use_cbor_discriminator: now works with imbricated classes.

src/cbor/decoder.cr

@@ -2,9 +2,17 @@ class CBOR::Decoder
   @lexer : Lexer
   getter current_token : Token::T?
 
-  def reset
+  # Decode until a certain point in the history (use_cbor_discriminator helper).
+  def reset(value : Int32 | Int64 = 0)
     @lexer.reset
-    @current_token = @lexer.next_token
+    while pos < value
+      @current_token = @lexer.next_token
+    end
+  end
+
+  # Give the current position in the decoder (use_cbor_discriminator helper).
+  def pos
+    @lexer.io.pos
   end
 
   def initialize(input)

src/cbor/lexer.cr

@@ -1,4 +1,5 @@
 class CBOR::Lexer
+  property io : IO
   def self.new(slice : Bytes)
     new IO::Memory.new(slice)
   end
@@ -8,8 +9,8 @@ class CBOR::Lexer
   def initialize(@io : IO)
   end
 
-  def reset
-    @io.seek 0
+  def reset(value : Int32 | Int64 = 0)
+    @io.seek value
     @eof = false
   end
 

src/cbor/serializable.cr

@@ -353,8 +353,9 @@ module CBOR
 
       # SLOW. Read everything, get the type, read everything again.
       def self.new(decoder : ::CBOR::Decoder)
+        current_offset = decoder.pos
         if v = decoder.read_value
-          decoder.reset
+          decoder.reset current_offset
           case v
           when Hash(CBOR::Type, CBOR::Type)
             discriminator_value = v[{{field.id.stringify}}]?