From d037e71bba60f23faaf55fdecaa19278415c13de Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Wed, 17 Jan 2018 20:10:26 +1100
Subject: [PATCH] encode/base: correct decode table indexing

---
 encode/base.hpp      | 65 ++++++++++++++++++++++----------------------
 test/encode/base.cpp | 29 +++++++++++++++++---
 2 files changed, 58 insertions(+), 36 deletions(-)

diff --git a/encode/base.hpp b/encode/base.hpp
index d4aad0f9..3a1157e5 100644
--- a/encode/base.hpp
+++ b/encode/base.hpp
@@ -200,47 +200,48 @@ namespace util::encode {
             if (src.size () % group_symbols)
                 throw std::invalid_argument ("base-encoded strings must be a proper multiple of symbols");
 
-            union {
-                uint_fast32_t num;
-                uint8_t bytes[group_bytes];
-            };
-
-            const bool padded = src.end ()[-1] == '=';
             auto cursor = std::cbegin (src);
-            for (size_t i = 0, last = std::size (src) / group_symbols - padded?1:0;
-                i != last;
-                 ++i)
-            {
-                num = std::accumulate (
-                    cursor, cursor + group_symbols,
-                    uint_fast32_t {0},
-                    [] (auto a, auto b) {
-                        return a << symbol_bits | dec_v<Size>[static_cast<unsigned>(b)];
-                    }
-                );
+            const int groups = std::size (src) / group_symbols;
+            const bool padded = std::cend (src)[-1] == '=';
 
-                cursor += group_symbols;
+            for (int i = 0; i < groups - (padded?1:0); ++i) {
+                uint64_t accum = 0;
 
-                dst = std::copy (std::crbegin (bytes), std::crend (bytes), dst);
+                for (int j = 0; j < group_symbols; ++j) {
+                    auto symbol = dec_v<Size>[unsigned(*cursor++)];
+                    accum <<= symbol_bits;
+                    accum  |= symbol;
+                }
+
+                for (int j = group_bytes - 1; j >= 0; --j) {
+                    const uint8_t byte = accum >> (j*8);
+                    *dst++ = byte;
+                }
             }
 
-            if (cursor != std::end (src)) {
-                auto last = std::find (cursor, std::cend (src), '=');
-                num = std::accumulate (
-                    cursor, last,
-                    uint_fast32_t{0},
-                    [] (auto a, auto b) {
-                        return a << symbol_bits | dec_v<Size>[static_cast<unsigned> (b)];
-                    }
-                );
+            if (padded) {
+                uint64_t accum = 0;
+                int symbols = 0;
+
+                for (int j = 0; j < group_symbols; ++j) {
+                    auto symbol = *cursor++;
+                    if (symbol == '=')
+                        break;
+
+                    symbols++;
+
+                    accum <<= symbol_bits;
+                    accum  |= dec_v<Size>[unsigned(symbol)];
+                }
 
-                auto symbols = last - cursor;
                 auto bits = symbols * symbol_bits;
                 auto shift = bits%8;
-                num >>= shift;
+                accum >>= shift;
 
-                for (auto i = bits / 8; i; )
-                    *dst++ = bytes[--i];
+                for (int j = bits / 8 - 1; j >= 0; --j) {
+                    auto byte = (accum >> (j*8)) & 0xff;
+                    *dst++ = byte;
+                }
             }
 
             return dst;
diff --git a/test/encode/base.cpp b/test/encode/base.cpp
index 621da75b..0a6c9d61 100644
--- a/test/encode/base.cpp
+++ b/test/encode/base.cpp
@@ -8,24 +8,39 @@
 static constexpr char input[] = "foobar";
 
 
-//-----------------------------------------------------------------------------
+///////////////////////////////////////////////////////////////////////////////
 // test vectors from rfc4648
 template <int Size>
 struct output { static const char *value[std::size (input)]; };
 
+
 //-----------------------------------------------------------------------------
 template <>
 const char* output<64>::value[] = {
-    "", "Zg==", "Zm8=", "Zm9v", "Zm9vYg==", "Zm9vYmE=", "Zm9vYmFy",
+    "",
+    "Zg==",
+    "Zm8=",
+    "Zm9v",
+    "Zm9vYg==",
+    "Zm9vYmE=",
+    "Zm9vYmFy",
 };
 
 
+//-----------------------------------------------------------------------------
 template <>
 const char* output<32>::value[]  {
-    "", "MY======", "MZXQ====", "MZXW6===", "MZXW6YQ=", "MZXW6YTB", "MZXW6YTBOI======"
+    "",
+    "MY======",
+    "MZXQ====",
+    "MZXW6===",
+    "MZXW6YQ=",
+    "MZXW6YTB",
+    "MZXW6YTBOI======"
 };
 
 
+//-----------------------------------------------------------------------------
 #if 0
 template <>
 const char* output<32h>::value[] {
@@ -37,7 +52,13 @@ const char* output<32h>::value[] {
 //-----------------------------------------------------------------------------
 template <>
 const char *output<16>::value[] = {
-    "", "66", "666F", "666F6F", "666F6F62", "666F6F6261", "666F6F626172"
+    "",
+    "66",
+    "666F",
+    "666F6F",
+    "666F6F62",
+    "666F6F6261",
+    "666F6F626172"
 };