feat: compatible with c++17 standard

CMakeLists.txt

@@ -4,7 +4,21 @@ project(md5sum LANGUAGES CXX)
 option(MD5_ENABLE_TESTING "Enable testing of the md5sum library." ON)
 option(MD5_ENABLE_BENCHMARK "Enable benchmark of the md5sum library." ON)
 
-set(CMAKE_CXX_STANDARD 20)
+set(CXX_STANDARDS)
+foreach (CXX_FEATURE ${CMAKE_CXX_COMPILE_FEATURES})
+    if (CXX_FEATURE MATCHES "^cxx_std_[0-9]+$")
+        string(REGEX MATCH "[0-9]+$" CXX_STANDARD ${CXX_FEATURE})
+        list(APPEND CXX_STANDARDS ${CXX_STANDARD})  # Get supported c++ standards
+    endif()
+endforeach()
+
+if (NOT 17 IN_LIST CXX_STANDARDS)
+    message(FATAL_ERROR "MD5 projects require at least C++17 support.")
+endif()
+
+list(GET CXX_STANDARDS -1 MD5_CXX_STANDARD)  # Select the latest C++ standard
+message("MD5 CXX STANDARD: ${MD5_CXX_STANDARD}")
+set(CMAKE_CXX_STANDARD ${MD5_CXX_STANDARD})
 add_compile_options(-Wall -Wextra)
 
 add_library(md5sum STATIC src/impl/core.cc src/impl/wrapper.cc)

src/impl/algorithm.inc

@@ -58,14 +58,14 @@ struct md5_ctx {
 };
 
 /// MD5 data block index, input between 0 and 63.
-consteval int K(int i) {
+constexpr int K(int i) {
     const int step[4] = {1, 5, 3, 7};
     const int begin[4] = {0, 1, 5, 0};
     return (begin[i >> 4] + step[i >> 4] * i) & 0b1111;
 }
 
 /// MD5 circular shift times, input between 0 and 63.
-consteval int S(int i) {
+constexpr int S(int i) {
     const int shift[4][4] = {
         {7, 12, 17, 22},
         {5, 9, 14, 20},
@@ -76,7 +76,7 @@ consteval int S(int i) {
 }
 
 /// MD5 T-table constant, input between 0 and 63.
-consteval uint32_t T(int i) {
+constexpr uint32_t T(int i) {
     auto val = ::md5::math::sin(i + 1);
     return static_cast<uint32_t>(::std::abs(val) * 0x100000000);
 }

src/impl/constexpr.inc

@@ -13,9 +13,8 @@ struct md5_ctx_ce {
     const char *data;
     uint64_t data_len, padded_len;
 
-    constexpr md5_ctx_ce(const char *ptr, uint64_t len) : data(ptr), data_len(len) {
-        padded_len = (data_len + 64 + 8) & ~0b111111ULL;
-    }
+    constexpr md5_ctx_ce(const char *data, uint64_t len)
+        : data(data), data_len(len), padded_len((len + 64 + 8) & ~0b111111ULL) {}
 };
 
 using Block = std::array<uint32_t, 16>; // single md5 block with 64 bytes
@@ -48,7 +47,7 @@ constexpr Block GetBlock(md5_ctx_ce *ctx, const uint64_t index) {
 /// Convert origin MD5 integers to hexadecimal character array.
 constexpr std::array<char, 32> DigestCE(std::array<uint32_t, 4> ctx) {
     std::array<char, 32> result {};
-    for (uint32_t i = 0, val; i < 32; val >>= 8) {
+    for (uint32_t i = 0, val = 0; i < 32; val >>= 8) {
         if (!(i & 0b111))
             val = ctx[i >> 3];
         result[i++] = HexTable[(val >> 4) & 0b1111];

src/impl/sine.inc

@@ -6,7 +6,7 @@ namespace md5::math {
 
 constexpr double PI = 3.14159265358979323846264338327950;
 
-consteval double pow(double x, int n) {
+constexpr double pow(double x, int n) {
     double res = 1;
     for (int i = 0; i < n; ++i) {
         res *= x;
@@ -14,7 +14,7 @@ consteval double pow(double x, int n) {
     return res;
 }
 
-consteval double factorial(int n) {
+constexpr double factorial(int n) {
     double res = 1;
     for (int i = 2 ; i <= n ; ++i) {
         res *= i;
@@ -23,7 +23,7 @@ consteval double factorial(int n) {
 }
 
 /// Calculate sin(x) value with Maclaurin series.
-consteval double sin_core(double x) {
+constexpr double sin_core(double x) {
     double res = x;
     for (int i = 1; i < 80; ++i) {
         const int n = i * 2 + 1;
@@ -34,7 +34,7 @@ consteval double sin_core(double x) {
 }
 
 /// Calculate the sin(x) value in radians.
-consteval double sin(double x) {
+constexpr double sin(double x) {
     x = std::fmod(x, 2 * PI); // -2PI < x < 2PI
 
     if (std::abs(x) > PI) {