[DCCP] tfrc: Binary search for reverse TFRC lookup

This replaces the linear search algorithm for reverse lookup with
binary search.

It has the advantage of better scalability: O(log2(N)) instead of O(N).
This means that the average number of iterations is reduced from 250
(linear search if each value appears equally likely) down to at most 9.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index 0a4a3d2..ddac2c5 100644 (file)
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -594,6 +594,21 @@ static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = {
        { 243315981, 271305 }
 };
 
+/* return largest index i such that fval <= lookup[i][small] */
+static inline u32 tfrc_binsearch(u32 fval, u8 small)
+{
+       u32 try, low = 0, high = TFRC_CALC_X_ARRSIZE - 1;
+
+       while (low < high) {
+               try = (low + high) / 2;
+               if (fval <= tfrc_calc_x_lookup[try][small])
+                       high = try;
+               else
+                       low  = try + 1;
+       }
+       return high;
+}
+
 /**
  * tfrc_calc_x - Calculate the send rate as per section 3.1 of RFC3448
  *
@@ -656,8 +671,7 @@ EXPORT_SYMBOL_GPL(tfrc_calc_x);
  */
 u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
 {
-       int ctr = 0;
-       int small;
+       int index;
 
        if (fvalue == 0)        /* f(p) = 0  whenever  p = 0 */
                return 0;
@@ -672,18 +686,14 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
                return 1000000;
        }
 
-       if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1])
-               small = 1;
-       else
-               small = 0;
-
-       while (fvalue > tfrc_calc_x_lookup[ctr][small])
-               ctr++;
-
-       if (small)
-               return (ctr + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE;
-       else
-               return (ctr + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
+       if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1]) {
+               index = tfrc_binsearch(fvalue, 1);
+               return (index + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE;
+       }
+ 
+       /* else ... it must be in the coarse-grained column */
+       index = tfrc_binsearch(fvalue, 0);
+       return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
 }
 
 EXPORT_SYMBOL_GPL(tfrc_calc_x_reverse_lookup);