Solution to task 2

1 files changed, 83 insertions, 0 deletions

diff --git a/challenge-249/jo-37/perl/ch-2.pl b/challenge-249/jo-37/perl/ch-2.pl
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+#!/usr/bin/perl -s
+
+use v5.24;
+use Test2::V0;
+
+our ($tests, $examples);
+
+run_tests() if $tests || $examples;	# does not return
+
+die <<EOS unless @ARGV;
+usage: $0 [-examples] [-tests] [DI]
+
+-examples
+    run the examples from the challenge
+ 
+-tests
+    run some tests
+
+DI
+    a string consisting of 'D's and 'I's
+
+EOS
+
+
+### Input and Output
+
+say "(@{[arrange_di(shift)]})";
+
+
+### Implementation
+
+# First we transform the DI-string into an array of zeroes and ones
+# where a zero represents a decrement and a one an increment.  Lets
+# assume the DI-string and the corresponding array have a length of L,
+# there are ND 'D's / zeroes and NI 'I's / ones.
+# Then we create two sequences:
+# - SD as the numbers 0 .. ND - 1 in descending order
+# - SI the numbers ND .. L - 1 in ascending order.
+# It can be seen that:
+# - Every element of SI is larger than any element of SD.
+# - Every element of SD is smaller than any element of SI.
+# - Every but the last element of SI is smaller than its successor.
+# - Every but the last element of DI is larger than its successor.
+# Now we can construct a permutation with the required incremental /
+# decremental behaviour by successively picking the first element from
+# the corresponding sequence - except for the very first.
+# The first element of our permutation may be chosen as follows:
+# - If the first step is a decrement, we just take L as the first
+#   element.
+# - Otherwise, we pick the first element from the incremental sequence SI
+#   and append L to SI
+
+sub arrange_di {
+	my @di = map tr/DI/01/r, split //, shift;
+    my @count = (0, 0);
+    $count[$_]++ for @di;
+    my @pool = ([reverse 0 .. $count[0] - 1], [$count[0] .. $#di]);
+
+    ($di[0] ?
+        do {push $pool[1]->@*, scalar @di; shift $pool[1]->@*} :
+        scalar @di,
+        map shift $pool[$_]->@*, @di);
+}
+
+
+### Examples and tests
+
+sub run_tests {
+    SKIP: {
+        skip "examples" unless $examples;
+
+        is [arrange_di('IDID')], [2, 3, 1, 4, 0], 'example 1';
+        is [arrange_di('III')], [0, 1, 2, 3], 'example 2';
+        is [arrange_di('DDI')], [3, 1, 0, 2], 'example 3';
+    }
+
+    SKIP: {
+        skip "tests" unless $tests;
+	}
+
+    done_testing;
+    exit;
+}