parser: implement DecodeLayerContainer

Changes:
* add DecodingLayerContainer to contain/search DecodingLayer values:
  * add DecodingLayerFunc type as a decoder closure;
  * add DecodingLayerMap, DecodingLayerSparse, DecodingLayerArray
    as implementations of DecodingLayerContainer;
* add code generation for the reference LayersDecoder method
  implementation.
* refactor DecodingLayerParser:
  * use DecodingLayerMap by default;
  * add SetDecodingLayerContainer method to initialize parser.

Signed-off-by: Yerden Zhumabekov <[email protected]>

gen.go

@@ -0,0 +1,75 @@
+// Copyright 2019 The GoPacket Authors. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the LICENSE file in the root of the source
+// tree.
+
+// +build ignore
+
+// This file generates LayersDecoder function for DecodingLayerContainer
+// go run gen.go | gofmt > layers_decoder.go
+package main
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+const headerFmt = `// Copyright 2019 The GoPacket Authors. All rights reserved.
+
+package gopacket
+
+// Created by gen.go, don't edit manually
+// Generated at %s
+
+// LayersDecoder returns DecodingLayerFunc for specified
+// DecodingLayerContainer, LayerType value to start decoding with and
+// some DecodeFeedback.
+func LayersDecoder(dl DecodingLayerContainer, first LayerType, df DecodeFeedback) DecodingLayerFunc {
+  firstDec, ok := dl.Decoder(first)
+  if !ok {
+    return func([]byte, *[]LayerType) (LayerType, error) {
+      return first, nil
+    }
+  }
+`
+
+var funcBody = `return func(data []byte, decoded *[]LayerType) (LayerType, error) {
+  *decoded = (*decoded)[:0] // Truncated decoded layers.
+  typ := first
+  decoder := firstDec
+  for {
+    if err := decoder.DecodeFromBytes(data, df); err != nil {
+      return LayerTypeZero, err
+    }
+    *decoded = append(*decoded, typ)
+    typ = decoder.NextLayerType()
+    if data = decoder.LayerPayload(); len(data) == 0 {
+      break
+    }
+    if decoder, ok = dlc.Decoder(typ); !ok {
+      return typ, nil
+    }
+  }
+  return LayerTypeZero, nil
+}`
+
+func main() {
+	fmt.Fprintf(os.Stderr, "Writing results to stdout\n")
+	types := []string{
+		"DecodingLayerSparse",
+		"DecodingLayerArray",
+		"DecodingLayerMap",
+	}
+
+	fmt.Printf(headerFmt, time.Now())
+	for _, t := range types {
+		fmt.Printf("if dlc, ok := dl.(%s); ok {", t)
+		fmt.Println(funcBody)
+		fmt.Println("}")
+	}
+	fmt.Println("dlc := dl")
+	fmt.Println(funcBody)
+	fmt.Println("}")
+}

layers_decoder.go

@@ -0,0 +1,101 @@
+// Copyright 2019 The GoPacket Authors. All rights reserved.
+
+package gopacket
+
+// Created by gen.go, don't edit manually
+// Generated at 2019-06-18 11:37:31.308731293 +0600 +06 m=+0.000842599
+
+// LayersDecoder returns DecodingLayerFunc for specified
+// DecodingLayerContainer, LayerType value to start decoding with and
+// some DecodeFeedback.
+func LayersDecoder(dl DecodingLayerContainer, first LayerType, df DecodeFeedback) DecodingLayerFunc {
+	firstDec, ok := dl.Decoder(first)
+	if !ok {
+		return func([]byte, *[]LayerType) (LayerType, error) {
+			return first, nil
+		}
+	}
+	if dlc, ok := dl.(DecodingLayerSparse); ok {
+		return func(data []byte, decoded *[]LayerType) (LayerType, error) {
+			*decoded = (*decoded)[:0] // Truncated decoded layers.
+			typ := first
+			decoder := firstDec
+			for {
+				if err := decoder.DecodeFromBytes(data, df); err != nil {
+					return LayerTypeZero, err
+				}
+				*decoded = append(*decoded, typ)
+				typ = decoder.NextLayerType()
+				if data = decoder.LayerPayload(); len(data) == 0 {
+					break
+				}
+				if decoder, ok = dlc.Decoder(typ); !ok {
+					return typ, nil
+				}
+			}
+			return LayerTypeZero, nil
+		}
+	}
+	if dlc, ok := dl.(DecodingLayerArray); ok {
+		return func(data []byte, decoded *[]LayerType) (LayerType, error) {
+			*decoded = (*decoded)[:0] // Truncated decoded layers.
+			typ := first
+			decoder := firstDec
+			for {
+				if err := decoder.DecodeFromBytes(data, df); err != nil {
+					return LayerTypeZero, err
+				}
+				*decoded = append(*decoded, typ)
+				typ = decoder.NextLayerType()
+				if data = decoder.LayerPayload(); len(data) == 0 {
+					break
+				}
+				if decoder, ok = dlc.Decoder(typ); !ok {
+					return typ, nil
+				}
+			}
+			return LayerTypeZero, nil
+		}
+	}
+	if dlc, ok := dl.(DecodingLayerMap); ok {
+		return func(data []byte, decoded *[]LayerType) (LayerType, error) {
+			*decoded = (*decoded)[:0] // Truncated decoded layers.
+			typ := first
+			decoder := firstDec
+			for {
+				if err := decoder.DecodeFromBytes(data, df); err != nil {
+					return LayerTypeZero, err
+				}
+				*decoded = append(*decoded, typ)
+				typ = decoder.NextLayerType()
+				if data = decoder.LayerPayload(); len(data) == 0 {
+					break
+				}
+				if decoder, ok = dlc.Decoder(typ); !ok {
+					return typ, nil
+				}
+			}
+			return LayerTypeZero, nil
+		}
+	}
+	dlc := dl
+	return func(data []byte, decoded *[]LayerType) (LayerType, error) {
+		*decoded = (*decoded)[:0] // Truncated decoded layers.
+		typ := first
+		decoder := firstDec
+		for {
+			if err := decoder.DecodeFromBytes(data, df); err != nil {
+				return LayerTypeZero, err
+			}
+			*decoded = append(*decoded, typ)
+			typ = decoder.NextLayerType()
+			if data = decoder.LayerPayload(); len(data) == 0 {
+				break
+			}
+			if decoder, ok = dlc.Decoder(typ); !ok {
+				return typ, nil
+			}
+		}
+		return LayerTypeZero, nil
+	}
+}