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+/*
+ * Copyright 1997 Phil Burk, Mobileer Inc
+ *
+ * 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 com.jsyn.unitgen;
+
+import com.jsyn.ports.UnitInputPort;
+
+/**
+ * InterpolatingDelay
+ * <P>
+ * InterpolatingDelay provides a variable time delay with an input and output. The internal data
+ * format is 32-bit floating point. The amount of delay can be varied from 0.0 to a time in seconds
+ * corresponding to the numFrames allocated. The fractional delay values are calculated by linearly
+ * interpolating between adjacent values in the delay line.
+ * <P>
+ * This unit can be used to implement time varying delay effects such as a flanger or a chorus. It
+ * can also be used to implement physical models of acoustic instruments, or other tunable delay
+ * based resonant systems.
+ * <P>
+ * 
+ * @author (C) 1997-2011 Phil Burk, Mobileer Inc
+ * @see Delay
+ */
+
+public class InterpolatingDelay extends UnitFilter {
+    /**
+     * Delay time in seconds. This value will converted to frames and clipped between zero and the
+     * numFrames value passed to allocate(). The minimum and default delay time is 0.0.
+     */
+    public UnitInputPort delay;
+
+    private float[] buffer;
+    private int cursor;
+    private int numFrames;
+
+    public InterpolatingDelay() {
+        addPort(delay = new UnitInputPort("Delay"));
+    }
+
+    /**
+     * Allocate memory for the delay buffer. For a 2 second delay at 44100 Hz sample rate you will
+     * need at least 88200 samples.
+     * 
+     * @param numFrames size of the float array to hold the delayed samples
+     */
+    public void allocate(int numFrames) {
+        this.numFrames = numFrames;
+        // Allocate extra frame for guard point to speed up interpolation.
+        buffer = new float[numFrames + 1];
+    }
+
+    @Override
+    public void generate(int start, int limit) {
+        double[] inputs = input.getValues();
+        double[] outputs = output.getValues();
+        double[] delays = delay.getValues();
+
+        for (int i = start; i < limit; i++) {
+            // This should be at the beginning of the loop
+            // because the guard point should == buffer[0].
+            if (cursor == numFrames) {
+                // Write guard point! Must allocate one extra sample.
+                buffer[numFrames] = (float) inputs[i];
+                cursor = 0;
+            }
+
+            buffer[cursor] = (float) inputs[i];
+
+            /* Convert delay time to a clipped frame offset. */
+            double delayFrames = delays[i] * getFrameRate();
+
+            // Clip to zero delay.
+            if (delayFrames <= 0.0) {
+                outputs[i] = buffer[cursor];
+            } else {
+                // Clip to maximum delay.
+                if (delayFrames >= numFrames) {
+                    delayFrames = numFrames - 1;
+                }
+
+                // Calculate fractional index into delay buffer.
+                double readIndex = cursor - delayFrames;
+                if (readIndex < 0.0) {
+                    readIndex += numFrames;
+                }
+                // setup for interpolation.
+                // We know readIndex is > 0 so we do not need to call floor().
+                int iReadIndex = (int) readIndex;
+                double frac = readIndex - iReadIndex;
+
+                // Get adjacent values relying on guard point to prevent overflow.
+                double val0 = buffer[iReadIndex];
+                double val1 = buffer[iReadIndex + 1];
+
+                // Interpolate new value.
+                outputs[i] = val0 + (frac * (val1 - val0));
+            }
+
+            cursor += 1;
+        }
+
+    }
+
+}