ensure coeffs ptr is writable before using

biquad.h

@@ -31,6 +31,7 @@
 
 #include "Arduino.h"
 #include <arm_math.h>
+#include <type_traits>
 
 enum class BiquadType {
 	LOW_PASS, HIGH_PASS, BAND_PASS, NOTCH, ALL_PASS, PEAKING, LOW_SHELF, HIGH_SHELF
@@ -39,7 +40,7 @@ enum class BiquadType {
 template <typename T>
 void getCoefficients(T* coeffs, BiquadType type, double dbGain, double freq, double srate, double bandwidthOrQOrS, bool isBandwidthOrS=false){
 	const double A =(type == BiquadType::PEAKING || type == BiquadType::LOW_SHELF || type == BiquadType::HIGH_SHELF) ? pow(10., dbGain / 40.) : pow(10, dbGain / 20);
-	
+
 	const double omega = 2 * M_PI * freq / srate;
 	const double sn = sin(omega);
 	const double cs = cos(omega);
@@ -124,11 +125,13 @@ void getCoefficients(T* coeffs, BiquadType type, double dbGain, double freq, dou
 		break;
 	}
 
-	*coeffs++=(T)(b0 * a0Inv);
-	*coeffs++=(T)(b1 * a0Inv);
-	*coeffs++=(T)(b2 * a0Inv);
-	*coeffs++=(T)(-a1 * a0Inv);
-	*coeffs=(T)(-a2 * a0Inv);
+	using writable_type = typename std::remove_const<T>::type;
+	writable_type* coeffs_ptr = const_cast<writable_type*>(coeffs);
+	*coeffs_ptr++ = (T)(b0 * a0Inv);
+	*coeffs_ptr++ = (T)(b1 * a0Inv);
+	*coeffs_ptr++ = (T)(b2 * a0Inv);
+	*coeffs_ptr++ = (T)(-a1 * a0Inv);
+	*coeffs_ptr = (T)(-a2 * a0Inv);
 }
 
 	template <typename T, typename BIQUAD, typename BTYPE>
@@ -189,7 +192,7 @@ void getCoefficients(T* coeffs, BiquadType type, double dbGain, double freq, dou
                 *pDst++=(T)yn;
             }
         }
-    }	
+    }
 
 	template <typename B>
 	void preload(const B*  S, double val=0.){
@@ -201,4 +204,4 @@ void getCoefficients(T* coeffs, BiquadType type, double dbGain, double freq, dou
 		*(S->pState) = (*(S->pCoeffs+1)  + *(S->pCoeffs+3)) * val + *(S->pState+1);
 	}
 
-#endif
+#endif