Change target duration alg from 'ceiling' to 'round'

fixes grafov#108

round.go

@@ -0,0 +1,56 @@
+package m3u8
+
+import (
+	"math"
+)
+
+// some constants copied from https://github.com/golang/go/blob/master/src/math/bits.go
+const (
+	shift = 64 - 11 - 1
+	bias  = 1023
+	mask  = 0x7FF
+)
+
+// round returns the nearest integer, rounding half away from zero.
+// This function is available natively in Go 1.10
+//
+// Special cases are:
+//	round(±0) = ±0
+//	round(±Inf) = ±Inf
+//	round(NaN) = NaN
+func round(x float64) float64 {
+	// Round is a faster implementation of:
+	//
+	// func Round(x float64) float64 {
+	//   t := Trunc(x)
+	//   if Abs(x-t) >= 0.5 {
+	//     return t + Copysign(1, x)
+	//   }
+	//   return t
+	// }
+	const (
+		signMask = 1 << 63
+		fracMask = 1<<shift - 1
+		half     = 1 << (shift - 1)
+		one      = bias << shift
+	)
+
+	bits := math.Float64bits(x)
+	e := uint(bits>>shift) & mask
+	if e < bias {
+		// Round abs(x) < 1 including denormals.
+		bits &= signMask // +-0
+		if e == bias-1 {
+			bits |= one // +-1
+		}
+	} else if e < bias+shift {
+		// Round any abs(x) >= 1 containing a fractional component [0,1).
+		//
+		// Numbers with larger exponents are returned unchanged since they
+		// must be either an integer, infinity, or NaN.
+		e -= bias
+		bits += half >> e
+		bits &^= fracMask >> e
+	}
+	return math.Float64frombits(bits)
+}

writer.go

@@ -305,7 +305,11 @@ func (p *MediaPlaylist) AppendSegment(seg *MediaSegment) error {
 	p.tail = (p.tail + 1) % p.capacity
 	p.count++
 	if p.TargetDuration < seg.Duration {
-		p.TargetDuration = math.Ceil(seg.Duration)
+		if p.durationAsInt {
+			p.TargetDuration = math.Ceil(seg.Duration)
+		} else {
+			p.TargetDuration = round(seg.Duration)
+		}
 	}
 	p.buf.Reset()
 	return nil

writer_test.go

@@ -167,8 +167,8 @@ func TestTargetDurationForMediaPlaylist(t *testing.T) {
 	if e != nil {
 		t.Errorf("Add 2nd segment to a media playlist failed: %s", e)
 	}
-	if p.TargetDuration < 10.0 {
-		t.Errorf("Target duration must = 10 (nearest greater integer to durations 9.0 and 9.1)")
+	if p.TargetDuration != 9.0 {
+		t.Errorf("Target duration must = 9 (nearest integer to durations 9.0 and 9.1)")
 	}
 }