day6, from N^3 to N^2

d06/exec.awk

@@ -1,17 +1,17 @@
-BEGIN { W = 14 }
+{ # use -vW=4 or 14
-{
+	for (i=1; ; i++) {
-	for (i=1; i<=length($1); i++) {
 		window = substr($1,i,W)
 		# compare each char of the window to rest of the window
 		S = 0  # to count match
-		for (j=1; j<=W; j++) {
+		for (j=1; j<W; j++) {
 			for (k=j+1; k<=W; k++) {
+				N++
 				S+= (substr(window,j,1) == substr(window,k,1)) ? 1 : 0
 			}
 		}
 		if (S == 0) {
-			print i + W - 1
+			print i + W - 1, "(" N " loops)"
 			next
 		}
 	}
-}
+} # O(N*W^2)

d06/exec2.awk

@@ -0,0 +1,17 @@
+{ j=0; # use -vW=4 or 14
+	for (i=1; ; i++) {
+		c = substr($0,i,1);
+		# look backward for a "new j" > j
+		for (k=i-1; k>j; k--) {
+			N++
+			if (substr($0,k,1) == c) {
+				j = k
+				break
+			}
+		}
+		if ((i-j) == W) {
+			print i, "(" N " loops)"
+			next
+		}
+	}
+} # O(N*W)