add xrealloc_vector.c

libbb/xrealloc_vector.c

@@ -0,0 +1,36 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Utility routines.
+ *
+ * Copyright (C) 2008 Denys Vlasenko
+ *
+ * Licensed under GPLv2, see file LICENSE in this tarball for details.
+ */
+
+#include "libbb.h"
+
+/* Resize (grow) malloced vector.
+ *
+ *  #define magic packed two parameters into one:
+ *  sizeof = sizeof_and_shift >> 8
+ *  shift  = (sizeof_and_shift) & 0xff
+ *  (TODO: encode "I want it zeroed" in lowest bit?)
+ *
+ * Lets say shift = 4. 1 << 4 == 0x10.
+ * If idx == 0, 0x10, 0x20 etc, vector[] is resized to next higher
+ * idx step, plus one: if idx == 0x20, vector[] is resized to 0x31,
+ * thus last usable element is vector[0x30].
+ *
+ * In other words: after xrealloc_vector(v, 4, idx) it's ok to use
+ * at least v[idx] and v[idx+1], for all idx values.
+ */
+void* FAST_FUNC xrealloc_vector_helper(void *vector, unsigned sizeof_and_shift, int idx)
+{
+	int mask = 1 << (uint8_t)sizeof_and_shift;
+
+	if (!(idx & (mask - 1))) {
+		sizeof_and_shift >>= 8;
+		vector = xrealloc(vector, sizeof_and_shift * (idx + mask + 1));
+	}
+	return vector;
+}