[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>

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);