Early concept for rendering the frequency waveform

src/components/views/voice_messages/FrequencyBars.tsx

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+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from "react";
+import {IFrequencyPackage, VoiceRecorder} from "../../../voice/VoiceRecorder";
+import {replaceableComponent} from "../../../utils/replaceableComponent";
+import {arrayFastResample, arraySeed} from "../../../utils/arrays";
+import {percentageOf} from "../../../utils/numbers";
+
+interface IProps {
+    recorder: VoiceRecorder
+}
+
+interface IState {
+    heights: number[];
+}
+
+const DOWNSAMPLE_TARGET = 35; // number of bars
+
+@replaceableComponent("views.voice_messages.FrequencyBars")
+export default class FrequencyBars extends React.PureComponent<IProps, IState> {
+    public constructor(props) {
+        super(props);
+
+        this.state = {heights: arraySeed(0, DOWNSAMPLE_TARGET)};
+        this.props.recorder.frequencyData.onUpdate(this.onFrequencyData);
+    }
+
+    private onFrequencyData = (freq: IFrequencyPackage) => {
+        // We're downsampling from about 1024 points to about 35, so this function is fine (see docs/impl)
+        const bars = arrayFastResample(Array.from(freq.dbBars), DOWNSAMPLE_TARGET);
+        this.setState({
+            // Values are somewhat arbitrary, but help decide what shape the graph should be
+            heights: bars.map(b => percentageOf(b, -150, -70) * 100),
+        });
+    };
+
+    public render() {
+        return <div className='mx_FrequencyBars'>
+            {this.state.heights.map((h, i) => {
+                return <span key={i} style={{height: h + '%'}} className='mx_FrequencyBars_bar' />;
+            })}
+        </div>;
+    }
+}