From 9efe8d1ed5ee210ae644b7118f1d674a2de5f3e8 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Sat, 22 Jul 2017 14:16:10 +0200
Subject: Add fast-path SHA256 functions for integral types

---
 libbutl/sha256.hxx      | 38 ++++++++++++++++++++++++++++++++++++--
 tests/sha256/driver.cxx | 37 +++++++++++++++++++++++++------------
 2 files changed, 61 insertions(+), 14 deletions(-)

diff --git a/libbutl/sha256.hxx b/libbutl/sha256.hxx
index e81b87b..459b46c 100644
--- a/libbutl/sha256.hxx
+++ b/libbutl/sha256.hxx
@@ -6,9 +6,10 @@
 #define LIBBUTL_SHA256_HXX
 
 #include <string>
-#include <cstring> // strlen()
+#include <cstring>     // strlen(), memcpy()
 #include <cstdint>
-#include <cstddef> // size_t
+#include <cstddef>     // size_t
+#include <type_traits> // enable_if, is_integral
 
 #include <libbutl/export.hxx>
 
@@ -50,6 +51,39 @@ namespace butl
     explicit
     sha256 (const char* s): sha256 () {append (s);}
 
+    // Append an integral type with a fast path optimization (see
+    // SHA256_Update() for details).
+    //
+    void
+    append (char c)
+    {
+      std::uint32_t r ((ctx_.count >> 3) & 0x3f);
+
+      if (1 < 64 - r)
+      {
+        ctx_.buf[r] = static_cast<std::uint8_t> (c);
+        ctx_.count += 8;
+      }
+      else
+        append (&c, 1);
+    }
+
+    template <typename T>
+    typename std::enable_if<std::is_integral<T>::value>::type
+    append (T x)
+    {
+      const std::size_t len (sizeof (x));
+      std::uint32_t r ((ctx_.count >> 3) & 0x3f);
+
+      if (len < 64 - r)
+      {
+        std::memcpy (&ctx_.buf[r], &x, sizeof (x));
+        ctx_.count += len << 3;
+      }
+      else
+        append (&x, len);
+    }
+
     // Extract result.
     //
     // It can be obtained as either a 32-byte binary digest or as a 64-
diff --git a/tests/sha256/driver.cxx b/tests/sha256/driver.cxx
index 5686898..2d2f963 100644
--- a/tests/sha256/driver.cxx
+++ b/tests/sha256/driver.cxx
@@ -26,18 +26,31 @@ main ()
   assert (string (sha256 ("123", 3).string ()) ==
           "a665a45920422f9d417e4867efdc4fb8a04a1f3fff1fa07e998e86f7f7a27ae3");
 
-  sha256 h;
-  h.append ("1");
-  h.append (string ("2"));
-  h.append ("3", 1);
-
-  auto& b (h.binary ());
-  assert (b[0] == 0x20 && b[31] == 0x9d);
-
-  string s (h.string ());
-  assert (s ==
-          "204d9db65789fbede7829ed77f72ba1f0fe21a833d95abad4849b82f33a69b9d");
-
+  {
+    sha256 h;
+    h.append ("1");
+    h.append (string ("2"));
+    h.append ("3", 1);
+
+    auto& b (h.binary ());
+    assert (b[0] == 0x20 && b[31] == 0x9d);
+
+    string s (h.string ());
+    assert (s ==
+            "204d9db65789fbede7829ed77f72ba1f0fe21a833d95abad4849b82f33a69b9d");
+  }
+
+  // Test fast path.
+  //
+  {
+    char c ('X');
+    sha256 h;
+    h.append (c);
+    assert (string (h.string ()) == sha256 (&c, 1).string ());
+  }
+
+  //
+  //
   string fp ("F4:9D:C0:02:C6:B6:62:06:A5:48:AE:87:35:32:95:64:C2:B8:C9:6D:9B:"
              "28:85:6D:EF:CA:FA:7F:04:B5:4F:A6");
 
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