differentiator is fir, also fixed it

gui/templates/5_fir_differentiator.cpp

@@ -1,8 +1,8 @@
 /**
- * 5_iir_differentiator.cpp
+ * 5_fir_differentiator.cpp
  * Written by Clyne Sullivan.
  *
- * Does an IIR differentiation on the incoming signal, so that the output is representative of the
+ * Does an FIR differentiation on the incoming signal, so that the output is representative of the
  * rate of change of the input.
  * A scaling factor is applied so that the output's form is more clearly visible.
  */
@@ -10,19 +10,21 @@
 adcsample_t *process_data(adcsample_t *samples, unsigned int size)
 {
     constexpr int scaling_factor = 4;
+	static adcsample_t output[SIZE];
     static adcsample_t prev = 2048;
 
     // Compute the first output value using the saved sample.
-    samples[0] = 2048 + ((samples[0] - prev) * scaling_factor;
-    // Save the last sample for the next iteration.
-    prev = samples[size - 1];
+    output[0] = 2048 + ((samples[0] - prev) * scaling_factor);
 
 	for (unsigned int i = 1; i < size; i++) {
         // Take the rate of change and scale it.
         // 2048 is added as the output should be centered in the voltage range.
-		samples[i] = 2048 + ((samples[i] - samples[i - 1]) * scaling_factor);
+		output[i] = 2048 + ((samples[i] - samples[i - 1]) * scaling_factor);
     }
 
-    return samples;
+	// Save the last sample for the next iteration.
+    prev = samples[size - 1];
+
+    return output;
 }