query MPV astats node directly in C for efficiency

backend/player/mpv/peaks.c

@@ -0,0 +1,49 @@
+#include <mpv/client.h>
+#include <stdlib.h>
+#include <string.h>
+
+int mpv_get_peaks(mpv_handle* handle, double* lPeak, double* rPeak, double* lRMS, double* rRMS) {
+    mpv_node result;
+    int ret = mpv_get_property(handle, "af-metadata/astats", MPV_FORMAT_NODE, &result);
+    if (ret != MPV_ERROR_SUCCESS) {
+        return ret;
+    }
+    if (result.format != MPV_FORMAT_NODE_MAP) {
+        return MPV_ERROR_PROPERTY_FORMAT;
+    }
+
+    int found = 0;
+    for (int i = 0; found < 4 && i < result.u.list->num; i++) {
+        if (strcmp("lavfi.astats.1.Peak_level", result.u.list->keys[i]) == 0) {
+            if (result.u.list->values[i].format != MPV_FORMAT_STRING) {
+                return MPV_ERROR_PROPERTY_FORMAT;
+            }
+            *lPeak = atof(result.u.list->values[i].u.string);
+            found++;
+        }
+        if (strcmp("lavfi.astats.2.Peak_level", result.u.list->keys[i]) == 0) {
+            if (result.u.list->values[i].format != MPV_FORMAT_STRING) {
+                return MPV_ERROR_PROPERTY_FORMAT;
+            }
+            *rPeak = atof(result.u.list->values[i].u.string);
+            found++;
+        }
+        if (strcmp("lavfi.astats.1.RMS_level", result.u.list->keys[i]) == 0) {
+            if (result.u.list->values[i].format != MPV_FORMAT_STRING) {
+                return MPV_ERROR_PROPERTY_FORMAT;
+            }
+            *lRMS = atof(result.u.list->values[i].u.string);
+            found++;
+        }
+        if (strcmp("lavfi.astats.2.RMS_level", result.u.list->keys[i]) == 0) {
+            if (result.u.list->values[i].format != MPV_FORMAT_STRING) {
+                return MPV_ERROR_PROPERTY_FORMAT;
+            }
+            *rRMS = atof(result.u.list->values[i].u.string);
+            found++;
+        }
+    }
+    mpv_free_node_contents(&result);
+
+    return MPV_ERROR_SUCCESS;
+}

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
+}

backend/player/mpv/player.go

@@ -455,19 +455,11 @@ func (p *Player) GetPeaks() (float64, float64, float64, float64) {
 	if p.status.State != player.Playing {
 		return nInf, nInf, nInf, nInf
 	}
-	prop, err := p.mpv.GetProperty("af-metadata/astats", mpv.FORMAT_NODE)
+	lPeak, rPeak, lRMS, rRMS, err := p.getPeaks()
 	if err != nil {
 		return nInf, nInf, nInf, nInf
 	}
-	m := prop.(*mpv.Node).Data.(map[string]*mpv.Node)
-	if lPeakNode, ok := m["lavfi.astats.1.Peak_level"]; ok {
-		lPeak, _ := strconv.ParseFloat(lPeakNode.Data.(string), 64)
-		rPeak, _ := strconv.ParseFloat(m["lavfi.astats.2.Peak_level"].Data.(string), 64)
-		lRMS, _ := strconv.ParseFloat(m["lavfi.astats.1.RMS_level"].Data.(string), 64)
-		rRMS, _ := strconv.ParseFloat(m["lavfi.astats.2.RMS_level"].Data.(string), 64)
-		return lPeak, rPeak, lRMS, rRMS
-	}
-	return nInf, nInf, nInf, nInf
+	return lPeak, rPeak, lRMS, rRMS
 }
 
 // sets the state and invokes callbacks, if triggered