av/stream/mts/meta/meta.go

/*
NAME
  meta.go

DESCRIPTION
  See Readme.md

AUTHOR
  Saxon Nelson-Milton <saxon@ausocean.org>

LICENSE
  meta.go is Copyright (C) 2017-2019 the Australian Ocean Lab (AusOcean)

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It 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 General Public License
 for more details.

  You should have received a copy of the GNU General Public License
  along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
*/

package meta

import (
	"errors"
	"strings"
	"sync"
)

const headSize = 4

const (
	majVer = 1
	minVer = 0
)

const (
	dataLenIdx1 = 2
	dataLenIdx2 = 3
)

var (
	errKeyAbsent = errors.New("Key does not exist in map")
)

type Metadata struct {
	mu   sync.RWMutex
	data map[string]string
	enc  []byte
}

func New() *Metadata {
	return &Metadata{
		data: make(map[string]string),
		enc: []byte{
			0x00,                   // Reserved byte
			(majVer << 4) | minVer, // MS and LS versions
			0x00,                   // Data len byte1
			0x00,                   // Data len byte2
		},
	}
}

// Add adds metadata with key and val
func (m *Metadata) Add(key, val string) {
	m.mu.Lock()
	m.data[key] = val
	m.mu.Unlock()
}

// All returns the a copy of the map containing the meta data
func (m *Metadata) All() map[string]string {
	m.mu.Lock()
	cpy := make(map[string]string)
	for k, v := range m.data {
		cpy[k] = v
	}
	m.mu.Unlock()
	return cpy
}

// Get returns the meta data for the passed key
func (m *Metadata) Get(key string) (string, error) {
	m.mu.Lock()
	val, ok := m.data[key]
	m.mu.Unlock()
	if !ok {
		return "", errKeyAbsent
	}

	return val, nil
}

// Remove deletes a meta entry in the map and returns error if it doesn’t exist
func (m *Metadata) Delete(key string) error {
	m.mu.Lock()
	defer m.mu.Unlock()
	if _, ok := m.data[key]; ok {
		delete(m.data, key)
		return nil
	}
	return errKeyAbsent
}

// Encode takes the meta data map and encods into a byte slice with header
// describing the version, length of data and data in TSV format.
func (m *Metadata) Encode() []byte {
	m.enc = m.enc[:headSize]

	// Iterate over map and append entries, only adding tab if we're not on the last entry
	var i int
	var entry string
	for k, v := range m.data {
		i++
		entry += k + "=" + v
		if i < len(m.data) {
			entry += "\t"
		}
	}
	m.enc = append(m.enc, []byte(entry)...)

	// Calculate and set data length in encoded meta header.
	dataLen := len(m.enc[headSize:])
	m.enc[dataLenIdx1] = byte(dataLen >> 8)
	m.enc[dataLenIdx2] = byte(dataLen)

	return m.enc
}

func ReadFrom(d []byte, key string) (string, error) {
	entries := strings.Split(string(d), "\t")
	for _, entry := range entries {
		kv := strings.Split(entry, "=")
		if kv[0] == key {
			return kv[1], nil
		}
	}
	return "", errors.New("could not find key in metadata")
}