Compute function tables at compile time

This requires C++ 14. Older compilers still use the old code.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/lstm/functions.cpp

@@ -20,6 +20,8 @@
 
 namespace tesseract {
 
+#if __cplusplus < 201402 // C++11
+
 double TanhTable[kTableSize];
 double LogisticTable[kTableSize];
 
@@ -35,4 +37,11 @@ class TableInit {
 
 TableInit TableInit::tableInit;
 
+#else // C++14 or newer
+
+constexpr LUTTempl<kTableSize, LUTFuncTanh> TanhTable;
+constexpr LUTTempl<kTableSize, LUTFuncLog>  LogisticTable;
+
+#endif
+
 }  // namespace tesseract.

src/lstm/functions.h

@@ -31,13 +31,45 @@
 namespace tesseract {
 
 // Size of static tables.
-const int kTableSize = 4096;
+constexpr int kTableSize = 4096;
 // Scale factor for float arg to int index.
-const double kScaleFactor = 256.0;
+constexpr double kScaleFactor = 256.0;
+
+#if __cplusplus < 201402 // C++11
 
 extern double TanhTable[];
 extern double LogisticTable[];
 
+#else // C++14 or newer
+
+typedef double (*LUT_FUNCTION)(int i);
+
+constexpr double LUTFuncTanh(int i) {
+  return std::tanh(i / kScaleFactor);
+}
+
+constexpr double LUTFuncLog(int i) {
+  return 1 / (1 + std::exp(-i / kScaleFactor));
+}
+
+template<int n, LUT_FUNCTION f>
+struct LUTTempl {
+  constexpr LUTTempl() : table_() {
+    for (auto i = 0; i < n; ++i) {
+      table_[i] = f(i);
+    }
+  }
+  const double& operator[](size_t i) const {
+    return table_[i];
+  }
+  double table_[n];
+};
+
+extern const LUTTempl<kTableSize, LUTFuncTanh> TanhTable;
+extern const LUTTempl<kTableSize, LUTFuncLog>  LogisticTable;
+
+#endif
+
 // Non-linearity (sigmoid) functions with cache tables and clipping.
 inline double Tanh(double x) {
   if (x < 0.0) return -Tanh(-x);