Recently I’ve been watching a lot of find-in-pi source code

```
(require racket/generator)
(define (make-pi-spigot)
(generator ()
(let loop ([q 1] [r 0] [t 1] [k 1] [n 3] [l 3])
(if (< (- (+ (* 4 q) r) t) (* n t))
(begin
(yield n)
(loop (* 10 q)
(* 10 (- r (* n t)))
t
k
(- (quotient (* 10 (+ (* 3 q) r)) t) (* 10 n))
l))
(loop (* q k)
(* (+ (* 2 q) r) l)
(* t l)
(+ k 1)
(quotient (+ (* q (+ (* 7 k) 2)) (* r l)) (* t l))
(+ l 2))))))
```

Simple enough to use, we can use Racket’s `for`

to generate a list of n digits of pi or to convert the same to a string:

```
(define (digits-of-pi n)
(for/list ([i (in-range (+ n 1))]
[d (in-producer (make-pi-spigot) #f)])
d))
(define (pi->string n)
(string-append
"3."
(apply
string-append
(map number->string (cdr (digits-of-pi n))))))
> (digits-of-pi 10)
'(3 1 4 1 5 9 2 6 5 3 5)
> (pi->string 10)
"3.1415926535"
```

In any case, it seems to be working well enough so far, so let’s write some code that can look for any arbitrary list of digits in pi:

```
(define (find-in-pi ls)
(let ([pi (make-pi-spigot)])
(pi) ; skip the 3
(let loop ([i 1] [l* '()])
(let* ([d (pi)]
[l* (map cdr (filter (lambda (ea)
(and (not (null? ea))
(= d (car ea))))
(cons ls l*)))])
(if (any? null? l*) (- i (length ls)) (loop (+ i 1) l*))))))
```

Using this, we can find the first time each digit occurs in pi:

```
> (for/list ([i (in-range 10)]) (list i (find-in-pi (list i))))
'((0 31) (1 0) (2 5) (3 8) (4 1) (5 3) (6 6) (7 12) (8 10) (9 4))
```

Or the first time that longer sequences occur:

```
> (find-in-pi '(1 2 3))
1923
> (find-in-pi '(4 5 6))
250
> (find-in-pi '(0 0 0))
600
```

Now, let’s get crazy and see when Jenny's number shows up:

```
┌ ☺ verkampj@minty ~/Projects/SmallProjects
└ time ./find-in-pi 8675309
8675309 is 9202591 digits into pi
```

I added a few more lines to the full source that lets it run from the command line and compiled it with `raco exe find-in-pi.rkt`

.

That took a while to run. 😄