Add support for url-safe base64 encoding

c++/src/kj/encoding-test.c++

@@ -475,5 +475,50 @@ KJ_TEST("base64 encoding/decoding") {
   }
 }
 
+KJ_TEST("base64 url encoding") {
+  {
+    // Handles empty.
+    auto encoded = encodeBase64Url(StringPtr("").asBytes());
+    KJ_EXPECT(encoded == "", encoded, encoded.size());
+  }
+
+  {
+    // Handles paddingless encoding.
+    auto encoded = encodeBase64Url(StringPtr("foo").asBytes());
+    KJ_EXPECT(encoded == "Zm9v", encoded, encoded.size());
+  }
+
+  {
+    // Handles padded encoding.
+    auto encoded1 = encodeBase64Url(StringPtr("quux").asBytes());
+    KJ_EXPECT(encoded1 == "cXV1eA", encoded1, encoded1.size());
+    auto encoded2 = encodeBase64Url(StringPtr("corge").asBytes());
+    KJ_EXPECT(encoded2 == "Y29yZ2U", encoded2, encoded2.size());
+  }
+
+  {
+    // No line breaks.
+    StringPtr fullLine = "012345678901234567890123456789012345678901234567890123";
+    auto encoded = encodeBase64Url(StringPtr(fullLine).asBytes());
+    KJ_EXPECT(
+        encoded == "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIz",
+        encoded);
+  }
+
+  {
+    // Replaces plusses.
+    ArrayPtr<const byte> data = { 0b11111011, 0b11101111, 0b10111110 };
+    auto encoded = encodeBase64Url(data);
+    KJ_EXPECT(encoded == "----", encoded, encoded.size());
+  }
+
+  {
+    // Replaces slashes.
+    ArrayPtr<const byte> data = { 0b11111111, 0b11111111, 0b11111111 };
+    auto encoded = encodeBase64Url(data);
+    KJ_EXPECT(encoded == "____", encoded, encoded.size());
+  }
+}
+
 }  // namespace
 }  // namespace kj

c++/src/kj/encoding.c++

@@ -1001,4 +1001,24 @@ EncodingResult<Array<byte>> decodeBase64(ArrayPtr<const char> input) {
   return EncodingResult<Array<byte>>(kj::mv(output), state.hadErrors);
 }
 
+String encodeBase64Url(ArrayPtr<const byte> bytes) {
+  // TODO(perf): Rewrite as single pass?
+  // TODO(someday): Write decoder?
+
+  auto base64 = kj::encodeBase64(bytes);
+
+  for (char& c: base64) {
+    if (c == '+') c = '-';
+    if (c == '/') c = '_';
+  }
+
+  // Remove trailing '='s.
+  kj::ArrayPtr<const char> slice = base64;
+  while (slice.size() > 0 && slice.back() == '=') {
+    slice = slice.slice(0, slice.size() - 1);
+  }
+
+  return kj::str(slice);
+}
+
 } // namespace kj

c++/src/kj/encoding.h

@@ -213,6 +213,9 @@ EncodingResult<Array<byte>> decodeBase64(ArrayPtr<const char> text);
 // Decode base64 text. This function reports errors required by the WHATWG HTML/Infra specs: see
 // https://html.spec.whatwg.org/multipage/webappapis.html#atob for details.
 
+String encodeBase64Url(ArrayPtr<const byte> bytes);
+// Encode the given bytes as URL-safe base64 text. (RFC 4648, section 5)
+
 // =======================================================================================
 // inline implementation details