MP3File: Add rms_level property

This lets a music player show it vu-meter style

shared-bindings/audiomp3/MP3File.c

@@ -224,6 +224,24 @@ const mp_obj_property_t audiomp3_mp3file_channel_count_obj = {
               (mp_obj_t)&mp_const_none_obj},
 };
 
+//|   .. attribute:: rms_level
+//|
+//|     The RMS audio level of a recently played moment of audio. (read only)
+//|
+STATIC mp_obj_t audiomp3_mp3file_obj_get_rms_level(mp_obj_t self_in) {
+    audiomp3_mp3file_obj_t *self = MP_OBJ_TO_PTR(self_in);
+    check_for_deinit(self);
+    return mp_obj_new_float(common_hal_audiomp3_mp3file_get_rms_level(self));
+}
+MP_DEFINE_CONST_FUN_OBJ_1(audiomp3_mp3file_get_rms_level_obj, audiomp3_mp3file_obj_get_rms_level);
+
+const mp_obj_property_t audiomp3_mp3file_rms_level_obj = {
+    .base.type = &mp_type_property,
+    .proxy = {(mp_obj_t)&audiomp3_mp3file_get_rms_level_obj,
+              (mp_obj_t)&mp_const_none_obj,
+              (mp_obj_t)&mp_const_none_obj},
+};
+
 
 STATIC const mp_rom_map_elem_t audiomp3_mp3file_locals_dict_table[] = {
     // Methods
@@ -236,6 +254,7 @@ STATIC const mp_rom_map_elem_t audiomp3_mp3file_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_sample_rate), MP_ROM_PTR(&audiomp3_mp3file_sample_rate_obj) },
     { MP_ROM_QSTR(MP_QSTR_bits_per_sample), MP_ROM_PTR(&audiomp3_mp3file_bits_per_sample_obj) },
     { MP_ROM_QSTR(MP_QSTR_channel_count), MP_ROM_PTR(&audiomp3_mp3file_channel_count_obj) },
+    { MP_ROM_QSTR(MP_QSTR_rms_level), MP_ROM_PTR(&audiomp3_mp3file_rms_level_obj) },
 };
 STATIC MP_DEFINE_CONST_DICT(audiomp3_mp3file_locals_dict, audiomp3_mp3file_locals_dict_table);
 

shared-bindings/audiomp3/MP3File.h

@@ -45,5 +45,6 @@ uint32_t common_hal_audiomp3_mp3file_get_sample_rate(audiomp3_mp3file_obj_t* sel
 void common_hal_audiomp3_mp3file_set_sample_rate(audiomp3_mp3file_obj_t* self, uint32_t sample_rate);
 uint8_t common_hal_audiomp3_mp3file_get_bits_per_sample(audiomp3_mp3file_obj_t* self);
 uint8_t common_hal_audiomp3_mp3file_get_channel_count(audiomp3_mp3file_obj_t* self);
+float common_hal_audiomp3_mp3file_get_rms_level(audiomp3_mp3file_obj_t* self);
 
 #endif // MICROPY_INCLUDED_SHARED_BINDINGS_AUDIOIO_MP3FILE_H

shared-module/audiomp3/MP3File.c

@@ -29,6 +29,7 @@
 
 #include <stdint.h>
 #include <string.h>
+#include <math.h>
 
 #include "py/mperrno.h"
 #include "py/runtime.h"
@@ -339,3 +340,13 @@ void audiomp3_mp3file_get_buffer_structure(audiomp3_mp3file_obj_t* self, bool si
         *spacing = 1;
     }
 }
+
+float common_hal_audiomp3_mp3file_get_rms_level(audiomp3_mp3file_obj_t* self) {
+    float sumsq = 0.f;
+    // Assumes no DC component to the audio.  Is that a safe assumption?
+    int16_t *buffer = (int16_t *)(void *)self->buffers[self->buffer_index];
+    for(size_t i=0; i<self->frame_buffer_size / sizeof(int16_t); i++) {
+        sumsq += (float)buffer[i] * buffer[i];
+    }
+    return sqrtf(sumsq) / (self->frame_buffer_size / sizeof(int16_t));
+}

shared-module/audiomp3/MP3File.h

@@ -67,4 +67,6 @@ void audiomp3_mp3file_get_buffer_structure(audiomp3_mp3file_obj_t* self, bool si
                                            bool* single_buffer, bool* samples_signed,
                                            uint32_t* max_buffer_length, uint8_t* spacing);
 
+float audiomp3_mp3file_get_rms_level(audiomp3_mp3file_obj_t* self);
+
 #endif // MICROPY_INCLUDED_SHARED_MODULE_AUDIOIO_MP3FILE_H