path: root/swarm/multihash/multihash.go

// Copyright 2018 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/>.

package multihash

import (
    "bytes"
    "encoding/binary"
    "errors"
    "fmt"
)

const (
    defaultMultihashLength   = 32
    defaultMultihashTypeCode = 0x1b
)

var (
    multihashTypeCode uint8
    MultihashLength   = defaultMultihashLength
)

func init() {
    multihashTypeCode = defaultMultihashTypeCode
    MultihashLength = defaultMultihashLength
}

// check if valid swarm multihash
func isSwarmMultihashType(code uint8) bool {
    return code == multihashTypeCode
}

// GetMultihashLength returns the digest length of the provided multihash
// It will fail if the multihash is not a valid swarm mulithash
func GetMultihashLength(data []byte) (int, int, error) {
    cursor := 0
    typ, c := binary.Uvarint(data)
    if c <= 0 {
        return 0, 0, errors.New("unreadable hashtype field")
    }
    if !isSwarmMultihashType(uint8(typ)) {
        return 0, 0, fmt.Errorf("hash code %x is not a swarm hashtype", typ)
    }
    cursor += c
    hashlength, c := binary.Uvarint(data[cursor:])
    if c <= 0 {
        return 0, 0, errors.New("unreadable length field")
    }
    cursor += c

    // we cheekily assume hashlength < maxint
    inthashlength := int(hashlength)
    if len(data[c:]) < inthashlength {
        return 0, 0, errors.New("length mismatch")
    }
    return inthashlength, cursor, nil
}

// FromMulithash returns the digest portion of the multihash
// It will fail if the multihash is not a valid swarm multihash
func FromMultihash(data []byte) ([]byte, error) {
    hashLength, _, err := GetMultihashLength(data)
    if err != nil {
        return nil, err
    }
    return data[len(data)-hashLength:], nil
}

// ToMulithash wraps the provided digest data with a swarm mulithash header
func ToMultihash(hashData []byte) []byte {
    buf := bytes.NewBuffer(nil)
    b := make([]byte, 8)
    c := binary.PutUvarint(b, uint64(multihashTypeCode))
    buf.Write(b[:c])
    c = binary.PutUvarint(b, uint64(len(hashData)))
    buf.Write(b[:c])
    buf.Write(hashData)
    return buf.Bytes()
}