Ruby solution for week 100, part 1

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diff --git a/challenge-100/abigail/ruby/ch-1.rb b/challenge-100/abigail/ruby/ch-1.rb
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+#!/usr/bin/ruby
+
+#
+# See ../README.md
+#
+ 
+#
+# Run as: ruby ch-1.rb < input-file
+#
+
+ARGF . each_line do |_|
+    #
+    # Parse data. Note that we have to map the hours and
+    # minutes from strings to integers.
+    #
+    m = _ . match /(?<hour>[0-9]+):(?<minute>[0-9]+)\s*(?<ampm>[ap]?)/
+    hour   = m [:hour]   . to_i
+    minute = m [:minute] . to_i
+    ampm   = m [:ampm]
+
+    #
+    # Calculate the new AM/PM marker
+    #
+    new_ampm = ampm == "" ? hour >= 12 ? "pm" : "am" : ""
+
+    #
+    # Calculate the new hour
+    #
+    hour %= 12
+
+    if   ampm == "" && hour == 0
+    then hour =  12
+    end
+
+    if   ampm == "p"
+    then hour += 12
+    end
+
+    #
+    # Print the result
+    #
+    puts sprintf("%02d:%02d%s", hour, minute, new_ampm)
+end