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+#!env perl6
+
+# see <https://perlweeklychallenge.org/blog/perl-weekly-challenge-107/>
+# Task 1: Descriptive Numbers
+# Write a script to generate Self-descriptive Numbers in a given base.
+
+# In mathematics, a self-descriptive number
+# is an integer m that in a given base b is b digits long
+# in which each digit d at position n (the most significant digit being at position 0 and the least significant at position b - 1) counts how many instances of digit n are in m.
+
+# For example, if the given base is 10, then script should print 6210001000.
+#
+# Invocation example:
+
+# % perl6 weekly-challenge/pwc_43_2.p6 --base=10
+# Starting generation for base 10
+# Batch between 6210001000 and 6219991000
+# Batch between 7210001000 and 7219991000
+# Batch between 8210001000 and 8219991000
+# Batch between 9210001000 and 9219991000
+
+# What did we generate?
+
+# [6210001000]
+
+# Bye bye!
+
+
+
+my $m = 6210001000;
+
+
+sub MAIN( :$base? where { (10,4,5,7,8,9,11,12,16,36).grep: $base } = 10 ){
+    say "Starting generation for base $base";
+
+    # I need to do something for other bases!
+    die "Not implemented!" if $base > 10;
+
+    if ( $base <= 5 ) {
+        my $end = ( $base - 1 ).Str x $base;
+        my @nums = grep { validate $_.Int, $base }, 0 .. $end.Int;
+        "\nWhat did we generate?\n".say;
+        for @nums {
+            .say
+        }
+    }
+    else {
+        # if the base is greater than 5 do a parallel scan
+
+        my @tasks;
+        for ( $base - 4 ) .. ( $base - 1 ) {
+            @tasks.push:  Promise.start( {
+                 # see <https://medium.com/@divyangrpatel/self-descriptive-number-best-algorithm-95b281e6de05>
+                 my $start = '%d21%s1000'.sprintf: $_, '0' x ( $base - 4 - 3 ).Int;
+                 my $end   = '%d21%s1000'.sprintf: $_, ( $base - 1 ) x ( $base - 4 - 3 ).Int;
+                 say "\tBatch between $start and $end";
+
+                my @nums = grep { validate $_.Int, $base }, $start .. $end;
+             } );
+        }
+
+        await @tasks;
+        "\nWhat did we generate?\n".say;
+        for @tasks {
+            .result.say if .result;
+        }
+
+    }
+
+    say "\nBye bye!";
+}
+
+
+# Splits the number into an array of single digits, so that the first element is
+# the most important digit of the number.
+# Then greps the digit in the whole array and counts how many times it appears.
+# If the digit appears the right number of times, the number is fine.
+sub validate( Int $number, Int $base ){
+    my @digits = $number.Str.split( '', :skip-empty );
+
+    # the number must have the same length
+    return False if @digits.elems != $base;
+
+    # the number must have at least one zero!
+    return False if ! @digits.grep: 0;
+
+    my $ok = True;
+    for @digits {
+        my $digit    = $_;
+        my $position = $++;
+        $ok &&= @digits.grep( * == $position ).elems == $digit;
+        return False if ! $ok;
+    }
+
+    return True;
+}