hyperstone/parser.go

package main

import (
	"bufio"
	"fmt"
	"io"
	"reflect"

	"github.com/golang/protobuf/proto"
	"github.com/golang/snappy"
)

type parser struct {
	// the source of replay bytes. Must NOT be compressed.
	source *bufio.Reader

	dumpDatagrams bool
	dumpPackets   bool
}

func newParser(r io.Reader) *parser {
	br := bufio.NewReaderSize(r, 1<<16)
	return &parser{source: br}
}

func (p *parser) start() error {
	ok, err := p.checkHeader()
	if err != nil {
		return wrap(err, "parser start error")
	}
	if !ok {
		return fmt.Errorf("parser start error: invalid header")
	}
	if _, err := p.source.Discard(8); err != nil {
		return wrap(err, "parser start error")
	}
	return nil
}

func (p *parser) run() error {
	for {
		gram, err := p.readDatagram()
		if err != nil {
			return wrap(err, "read datagram error in run loop")
		}
		if p.dumpDatagrams {
			fmt.Println(gram)
		}

		if len(gram.body) == 0 {
			continue
		}

		switch gram.cmd {
		case EDemoCommands_DEM_Packet:
			if err := gram.check(p.dumpPackets); err != nil {
				fmt.Printf("error: %v\n", err)
			}
		default:
			m := cmdFactory.BuildMessage(int(gram.cmd))
			if m != nil {
				err := proto.Unmarshal(gram.body, m)
				if err != nil {
					fmt.Printf("cmd unmarshal error unpacking data of length %d with cmd type %s into message type %v: %v\n", len(gram.body), gram.cmd, reflect.TypeOf(m), err)
				}
			}
		}
	}
}

// DecodeVarint reads a varint-encoded integer from the source reader.
// It returns the value as a uin64 and any errors encountered. The reader will
// be advanced by the number of bytes needed to consume this value. On error,
// the reader will not be advanced.
//
// This is the format for the
// int32, int64, uint32, uint64, bool, and enum
func (p *parser) decodeVarint() (uint64, error) {
	// protobuf defines values that are up to 64 bits wide. The largest value
	// stored in a protobuf varint is 64 data bits, which in varint encoding,
	// would require 10 bytes.
	buf, err := p.source.Peek(10)
	if err != nil {
		return 0, wrap(err, "decode varint couldn't peek 10 bytes")
	}

	var x uint64
	var s uint
	for i, b := range buf {
		// when msb is 0, we're at the last byte of the value
		if b < 0x80 {
			if _, err := p.source.Discard(i + 1); err != nil {
				return 0, wrap(err, "decode varint couldn't discard %d bytes", i)
			}
			return x | uint64(b)<<s, nil
		}

		// otherwise, include the 7 least significant bits in our value
		x |= uint64(b&0x7f) << s
		s += 7
	}

	return 0, fmt.Errorf("decode varint never saw the end of varint")
}

// reads n bytes from the source into the scratch buffer. the returned slice is
// the beginning of the scratch buffer. it will be corrupted on the next call
// to readn or the next operation that utilizes the scratch buffer.
func (p *parser) readn(n int) ([]byte, error) {
	buf := make([]byte, n)
	if _, err := io.ReadFull(p.source, buf); err != nil {
		return nil, wrap(err, "error reading %d bytes", n)
	}
	return buf, nil
}

// checks whether we have an acceptable header at the current reader position.
func (p *parser) checkHeader() (bool, error) {
	buf, err := p.readn(8)
	if err != nil {
		return false, wrap(err, "unable to read header bytes")
	}
	return string(buf) == replayHeader, nil
}

func (p *parser) readCommand() (datagramType, bool, error) {
	n, err := p.decodeVarint()
	if err != nil {
		return EDemoCommands_DEM_Error, false, wrap(err, "readCommand couldn't read varint")
	}

	compressed := false
	if n&0x40 > 0 {
		compressed = true
		n &^= 0x40
	}
	return datagramType(n), compressed, nil
}

func (p *parser) readDatagram() (*dataGram, error) {
	cmd, compressed, err := p.readCommand()
	if err != nil {
		return nil, wrap(err, "readDatagram couldn't get a command")
	}

	tick, err := p.decodeVarint()
	if err != nil {
		return nil, wrap(err, "readDatagram couldn't read the tick value")
	}

	size, err := p.decodeVarint()
	if err != nil {
		return nil, wrap(err, "readDatagram couldn't read the size value")
	}

	if size > 0 {
		buf := make([]byte, int(size))
		if _, err := io.ReadFull(p.source, buf); err != nil {
			return nil, wrap(err, "readDatagram couldn't read datagram body")
		}

		if compressed {
			var err error
			buf, err = snappy.Decode(nil, buf)
			if err != nil {
				return nil, wrap(err, "readDatagram couldn't snappy decode body")
			}
		}
		// TODO: pool these!
		return &dataGram{cmd, int64(tick), buf}, nil
	}

	// TODO: pool these!
	return &dataGram{cmd, int64(tick), nil}, nil
}