From b39f80e606ed4bfec09064aaba50d77f82f1ff3f Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Mon, 13 Feb 2017 17:13:46 +1100
Subject: [PATCH] hash/crc: parameterise on type and polynomial generator

---
 hash/crc.cpp   | 139 +++++++++++++++++++++++++++----------------------
 hash/crc.hpp   |  23 ++++++--
 hash/fwd.hpp   |   2 +-
 tools/hash.cpp |   2 +-
 4 files changed, 97 insertions(+), 69 deletions(-)

diff --git a/hash/crc.cpp b/hash/crc.cpp
index 8bc7b27b..5fb659da 100644
--- a/hash/crc.cpp
+++ b/hash/crc.cpp
@@ -20,22 +20,90 @@
 
 #include <array>
 
-using util::hash::crc32;
+using util::hash::crc;
 
 
 ///////////////////////////////////////////////////////////////////////////////
-static
-std::array<uint32_t,256>
-make_table (void)
+template <typename DigestT, DigestT Generator>
+const std::array<DigestT,256>
+crc<DigestT,Generator>::s_table = crc<DigestT,Generator>::table ();
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename DigestT, DigestT Generator>
+crc<DigestT,Generator>::crc () noexcept
 {
-    std::array<uint32_t,256> value {};
+    reset ();
+}
+
+
+//-----------------------------------------------------------------------------
+template <typename DigestT, DigestT Generator>
+void
+crc<DigestT,Generator>::reset (void) noexcept
+{
+    m_digest = 0;
+    m_digest = ~m_digest;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename DigestT, DigestT Generator>
+void
+crc<DigestT,Generator>::update (const uint8_t *restrict first,
+                                 const uint8_t *restrict last) noexcept
+{
+    CHECK_LE (first, last);
+
+    for (auto cursor = first; cursor != last; ++cursor) {
+        m_digest = s_table[*cursor ^ (m_digest & 0xFF)] ^ (m_digest >> 8u);
+    }
+}
+
+
+//-----------------------------------------------------------------------------
+template <typename DigestT, DigestT Generator>
+void
+crc<DigestT,Generator>::update (const void *restrict _data, size_t len) noexcept
+{
+    auto data = reinterpret_cast<const uint8_t *restrict> (_data);
+    return update(data, data + len);
+}
+
+
+//-----------------------------------------------------------------------------
+template <typename DigestT, DigestT Generator>
+void
+crc<DigestT,Generator>::finish (void)
+{
+    ;
+}
+
+
+//-----------------------------------------------------------------------------
+template <typename DigestT, DigestT Generator>
+DigestT
+crc<DigestT,Generator>::digest (void) const
+{
+    return ~m_digest;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename DigestT, DigestT Generator>
+std::array<DigestT,256>
+util::hash::crc<DigestT,Generator>::table (void)
+{
+    auto gen = Generator;
+
+    std::array<digest_t,256> value {};
 
     for (int i = 0; i < 256; ++i) {
-        std::uint32_t c = i;
+        digest_t c = i;
 
         for (int k = 0; k < 8; ++k) {
             if (c & 1)
-                c = 0xEDB88320L ^ (c >> 1);
+                c = gen ^ (c >> 1);
             else
                 c >>= 1;
         }
@@ -47,59 +115,6 @@ make_table (void)
 };
 
 
-//-----------------------------------------------------------------------------
-static std::array<uint32_t,256> table = make_table ();
-
-
 ///////////////////////////////////////////////////////////////////////////////
-crc32::crc32 () noexcept
-{
-    reset ();
-}
-
-
-//-----------------------------------------------------------------------------
-void
-crc32::reset (void) noexcept
-{
-    m_digest = 0;
-    m_digest = ~m_digest;
-}
-
-
-///////////////////////////////////////////////////////////////////////////////
-void
-crc32::update (const uint8_t *restrict first,
-               const uint8_t *restrict last) noexcept
-{
-    CHECK_LE (first, last);
-
-    for (auto cursor = first; cursor != last; ++cursor) {
-        m_digest = table[*cursor ^ (m_digest & 0xFF)] ^ (m_digest >> 8u);
-    }
-}
-
-
-//-----------------------------------------------------------------------------
-void
-crc32::update (const void *restrict _data, size_t len) noexcept
-{
-    auto data = reinterpret_cast<const uint8_t *restrict> (_data);
-    return update(data, data + len);
-}
-
-
-//-----------------------------------------------------------------------------
-void
-crc32::finish (void)
-{
-    ;
-}
-
-
-//-----------------------------------------------------------------------------
-typename crc32::digest_t
-crc32::digest (void) const
-{
-    return ~m_digest;
-}
\ No newline at end of file
+template class util::hash::crc<uint32_t, 0xEDB88320>; // png
+//template class util::hash::crc<uint32_t, 0X04C11DB7>; // ogg
diff --git a/hash/crc.hpp b/hash/crc.hpp
index 57ec9031..76e36228 100644
--- a/hash/crc.hpp
+++ b/hash/crc.hpp
@@ -17,19 +17,25 @@
 #ifndef __UTIL_HASH_CRC_HPP
 #define __UTIL_HASH_CRC_HPP
 
+#include <array>
 #include <cstdint>
 #include <cstdlib>
+#include <type_traits>
 
 
 ///////////////////////////////////////////////////////////////////////////////
 namespace util::hash {
-    // Implements the crc32 checksum (from ethernet, png, etc).
+    // Implements the crc checksum (from ethernet, png, etc).
     // Adapted from the PNG specification (ISO/IEC 15948:2003), appendix D.
-    class crc32 {
+    template <typename DigestT = uint32_t, DigestT Generator = 0xEDB88320>
+    class crc {
     public:
-        using digest_t = uint32_t;
+        static_assert (std::is_same<DigestT, std::uint32_t>::value,
+                       "only 32 bit crc is supported at this time");
 
-        crc32 () noexcept;
+        using digest_t = DigestT;
+
+        crc () noexcept;
 
         void reset (void) noexcept;
 
@@ -40,10 +46,17 @@ namespace util::hash {
 
         digest_t digest (void) const;
 
+        static
+        std::array<DigestT,256>
+        table (void);
+
     private:
         digest_t m_digest;
+
+        static const std::array<DigestT,256> s_table;
     };
+
+    using crc32 = crc<uint32_t, 0xEDB88320>;
 }
 
 #endif
-
diff --git a/hash/fwd.hpp b/hash/fwd.hpp
index e8703dc2..b4fd9bee 100644
--- a/hash/fwd.hpp
+++ b/hash/fwd.hpp
@@ -34,7 +34,7 @@ namespace util::hash {
     class adler32;
     class bsdsum;
     class fletcher;
-    class crc32;
+    class crc;
 
     class xxhash;
 
diff --git a/tools/hash.cpp b/tools/hash.cpp
index 91c92556..9b47a04d 100644
--- a/tools/hash.cpp
+++ b/tools/hash.cpp
@@ -40,7 +40,7 @@ static
 const char* NAMES[] = {
     "adler32",
     "bsdsum",
-    "crc32",
+    "crc",
     "MD2",
     "MD4",
     "MD5",