https://projecteuler.net/problem=24

A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are:

012   021   102   120   201   210

What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9?

use v6;



# idea: the last 9 digits can be permuted in 9! = 362880 ways.  so there are
# 9! numbers that start with a 0, 9! numbers that start with a 1 etc.
#
# So to get the first digit, divide our target by 9!, and the rounded result
# is the first digit.
#
# then we remove the first digit from the pool of available digits, divide
# the rest by 8!, round, store result in $n. Then the $n'th lowest available
# digit is the second digit that we search.

my $target = 1e6;
my $t = $target;

sub f(Int $x){
    [*] 1..$x;
}

my @f = map &f, 0..9;
my @available = 0 .. 9;

say gather {
    for reverse(0..9) -> $marker {
        my $n = ceiling($t / @f[$marker])- 1;
        $t -= $n * @f[$marker];
        take @available[$n];
        @available.splice($n, 1);
    }
}.join('')

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