Aurora-SpotiRip/helpers/audioAnalyzer.tsx

import FFT from "fft.js";
import { decodeAudio } from "./audioProcessing";

export type AudioAnalysis = {
  durationSeconds: number;
  sampleRate: number;
  channels: number;
  peakDbfs: number;
  rmsDbfs: number;
  crestFactorDb: number;
  estimatedCutoffHz: number;
  qualityLabel: "Full Spectrum" | "Suspect Quality" | "Likely Transcode" | "Fake Lossless";
  spectrogram: number[][];
};

function db(value: number) {
  return 20 * Math.log10(Math.max(value, 1e-12));
}

function hann(index: number, size: number) {
  return 0.5 * (1 - Math.cos((2 * Math.PI * index) / Math.max(1, size - 1)));
}

function monoAt(buffer: AudioBuffer, index: number) {
  let value = 0;
  for (let channel = 0; channel < buffer.numberOfChannels; channel += 1) {
    value += buffer.getChannelData(channel)[index] || 0;
  }
  return value / Math.max(1, buffer.numberOfChannels);
}

function qualityFromCutoff(cutoff: number, nyquist: number): AudioAnalysis["qualityLabel"] {
  const effective = Math.min(cutoff, nyquist);
  if (effective < 14_000) return "Fake Lossless";
  if (effective < 17_000) return "Likely Transcode";
  if (effective < 19_500 && nyquist >= 20_000) return "Suspect Quality";
  return "Full Spectrum";
}

export async function analyzeAudioFile(file: File): Promise<AudioAnalysis> {
  const buffer = await decodeAudio(file);
  let peak = 0;
  let sumSquares = 0;
  let samples = 0;
  const stride = Math.max(1, Math.floor(buffer.length / 2_000_000));

  for (let index = 0; index < buffer.length; index += stride) {
    const sample = monoAt(buffer, index);
    const abs = Math.abs(sample);
    if (abs > peak) peak = abs;
    sumSquares += sample * sample;
    samples += 1;
  }

  const rms = Math.sqrt(sumSquares / Math.max(1, samples));
  const peakDbfs = db(peak);
  const rmsDbfs = db(rms);

  const fftSize = 4096;
  const fft = new FFT(fftSize);
  const input = new Array(fftSize).fill(0);
  const output = fft.createComplexArray();
  const slices = Math.min(96, Math.max(24, Math.floor(buffer.duration * 2)));
  const bins = 128;
  const spectrogram: number[][] = [];
  const aggregate = new Float64Array(fftSize / 2);

  for (let slice = 0; slice < slices; slice += 1) {
    const center = Math.floor(((slice + 0.5) / slices) * buffer.length);
    const start = Math.max(0, Math.min(buffer.length - fftSize, center - fftSize / 2));
    for (let index = 0; index < fftSize; index += 1) {
      input[index] = monoAt(buffer, start + index) * hann(index, fftSize);
    }
    fft.realTransform(output, input);
    fft.completeSpectrum(output);

    const magnitudes = new Float64Array(fftSize / 2);
    let maxMagnitude = 1e-12;
    for (let bin = 0; bin < fftSize / 2; bin += 1) {
      const real = output[2 * bin];
      const imag = output[2 * bin + 1];
      const magnitude = Math.sqrt(real * real + imag * imag);
      magnitudes[bin] = magnitude;
      aggregate[bin] += magnitude;
      if (magnitude > maxMagnitude) maxMagnitude = magnitude;
    }

    const row: number[] = [];
    for (let displayBin = 0; displayBin < bins; displayBin += 1) {
      const sourceStart = Math.floor((displayBin / bins) * magnitudes.length);
      const sourceEnd = Math.max(
        sourceStart + 1,
        Math.floor(((displayBin + 1) / bins) * magnitudes.length),
      );
      let localMax = 0;
      for (let source = sourceStart; source < sourceEnd; source += 1) {
        localMax = Math.max(localMax, magnitudes[source]);
      }
      const relativeDb = db(localMax / maxMagnitude);
      row.push(Math.max(0, Math.min(1, (relativeDb + 72) / 72)));
    }
    spectrogram.push(row);
  }

  let aggregatePeak = 1e-12;
  for (const value of aggregate) aggregatePeak = Math.max(aggregatePeak, value);
  let cutoffBin = 0;
  for (let bin = 1; bin < aggregate.length; bin += 1) {
    const relativeDb = db(aggregate[bin] / aggregatePeak);
    if (relativeDb > -48) cutoffBin = bin;
  }
  const estimatedCutoffHz = Math.min(
    buffer.sampleRate / 2,
    (cutoffBin * buffer.sampleRate) / fftSize,
  );

  return {
    durationSeconds: buffer.duration,
    sampleRate: buffer.sampleRate,
    channels: buffer.numberOfChannels,
    peakDbfs,
    rmsDbfs,
    crestFactorDb: peakDbfs - rmsDbfs,
    estimatedCutoffHz,
    qualityLabel: qualityFromCutoff(estimatedCutoffHz, buffer.sampleRate / 2),
    spectrogram,
  };
}