rounds alsa HW mixer volume towards ±∞ depending on sgn(∆ vol)

This alleviates a problem in which 'volume +1' cannot be undo by 'volume -1' when using alsa hw mixer. Closes #104

rounds alsa HW mixer volume towards ±∞ depending on sgn(∆ vol)
967af603 · Stefano Miccoli · Max Kellermann · f1ef9f9d · 967af603 · 967af603
Commit 967af603 authored Dec 12, 2017 by Stefano Miccoli Committed by Max Kellermann Dec 18, 2017
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NEWS NEWS +2 -0

AlsaMixerPlugin.cxx src/mixer/plugins/AlsaMixerPlugin.cxx +3 -1

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--- a/NEWS
+++ b/NEWS
 ver 0.20.13 (not yet released)
 * output
  - osx: set up ring buffer to hold at least 100ms
+* mixer
+  - alsa: fix rounding errors
 * database
  - simple: don't purge mount points on update/rescan
  - simple: fix "mount" bug caused by bad compiler optimization

--- a/src/mixer/plugins/AlsaMixerPlugin.cxx
+++ b/src/mixer/plugins/AlsaMixerPlugin.cxx
@@ -292,7 +292,9 @@ AlsaMixer::SetVolume(unsigned volume)
 {
 	assert(handle != nullptr);

-	int err = set_normalized_playback_volume(elem, 0.01*volume, 1);
+	double cur = get_normalized_playback_volume(elem, SND_MIXER_SCHN_FRONT_LEFT);
+	int delta = volume - lrint(100.*cur);
+	int err = set_normalized_playback_volume(elem, cur + 0.01*delta, delta);
 	if (err < 0)
 		throw FormatRuntimeError("failed to set ALSA volume: %s",
 					 snd_strerror(err));