Decoding: use_cbor_discriminator

src/cbor/decoder.cr

@@ -2,6 +2,19 @@ class CBOR::Decoder
   @lexer : Lexer
   getter current_token : Token::T?
 
+  # Decode until a certain point in the history (use_cbor_discriminator helper).
+  def reset(value : Int32 | Int64 = 0)
+    @lexer.reset
+    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)
     @lexer = Lexer.new(input)
     @current_token = @lexer.next_token

src/cbor/from_cbor.cr

@@ -3,6 +3,10 @@ def Object.from_cbor(string_or_io)
   new(parser)
 end
 
+def Object.from_cbor(parser : CBOR::Decoder)
+  new(parser)
+end
+
 def String.new(decoder : CBOR::Decoder)
   decoder.read_string
 end

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
@@ -8,6 +9,11 @@ class CBOR::Lexer
   def initialize(@io : IO)
   end
 
+  def reset(value : Int32 | Int64 = 0)
+    @io.seek value
+    @eof = false
+  end
+
   def next_token : Token::T?
     return nil if @eof
 

src/cbor/serializable.cr

@@ -345,6 +345,36 @@ module CBOR
       end
     end
 
+    macro use_cbor_discriminator(field, mapping)
+      {% unless mapping.is_a?(HashLiteral) || mapping.is_a?(NamedTupleLiteral) %}
+        {% mapping.raise "mapping argument must be a HashLiteral or a NamedTupleLiteral, not #{mapping.class_name.id}" %}
+      {% end %}
+
+      # 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 current_offset
+          case v
+          when Hash(CBOR::Type, CBOR::Type)
+            discriminator_value = v[{{field.id.stringify}}]?
+            case discriminator_value
+            {% for key, value in mapping %}
+              when {{key.id.stringify}}
+                return {{value.id}}.from_cbor(decoder)
+            {% end %}
+            else
+              raise "Unknown '{{field.id}}' discriminator value: #{discriminator_value.inspect}"
+            end
+          else
+            raise "cannot get cbor discriminator #{ {{ field.id.stringify }} }"
+          end
+        else
+          raise "cannot decode cbor value"
+        end
+      end
+    end
+
     # Tells this class to decode CBOR by using a field as a discriminator.
     #
     # - *field* must be the field name to use as a discriminator