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      1 package trace
      2 
      3 import "bytes"
      4 
      5 // Writer is a test trace writer.
      6 type Writer struct {
      7 	bytes.Buffer
      8 }
      9 
     10 func NewWriter() *Writer {
     11 	w := new(Writer)
     12 	w.Write([]byte("go 1.7 trace\x00\x00\x00\x00"))
     13 	return w
     14 }
     15 
     16 // Emit writes an event record to the trace.
     17 // See Event types for valid types and required arguments.
     18 func (w *Writer) Emit(typ byte, args ...uint64) {
     19 	nargs := byte(len(args)) - 1
     20 	if nargs > 3 {
     21 		nargs = 3
     22 	}
     23 	buf := []byte{typ | nargs<<6}
     24 	if nargs == 3 {
     25 		buf = append(buf, 0)
     26 	}
     27 	for _, a := range args {
     28 		buf = appendVarint(buf, a)
     29 	}
     30 	if nargs == 3 {
     31 		buf[1] = byte(len(buf) - 2)
     32 	}
     33 	n, err := w.Write(buf)
     34 	if n != len(buf) || err != nil {
     35 		panic("failed to write")
     36 	}
     37 }
     38 
     39 func appendVarint(buf []byte, v uint64) []byte {
     40 	for ; v >= 0x80; v >>= 7 {
     41 		buf = append(buf, 0x80|byte(v))
     42 	}
     43 	buf = append(buf, byte(v))
     44 	return buf
     45 }
     46