Add step 1 (extract bits from IEEE representation)

Some solvers, e.g., dragon4, assume the sign, mantissa and exponent are
known before the algorithm is called.

Author: jaja360

benchmarks/CMakeLists.txt

@@ -1,6 +1,6 @@
 add_executable(benchmark
     benchmark.cpp
-    decimalToString.cpp
+    ieeeToString.cpp
 )
 
 include(CheckSourceCompiles)

benchmarks/benchmark.cpp

@@ -29,7 +29,7 @@
 
 #include "grisu2.h"
 #include "random_generators.h"
-#include "decimalToString.h"
+#include "ieeeToString.h"
 
 #include <charconv>
 #include <climits>

benchmarks/decimalToString.h

@@ -1,10 +1,30 @@
-#include "decimalToString.h"
+#include "ieeeToString.h"
 
 #include <cassert>
 #include <cstring>
 
 #include "ryu/digit_table.h"
 
+IEEE754f decode_ieee754(float f) {
+  const uint32_t& bits = reinterpret_cast<const uint32_t&>(f);
+
+  IEEE754f decomposed;
+  decomposed.exponent = (bits >> FloatMantissaBits) & ((1 << FloatExponentBits) - 1);
+  decomposed.mantissa = bits & ((1 << FloatMantissaBits) - 1);
+  decomposed.sign = bits >> 31;
+  return decomposed;
+}
+
+IEEE754d decode_ieee754(double f) {
+  const uint64_t& bits = reinterpret_cast<const uint64_t&>(f);
+
+  IEEE754d decomposed;
+  decomposed.exponent = (bits >> DoubleMantissaBits) & ((1ull << DoubleExponentBits) - 1);
+  decomposed.mantissa = bits & ((1ull << DoubleMantissaBits) - 1);
+  decomposed.sign = bits >> 63;
+  return decomposed;
+}
+
 // Extracted from the Ryu implementation.
 static inline uint32_t decimalLength17(const uint64_t v) {
   // Function precondition: v is not an 18, 19, or 20-digit number.

benchmarks/decimalToString.cpp benchmarks/ieeeToString.cppbenchmarks/decimalToString.cpp renamed to benchmarks/ieeeToString.cpp

@@ -0,0 +1,31 @@
+#ifndef IEEETOSTRING_H
+#define IEEETOSTRING_H
+
+#include <cstdint>
+
+constexpr uint8_t FloatMantissaBits = 23;
+constexpr uint8_t FloatExponentBits = 8;
+
+constexpr uint8_t DoubleMantissaBits = 52;
+constexpr uint8_t DoubleExponentBits = 11;
+
+// Step 1: extract the sign, mantissa and exponent from an IEEE 754 number
+struct IEEE754f {
+  uint32_t mantissa;
+  uint32_t exponent;
+  bool sign;
+};
+
+struct IEEE754d {
+  uint64_t mantissa;
+  uint32_t exponent;
+  bool sign;
+};
+
+IEEE754f decode_ieee754(float f);
+IEEE754d decode_ieee754(double f);
+
+// Step 3: convert a decimal exponent and mantissa to a string representation
+int to_chars(uint64_t mantissa, int32_t exponent, bool sign, char* const result);
+
+#endif