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#REDIRECT [[Joel Spolsky#Schlemiel the Painter's algorithm]]
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== Introduction ==
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In computer science "Schlemiel The Painter's Algorithm" denotes an algorithm which repeatedly reprocesses content. This '''re'''processing is extremely inefficient, which becomes problematic with repetition. The problem most commonly appears in string and buffer processing.
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== Who is Schlemiel? ==
Schlemiel got a job painting the white-lines down the middle of the road. On his first day Schlemiel painted 300 yards of road, which is good. On his second day Schlemiel painted 150 yards, which isn't too bad. On the third day Schlemiel painted only 30 yards, which is grounds for dismissal.
The boss called Schlemiel into the office for some counselling. The boss said "Schlemiel, I think you are good worker, so why is that the longer you work the less you get done?". "Well" Schlemiel replied "the further I get from my can of paint the further I have to run to the end of the line, and all the way back again to replenish my brush." The boss replied "Schlemiel you need to '''take your paint with you!'''"
The principle is childishly simple, but it is too easy to miss this scenario when implementing a complex software product, especially when it spans modules.
== The Problem ==
Repeated calls to [[ANSI_C|ANSI-C]]'s [[strcat]] function is a classic example of Schlemiel's algorithm.
An implementation of ''strcat'' might look something like this:
<source lang="c">
char *strcat(char *dest, const char *src) {
// remember the start of the destination string
char *startOfDest = dest;
// make dest point at the null terminator at the end of the destination string
while(*dest) { // <-- EXPENSIVE
dest++;
}
// append the source string to the destination string
while(*src) {
dest++ = src++;
}
// null terminate the destination string
dest = '\0';
// return the start of the destination string
return startOfDest;
}
</source>
The ''strcat'' function is innocious, unless it is called '''repeatedly'''.
Here is an example of how '''not''' to use ''strcat'':
<source lang="c">
// builds a 1 Megabyte string of "word, word, word, ....";
void testPerformanceOfStrcat() {
final int BUFFER_SIZE = 1024*1024;
char word[] = "word";
char buffer[BUFFER_SIZE] = "";
while( strlen(buffer) < BUFFER_SIZE-strlen(word) ) { // <-- EXPENSIVE
if(*buffer) strcat(buffer, ", "); // <-- EXPENSIVE
strcat(buffer, word); // <-- EXPENSIVE
}
}
</source>
The estimated cost of ''testPerformanceOfStrcat'' is:
* NOTE: "Ancillary costs" like the cost of making a function call are ignored.
* The string "word, " is 6 characters in length.
* The number of times through the ''while'' loop is int(1024*1024/6) = 174,762
* The cost of ''strlen(buffer)'' in the ''while'' condition is [[Fibonacci_number|fib]](174,762) = 15,270,965,703 cycles.
* The cost of ''strlen(word)'' in the ''while'' condition is 4 cycles per loop = 174,762 * 4 = 699,048
* The cost of finding the-end-dest in ''strcat(buffer, ", ")'' is 15,270,965,703 cycles.
* The cost of finding the-end-dest in ''strcat(buffer, word)'' is 15,270,965,703 cycles.
* The cost of copying the string is 6 cycles per loop = 174,762 * 6 = 1,048,572
* The TOTAL COST of ''testPerformanceOfStrcat'' is 15,270,965,710 * 3 + 699,048 + 1,048,572 = 45,814,644,750 or about 46 billion cycles.
* 46 billion cycles on a 2GHz CPU is about 23 seconds, which is totally unaceptable.
== The Solution ==
The solution to Schlemiel's performance degradation is obvious: "Schlemiel, take your paint with you". The same principle can be applied to ''strcat''. The program needs to remember the-___location-of-end-of-the-string, to avoid the expense of repeatedlt finding it.
The below ''stringcat'' method returns the-new-end-of-the-destination-string.
<source lang="c">
char *stringcat(char *dest, const char *src) {
// make dest point at the null terminator at the end of the destination string
while(*dest) { // <-- EXPENSIVE, SO AVOID UYSING IT REPEATEDLY.
dest++;
}
// append the source string to the destination string
while(*src) {
dest++ = src++;
}
// null terminate the destination string
dest = '\0';
// return a pointer to the null terminator at the end of the destination string
return dest;
}
</source>
The ''stringcat'' function is the same as the traditional ''strcat'', except that it returns the-new-end-of-the-destination-string which may then be passed to the next invocation of ''stringcat'', eliminating the overhead of repeatedly finding the-end-of-the-destination-string.
The following test harness excersises the new ''stringcat'' function.
<source lang="c">
// builds a 1Meg string of "word, word, word, ...."
char *testPerformanceOfStringcat() {
final int BUFFER_SIZE = 1024*1024;
char word[] = "word";
char nextWord[] = ", word";
char buffer[BUFFER_SIZE] = "";
char *last = buffer+BUFFER_SIZE-strlen(word)-strlen(", "); // <-- Do this once!
char *endOfBuffer = stringcat(buffer, word); // <-- Do this once!
while( endOfBuffer < last ) { // <-- 15 billion times cheaper
endOfBuffer = strcat(endOfBuffer, nextWord); // <-- 30 billion times cheaper
}
}
</source>
The estimated cost of ''testPerformanceOfStringcat'' is:
* The number of times through the loop is still int(1024*1024/6) = 174,762
* The overhead of finding the-end-of-dest in the ''while'' condition has been eliminated.
* The overhead of finding the-end-of-dest in ''strcat(endOfBuffer, word)'' has been eliminated
* The cost of copying the string, which is 6 cycles per loop = 174,762 * 6 = 1,048,572
* The TOTAL COST of ''testPerformanceOfStringcat'' is down to 174,762 + 1,048,572 = 1,223,334
* 1.2 million cycles on a 2GHz CPU is 0.6 milliseconds, which is acceptable.
== Conclusion ==
Schlemiel's algorithm is a common cause of nonperformant, unscalable software implementations. The solution is to eliminate content '''re'''processing.
== References ==
* [http://www.joelonsoftware.com/articles/fog0000000319.html] Joel Spolsky's article "Back to Basics"
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