Butterworth Filter

1class ButterworthFilter {
2  private a: number[];
3  private b: number[];
4  
5  constructor(order: number, cutoff: number, sampleRate: number, type: 'lowpass' | 'highpass') {
6    const wc = 2 * Math.PI * cutoff / sampleRate;
7    const { a, b } = this.designFilter(order, wc, type);
8    this.a = a;
9    this.b = b;
10  }
11  
12  filter(signal: number[]): number[] {
13    const output: number[] = new Array(signal.length);
14    const x: number[] = new Array(this.b.length).fill(0); // Input history
15    const y: number[] = new Array(this.a.length).fill(0); // Output history
16    
17    for (let i = 0; i < signal.length; i++) {
18      // Shift input history
19      x.unshift(signal[i]);
20      x.pop();
21      
22      // Apply difference equation
23      let sum = 0;
24      for (let j = 0; j < this.b.length; j++) {
25        sum += this.b[j] * x[j];
26      }
27      for (let j = 1; j < this.a.length; j++) {
28        sum -= this.a[j] * y[j - 1];
29      }
30      
31      output[i] = sum;
32      
33      // Shift output history
34      y.unshift(sum);
35      y.pop();
36    }
37    
38    return output;
39  }
40  
41  private designFilter(order: number, wc: number, type: string): { a: number[], b: number[] } {
42    // Simplified coefficient calculation
43    // Real implementation uses scipy.signal or similar
44    return { a: [1], b: [1] };
45  }
46}