AK: Add a forgiving_base64_decode helper

According to the specification at
https://infra.spec.whatwg.org/#forgiving-base64

AK/Base64.cpp

@@ -5,8 +5,10 @@
  */
 
 #include <AK/Array.h>
+#include <AK/Assertions.h>
 #include <AK/Base64.h>
 #include <AK/CharacterTypes.h>
+#include <AK/Error.h>
 #include <AK/StringBuilder.h>
 #include <AK/Types.h>
 #include <AK/Vector.h>
@@ -140,4 +142,100 @@ ErrorOr<String> encode_base64(ReadonlyBytes input)
     return output.to_string();
 }
 
+// https://infra.spec.whatwg.org/#forgiving-base64
+ErrorOr<ByteBuffer> decode_forgiving_base64(StringView input)
+{
+    // 1. Remove all ASCII whitespace from data.
+    auto data = input.trim_whitespace();
+
+    // 2. If data’s code point length divides by 4 leaving no remainder, then:
+    if (data.length() % 4 == 0) {
+        // If data ends with one or two U+003D (=) code points, then remove them from data.
+        if (data.ends_with("=="sv))
+            data = data.substring_view(0, data.length() - 2);
+        else if (data.ends_with('='))
+            data = data.substring_view(0, data.length() - 1);
+    }
+
+    // 3. If data’s code point length divides by 4 leaving a remainder of 1, then return failure.
+    if (data.length() % 4 == 1)
+        return Error::from_string_literal("Invalid input length in forgiving base64 decode");
+
+    // 4. If data contains a code point that is not one of
+    //     U+002B (+), U+002F (/), ASCII alphanumeric
+    // then return failure.
+    for (auto point : data) {
+        if (point != '+' && point != '/' && !is_ascii_alphanumeric(point))
+            return Error::from_string_literal("Invalid character in forgiving base64 decode");
+    }
+
+    // 5. Let output be an empty byte sequence.
+    // 6. Let buffer be an empty buffer that can have bits appended to it.
+    Vector<u8> output;
+    u32 buffer = 0;
+    auto accumulated_bits = 0;
+
+    auto add_to_buffer = [&](u8 number) {
+        VERIFY(number < 64);
+        u32 buffer_mask = number;
+
+        if (accumulated_bits == 0)
+            buffer_mask <<= 18;
+        else if (accumulated_bits == 6)
+            buffer_mask <<= 12;
+        else if (accumulated_bits == 12)
+            buffer_mask <<= 6;
+        else if (accumulated_bits == 18)
+            buffer_mask <<= 0;
+
+        buffer |= buffer_mask;
+
+        accumulated_bits += 6;
+    };
+
+    auto append_bytes = [&]() {
+        output.append(static_cast<u8>((buffer & 0xff0000) >> 16));
+        output.append(static_cast<u8>((buffer & 0xff00) >> 8));
+        output.append(static_cast<u8>(buffer & 0xff));
+
+        buffer = 0;
+        accumulated_bits = 0;
+    };
+
+    // 7. Let position be a position variable for data, initially pointing at the start of data.
+    // 8. While position does not point past the end of data:
+    for (auto point : data) {
+        // 1. Find the code point pointed to by position in the second column of Table 1: The Base 64 Alphabet of RFC 4648.
+        //    Let n be the number given in the first cell of the same row. [RFC4648]
+        auto n = alphabet_lookup_table[point];
+        VERIFY(n >= 0);
+
+        // 2. Append the six bits corresponding to n, most significant bit first, to buffer.
+        add_to_buffer(static_cast<u8>(n));
+
+        // 3. buffer has accumulated 24 bits,
+        if (accumulated_bits == 24) {
+            // interpret them as three 8-bit big-endian numbers.
+            // Append three bytes with values equal to those numbers to output, in the same order, and then empty buffer
+            append_bytes();
+        }
+    }
+
+    // 9. If buffer is not empty, it contains either 12 or 18 bits.
+    VERIFY(accumulated_bits == 0 || accumulated_bits == 12 || accumulated_bits == 18);
+
+    // If it contains 12 bits, then discard the last four and interpret the remaining eight as an 8-bit big-endian number.
+    if (accumulated_bits == 12)
+        output.append(static_cast<u8>((buffer & 0xff0000) >> 16));
+
+    // If it contains 18 bits, then discard the last two and interpret the remaining 16 as two 8-bit big-endian numbers.
+    // Append the one or two bytes with values equal to those one or two numbers to output, in the same order.
+    if (accumulated_bits == 18) {
+        output.append(static_cast<u8>((buffer & 0xff0000) >> 16));
+        output.append(static_cast<u8>((buffer & 0xff00) >> 8));
+    }
+
+    return ByteBuffer::copy(output);
+}
+
 }