1 files changed, 37 insertions, 0 deletions
diff --git a/src/common/pg_lzcompress.c b/src/common/pg_lzcompress.c
index 988b3987d04..3adad62d9a4 100644
--- a/src/common/pg_lzcompress.c
+++ b/src/common/pg_lzcompress.c
@@ -771,3 +771,40 @@ pglz_decompress(const char *source, int32 slen, char *dest,
 	 */
 	return (char *) dp - dest;
 }
+
+
+/* ----------
+ * pglz_max_compressed_size -
+ *
+ *		Calculate the maximum compressed size for a given amount of raw data.
+ *		Return the maximum size, or total compressed size if maximum size is
+ *		larger than total compressed size.
+ *
+ * We can't use PGLZ_MAX_OUTPUT for this purpose, because that's used to size
+ * the compression buffer (and abort the compression). It does not really say
+ * what's the maximum compressed size for an input of a given length, and it
+ * may happen that while the whole value is compressible (and thus fits into
+ * PGLZ_MAX_OUTPUT nicely), the prefix is not compressible at all.
+ * ----------
+ */
+int32
+pglz_maximum_compressed_size(int32 rawsize, int32 total_compressed_size)
+{
+	int32 compressed_size;
+
+	/*
+	 * pglz uses one control bit per byte, so we need (rawsize * 9) bits. We
+	 * care about bytes though, so we add 7 to make sure we include the last
+	 * incomplete byte (integer division rounds down).
+	 *
+	 * XXX Use int64 to prevent overflow during calculation.
+	 */
+	compressed_size = (int32) ((int64) rawsize * 9 + 7) / 8;
+
+	/*
+	 * Maximum compressed size can't be larger than total compressed size.
+	 */
+	compressed_size = Min(compressed_size, total_compressed_size);
+
+	return compressed_size;
+}