examples : refactor in order to reuse code and reduce duplication (#482)

* examples : refactor common code into a library

* examples : refactor common SDL code into a library

* make : update Makefile to use common libs

* common : fix MSVC M_PI ..

* addon.node : link common lib

examples/common.h

@@ -0,0 +1,40 @@
+#pragma once
+
+// needs to match WHISPER_SAMPLE_RATE
+#define COMMON_SAMPLE_RATE 16000
+
+#include <vector>
+#include <string>
+
+std::string trim(const std::string & s);
+
+std::string replace(
+        const std::string & s,
+        const std::string & from,
+        const std::string & to);
+
+// Read WAV audio file and store the PCM data into pcmf32
+// The sample rate of the audio must be equal to COMMON_SAMPLE_RATE
+// If stereo flag is set and the audio has 2 channels, the pcmf32s will contain 2 channel PCM
+bool read_wav(
+        const std::string & fname,
+        std::vector<float> & pcmf32,
+        std::vector<std::vector<float>> & pcmf32s,
+        bool stereo);
+
+// Apply a high-pass frequency filter to PCM audio
+// Suppresses frequencies below cutoff Hz
+void high_pass_filter(
+        std::vector<float> & data,
+        float cutoff,
+        float sample_rate);
+
+// Basic voice activity detection (VAD) using audio energy adaptive threshold
+bool vad_simple(
+        std::vector<float> & pcmf32,
+        int   sample_rate,
+        int   last_ms,
+        float vad_thold,
+        float freq_thold,
+        bool  verbose);
+