waveform.js

var probe = require("./lib/probe.js"),
  d3 = require("d3"),
  pcmStream = require("./lib/pcm.js");

function getWaveform(filename, options, cb) {
  var stream = pcmStream(filename, {
      channels: options.channels,
    }),
    samples = [];

  stream.on("data", function (sample, channel) {
    // Average multiple channels
    if (channel > 0) {
      samples[samples.length - 1] =
        (samples[samples.length - 1] * channel + sample) / (channel + 1);
    } else {
      samples.push(sample);
    }
  });

  stream.on("error", cb);

  stream.on("end", function (output) {
    var processed = processSamples(
      samples,
      options.numFrames,
      options.samplesPerFrame
    );
    return cb(null, processed);
  });
}

function processSamples(samples, numFrames, samplesPerFrame) {
  // TODO spread out slop across frames
  var perFrame = Math.floor(samples.length / numFrames),
    perPoint = Math.floor(perFrame / samplesPerFrame),
    range = d3.range(samplesPerFrame),
    maxFrame,
    maxRms = (maxMid = 0);

  var unadjusted = d3.range(numFrames).map(function (frame) {
    var frameSamples = samples.slice(frame * perFrame, (frame + 1) * perFrame),
      points = range.map(function (point) {
        var pointSamples = frameSamples.slice(
            point * perPoint,
            (point + 1) * perPoint
          ),
          midpoint = pointSamples[Math.floor(pointSamples.length / 2)];

        var rms = Math.sqrt(
          d3.sum(
            pointSamples.map(function (d) {
              return d * d;
            })
          ) / perPoint
        );

        if (rms > maxRms) {
          maxRms = rms;
          maxFrame = frame;
        }

        if (Math.abs(midpoint) > maxMid) {
          maxMid = Math.abs(midpoint);
        }

        // Min value, max value, and midpoint value
        return [rms, midpoint];
      });

    return points;
  });

  var adjusted = unadjusted.map(function (frame) {
    return frame.map(function (point) {
      return [point[0] / maxRms, point[1] / maxMid];
    });
  });

  // Make first and last frame peaky
  adjusted[0] = adjusted[numFrames - 1] = adjusted[maxFrame];

  return adjusted;
}

module.exports = getWaveform;