span: Add std::byte helpers

Also, add Span<std::byte> interface to strencondings.

src/span.h

@@ -180,6 +180,7 @@ public:
         return m_data[m_size - 1];
     }
     constexpr std::size_t size() const noexcept { return m_size; }
+    constexpr std::size_t size_bytes() const noexcept { return sizeof(C) * m_size; }
     constexpr bool empty() const noexcept { return size() == 0; }
     CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
     {
@@ -236,11 +237,35 @@ T& SpanPopBack(Span<T>& span)
     return back;
 }
 
+// From C++20 as_bytes and as_writeable_bytes
+template <typename T>
+Span<const std::byte> AsBytes(Span<T> s) noexcept
+{
+    return {reinterpret_cast<const std::byte*>(s.data()), s.size_bytes()};
+}
+template <typename T>
+Span<std::byte> AsWritableBytes(Span<T> s) noexcept
+{
+    return {reinterpret_cast<std::byte*>(s.data()), s.size_bytes()};
+}
+
+template <typename V>
+Span<const std::byte> MakeByteSpan(V&& v) noexcept
+{
+    return AsBytes(MakeSpan(std::forward<V>(v)));
+}
+template <typename V>
+Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
+{
+    return AsWritableBytes(MakeSpan(std::forward<V>(v)));
+}
+
 // Helper functions to safely cast to unsigned char pointers.
 inline unsigned char* UCharCast(char* c) { return (unsigned char*)c; }
 inline unsigned char* UCharCast(unsigned char* c) { return c; }
 inline const unsigned char* UCharCast(const char* c) { return (unsigned char*)c; }
 inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
+inline const unsigned char* UCharCast(const std::byte* c) { return reinterpret_cast<const unsigned char*>(c); }
 
 // Helper function to safely convert a Span to a Span<[const] unsigned char>.
 template <typename T> constexpr auto UCharSpanCast(Span<T> s) -> Span<typename std::remove_pointer<decltype(UCharCast(s.data()))>::type> { return {UCharCast(s.data()), s.size()}; }

src/test/base64_tests.cpp

@@ -23,6 +23,16 @@ BOOST_AUTO_TEST_CASE(base64_testvectors)
         BOOST_CHECK_EQUAL(strDec, vstrIn[i]);
     }
 
+    {
+        const std::vector<uint8_t> in_u{0xff, 0x01, 0xff};
+        const std::vector<std::byte> in_b{std::byte{0xff}, std::byte{0x01}, std::byte{0xff}};
+        const std::string in_s{"\xff\x01\xff"};
+        const std::string out_exp{"/wH/"};
+        BOOST_CHECK_EQUAL(EncodeBase64(in_u), out_exp);
+        BOOST_CHECK_EQUAL(EncodeBase64(in_b), out_exp);
+        BOOST_CHECK_EQUAL(EncodeBase64(in_s), out_exp);
+    }
+
     // Decoding strings with embedded NUL characters should fail
     bool failure;
     (void)DecodeBase64("invalid\0"s, &failure);

src/test/util_tests.cpp

@@ -151,12 +151,25 @@ BOOST_AUTO_TEST_CASE(util_HexStr)
         HexStr(Span<const unsigned char>(ParseHex_expected, ParseHex_expected)),
         "");
 
-    std::vector<unsigned char> ParseHex_vec(ParseHex_expected, ParseHex_expected + 5);
+    {
+        const std::vector<char> in_s{ParseHex_expected, ParseHex_expected + 5};
+        const Span<const uint8_t> in_u{MakeUCharSpan(in_s)};
+        const Span<const std::byte> in_b{MakeByteSpan(in_s)};
+        const std::string out_exp{"04678afdb0"};
 
-    BOOST_CHECK_EQUAL(
-        HexStr(ParseHex_vec),
-        "04678afdb0"
-    );
+        BOOST_CHECK_EQUAL(HexStr(in_u), out_exp);
+        BOOST_CHECK_EQUAL(HexStr(in_s), out_exp);
+        BOOST_CHECK_EQUAL(HexStr(in_b), out_exp);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(span_write_bytes)
+{
+    std::array mut_arr{uint8_t{0xaa}, uint8_t{0xbb}};
+    const auto mut_bytes{MakeWritableByteSpan(mut_arr)};
+    mut_bytes[1] = std::byte{0x11};
+    BOOST_CHECK_EQUAL(mut_arr.at(0), 0xaa);
+    BOOST_CHECK_EQUAL(mut_arr.at(1), 0x11);
 }
 
 BOOST_AUTO_TEST_CASE(util_Join)

src/util/strencodings.cpp

@@ -138,11 +138,6 @@ std::string EncodeBase64(Span<const unsigned char> input)
     return str;
 }
 
-std::string EncodeBase64(const std::string& str)
-{
-    return EncodeBase64(MakeUCharSpan(str));
-}
-
 std::vector<unsigned char> DecodeBase64(const char* p, bool* pf_invalid)
 {
     static const int decode64_table[256] =

src/util/strencodings.h

@@ -50,7 +50,8 @@ bool IsHexNumber(const std::string& str);
 std::vector<unsigned char> DecodeBase64(const char* p, bool* pf_invalid = nullptr);
 std::string DecodeBase64(const std::string& str, bool* pf_invalid = nullptr);
 std::string EncodeBase64(Span<const unsigned char> input);
-std::string EncodeBase64(const std::string& str);
+inline std::string EncodeBase64(Span<const std::byte> input) { return EncodeBase64(MakeUCharSpan(input)); }
+inline std::string EncodeBase64(const std::string& str) { return EncodeBase64(MakeUCharSpan(str)); }
 std::vector<unsigned char> DecodeBase32(const char* p, bool* pf_invalid = nullptr);
 std::string DecodeBase32(const std::string& str, bool* pf_invalid = nullptr);
 
@@ -189,6 +190,7 @@ std::optional<T> ToIntegral(const std::string& str)
  */
 std::string HexStr(const Span<const uint8_t> s);
 inline std::string HexStr(const Span<const char> s) { return HexStr(MakeUCharSpan(s)); }
+inline std::string HexStr(const Span<const std::byte> s) { return HexStr(MakeUCharSpan(s)); }
 
 /**
  * Format a paragraph of text to a fixed width, adding spaces for