Extract method for calculating expected accumulated truncation error

This is prework for implementing
`RandomParameterizedSpeedChangingAudioProcessorTest`, which depends on
Sonic's resampling algorithm's behaviour.

This is a non-functional refactor.

PiperOrigin-RevId: 686874593

libraries/common/src/test/java/androidx/media3/common/audio/RandomParameterizedSonicTest.java

@@ -15,6 +15,7 @@
  */
 package androidx.media3.common.audio;
 
+import static androidx.media3.common.audio.SonicTestingUtils.calculateAccumulatedTruncationErrorForResampling;
 import static androidx.media3.test.utils.TestUtil.generateFloatInRange;
 import static com.google.common.truth.Truth.assertThat;
 import static java.lang.Math.max;
@@ -163,31 +164,18 @@ public final class RandomParameterizedSonicTest {
     }
     sonic.flush();
 
-    BigDecimal bigSampleRate = new BigDecimal(SAMPLE_RATE);
     BigDecimal bigLength = new BigDecimal(String.valueOf(streamLength));
     // The scale of expectedSize will be bigLength.scale() - speed.scale(). Thus, the result should
     // always yield an integer.
     BigDecimal expectedSize = bigLength.divide(speed, RoundingMode.HALF_EVEN);
 
-    // Calculate number of times that Sonic accumulates truncation error. Set scale to 20 decimal
-    // places, so that division doesn't return an integral.
-    BigDecimal errorCount =
-        bigLength.divide(bigSampleRate, /* scale= */ 20, RoundingMode.HALF_EVEN);
-
-    // Calculate what truncation error Sonic is accumulating, calculated as:
-    // inputSampleRate / speed - (int) inputSampleRate / speed. Set scale to 20 decimal places, so
-    // that division doesn't return an integral.
-    BigDecimal individualError =
-        bigSampleRate.divide(speed, /* scale */ 20, RoundingMode.HALF_EVEN);
-    individualError =
-        individualError.subtract(individualError.setScale(/* newScale= */ 0, RoundingMode.FLOOR));
-    // Calculate total accumulated error = (int) floor(errorCount * individualError).
-    BigDecimal accumulatedError =
-        errorCount.multiply(individualError).setScale(/* newScale= */ 0, RoundingMode.FLOOR);
+    long accumulatedTruncationError =
+        calculateAccumulatedTruncationErrorForResampling(
+            bigLength, new BigDecimal(SAMPLE_RATE), speed);
 
     assertThat(readSampleCount)
         .isWithin(1)
-        .of(expectedSize.longValueExact() - accumulatedError.longValueExact());
+        .of(expectedSize.longValueExact() - accumulatedTruncationError);
   }
 
   @Test

libraries/common/src/test/java/androidx/media3/common/audio/SonicTestingUtils.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * 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.
+ */
+package androidx.media3.common.audio;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+
+/** Testing utils class related to {@link Sonic} */
+/* package */ final class SonicTestingUtils {
+  /**
+   * Returns expected accumulated truncation error for {@link Sonic}'s resampling algorithm, given
+   * an input length, input sample rate, and resampling rate.
+   *
+   * <p><b>Note:</b> This method is only necessary until we address b/361768785 and fix the
+   * underlying truncation issue.
+   *
+   * <p>The accumulated truncation error is calculated as follows:
+   *
+   * <ol>
+   *   <li>Individual truncation error: Divide sample rate by resampling rate, and calculate delta
+   *       between floating point result and truncated int representation.
+   *   <li>Truncation accumulation count: Divide length by sample rate to obtain number of times
+   *       that truncation error accumulates.
+   *   <li>Accumulated truncation error: Multiply results of 1 and 2.
+   * </ol>
+   *
+   * @param length Length of input in frames.
+   * @param sampleRate Input sample rate of {@link Sonic} instance.
+   * @param resamplingRate Resampling rate given by {@code pitch * (inputSampleRate /
+   *     outputSampleRate)}.
+   */
+  public static long calculateAccumulatedTruncationErrorForResampling(
+      BigDecimal length, BigDecimal sampleRate, BigDecimal resamplingRate) {
+    // Calculate number of times that Sonic accumulates truncation error. Set scale to 20 decimal
+    // places, so that division doesn't return an integer.
+    BigDecimal errorCount = length.divide(sampleRate, /* scale= */ 20, RoundingMode.HALF_EVEN);
+
+    // Calculate what truncation error Sonic is accumulating, calculated as:
+    // inputSampleRate / resamplingRate - (int) inputSampleRate / resamplingRate. Set scale to 20
+    // decimal places, so that division doesn't return an integer.
+    BigDecimal individualError =
+        sampleRate.divide(resamplingRate, /* scale */ 20, RoundingMode.HALF_EVEN);
+    individualError =
+        individualError.subtract(individualError.setScale(/* newScale= */ 0, RoundingMode.FLOOR));
+    // Calculate total accumulated error = (int) floor(errorCount * individualError).
+    BigDecimal accumulatedError =
+        errorCount.multiply(individualError).setScale(/* newScale= */ 0, RoundingMode.FLOOR);
+
+    return accumulatedError.longValueExact();
+  }
+
+  private SonicTestingUtils() {}
+}