query MPV astats node directly in C for efficiency

backend/player/mpv/peaks.go

@@ -0,0 +1,19 @@
+package mpv
+
+// #include <mpv/client.h>
+// int mpv_get_peaks(mpv_handle* handle, double* lPeak, double* rPeak, double* lRMS, double* rRMS);
+import "C"
+import (
+	"github.com/dweymouth/go-mpv"
+)
+
+// rather than querying peaks through m.mpv.GetProperty which necessitates
+// converting the MPV node to a Go map, we can do so in C with no Go allocations
+func (m *Player) getPeaks() (float64, float64, float64, float64, error) {
+	var lPeak, rPeak, lRMS, rRMS C.double
+	ret := int(C.mpv_get_peaks((*C.mpv_handle)(m.mpv.MPVHandle()), &lPeak, &rPeak, &lRMS, &rRMS))
+	if err := mpv.NewError(ret); err != nil {
+		return 0, 0, 0, 0, err
+	}
+	return float64(lPeak), float64(rPeak), float64(lRMS), float64(rRMS), nil
+}