audio-ui/lib/audio/effects/fade.ts

/**
 * Fade in/out effects
 */

import { getAudioContext } from '../context';

export type FadeType = 'linear' | 'exponential' | 'logarithmic';

/**
 * Apply fade in to audio buffer
 * @param buffer - Source audio buffer
 * @param duration - Fade duration in seconds
 * @param type - Fade curve type
 * @returns New audio buffer with fade in applied
 */
export function applyFadeIn(
  buffer: AudioBuffer,
  duration: number,
  type: FadeType = 'linear'
): AudioBuffer {
  const audioContext = getAudioContext();
  const fadeSamples = Math.min(
    Math.floor(duration * buffer.sampleRate),
    buffer.length
  );

  const outputBuffer = audioContext.createBuffer(
    buffer.numberOfChannels,
    buffer.length,
    buffer.sampleRate
  );

  for (let channel = 0; channel < buffer.numberOfChannels; channel++) {
    const inputData = buffer.getChannelData(channel);
    const outputData = outputBuffer.getChannelData(channel);

    for (let i = 0; i < buffer.length; i++) {
      if (i < fadeSamples) {
        const progress = i / fadeSamples;
        const gain = calculateFadeGain(progress, type);
        outputData[i] = inputData[i] * gain;
      } else {
        outputData[i] = inputData[i];
      }
    }
  }

  return outputBuffer;
}

/**
 * Apply fade out to audio buffer
 * @param buffer - Source audio buffer
 * @param duration - Fade duration in seconds
 * @param type - Fade curve type
 * @returns New audio buffer with fade out applied
 */
export function applyFadeOut(
  buffer: AudioBuffer,
  duration: number,
  type: FadeType = 'linear'
): AudioBuffer {
  const audioContext = getAudioContext();
  const fadeSamples = Math.min(
    Math.floor(duration * buffer.sampleRate),
    buffer.length
  );
  const fadeStartSample = buffer.length - fadeSamples;

  const outputBuffer = audioContext.createBuffer(
    buffer.numberOfChannels,
    buffer.length,
    buffer.sampleRate
  );

  for (let channel = 0; channel < buffer.numberOfChannels; channel++) {
    const inputData = buffer.getChannelData(channel);
    const outputData = outputBuffer.getChannelData(channel);

    for (let i = 0; i < buffer.length; i++) {
      if (i >= fadeStartSample) {
        const progress = (i - fadeStartSample) / fadeSamples;
        const gain = calculateFadeGain(1 - progress, type);
        outputData[i] = inputData[i] * gain;
      } else {
        outputData[i] = inputData[i];
      }
    }
  }

  return outputBuffer;
}

/**
 * Calculate fade gain based on progress and curve type
 * @param progress - Progress from 0 to 1
 * @param type - Fade curve type
 * @returns Gain value from 0 to 1
 */
function calculateFadeGain(progress: number, type: FadeType): number {
  switch (type) {
    case 'linear':
      return progress;

    case 'exponential':
      // Exponential curve: faster at the start, slower at the end
      return progress * progress;

    case 'logarithmic':
      // Logarithmic curve: slower at the start, faster at the end
      return Math.sqrt(progress);

    default:
      return progress;
  }
}