Go version of streaming base64 decoder

tools/utils/streaming_base64/api.go

@@ -0,0 +1,106 @@
+package streaming_base64
+
+import (
+	"encoding/base64"
+	"fmt"
+	"iter"
+)
+
+var _ = fmt.Print
+
+type StreamingBase64Decoder struct {
+	leftover     [4]byte
+	num_leftover int
+	total_read   int64
+}
+
+func wrap_error(err error, chunkOffset int64) error {
+	if e, ok := err.(base64.CorruptInputError); ok {
+		// CorruptInputError is an int64 representing the relative byte offset
+		return base64.CorruptInputError(int64(e) + chunkOffset)
+	}
+	return err
+}
+
+func (s *StreamingBase64Decoder) Decode(input []byte, output []byte) iter.Seq2[[]byte, error] {
+	// Base64 decoding: 4 input bytes -> 3 output bytes.
+	// We check if output is large enough for this chunk + any buffered data.
+	maxPossibleOutput := ((len(input) + s.num_leftover) / 4) * 3
+	return func(yield func([]byte, error) bool) {
+		if len(output) < maxPossibleOutput {
+			yield(nil, fmt.Errorf("output slice too small: need at least %d, got %d", maxPossibleOutput, len(output)))
+			return
+		}
+		currIn := input
+		outOffset := 0
+
+		// 1. Handle leftover bytes from previous call
+		if s.num_leftover > 0 {
+			need := 4 - s.num_leftover
+			if len(currIn) >= need {
+				copy(s.leftover[s.num_leftover:], currIn[:need])
+
+				// Decode the bridge block
+				n, err := base64.StdEncoding.Decode(output[outOffset:], s.leftover[:4])
+				if err != nil {
+					yield(nil, wrap_error(err, s.total_read-int64(s.num_leftover)))
+					return
+				}
+
+				if !yield(output[outOffset:outOffset+n], nil) {
+					return
+				}
+				outOffset += n
+				currIn = currIn[need:]
+				s.total_read += int64(need)
+				s.num_leftover = 0
+			} else {
+				// Still not enough to complete a block
+				copy(s.leftover[s.num_leftover:], currIn)
+				s.num_leftover += len(currIn)
+				s.total_read += int64(len(currIn))
+				return
+			}
+		}
+
+		// 2. Decode the bulk of the current chunk
+		processableLen := (len(currIn) / 4) * 4
+		if processableLen > 0 {
+			if n, err := base64.StdEncoding.Decode(output[outOffset:], currIn[:processableLen]); err != nil {
+				yield(nil, wrap_error(err, s.total_read))
+				return
+			} else if n > 0 {
+				if !yield(output[outOffset:outOffset+n], nil) {
+					return
+				}
+				outOffset += n
+			}
+			currIn = currIn[processableLen:]
+			s.total_read += int64(processableLen)
+		}
+
+		// 3. Buffer remaining bytes (1-3) for the next Decode call
+		if len(currIn) > 0 {
+			copy(s.leftover[:], currIn)
+			s.num_leftover = len(currIn)
+			s.total_read += int64(len(currIn))
+		}
+	}
+}
+
+func (s *StreamingBase64Decoder) Finish() ([]byte, error) {
+	switch s.num_leftover {
+	case 0:
+		return nil, nil
+	case 1:
+		return nil, base64.CorruptInputError(s.total_read - 1)
+	case 2:
+		s.leftover[2] = '='
+		s.leftover[3] = '='
+	case 3:
+		s.leftover[3] = '='
+	}
+	output := [3]byte{}
+	n, err := base64.StdEncoding.Decode(output[:3], s.leftover[:4])
+	return output[:n], wrap_error(err, s.total_read-int64(s.num_leftover))
+}