From 106dd6ce6a1e55f104d90ab85946a63008a50f7e Mon Sep 17 00:00:00 2001
From: Ken Cooke <ken@highfidelity.io>
Date: Thu, 18 May 2017 14:15:10 -0700
Subject: [PATCH] Fast fixed-point log2 approximation

---
 libraries/audio/src/AudioDynamics.h | 63 +++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/libraries/audio/src/AudioDynamics.h b/libraries/audio/src/AudioDynamics.h
index 2306fecf24..4c2f890c67 100644
--- a/libraries/audio/src/AudioDynamics.h
+++ b/libraries/audio/src/AudioDynamics.h
@@ -252,6 +252,69 @@ static inline int32_t peaklog2(float* input0, float* input1, float* input2, floa
     return (e << LOG2_FRACBITS) - (c2 >> 3);
 }
 
+//
+// Count Leading Zeros
+// Emulates the CLZ (ARM) and LZCNT (x86) instruction
+//
+static inline int CLZ(uint32_t x) {
+
+    if (x == 0) {
+        return 32;
+    }
+
+    int e = 0;
+    if (x < 0x00010000) {
+        x <<= 16;
+        e += 16;
+    }
+    if (x < 0x01000000) {
+        x <<= 8;
+        e += 8;
+    }
+    if (x < 0x10000000) {
+        x <<= 4;
+        e += 4;
+    }
+    if (x < 0x40000000) {
+        x <<= 2;
+        e += 2;
+    }
+    if (x < 0x80000000) {
+        e += 1;
+    }
+    return e;
+}
+
+//
+// Compute -log2(x) for x=[0,1] in Q31, result in Q26
+// x = 0 returns 0x7fffffff
+// x < 0 undefined
+//
+static inline int32_t fixlog2(int32_t x) {
+
+    if (x == 0) {
+        return 0x7fffffff;
+    }
+
+    // split into e and x - 1.0
+    int e = CLZ((uint32_t)x);
+    x <<= e;            // normalize to [0x80000000, 0xffffffff]
+    x &= 0x7fffffff;    // x - 1.0
+
+    int k = x >> (31 - LOG2_TABBITS);
+
+    // polynomial for log2(1+x) over x=[0,1]
+    int32_t c0 = log2Table[k][0];
+    int32_t c1 = log2Table[k][1];
+    int32_t c2 = log2Table[k][2];
+
+    c1 += MULHI(c0, x);
+    c2 += MULHI(c1, x);
+
+    // reconstruct result in Q26
+    return (e << LOG2_FRACBITS) - (c2 >> 3);
+}
+
 //
 // Compute exp2(-x) for x=[0,32] in Q26, result in Q31
 // x < 0 undefined