sashazykov · January 9, 2013 09:04 · Jan 9, 2013
diff --git a/bitcoin_address_validator.rb b/bitcoin_address_validator.rb
@@ -0,0 +1,55 @@
+require 'digest'
+
+class BitcoinAddressValidator < ActiveModel::EachValidator
+  def validate_each(record, field, value)
+    unless value.blank? || valid_bitcoin_address?(value)
+      record.errors[field] << "Bitcoin address is invalid"
+    end
+  end
+
+  private
+
+  B58Chars = '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz'
+  B58Base = B58Chars.length
+
+  def valid_bitcoin_address?(address)
+    (address =~ /^[a-zA-Z1-9]{33,35}$/) and version(address)
+  end
+
+  def version(address)
+    decoded = b58_decode(address, 25)
+
+    version = decoded[0, 1]
+    checksum = decoded[-4, decoded.length]
+    vh160 = decoded[0, decoded.length - 4]
+
+    hashed = (Digest::SHA2.new << (Digest::SHA2.new << vh160).digest).digest
+
+    hashed[0, 4] == checksum ? version[0] : nil
+  end
+
+  def b58_decode(value, length)
+    long_value = 0
+    index = 0
+    result = ""
+
+    value.reverse.each_char do |c|
+      long_value += B58Chars.index(c) * (B58Base ** index)
+      index += 1
+    end
+
+    while long_value >= 256 do
+      div, mod = long_value.divmod 256
+      result = mod.chr + result
+      long_value = div
+    end
+
+    result = long_value.chr + result
+
+    if result.length < length
+      result = 0.chr * (length - result.length) + result
+    end
+
+    result
+  end
+end
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