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reader.go
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reader.go
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package mcap
import (
"encoding/binary"
"errors"
"fmt"
"io"
"math"
)
var ErrMetadataNotFound = errors.New("metadata not found")
func getPrefixedString(data []byte, offset int) (s string, newoffset int, err error) {
if len(data[offset:]) < 4 {
return "", 0, io.ErrShortBuffer
}
length := int(binary.LittleEndian.Uint32(data[offset : offset+4]))
if len(data[offset+4:]) < length {
return "", 0, io.ErrShortBuffer
}
return string(data[offset+4 : offset+length+4]), offset + 4 + length, nil
}
func getPrefixedBytes(data []byte, offset int) (s []byte, newoffset int, err error) {
if len(data[offset:]) < 4 {
return nil, 0, io.ErrShortBuffer
}
length := int(binary.LittleEndian.Uint32(data[offset : offset+4]))
if len(data[offset+4:]) < length {
return nil, 0, io.ErrShortBuffer
}
return data[offset+4 : offset+length+4], offset + 4 + length, nil
}
func getPrefixedMap(data []byte, offset int) (result map[string]string, newoffset int, err error) {
var key, value string
var inset int
m := make(map[string]string)
maplen, offset, err := getUint32(data, offset)
if err != nil {
return nil, 0, fmt.Errorf("failed to read map length: %w", err)
}
for uint32(offset+inset) < uint32(offset)+maplen {
key, inset, err = getPrefixedString(data[offset:], inset)
if err != nil {
return nil, 0, fmt.Errorf("failed to read map key: %w", err)
}
value, inset, err = getPrefixedString(data[offset:], inset)
if err != nil {
return nil, 0, fmt.Errorf("failed to read map value: %w", err)
}
m[key] = value
}
return m, offset + inset, nil
}
type Reader struct {
l *Lexer
r io.Reader
rs io.ReadSeeker
header *Header
channels map[uint16]*Channel
info *Info
}
type MessageIterator interface {
// Deprecated: use NextInto to avoid repeatedly heap-allocating Message structs while iterating.
Next([]byte) (*Schema, *Channel, *Message, error)
// NextInto returns the next message from the MCAP. If the returned error is io.EOF,
// this signals the end of the MCAP.
// If `msg` is not nil, NextInto will populate it with new data and
// return the same pointer, re-using or resizing `msg.Data` as needed.
// If `msg` is nil, NextInto will allocate and return a new Message on the heap.
NextInto(msg *Message) (*Schema, *Channel, *Message, error)
}
func Range(it MessageIterator, f func(*Schema, *Channel, *Message) error) error {
for {
schema, channel, message, err := it.Next(nil)
if err != nil {
if errors.Is(err, io.EOF) {
return nil
}
return fmt.Errorf("failed to read record: %w", err)
}
err = f(schema, channel, message)
if err != nil {
return fmt.Errorf("failed to process record: %w", err)
}
}
}
func (r *Reader) unindexedIterator(opts *ReadOptions) *unindexedMessageIterator {
opts.Finalize()
topicMap := make(map[string]bool)
for _, topic := range opts.Topics {
topicMap[topic] = true
}
r.l.emitChunks = false
return &unindexedMessageIterator{
lexer: r.l,
topics: topicMap,
start: opts.StartNanos,
end: opts.EndNanos,
metadataCallback: opts.MetadataCallback,
}
}
func (r *Reader) indexedMessageIterator(
opts *ReadOptions,
) *indexedMessageIterator {
opts.Finalize()
topicMap := make(map[string]bool)
for _, topic := range opts.Topics {
topicMap[topic] = true
}
r.l.emitChunks = true
return &indexedMessageIterator{
lexer: r.l,
rs: r.rs,
topics: topicMap,
start: opts.StartNanos,
end: opts.EndNanos,
order: opts.Order,
metadataCallback: opts.MetadataCallback,
}
}
func (r *Reader) Messages(
opts ...ReadOpt,
) (MessageIterator, error) {
options := ReadOptions{
StartNanos: 0,
EndNanos: math.MaxUint64,
Topics: nil,
UseIndex: true,
Order: FileOrder,
}
for _, opt := range opts {
err := opt(&options)
if err != nil {
return nil, err
}
}
options.Finalize()
if options.UseIndex {
if rs, ok := r.r.(io.ReadSeeker); ok {
r.rs = rs
} else {
return nil, fmt.Errorf("indexed reader requires a seekable reader")
}
startPos, err := r.rs.Seek(0, io.SeekCurrent)
if err != nil {
return nil, fmt.Errorf("failed to get current stream position: %w", err)
}
info, err := r.Info()
if err != nil {
return nil, fmt.Errorf("could not get info: %w", err)
}
// if there are no chunk index records present, but there are messages, we need to
// scan the file linearly to find them.
if len(info.ChunkIndexes) == 0 && info.Statistics != nil && info.Statistics.MessageCount > 0 {
_, err = r.rs.Seek(startPos, io.SeekStart)
if err != nil {
return nil, fmt.Errorf("failed to seek to start: %w", err)
}
return r.unindexedIterator(&options), nil
}
return r.indexedMessageIterator(&options), nil
}
return r.unindexedIterator(&options), nil
}
// Get the Header record from this MCAP.
func (r *Reader) Header() *Header {
return r.header
}
// Info scans the summary section to form a structure describing characteristics
// of the underlying mcap file.
func (r *Reader) Info() (*Info, error) {
if r.info != nil {
return r.info, nil
}
if r.rs == nil {
return nil, fmt.Errorf("cannot get info from non-seekable reader")
}
it := r.indexedMessageIterator(&ReadOptions{
UseIndex: true,
})
err := it.parseSummarySection()
if err != nil {
return nil, err
}
info := &Info{
Statistics: it.statistics,
Channels: it.channels.ToMap(),
ChunkIndexes: it.chunkIndexes,
AttachmentIndexes: it.attachmentIndexes,
MetadataIndexes: it.metadataIndexes,
Schemas: it.schemas.ToMap(),
Footer: it.footer,
Header: r.header,
}
r.info = info
return info, nil
}
// GetAttachmentReader returns an attachment reader located at the specific offset.
// The reader must be consumed before the base reader is used again.
func (r *Reader) GetAttachmentReader(offset uint64) (*AttachmentReader, error) {
_, err := r.rs.Seek(int64(offset+9), io.SeekStart)
if err != nil {
return nil, err
}
ar, err := parseAttachmentReader(r.rs, true)
if err != nil {
return nil, err
}
return ar, nil
}
func (r *Reader) GetMetadata(offset uint64) (*Metadata, error) {
_, err := r.rs.Seek(int64(offset), io.SeekStart)
if err != nil {
return nil, err
}
token, data, err := r.l.Next(nil)
if err != nil {
return nil, err
}
if token != TokenMetadata {
return nil, fmt.Errorf("expected metadata record, found %v", data)
}
metadata, err := ParseMetadata(data)
if err != nil {
return nil, fmt.Errorf("failed to parse metadata record: %w", err)
}
return metadata, nil
}
// Close the reader.
func (r *Reader) Close() {
r.l.Close()
}
func NewReader(r io.Reader) (*Reader, error) {
var rs io.ReadSeeker
if readseeker, ok := r.(io.ReadSeeker); ok {
rs = readseeker
}
lexer, err := NewLexer(r, &LexerOptions{
EmitChunks: true,
})
if err != nil {
return nil, err
}
defer lexer.Close()
token, headerData, err := lexer.Next(nil)
if err != nil {
return nil, fmt.Errorf("could not read MCAP header when opening reader: %w", err)
}
if token != TokenHeader {
return nil, fmt.Errorf("expected first record in MCAP to be a Header, found %v", headerData)
}
header, err := ParseHeader(headerData)
if err != nil {
return nil, err
}
return &Reader{
l: lexer,
r: r,
rs: rs,
header: header,
channels: make(map[uint16]*Channel),
}, nil
}