reader: refactor reader_fill_buffer()

This changes reader_fill_buffer() to calculate the remaining size to be
copied to the buffer instead of using the size that is already copied to
the buffer. Also this commit adds assertions to check if the result of
reader_fill_buffer() is sound.

lib/reader/reader.c

@@ -114,36 +114,38 @@ reader_fill_buffer(struct SqshReader *reader, size_t size) {
 	const struct SqshReaderIteratorImpl *impl = reader->iterator_impl;
 	struct SqshBuffer *buffer = &reader->buffer;
 	sqsh_index_t offset = reader->offset;
+	sqsh_index_t iterator_offset = reader->iterator_offset;
 
-	size_t buffer_size = sqsh__buffer_size(buffer);
+	size_t remaining_size = size - sqsh__buffer_size(buffer);
 	for (;;) {
 		const uint8_t *data = impl->data(iterator);
 		const size_t data_size = impl->size(iterator);
-		const size_t copy_size =
-				SQSH_MIN(data_size - offset, size - buffer_size);
+		const size_t copy_size = SQSH_MIN(data_size - offset, remaining_size);
 		rv = sqsh__buffer_append(buffer, &data[offset], copy_size);
 		if (rv < 0) {
 			goto out;
 		}
 
 		offset = 0;
-		buffer_size += copy_size;
-		if (size <= buffer_size) {
-			assert(size == buffer_size);
+		remaining_size -= copy_size;
+		if (remaining_size == 0) {
 			break;
 		}
 
-		rv = reader_iterator_next(reader, size);
+		iterator_offset = size - remaining_size;
+		rv = reader_iterator_next(reader, remaining_size);
 		if (rv < 0) {
 			goto out;
 		}
-		reader->iterator_offset = buffer_size;
 	}
 
+	assert(size == sqsh__buffer_size(buffer));
+	assert(iterator_offset != 0);
+
+	reader->iterator_offset = iterator_offset;
 	reader->offset = 0;
 	reader->data = sqsh__buffer_data(buffer);
 	reader->size = size;
-	assert(reader->iterator_offset != 0);
 out:
 	return rv;
 }