abtract logmod as MSB

xiachonga · May 4, 2021 · fde5149 · fde5149
1 parent 3bf6ebe
commit fde5149
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Showing 3 changed files with 8 additions and 8 deletions.
diff --git a/hexl/number-theory/number-theory.hpp b/hexl/number-theory/number-theory.hpp
@@ -56,9 +56,7 @@ inline bool IsPowerOfTwo(uint64_t num) { return num && !(num & (num - 1)); }
 // Returns log2(x) for x a power of 2
 inline uint64_t Log2(uint64_t x) {
   HEXL_CHECK(IsPowerOfTwo(x), x << " not a power of 2");
-  uint64_t ret = 0;
-  while (x >>= 1) ++ret;
-  return ret;
+  return MSB(x);
 }
 
 // Returns the maximum value that can be represented using bits bits

diff --git a/hexl/util/msvc.hpp b/hexl/util/msvc.hpp
@@ -151,7 +151,7 @@ inline void SubWithCarry128(uint64_t* result_hi, uint64_t* result_lo,
 }
 
 /// @brief Computes and returns significant bit count
-/// @param[in] value Input element
+/// @param[in] value Input element at most 128 bits long
 inline uint64_t SignificantBitLength(const uint64_t* value) {
   HEXL_CHECK(value != nullptr, "Require value != nullptr");
 
@@ -191,7 +191,7 @@ inline void DivideUInt128UInt64(uint64_t* quotient, const uint64_t* numerator,
   HEXL_CHECK(numerator != nullptr, "Require numerator != nullptr");
   HEXL_CHECK(denominator == 128, "denominator cannot be 0 " << denominator);
 
-  // get bit count of Divisor
+  // get bit count of divisor
   uint64_t numerator_bits = SignificantBitLength(numerator);
   const uint64_t numerator_bits_const = numerator_bits;
   const uint64_t uint_128_bit = 128ULL;

diff --git a/test/test-number-theory.cpp b/test/test-number-theory.cpp
@@ -425,9 +425,11 @@ TEST(NumberTheory, DivideUInt128UInt64Lo) {
 }
 
 TEST(NumberTheory, MSB) {
-  EXPECT_EQ(60ULL, MSB(2305843009213689601));
-  EXPECT_EQ(59ULL, MSB(1152921504606844417));
-  EXPECT_EQ(59ULL, MSB(1152921504606844289));
+  EXPECT_EQ(60ULL, MSB(2305843009213689601));  // 2**61 - 4351
+  EXPECT_EQ(59ULL, MSB(1152921504606844417));  // 2**60 - 2559
+  EXPECT_EQ(59ULL, MSB(1152921504606844289));  // 2**60 - 2687
+  EXPECT_EQ(8ULL, MSB(256));
+  EXPECT_EQ(0ULL, MSB(1));
 }
 
 }  // namespace hexl