[Audio] Add equalizer (#2787)

* [Audio] Add equalizer

* [Audio] Add equalizer

redbot/cogs/audio/equalizer.py

@@ -0,0 +1,50 @@
+# The equalizer class and some audio eq functions are derived from
+# 180093157554388993's work, with his permission
+
+
+class Equalizer:
+    def __init__(self):
+        self._band_count = 15
+        self.bands = [0.0 for x in range(self._band_count)]
+
+    def set_gain(self, band: int, gain: float):
+        if band < 0 or band >= self._band_count:
+            raise IndexError(f"Band {band} does not exist!")
+
+        gain = min(max(gain, -0.25), 1.0)
+
+        self.bands[band] = gain
+
+    def get_gain(self, band: int):
+        if band < 0 or band >= self._band_count:
+            raise IndexError(f"Band {band} does not exist!")
+        return self.bands[band]
+
+    def visualise(self):
+        block = ""
+        bands = [str(band + 1).zfill(2) for band in range(self._band_count)]
+        bottom = (" " * 8) + " ".join(bands)
+        gains = [1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.0, -0.1, -0.2, -0.25]
+
+        for gain in gains:
+            prefix = " "
+
+            if gain > 0:
+                prefix = "+"
+            elif gain == 0:
+                prefix = " "
+            else:
+                prefix = ""
+
+            block += f"{prefix}{gain:.2f} | "
+
+            for value in self.bands:
+                if value >= gain:
+                    block += "[] "
+                else:
+                    block += "   "
+
+            block += "\n"
+
+        block += bottom
+        return block