Make it more obvious why no bounds checks are necessary for 11n.

For unsigned x, x/8 is x >> 3, and x*8 is x << 3, so 8*(x/8) is the result
of shifting x right 3 bits and then left 3 bits, which is the same as
masking out the low-order 3 bits, and x - (8*x/8) is the result of
subtracting all but the low-order 3 bits from x, thus it's the lower 3
bits, so you can just mask it with 0x7.  That means the result is in the
range 0 through 7, so as long as the array has 8 elements, you're OK; it
does, but explicitly declare it as such, to make it clearer that it is,
and to get compiler warnings if not all 8 elements are initialized.

Change-Id: Iff9c0626b9bdc012cca52e4160dda9e947315bc4
Reviewed-on: https://code.wireshark.org/review/12264
Reviewed-by: Guy Harris <guy@alum.mit.edu>

1 files changed, 7 insertions, 6 deletions

diff --git a/wiretap/vwr.c b/wiretap/vwr.c
index 3b9659322d..a699d8d6b9 100644
--- a/wiretap/vwr.c
+++ b/wiretap/vwr.c
@@ -2262,14 +2262,13 @@ static float getRate( guint8 plcpType, guint8 mcsIndex, guint16 rflags, guint8 n
     /* Rate conversion data */
     float canonical_rate_legacy[]  = {1.0f, 2.0f, 5.5f, 11.0f, 6.0f, 9.0f, 12.0f, 18.0f, 24.0f, 36.0f, 48.0f, 54.0f};
 
-    int   canonical_ndbps_20_ht[]  = {26, 52, 78, 104, 156, 208, 234, 260};
-    int   canonical_ndbps_40_ht[]  = {54, 108, 162, 216, 324, 432, 486, 540};
+    int   canonical_ndbps_20_ht[8]  = {26, 52, 78, 104, 156, 208, 234, 260};
+    int   canonical_ndbps_40_ht[8]  = {54, 108, 162, 216, 324, 432, 486, 540};
 
     int   canonical_ndbps_20_vht[] = {26,52, 78, 104, 156, 208, 234, 260, 312};
     int   canonical_ndbps_40_vht[] = {54, 108, 162, 216, 324, 432, 486, 540, 648, 720};
     int   canonical_ndbps_80_vht[] = {117, 234, 351, 468, 702, 936, 1053, 1170, 1404, 1560};
 
-    int   ndbps;
     float symbol_tx_time, bitrate  = 0.0f;
 
     if (plcpType == 0)
@@ -2279,17 +2278,19 @@ static float getRate( guint8 plcpType, guint8 mcsIndex, guint16 rflags, guint8 n
     }
     else if (plcpType == 1 || plcpType == 2)
     {
+        int   ndbps;
+
         if ( rflags & FLAGS_CHAN_SHORTGI)
             symbol_tx_time = 3.6f;
         else
             symbol_tx_time = 4.0f;
 
         if ( rflags & FLAGS_CHAN_40MHZ )
-            ndbps = canonical_ndbps_40_ht[ mcsIndex - 8*(int)(mcsIndex/8) ];
+            ndbps = canonical_ndbps_40_ht[mcsIndex & 0x07];
         else
-            ndbps = canonical_ndbps_20_ht[ mcsIndex - 8*(int)(mcsIndex/8) ];
+            ndbps = canonical_ndbps_20_ht[mcsIndex & 0x07];
 
-        bitrate = ( ndbps * (((int)(mcsIndex/8) + 1) )) / symbol_tx_time;
+        bitrate = ( ndbps * (((int)(mcsIndex >> 3) + 1) )) / symbol_tx_time;
     }
     else
     {