Integer log2

libpsk/include/dsp/log2_int.hpp

@@ -0,0 +1,60 @@
+/*
+ * log2_int.hpp
+ *
+ *  Created on: 16-05-2013
+ *      Author: lucck
+ */
+
+#ifndef DSP_LOG2_INT_HPP_
+#define DSP_LOG2_INT_HPP_
+
+/* ------------------------------------------------------------------------- */
+namespace dsp {
+namespace integer {
+/* ------------------------------------------------------------------------- */
+/* Taylor series log in range 1-2 */
+    template <typename N, unsigned SC>
+    inline unsigned log_1_2( N x )
+    {
+	if( x == SC ) return 0;
+	const  N x1 = ( (x-N(SC)) * N(SC) ) / ( x+N(SC) );
+	const  N x2 = N( N(x1)  * N(x1)) / N(SC);
+	const  N x3 = N( N(x2)  * N(x1)) / N(SC);
+	const  N x5 = N( N(x3)  * N(x2)) / N(SC);
+	const  N x7 = N( N(x5)  * N(x2)) / N(SC);
+	return N(2) * N( x1 + x3/N(3) + x5/N(5) + x7/N(7) );
+    }
+	/* Log2 without partials */
+	inline unsigned log2c( unsigned x )
+	{
+	  return (8*sizeof (unsigned ) - __builtin_clz(x))-1;
+	}
+
+	inline unsigned log2c(unsigned long long x)
+	{
+	  return (8*sizeof (unsigned long long) - __builtin_clzll(x))-1;
+	}
+
+	/* Log2 partial version */
+	template <typename N, unsigned SC>
+	inline unsigned log2_1_n( N x )
+	{
+		constexpr N LE2L2 = (std::log(10)/std::log2(10)) * double(SC);
+		const auto l2c = log2c( N(x / SC) );
+		const N lr = x / ( N(1)<< l2c );
+		const N ler = log_1_2<N,SC>( lr );
+		const N l2r = N(ler*SC) / LE2L2;
+		return l2r + l2c*N(SC);
+	}
+	 template <typename N, unsigned SC>
+	 inline int log2_0_1( N x )
+	 {
+	   constexpr N LN2SC = std::log2( SC ) * double(SC);
+	   return log2_1_n<N,SC>( x * N(SC) )  - LN2SC;
+	 }
+
+/* ------------------------------------------------------------------------- */
+}}
+/* ------------------------------------------------------------------------- */
+
+#endif /* LOG2_INT_HPP_ */