Implement unpad for ISO 7816 padding.

Chaos/Padding/PadderIso7816.hpp

@@ -1,9 +1,12 @@
 #ifndef CHAOS_PADDING_PADDERISO7816_HPP
 #define CHAOS_PADDING_PADDERISO7816_HPP
 
+#include <cstddef>
 #include <cstdint>
+#include <limits>
 
 #include "Padding/Padder.hpp"
+#include "Service/Branchless.hpp"
 
 namespace Chaos::Padding
 {
@@ -26,6 +29,52 @@ public:
             }
         }
     }
+
+    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 };
+        }
+
+        uint8_t padSizeIncrement = 1;
+        uint8_t padSize = 0;
+
+        uint8_t encountered0x80 = 0x00;
+        uint8_t onlyZerosPast0x80 = 0xFF;
+
+        InputIt it = end;
+        while (it != begin && padSize < std::numeric_limits<uint8_t>::max())
+        {
+            --it;
+
+            const uint8_t byte = *it;
+
+            padSizeIncrement = Branchless::Sel<uint8_t>(encountered0x80, 0, 1);
+            encountered0x80 |= Branchless::Eq<uint8_t, uint8_t>(byte, 0x80);
+            onlyZerosPast0x80 &= Branchless::IsZero<uint8_t, uint8_t>(byte) | encountered0x80;
+
+            padSize += padSizeIncrement;
+        }
+
+        uint8_t isOkay = encountered0x80 & onlyZerosPast0x80;
+
+        return
+        {
+            .IsOkay_ = static_cast<bool>(isOkay),
+            .PadSize_ = Branchless::Sel<uint8_t>(isOkay, padSize, 0)
+        };
+    }
+
+private:
+    using Branchless = Service::Branchless;
 };
 
 } // namespace Chaos::Padding