refactor: pure constexpr functions

src/impl/constexpr.inc

@@ -2,23 +2,25 @@
 
 namespace md5::impl {
 
-struct md5_ctx_ce {
+struct md5_ce_ctx {
     uint32_t A = value::kA;
     uint32_t B = value::kB;
     uint32_t C = value::kC;
     uint32_t D = value::kD;
+};
 
+struct md5_ce {
     const char *data;
     uint64_t data_len, padded_len;
 
-    constexpr md5_ctx_ce(const char *data, const uint64_t len)
+    constexpr md5_ce(const char *data, const 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
 
 /// Get the data and padding byte of the specified index.
-constexpr uint8_t GetByte(const md5_ctx_ce *ctx, const uint64_t index) {
+constexpr uint8_t GetByte(const md5_ce *ctx, const uint64_t index) {
     if (index < ctx->data_len) // message data
         return ctx->data[index];
     if (index == ctx->data_len) // padding flag
@@ -30,7 +32,7 @@ constexpr uint8_t GetByte(const md5_ctx_ce *ctx, const uint64_t index) {
 }
 
 /// Get the MD5 block content at the specified index.
-constexpr Block GetBlock(const md5_ctx_ce *ctx, const uint64_t index) {
+constexpr Block GetBlock(const md5_ce *ctx, const uint64_t index) {
     Block block {};
     for (int i = 0; i < 16; ++i) {
         const auto offset = index + i * 4;
@@ -54,15 +56,32 @@ constexpr std::array<char, 32> DigestCE(const std::array<uint32_t, 4> &ctx) {
     return result;
 }
 
-constexpr std::array<char, 32> MD5::HashCE(const char *data, uint64_t len) {
-    md5_ctx_ce ctx(data, len);
-    for (uint32_t index = 0; index < ctx.padded_len; index += 64) {
-        auto block = GetBlock(&ctx, index);
-        auto A = ctx.A;
-        auto B = ctx.B;
-        auto C = ctx.C;
-        auto D = ctx.D;
-        MD5_UPDATE
+constexpr uint32_t Calc(const md5_ce_ctx &ctx, const int i) {
+    if (i < 0x10)
+        return ctx.D ^ (ctx.B & (ctx.C ^ ctx.D));
+    if (i < 0x20)
+        return ctx.C ^ (ctx.D & (ctx.B ^ ctx.C));
+    if (i < 0x30)
+        return ctx.B ^ ctx.C ^ ctx.D;
+    return ctx.C ^ (ctx.B | ~ctx.D);
+}
+
+constexpr md5_ce_ctx Round(const Block &block, md5_ce_ctx ctx) {
+    for (int i = 0; i < 64; ++i) {
+        const auto a = ctx.A + Calc(ctx, i) + block[K(i)] + T(i);
+        ctx.A = ctx.D;
+        ctx.D = ctx.C;
+        ctx.C = ctx.B;
+        ctx.B += a << S(i) | a >> (32 - S(i));
+    }
+    return ctx;
+}
+
+constexpr std::array<char, 32> MD5::HashCE(const char *data, const uint64_t len) {
+    md5_ce_ctx ctx;
+    const md5_ce md5(data, len);
+    for (uint32_t index = 0; index < md5.padded_len; index += 64) {
+        const auto [A, B, C, D] = Round(GetBlock(&md5, index), ctx);
         ctx.A += A;
         ctx.B += B;
         ctx.C += C;
@@ -71,6 +90,6 @@ constexpr std::array<char, 32> MD5::HashCE(const char *data, uint64_t len) {
     return DigestCE({ctx.A, ctx.B, ctx.C, ctx.D});
 }
 
-static_assert(MD5::HashCE("")[0] == 'd', "invalid constexpr");
+static_assert(MD5::HashCE("")[0] == 'd');
 
 } // namespace md5::impl