances");
>     }
>
>     public static Iterable<BigInteger> nacci() {
>         return new Iterable<BigInteger>() {
>             @Override public Iterator<BigInteger> iterator() {
>                 return new Iterator<BigInteger>() {
>                     private BigInteger a = ONE, b = ZERO;
>
>                     @Override public boolean hasNext() { return true; }
>
>                     @Override public BigInteger next() {
>                         set(a.add(b), a); // Note 1
>                         return b;
>                     }
>
>                     private void set(BigInteger a, BigInteger b) {
>                         this.a = a; this.b = b;
>                     }
>                 };
>             }
>         };
>     }
> }
>
>
> import in.digo.func.Take;
> import java.math.BigInteger;
> import static java.math.BigInteger.ZERO;
> import static java.math.BigInteger.TEN;
>
> public class Fib {
>     public static void main(String[] args) {
>         BigInteger googol = TEN.pow(100);
>         System.out.printf("%,d%n",
>             Take.whileTrue(n -> n.compareTo(googol) <= 0, Fibo.nacci())
>             .reduce(ZERO, (a, b) -> a.add(b))); // Note 2
>     }
> }
>
> $ time java Fib
>
> 24,130,015,357,889,614,840,807,962,620,028,350,479,216,011,277,190,196,743,261,610,776,878,424,511,662,841,261,217,058,994,930,287,040
>
> real    0m0.234s
> user    0m0.030s
> sys     0m0.061s
>
> Sum of all terms of the Fibonacci sequence up to a googol.
>
> Note 1: What I really want to write is a, b = a + b, a, but Java doesn't
> have tuples (yet?)
> Is there a bug I can vote for on that? :)
>
> Note 2: Take.whileTrue omitted for brevity, but it does what the name
> implies.
>
> A plus, et bon weekend,
> David
>
>