[DCCP]: 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 <[EMAIL PROTECTED]>
---
net/dccp/ccids/lib/tfrc_equation.c | 36 ++++++++++++++++++++++--------------
1 file changed, 22 insertions(+), 14 deletions(-)
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -626,6 +626,21 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
EXPORT_SYMBOL_GPL(tfrc_calc_x);
+/* return largest index i such that fval <= lookup[i][small] */
+static inline u32 tfrc_binsearch(u32 fval, int 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_reverse_lookup - try to find p given f(p)
* @fvalue: function value to match, scaled by 1000000
@@ -634,8 +649,7 @@ 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 <=> p = 0 */
return 0;
@@ -650,19 +664,13 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalu
return 1000000;
}
- if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1])
- small = 1;
- else
- small = 0;
-
+ 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;
+ }
- 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;
+ index = tfrc_binsearch(fvalue, 0);
+ return (index + 1) * 1000000/TFRC_CALC_X_ARRSIZE;
}
EXPORT_SYMBOL_GPL(tfrc_calc_x_reverse_lookup);
-
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