lzx_compress: make lzx_cost_for_probability() more robust

diff --git a/src/lzx_compress.c b/src/lzx_compress.c
index 21636d49cedc790d176eee8513b4e880d0539b1e..ca25d92f2ae9e35416765577e59e5726bbf201c5 100644 (file)
--- a/src/lzx_compress.c
+++ b/src/lzx_compress.c
@@ -1920,12 +1920,17 @@ lzx_cost_for_probability(float prob)
         *      entropy = -log2(probability)
         *
         * Use this to get the cost in fractional bits.  Then multiply by our
         *      entropy = -log2(probability)
         *
         * Use this to get the cost in fractional bits.  Then multiply by our

-        * scaling factor of BIT_COST and truncate to a u32.
+        * scaling factor of BIT_COST and convert to an integer.

         *
         * In addition, the minimum cost is BIT_COST (one bit) because the
         * entropy coding method will be Huffman codes.
         *
         * In addition, the minimum cost is BIT_COST (one bit) because the
         * entropy coding method will be Huffman codes.

+        *
+        * Careful: even though 'prob' should be <= 1.0, 'log2f_fast(prob)' may
+        * be positive due to inaccuracy in our log2 approximation.  Therefore,
+        * we cannot, in general, assume the computed cost is non-negative, and
+        * we should make sure negative costs get rounded up correctly.

         */
         */

-       u32 cost = -log2f_fast(prob) * BIT_COST;
+       s32 cost = -log2f_fast(prob) * BIT_COST;

        return max(cost, BIT_COST);
 }
 
        return max(cost, BIT_COST);
 }