Merge pull request #1784 from evoskuil/master

Add constexpr Float power(Float base, Exponent exponent), style.

Author: evoskuil

include/bitcoin/system/impl/math/power.ipp

@@ -33,6 +33,7 @@ namespace system {
 
 // Called by bc::base85 (for number coding).
 // Called by wallet::electrum_v1 (for number coding).
+// Overflow is allowed behavior as this models a mathematical operator.
 template <typename Value, typename Base, typename Exponent,
     if_integer<Value> = true,
     if_integer<Base> = true,
@@ -42,19 +43,11 @@ constexpr Value power_(Base base, Exponent exponent) NOEXCEPT
     if (is_power_overflow(base, exponent))
         return 0;
 
-    if (is_zero(exponent))
-        return 1;
+    Value product{ 1 };
+    while (exponent-- > 0u)
+        product *= possible_narrow_and_sign_cast<Value>(base);
 
-    auto value = possible_narrow_and_sign_cast<Value>(base);
-
-    // Overflow is allowed behavior as this models a mathematical operator.
-    BC_PUSH_WARNING(NARROWING_CONVERSION)
-    BC_PUSH_WARNING(SIZE_NARROWING_CONVERSION)
-    while (--exponent > 0u) { value *= base; }
-    BC_POP_WARNING()
-    BC_POP_WARNING()
-
-    return value;
+    return product;
 }
 
 // published
@@ -112,6 +105,23 @@ constexpr Value power2(Exponent exponent) NOEXCEPT
         possible_narrow_and_sign_cast<unsigned>(exponent));
 }
 
+template <typename Float, typename Exponent,
+    if_floating_point<Float>,
+    if_unsigned_integer<Exponent>>
+constexpr Float power(Float base, Exponent exponent) NOEXCEPT
+{
+    // 0^0 is undefined, return 0.
+    if ((base == 0.0) && is_zero(exponent))
+        return 0.0;
+
+    Float product{ 1.0 };
+    while (exponent-- > 0u)
+        if ((product *= base) == 0.0)
+            break;
+
+    return product;
+}
+
 } // namespace system
 } // namespace libbitcoin
 

include/bitcoin/system/math/power.hpp

@@ -54,6 +54,12 @@ template <typename Value = size_t, typename Exponent,
     if_unsigned_integer<Exponent> = true>
 constexpr Value power2(Exponent exponent) NOEXCEPT;
 
+/// Floating point constexpr optimization for integer exponent.
+template <typename Float, typename Exponent,
+    if_floating_point<Float> = true,
+    if_unsigned_integer<Exponent> = true>
+constexpr Float power(Float base, Exponent exponent) NOEXCEPT;
+
 } // namespace system
 } // namespace libbitcoin
 

test/math/power.cpp

@@ -37,23 +37,21 @@ static_assert(power(3,   0u) == 1u);
 static_assert(power(3u,  0u) == 1u);
 static_assert(power(3,  10u) == 0xe6a9_size);
 static_assert(power(3u, 10u) == 0xe6a9_size);
-static_assert(power<uint16_t>(3, 11u) == 0xb3fb_u16);
 
 static_assert(power(-1, 0u) == 1u);
 static_assert(power(-1, 1u) == 1_nsize);
 
 static_assert(power(-3,  0u) == 1u);
 static_assert(power(-3,  1u) == 3_nsize);
 static_assert(power(-3, 10u) == 0xe6a9_size);
-static_assert(power<uint16_t>(-3, 11u) == 0x4c05_u16);
 
 // power2
 static_assert(power2(0u) == 1u);
 static_assert(power2(0u) == 1u);
 static_assert(power2(1u) == 2u);
 static_assert(power2(1u) == 2u);
 static_assert(power2<uint16_t>(15u) == 0b1000'0000'0000'0000_u16);
-static_assert(power2<uint16_t>(16u) == 0_u16);
+static_assert(power2<uint16_t>(16u) == 0_u16); // overflow
 
 // power<>
 static_assert(power<0>(16u) == power(0u, 16u));
@@ -66,10 +64,25 @@ static_assert(power<3, size_t>(0u) == 1u);
 // uint256_t
 static_assert(power<2u, uint256_t>(10u) == 1024u);
 
-// uintx is not constexpr.
+
+// floating point
+static_assert(is_same_type<decltype(power(0.0f, 0u)), float>);
+static_assert(is_same_type<decltype(power(0.0, 0u)), double>);
+
+static_assert(power(0.0f, 0u) == 0.0f);
+static_assert(power(0.0, 0u) == 0.0);
+static_assert(power(1.0, 0u) == 1.0);
+static_assert(power(1.0, 1u) == 1.0);
+static_assert(power(2.0, 1u) == 2.0);
+static_assert(power(2.0, 2u) == 4.0);
+static_assert(power(3.0, 3u) == 27.0);
+
+// Actually 311.1696 but this is the floating point representation.
+static_assert(power(4.2, 4u) == 311.16960000000006);
 
 BOOST_AUTO_TEST_SUITE(power_tests)
 
+// uintx is not constexpr.
 BOOST_AUTO_TEST_CASE(power_uintx_tests)
 {
     BOOST_REQUIRE(true);