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 (
	"encoding/binary"
	"errors"
	"strings"
	"sync"
)

// This is the headsize of our metadata string,
// which is encoded int the data body of a pmt descriptor.
const headSize = 4

const (
	majVer = 1
	minVer = 0
)

// Indices of bytes for uint16 metadata length.
const (
	dataLenIdx = 2
)

var (
	errKeyAbsent     = errors.New("Key does not exist in map")
	errNoHeader      = errors.New("Metadata string does not contain header")
	errInvalidHeader = errors.New("Metadata string does not contain valid header")
)

// Metadata provides functionality for the storage and encoding of metadata
// using a map.
type Metadata struct {
	mu    sync.RWMutex
	data  map[string]string
	order []string
	enc   []byte
}

// New returns a pointer to a new Metadata.
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()
	defer m.mu.Unlock()
	m.data[key] = val
	for _, k := range m.order {
		if k == key {
			return
		}
	}
	m.order = append(m.order, key)
	return
}

// 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
}

// Delete 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)
		for i, k := range m.order {
			if k == key {
				copy(m.order[:i], m.order[i+1:])
				m.order = m.order[:len(m.order)-1]
				break
			}
		}
		return nil
	}
	return errKeyAbsent
}

// Encode takes the meta data map and encodes 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 entry string
	for i, k := range m.order {
		v := m.data[k]
		entry += k + "=" + v
		if i+1 < 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:])
	binary.BigEndian.PutUint16(m.enc[dataLenIdx:dataLenIdx+2], uint16(dataLen))
	return m.enc
}

// ReadFrom extracts a value from a metadata string d, for the given key. If the
// key is not present in the metadata string, an error is returned. If the
// metadata header is not present in the string, an error is returned.
func ReadFrom(d []byte, key string) (string, error) {
	if d[0] != 0 {
		return "", errNoHeader
	} else if d[0] == 0 && binary.BigEndian.Uint16(d[2:headSize]) != uint16(len(d[headSize:])) {
		return "", errInvalidHeader
	}
	d = d[headSize:]
	entries := strings.Split(string(d), "\t")
	for _, entry := range entries {
		kv := strings.Split(entry, "=")
		if kv[0] == key {
			return kv[1], nil
		}
	}
	return "", errKeyAbsent
}