bin/base64: decode: ignore newlines

By popular demand, ignore newlines in the encoded input. This achieves
bug compatibility with GNU base64.

The algorithm is quite naive and slow because it checks each byte
independently. There is definitely room for improvement.

bin/base64.c

@@ -340,29 +340,75 @@ encode (const struct config *config, struct buffer *buf)
 	return true;
 }
 
-static int
+static inline size_t
+find_newline (const char *p, const size_t avail)
+{
+	// This is very naive and can probably be improved by vectorization.
+	for (size_t len = 0; len < avail; len++) {
+		if (p[len] == '\n') {
+			return len;
+		}
+	}
+
+	return avail;
+}
+
+static bool
 decode (const struct config *config, struct buffer *buf)
 {
-	size_t nread, nout;
+	size_t avail;
 	struct base64_state state;
 
 	// Initialize the decoder's state structure.
 	base64_stream_decode_init(&state, 0);
 
 	// Read encoded data into the buffer. Use the smallest buffer size to
 	// be on the safe side: the decoded output will fit the raw buffer.
-	while ((nread = fread(buf->enc, 1, BUFFER_RAW_SIZE, config->fp)) > 0) {
+	while ((avail = fread(buf->enc, 1, BUFFER_RAW_SIZE, config->fp)) > 0) {
+		char  *start  = buf->enc;
+		char  *outbuf = buf->raw;
+		size_t ototal = 0;
+
+		// By popular demand, this utility tries to be bug-compatible
+		// with GNU `base64'. That includes silently ignoring newlines
+		// in the input. Tokenize the input on newline characters.
+		while (avail > 0) {
+
+			// Find the offset of the next newline character, which
+			// is also the length of the next chunk.
+			size_t outlen, len = find_newline(start, avail);
+
+			// Ignore empty chunks.
+			if (len == 0) {
+				start++;
+				avail--;
+				continue;
+			}
 
-		// Decode the input into the raw buffer.
-		if (base64_stream_decode(&state, buf->enc, nread,
-		                         buf->raw, &nout) == 0) {
-			fprintf(stderr, "%s: %s: decoding error\n",
-			        config->name, config->file);
-			return false;
+			// Decode the chunk into the raw buffer.
+			if (base64_stream_decode(&state, start, len,
+			                         outbuf, &outlen) == 0) {
+				fprintf(stderr, "%s: %s: decoding error\n",
+				        config->name, config->file);
+				return false;
+			}
+
+			// Update the output buffer pointer and total size.
+			outbuf += outlen;
+			ototal += outlen;
+
+			// Bail out if the whole string has been consumed.
+			if (len == avail) {
+				break;
+			}
+
+			// Move the start pointer past the newline.
+			start += len + 1;
+			avail -= len + 1;
 		}
 
 		// Append the raw data to the output stream.
-		if (write_stdout(config, buf->raw, nout) == false) {
+		if (write_stdout(config, buf->raw, ototal) == false) {
 			return false;
 		}
 	}