Implement unpad for PKCS#7 padding.

Chaos/Padding/PadderPkcs7.hpp

@@ -1,9 +1,12 @@
 #ifndef CHAOS_PADDING_PADDERPKCS7_HPP
 #define CHAOS_PADDING_PADDERPKCS7_HPP
 
+#include <climits>
+#include <cstddef>
 #include <cstdint>
 #include <iterator>
 #include <limits>
+#include <type_traits>
 
 #include "Padding/Padder.hpp"
 #include "Service/ChaosException.hpp"
@@ -19,6 +22,7 @@ public:
     {
         auto dist = std::distance(begin, end);
 
+        // TODO: dist > 0
         if (dist >= 0 && dist <= std::numeric_limits<uint8_t>::max())
         {
             for (OutputIt it = begin; it != end; ++it)
@@ -31,6 +35,114 @@ public:
             throw Service::ChaosException("PadderPkcs7::Pad(): invalid range");
         }
     }
+
+    struct ComputeUnpadResult
+    {
+        bool IsOkay_;
+        uint8_t PadSize_;
+    };
+
+    template<typename InputIt>
+    static ComputeUnpadResult ComputeUnpad(InputIt begin, InputIt end) noexcept
+    {
+        if (begin == end)
+        {
+            return { .IsOkay_ = false, .PadSize_ = 0 };
+        }
+
+        const uint8_t padSize = *std::prev(end);
+        uint8_t isOkay = ~BlIsZero<uint8_t>(padSize);
+
+        size_t suffixSize = 0;
+        InputIt it = end;
+        while (it != begin && suffixSize < std::numeric_limits<uint8_t>::max())
+        {
+            --it;
+            ++suffixSize;
+
+            isOkay &= (BlEq<uint8_t>(*it, padSize) |
+                       BlGt<uint8_t, size_t>(suffixSize, padSize));
+        }
+
+        isOkay &= BlGe<uint8_t, size_t>(suffixSize, padSize);
+
+        return
+        {
+            .IsOkay_ = static_cast<bool>(isOkay),
+            .PadSize_ = BlSel<uint8_t>(isOkay, padSize, 0)
+        };
+    }
+
+private:
+    template<typename OutUInt, typename InUInt>
+    static constexpr OutUInt BlMsb(InUInt in) noexcept
+    {
+        static_assert(std::is_unsigned_v<OutUInt> &&
+                      std::is_unsigned_v<InUInt>);
+
+        constexpr uint8_t shift = (sizeof(InUInt) * CHAR_BIT) - 1;
+        return static_cast<OutUInt>(0) - (in >> shift);
+    }
+
+    template<typename OutUInt, typename InUInt>
+    static constexpr OutUInt BlLt(InUInt lhs, InUInt rhs) noexcept
+    {
+        static_assert(std::is_unsigned_v<OutUInt> &&
+                      std::is_unsigned_v<InUInt>);
+        static_assert(sizeof(InUInt) <= sizeof(uint64_t));
+
+        const uint64_t lhsEx = lhs;
+        const uint64_t rhsEx = rhs;
+
+        return BlMsb<OutUInt>(lhsEx ^ ((lhsEx ^ rhsEx) | ((lhsEx - rhsEx) ^ lhsEx)));
+    }
+
+    template<typename OutUInt, typename InUInt>
+    static constexpr OutUInt BlIsZero(InUInt in) noexcept
+    {
+        static_assert(std::is_unsigned_v<OutUInt> &&
+                      std::is_unsigned_v<InUInt>);
+        static_assert(sizeof(InUInt) <= sizeof(uint64_t));
+
+        const uint64_t inEx = in;
+
+        return BlMsb<OutUInt>(~inEx & (inEx - 1U));
+    }
+
+    template<typename OutUInt, typename InUInt>
+    static constexpr OutUInt BlEq(InUInt lhs, InUInt rhs) noexcept
+    {
+        static_assert(std::is_unsigned_v<OutUInt> &&
+                      std::is_unsigned_v<InUInt>);
+
+        return BlIsZero<OutUInt, InUInt>(lhs ^ rhs);
+    }
+
+    template<typename OutUInt, typename InUInt>
+    static constexpr OutUInt BlGe(InUInt lhs, InUInt rhs) noexcept
+    {
+        static_assert(std::is_unsigned_v<OutUInt> &&
+                      std::is_unsigned_v<InUInt>);
+
+        return ~BlLt<OutUInt>(lhs, rhs);
+    }
+
+    template<typename OutUInt, typename InUInt>
+    static constexpr OutUInt BlGt(InUInt lhs, InUInt rhs) noexcept
+    {
+        static_assert(std::is_unsigned_v<OutUInt> &&
+                      std::is_unsigned_v<InUInt>);
+
+        return BlLt<OutUInt>(rhs, lhs);
+    }
+
+    template<typename UInt>
+    static constexpr UInt BlSel(UInt mask, UInt onTrue, UInt onFalse) noexcept
+    {
+        static_assert(std::is_unsigned_v<UInt>);
+
+        return (mask & onTrue) | (~mask & onFalse);
+    }
 };
 
 } // namespace Chaos::Padding