topic.go

// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

// Contains the Whisper protocol Topic element.

package whisperv5

import (
	"fmt"
	"strings"

	"github.com/ethereum/go-ethereum/common"
)

// Topic represents a cryptographically secure, probabilistic partial
// classifications of a message, determined as the first (left) 4 bytes of the
// SHA3 hash of some arbitrary data given by the original author of the message.
type TopicType [TopicLength]byte

func BytesToTopic(b []byte) (t TopicType) {
	sz := TopicLength
	if x := len(b); x < TopicLength {
		sz = x
	}
	for i := 0; i < sz; i++ {
		t[i] = b[i]
	}
	return t
}

// String converts a topic byte array to a string representation.
func (topic *TopicType) String() string {
	return string(common.ToHex(topic[:]))
}

// UnmarshalJSON parses a hex representation to a topic.
func (t *TopicType) UnmarshalJSON(input []byte) error {
	length := len(input)
	if length >= 2 && input[0] == '"' && input[length-1] == '"' {
		input = input[1 : length-1]
	}
	// strip "0x" for length check
	if len(input) > 1 && strings.ToLower(string(input[:2])) == "0x" {
		input = input[2:]
	}
	// validate the length of the input
	if len(input) != TopicLength*2 {
		return fmt.Errorf("unmarshalJSON failed: topic must be exactly %d bytes", TopicLength)
	}
	b := common.FromHex(string(input))
	if b == nil {
		return fmt.Errorf("unmarshalJSON failed: wrong topic format")
	}
	*t = BytesToTopic(b)
	return nil
}