Merge pull request #405 from cppalliance/397

Add ipow function

Author: mborland

include/boost/int128/detail/int128_imp.hpp

@@ -2024,7 +2024,7 @@ BOOST_INT128_HOST_DEVICE BOOST_INT128_FORCE_INLINE constexpr int128_t library_mu
     const auto c {rhs.low >> 32U};
     const auto d {rhs.low & UINT32_MAX};
 
-    int128_t result { static_cast<std::int64_t>(static_cast<std::uint64_t>(lhs.high) * rhs.low + static_cast<std::uint64_t>(lhs.low) * rhs.high + a * c), b * d };
+    int128_t result { static_cast<std::int64_t>(static_cast<std::uint64_t>(lhs.high) * rhs.low + lhs.low * static_cast<std::uint64_t>(rhs.high) + a * c), b * d };
     result += signed_shift_left_32(a * d) + signed_shift_left_32(b * c);
 
     return result;

include/boost/int128/utilities.hpp

@@ -171,6 +171,53 @@ BOOST_INT128_EXPORT BOOST_INT128_HOST_DEVICE constexpr int128_t powm(const int12
     return static_cast<int128_t>(powm(ub, static_cast<uint128_t>(exp), um));
 }
 
+// Computes base^exp using exponentiation by squaring. The result is reduced
+// modulo 2^128, mirroring the wrap-around behavior of operator*.
+BOOST_INT128_EXPORT BOOST_INT128_HOST_DEVICE constexpr uint128_t ipow(uint128_t base, std::uint64_t exp) noexcept
+{
+    uint128_t result {1};
+
+    while (exp != 0U)
+    {
+        if (static_cast<bool>(exp & 1U))
+        {
+            result *= base;
+        }
+
+        exp >>= 1;
+
+        if (exp != 0U)
+        {
+            base *= base;
+        }
+    }
+
+    return result;
+}
+
+// Signed overload. Wraps modulo 2^128 on overflow, matching operator*.
+BOOST_INT128_EXPORT BOOST_INT128_HOST_DEVICE constexpr int128_t ipow(int128_t base, std::uint64_t exp) noexcept
+{
+    int128_t result {1};
+
+    while (exp != 0U)
+    {
+        if (static_cast<bool>(exp & 1U))
+        {
+            result *= base;
+        }
+
+        exp >>= 1;
+
+        if (exp != 0U)
+        {
+            base *= base;
+        }
+    }
+
+    return result;
+}
+
 } // namespace int128
 } // namespace boost
 

test/Jamfile

@@ -80,6 +80,7 @@ run test_x64_msvc_div.cpp ;
 run test_gcd_lcm.cpp ;
 run test_midpoint.cpp ;
 run test_powm.cpp ;
+run test_ipow.cpp ;
 
 run test_format.cpp ;
 run test_fmt_format.cpp ;

test/test_ipow.cpp

@@ -0,0 +1,238 @@
+// Copyright 2026 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#if defined(__GNUC__) && __GNUC__ == 7 && defined(__i386__)
+
+// 32-bit GCC-7 fails with: "error: constexpr loop iteration count exceeds limit of 262144"
+
+int main() { return 0; }
+
+#else
+
+#ifndef BOOST_INT128_BUILD_MODULE
+
+#include <boost/int128.hpp>
+
+#else
+
+import boost.int128;
+
+#endif
+
+#include <boost/core/lightweight_test.hpp>
+#include <cstdint>
+#include <limits>
+
+using namespace boost::int128;
+
+namespace {
+
+// Naive reference implementation used to cross-check the squaring loop.
+template <typename T>
+constexpr T ipow_ref(T base, std::uint64_t exp) noexcept
+{
+    T result {1};
+
+    for (std::uint64_t i {0}; i < exp; ++i)
+    {
+        result *= base;
+    }
+
+    return result;
+}
+
+} // namespace
+
+void test_uint128_ipow_basic()
+{
+    BOOST_TEST_EQ(ipow(uint128_t{0}, 0U), uint128_t{1});
+    BOOST_TEST_EQ(ipow(uint128_t{1}, 0U), uint128_t{1});
+    BOOST_TEST_EQ(ipow(uint128_t{42}, 0U), uint128_t{1});
+    BOOST_TEST_EQ(ipow(uint128_t{0}, 1U), uint128_t{0});
+    BOOST_TEST_EQ(ipow(uint128_t{0}, 5U), uint128_t{0});
+    BOOST_TEST_EQ(ipow(uint128_t{1}, 1000U), uint128_t{1});
+    BOOST_TEST_EQ(ipow(uint128_t{42}, 1U), uint128_t{42});
+
+    BOOST_TEST_EQ(ipow(uint128_t{2}, 0U), uint128_t{1});
+    BOOST_TEST_EQ(ipow(uint128_t{2}, 1U), uint128_t{2});
+    BOOST_TEST_EQ(ipow(uint128_t{2}, 2U), uint128_t{4});
+    BOOST_TEST_EQ(ipow(uint128_t{2}, 10U), uint128_t{1024});
+    BOOST_TEST_EQ(ipow(uint128_t{3}, 5U), uint128_t{243});
+    BOOST_TEST_EQ(ipow(uint128_t{10}, 9U), uint128_t{UINT64_C(1000000000)});
+    BOOST_TEST_EQ(ipow(uint128_t{10}, 18U), uint128_t{UINT64_C(1000000000000000000)});
+}
+
+void test_uint128_ipow_power_of_two()
+{
+    // 2^k fills bit k, so we can hit every bit position up to 127.
+    for (std::uint64_t k {0}; k < 64; ++k)
+    {
+        const uint128_t expected {static_cast<std::uint64_t>(1) << k};
+        BOOST_TEST_EQ(ipow(uint128_t{2}, k), expected);
+    }
+
+    for (std::uint64_t k {64}; k < 128; ++k)
+    {
+        const uint128_t expected {static_cast<std::uint64_t>(1) << (k - 64), 0U};
+        BOOST_TEST_EQ(ipow(uint128_t{2}, k), expected);
+    }
+
+    // 2^128 wraps to 0 in uint128 arithmetic.
+    BOOST_TEST_EQ(ipow(uint128_t{2}, 128U), uint128_t{0});
+    BOOST_TEST_EQ(ipow(uint128_t{2}, 200U), uint128_t{0});
+}
+
+void test_uint128_ipow_large()
+{
+    // 10^38 is the largest power of 10 that fits in 128 bits.
+    // 10^38 = 100000000000000000000000000000000000000.
+    const uint128_t ten_pow_38 {UINT64_C(0x4B3B4CA85A86C47A), UINT64_C(0x098A224000000000)};
+    BOOST_TEST_EQ(ipow(uint128_t{10}, 38U), ten_pow_38);
+
+    // Cross-check a range of bases against the naive reference for small
+    // exponents where the result is hand-verifiable through repeated mul.
+    for (std::uint64_t base {2}; base < 8; ++base)
+    {
+        for (std::uint64_t exp {0}; exp < 12; ++exp)
+        {
+            BOOST_TEST_EQ(ipow(uint128_t{base}, exp), ipow_ref(uint128_t{base}, exp));
+        }
+    }
+}
+
+void test_uint128_ipow_wrap()
+{
+    // Squaring 2^64 yields 2^128 which wraps to 0.
+    const uint128_t two_pow_64 {1U, 0U};
+    BOOST_TEST_EQ(ipow(two_pow_64, 2U), uint128_t{0});
+
+    // (2^64 - 1)^2 mod 2^128 = 2^128 - 2^65 + 1, which has a known bit pattern.
+    const uint128_t u64_max {(std::numeric_limits<std::uint64_t>::max)()};
+    const uint128_t expected {UINT64_C(0xFFFFFFFFFFFFFFFE), 1U};
+    BOOST_TEST_EQ(ipow(u64_max, 2U), expected);
+
+    // Anything to a sufficiently large power eventually wraps to 0 if the base
+    // shares a factor of 2 with 2^128.
+    BOOST_TEST_EQ(ipow(uint128_t{4}, 64U), uint128_t{0});
+    BOOST_TEST_EQ(ipow(uint128_t{6}, 200U), uint128_t{0});
+}
+
+void test_uint128_ipow_identities()
+{
+    // a^(b+c) == a^b * a^c (under wrap modulo 2^128).
+    const uint128_t a {UINT64_C(0xDEADBEEF)};
+    BOOST_TEST_EQ(ipow(a, 7U), ipow(a, 3U) * ipow(a, 4U));
+    BOOST_TEST_EQ(ipow(a, 20U), ipow(a, 13U) * ipow(a, 7U));
+
+    // (a*b)^e == a^e * b^e.
+    const uint128_t aa {7};
+    const uint128_t bb {11};
+    BOOST_TEST_EQ(ipow(aa * bb, 6U), ipow(aa, 6U) * ipow(bb, 6U));
+
+    // (a^b)^c == a^(b*c).
+    BOOST_TEST_EQ(ipow(ipow(uint128_t{3}, 4U), 5U), ipow(uint128_t{3}, 4U * 5U));
+}
+
+void test_int128_ipow_basic()
+{
+    BOOST_TEST_EQ(ipow(int128_t{0}, 0U), int128_t{1});
+    BOOST_TEST_EQ(ipow(int128_t{1}, 0U), int128_t{1});
+    BOOST_TEST_EQ(ipow(int128_t{-1}, 0U), int128_t{1});
+    BOOST_TEST_EQ(ipow(int128_t{0}, 5U), int128_t{0});
+    BOOST_TEST_EQ(ipow(int128_t{42}, 1U), int128_t{42});
+
+    BOOST_TEST_EQ(ipow(int128_t{2}, 10U), int128_t{1024});
+    BOOST_TEST_EQ(ipow(int128_t{3}, 5U), int128_t{243});
+    BOOST_TEST_EQ(ipow(int128_t{10}, 18U), int128_t{INT64_C(1000000000000000000)});
+}
+
+void test_int128_ipow_negative_base()
+{
+    // Even exponents are non-negative, odd exponents preserve the sign.
+    BOOST_TEST_EQ(ipow(int128_t{-2}, 0U), int128_t{1});
+    BOOST_TEST_EQ(ipow(int128_t{-2}, 1U), int128_t{-2});
+    BOOST_TEST_EQ(ipow(int128_t{-2}, 2U), int128_t{4});
+    BOOST_TEST_EQ(ipow(int128_t{-2}, 3U), int128_t{-8});
+    BOOST_TEST_EQ(ipow(int128_t{-2}, 10U), int128_t{1024});
+    BOOST_TEST_EQ(ipow(int128_t{-3}, 5U), int128_t{-243});
+
+    BOOST_TEST_EQ(ipow(int128_t{-1}, 100U), int128_t{1});
+    BOOST_TEST_EQ(ipow(int128_t{-1}, 101U), int128_t{-1});
+
+    BOOST_TEST_EQ(ipow(int128_t{-10}, 18U), int128_t{INT64_C(1000000000000000000)});
+    BOOST_TEST_EQ(ipow(int128_t{-10}, 17U), int128_t{INT64_C(-100000000000000000)});
+}
+
+void test_int128_ipow_large()
+{
+    // 10^38 still fits in int128_t (signed max is roughly 1.7e38).
+    const int128_t ten_pow_38 {static_cast<int128_t>(uint128_t{UINT64_C(0x4B3B4CA85A86C47A), UINT64_C(0x098A224000000000)})};
+    BOOST_TEST_EQ(ipow(int128_t{10}, 38U), ten_pow_38);
+    BOOST_TEST_EQ(ipow(int128_t{-10}, 38U), ten_pow_38);
+
+    // Cross-check small bases against the naive reference.
+    for (std::int64_t base {-7}; base < 8; ++base)
+    {
+        for (std::uint64_t exp {0}; exp < 12; ++exp)
+        {
+            BOOST_TEST_EQ(ipow(int128_t{base}, exp), ipow_ref(int128_t{base}, exp));
+        }
+    }
+}
+
+void test_int128_ipow_identities()
+{
+    const int128_t a {12345};
+    BOOST_TEST_EQ(ipow(a, 7U), ipow(a, 3U) * ipow(a, 4U));
+    BOOST_TEST_EQ(ipow(ipow(int128_t{3}, 4U), 5U), ipow(int128_t{3}, 4U * 5U));
+
+    // Sign behaves multiplicatively.
+    BOOST_TEST_EQ(ipow(int128_t{-7}, 3U) * ipow(int128_t{-7}, 4U), ipow(int128_t{-7}, 7U));
+}
+
+#ifdef _MSC_VER
+#  pragma warning(push)
+#  pragma warning(disable : 4307) // integral constant overflow
+#  pragma warning(disable : 4308) // negative integral constant converted to unsigned type
+#endif
+
+void test_constexpr_ipow()
+{
+    constexpr uint128_t r1 {ipow(uint128_t{2}, 10U)};
+    static_assert(r1 == uint128_t{1024}, "ipow constexpr uint128 small case");
+
+    constexpr uint128_t r2 {ipow(uint128_t{10}, 18U)};
+    static_assert(r2 == uint128_t{UINT64_C(1000000000000000000)}, "ipow constexpr uint128 18 digits");
+
+    constexpr int128_t r3 {ipow(int128_t{-3}, 5U)};
+    static_assert(r3 == int128_t{-243}, "ipow constexpr int128 negative base odd exp");
+
+    constexpr int128_t r4 {ipow(int128_t{-3}, 4U)};
+    static_assert(r4 == int128_t{81}, "ipow constexpr int128 negative base even exp");
+
+    constexpr uint128_t r5 {ipow(uint128_t{2}, 128U)};
+    static_assert(r5 == uint128_t{0}, "ipow constexpr uint128 wrap to zero");
+}
+
+#ifdef _MSC_VER
+#  pragma warning(pop)
+#endif
+
+int main()
+{
+    test_uint128_ipow_basic();
+    test_uint128_ipow_power_of_two();
+    test_uint128_ipow_large();
+    test_uint128_ipow_wrap();
+    test_uint128_ipow_identities();
+    test_int128_ipow_basic();
+    test_int128_ipow_negative_base();
+    test_int128_ipow_large();
+    test_int128_ipow_identities();
+    test_constexpr_ipow();
+
+    return boost::report_errors();
+}
+
+#endif