XP: add a pre-splitter

instead of ingesting only full blocks, make an analysis of data, and infer where to split.

lib/compress/zstd_preSplit.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
+
+#ifndef ZSTD_PRESPLIT_H
+#define ZSTD_PRESPLIT_H
+
+#include <stddef.h>  /* size_t */
+
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
+size_t ZSTD_splitBlock_4k(const void* src, size_t srcSize, size_t blockSizeMax);
+
+#if defined (__cplusplus)
+}
+#endif
+
+#endif /* ZSTD_PRESPLIT_H */