From 0c169039893c8e6e89c124715f3ca1c59c8c38b9 Mon Sep 17 00:00:00 2001
From: George F Murphy <gfmurphy@gmail.com>
Date: Thu, 17 Nov 2011 08:22:17 -0600
Subject: [PATCH] first attempt at recursive solution

---
 luhnybin.rb | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 mask.sh     |  2 +-
 2 files changed, 77 insertions(+), 1 deletion(-)
 create mode 100755 luhnybin.rb

diff --git a/luhnybin.rb b/luhnybin.rb
new file mode 100755
index 0000000..03fcc0c
--- /dev/null
+++ b/luhnybin.rb
@@ -0,0 +1,76 @@
+#!/usr/bin/env ruby
+class Luhnybin
+  MINIMUM = 14
+  SUMMED_DOUBLES = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9].freeze
+
+  def initialize(text)
+    @text = text.unpack('c*')
+    filter
+  end
+
+  def text
+    @text.pack('c*')
+  end
+
+  private
+  def filter(start=0, index=0, digits=[])
+    return 0 if index == @text.length || digits.length > 16
+
+    count = luhn(digits)
+    return count if count > 0
+
+    char = @text[index]
+    digit = digit?(char)
+
+    if digit
+      value = char - ?0
+      digits.unshift(value)
+    elsif !separator?(char)
+      start = index
+      digits.clear
+    end
+
+    lc = filter(start, index + 1, digits)
+    if lc > 0 && digit
+      @text[index] = ?X
+      lc -= 1
+    end
+    return lc
+  end
+
+  def luhn(digits)
+    length = digits.length
+    return 0 if length < MINIMUM
+
+    length.downto(MINIMUM) do |n|
+      i = 0
+      sum = digits[0,n].reduce(0) do |tot, d|
+        tot += i.even? ? d : SUMMED_DOUBLES[d]
+        i += 1
+        tot
+      end
+      return n if sum % 10 == 0
+    end
+    return 0
+  end
+
+  def digit?(char)
+    char.between?(?0, ?9)
+  end
+
+  def separator?(char)
+    char == ?- || char == 32
+  end
+end
+
+class String
+  def mask_cc_number
+    Luhnybin.new(self).text
+  end
+end
+
+if __FILE__ == $0
+  STDIN.each do |line|
+    STDOUT << line.mask_cc_number
+  end
+end
diff --git a/mask.sh b/mask.sh
index 652565b..9764432 100755
--- a/mask.sh
+++ b/mask.sh
@@ -1,4 +1,4 @@
 #!/bin/sh
 
 # Call your program here instead of cat.
-cat
+./luhnybin.rb