diff --git a/src/main/java/io/github/dsheirer/dsp/filter/channelizer/ComplexPolyphaseChannelizerM2.java b/src/main/java/io/github/dsheirer/dsp/filter/channelizer/ComplexPolyphaseChannelizerM2.java
index 707eea94..28b9a9ee 100644
--- a/src/main/java/io/github/dsheirer/dsp/filter/channelizer/ComplexPolyphaseChannelizerM2.java
+++ b/src/main/java/io/github/dsheirer/dsp/filter/channelizer/ComplexPolyphaseChannelizerM2.java
@@ -27,7 +27,7 @@ import java.util.Arrays;
import java.util.Queue;
import java.util.concurrent.ArrayBlockingQueue;
import org.apache.commons.math3.util.FastMath;
-import org.jtransforms.fft.FloatFFT_1D;
+import org.jtransforms.fft.ReusableFloatInverseFFT;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
@@ -85,7 +85,7 @@ public class ComplexPolyphaseChannelizerM2 extends AbstractComplexPolyphaseChann
//Sized to process 40 times per second
private IFFTProcessorDispatcher mIFFTProcessorDispatcher = new IFFTProcessorDispatcher(25);
- private FloatFFT_1D mFFT;
+ private ReusableFloatInverseFFT mFFT;
private ChannelDFT mChannelDFT;
private float[] mInlineSamples;
private float[] mInlineFilter;
@@ -453,7 +453,7 @@ public class ComplexPolyphaseChannelizerM2 extends AbstractComplexPolyphaseChann
private void init(float[] coefficients)
{
int channelCount = getChannelCount();
- mFFT = new FloatFFT_1D(channelCount);
+ mFFT = new ReusableFloatInverseFFT(channelCount);
mChannelDFT = ChannelDFTFactory.getChannelDFT(channelCount);
int bufferLength = getSubChannelCount() * mTapsPerChannel;
mSamplesPerBlock = channelCount; //Same as subChannelCount / 2
@@ -505,7 +505,7 @@ public class ComplexPolyphaseChannelizerM2 extends AbstractComplexPolyphaseChann
for(int offset = 0; offset < samples.length; offset += blockLength)
{
//Rotate each of the channels to the correct phase using the IFFT
- mFFT.complexInverse(samples, offset, true);
+ mFFT.complexInverse(samples, offset);
}
}
diff --git a/src/main/java/org/jtransforms/fft/ReusableFloatInverseFFT.java b/src/main/java/org/jtransforms/fft/ReusableFloatInverseFFT.java
new file mode 100644
index 00000000..abafce1e
--- /dev/null
+++ b/src/main/java/org/jtransforms/fft/ReusableFloatInverseFFT.java
@@ -0,0 +1,156 @@
+/*
+ * *****************************************************************************
+ * Copyright (C) 2014-2026 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see
+ * ****************************************************************************
+ */
+package org.jtransforms.fft;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Scaled complex inverse FFT that produces the same values as FloatFFT_1D.complexInverse(samples, offset, true) without
+ * allocating on each call.
+ *
+ * FloatFFT_1D allocates a scratch array of two floats per channel on every mixed-radix transform. This class runs the
+ * same mixed-radix passes using a single reused scratch array. It's in this package because those passes are package
+ * private. Sizes that FloatFFT_1D doesn't transform as mixed-radix (powers of two and sizes with other prime factors)
+ * are delegated to FloatFFT_1D.
+ *
+ * Note: this class isn't thread safe because the scratch array is shared across calls.
+ */
+public class ReusableFloatInverseFFT
+{
+ private static final int[] SUPPORTED_RADIX = {3, 5};
+ private final FloatFFT_1D mFFT;
+ private final int mSize;
+ private final int[] mFactors;
+ private final float[] mScratch;
+ private final float mScale;
+
+ /**
+ * Constructs an instance
+ * @param size of the transform in complex samples
+ */
+ public ReusableFloatInverseFFT(int size)
+ {
+ mSize = size;
+ mFFT = new FloatFFT_1D(size);
+ mFactors = getFactors(size);
+ mScratch = mFactors != null ? new float[2 * size] : null;
+ mScale = 1.0f / size;
+ }
+
+ /**
+ * Identifies the order of radix passes for the size, matching the order used by FloatFFT_1D.
+ * @param size of the transform
+ * @return radix values in pass order, or null if the size should be delegated to FloatFFT_1D.
+ */
+ private static int[] getFactors(int size)
+ {
+ if(size < 2 || Integer.bitCount(size) == 1)
+ {
+ return null;
+ }
+
+ List factors = new ArrayList<>();
+ int remaining = size;
+
+ while(remaining % 4 == 0)
+ {
+ factors.add(4);
+ remaining /= 4;
+ }
+
+ //FloatFFT_1D moves a radix-2 pass to the front
+ if(remaining % 2 == 0)
+ {
+ factors.addFirst(2);
+ remaining /= 2;
+ }
+
+ for(int radix : SUPPORTED_RADIX)
+ {
+ while(remaining % radix == 0)
+ {
+ factors.add(radix);
+ remaining /= radix;
+ }
+ }
+
+ if(remaining != 1)
+ {
+ return null;
+ }
+
+ return factors.stream().mapToInt(Integer::intValue).toArray();
+ }
+
+ /**
+ * Performs a scaled inverse transform in place.
+ * @param samples containing interleaved I/Q values
+ * @param offset to the start of the block in the samples array
+ */
+ public void complexInverse(float[] samples, int offset)
+ {
+ if(mFactors == null)
+ {
+ mFFT.complexInverse(samples, offset, true);
+ return;
+ }
+
+ final float[] scratch = mScratch;
+ final int size = mSize;
+ boolean resultsInScratch = false;
+ int subTransforms = 1;
+ int twiddleOffset = 2 * size;
+
+ for(int radix : mFactors)
+ {
+ int next = radix * subTransforms;
+ int stride = 2 * (size / next);
+ float[] in = resultsInScratch ? scratch : samples;
+ int inOffset = resultsInScratch ? 0 : offset;
+ float[] out = resultsInScratch ? samples : scratch;
+ int outOffset = resultsInScratch ? offset : 0;
+
+ switch(radix)
+ {
+ case 2 -> mFFT.passf2(stride, subTransforms, in, inOffset, out, outOffset, twiddleOffset, 1);
+ case 3 -> mFFT.passf3(stride, subTransforms, in, inOffset, out, outOffset, twiddleOffset, 1);
+ case 4 -> mFFT.passf4(stride, subTransforms, in, inOffset, out, outOffset, twiddleOffset, 1);
+ default -> mFFT.passf5(stride, subTransforms, in, inOffset, out, outOffset, twiddleOffset, 1);
+ }
+
+ resultsInScratch = !resultsInScratch;
+ subTransforms = next;
+ twiddleOffset += (radix - 1) * stride;
+ }
+
+ if(resultsInScratch)
+ {
+ System.arraycopy(scratch, 0, samples, offset, 2 * size);
+ }
+
+ final float scale = mScale;
+ final int end = offset + 2 * size;
+
+ for(int x = offset; x < end; x++)
+ {
+ samples[x] *= scale;
+ }
+ }
+}
diff --git a/src/test/java/org/jtransforms/fft/ReusableFloatInverseFFTTest.java b/src/test/java/org/jtransforms/fft/ReusableFloatInverseFFTTest.java
new file mode 100644
index 00000000..44b8a847
--- /dev/null
+++ b/src/test/java/org/jtransforms/fft/ReusableFloatInverseFFTTest.java
@@ -0,0 +1,134 @@
+/*
+ * *****************************************************************************
+ * Copyright (C) 2014-2026 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see
+ * ****************************************************************************
+ */
+package org.jtransforms.fft;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.lang.management.ManagementFactory;
+import java.util.Random;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests that the reusable inverse FFT is an exact replacement for FloatFFT_1D.
+ */
+class ReusableFloatInverseFFTTest
+{
+ private static final int OFFSET = 6;
+ private static final int LARGEST_SIZE = 5000;
+
+ private static float[] noise(int length, Random random)
+ {
+ float[] samples = new float[length];
+
+ for(int x = 0; x < length; x++)
+ {
+ samples[x] = random.nextFloat() * 2.0f - 1.0f;
+ }
+
+ return samples;
+ }
+
+ /**
+ * Test: output values are bit-identical to FloatFFT_1D for every size, including powers of two and sizes with other
+ * prime factors, and samples outside of the block aren't changed. Each instance is used twice to show that
+ * scratch contents don't carry over between calls.
+ */
+ @Test
+ void matchesFloatFFT1D()
+ {
+ Random random = new Random(0xC0FFEEL);
+
+ for(int size = 1; size <= LARGEST_SIZE; size += size < 512 ? 1 : 53)
+ {
+ ReusableFloatInverseFFT reusable = new ReusableFloatInverseFFT(size);
+ FloatFFT_1D reference = new FloatFFT_1D(size);
+
+ for(int pass = 0; pass < 2; pass++)
+ {
+ float[] expected = noise(2 * size + 2 * OFFSET, random);
+ float[] actual = expected.clone();
+ reference.complexInverse(expected, OFFSET, true);
+ reusable.complexInverse(actual, OFFSET);
+ assertArrayEquals(expected, actual, 0.0f, "Size " + size);
+ }
+ }
+ }
+
+ /**
+ * Test: every channel count from 2 to 4800 with only factors of 2, 3 and 5 matches FloatFFT_1D. These are the only
+ * sizes the channelizer can select.
+ */
+ @Test
+ void matchesFloatFFT1DForChannelizerSizes()
+ {
+ Random random = new Random(0xBEEFL);
+
+ for(int size = 2; size <= 4800; size += 2)
+ {
+ int remaining = size;
+
+ for(int factor : new int[]{2, 3, 5})
+ {
+ while(remaining % factor == 0)
+ {
+ remaining /= factor;
+ }
+ }
+
+ if(remaining == 1)
+ {
+ float[] expected = noise(2 * size, random);
+ float[] actual = expected.clone();
+ new FloatFFT_1D(size).complexInverse(expected, 0, true);
+ new ReusableFloatInverseFFT(size).complexInverse(actual, 0);
+ assertArrayEquals(expected, actual, 0.0f, "Size " + size);
+ }
+ }
+ }
+
+ /**
+ * Test: transforms don't allocate, for a mixed-radix size with an odd pass count and an even pass count.
+ */
+ @Test
+ void doesNotAllocate()
+ {
+ com.sun.management.ThreadMXBean threads = (com.sun.management.ThreadMXBean)ManagementFactory.getThreadMXBean();
+ long threadId = Thread.currentThread().threadId();
+
+ for(int size : new int[]{96, 400})
+ {
+ ReusableFloatInverseFFT fft = new ReusableFloatInverseFFT(size);
+ float[] samples = noise(2 * size, new Random(1L));
+ fft.complexInverse(samples, 0);
+
+ long before = threads.getThreadAllocatedBytes(threadId);
+
+ for(int x = 0; x < 1000; x++)
+ {
+ fft.complexInverse(samples, 0);
+ }
+
+ long allocated = threads.getThreadAllocatedBytes(threadId) - before;
+
+ //FloatFFT_1D would allocate at least 8 bytes per channel, per call. Allow for measurement overhead.
+ assertEquals(0, allocated / (1000L * size), "Size " + size + " allocated " + allocated + " bytes");
+ }
+ }
+}