Scanner for LP hill
Created by Christian Schmidt, 2002,2003,2004
by John Metcalf
A battle program which is continually creating new copies of itself is described as a replicator. No program can hope to attack copies of a replicating program sufficiently fast enough to keep up with the new copies which are constantly being spawned.
The only practical solution is to use a two stage attack. First, slow the rate at which the replicator is producing new copies. Next, destroy all copies of the opponent program while they are incapacitated.
The manner in which a replicator can be slowed is by making it split off useless processes. We achive this by overwriting it with, or otherwise forcing it to execute, SPL instructions.
Modern replicators spend approaching 50% of their time splitting off new processes, so we will need several consecutive SPL instructions to have any chance of being effective. The number of cycles to place these is considerable - compared, for example, to the time taken to drop single bombs.
Before spending a substantial number of cycles, it would make sense to check the area we are about to wipe contains an opponent, rather than empty core. This is precisely what a scanner is designed to do:
1) look for an area of core which potentially contains an opponent,
2) attack the selected area with a stun attack to slow the opponent,
3) repeat from step 1, until an implementation determined point,
4) the end-game - change the means of attack from stun to destroy.
Let us take a closer look at each of these stages:
Scan loops come in a variety of speeds. The ratio of scanner size to scan speed can make a big difference to the effectiveness of a warrior. A small/fast scanner will find and attack a bigger/slower opponent more frequently than that opponent finds and attacks it. It is therefore desirable to have the lowest possible value of size/speed.
The example code in this section is labelled in a manner consistent with a great deal of published code. Conventions used are:
* scan - the top of the scan loop, and the entry point for our examples. Would ptr be a better entry point to the scan loops below? Why? * ptr - the instruction holding either one or two pointers to the location(s) to be scanned. * inc - an instruction holding the constant 'step' in both fields, so a single ADD instruction can update two pointers. * step - the distance between two scanned locations. Some important literature on 'good' scan steps is listed below in Suggested Reading. * hop - certain scanners look at two locations which are close to each other. When something is found, they use an attack which covers both locations. Hop is the distance between such a pair of locations. * start - the first location to be scanned. Often chosen to maximise the length of time before the scanner detects itself.Some examples of the most common forms of scan loop follow:
scanA ADD #step, ptrA ptrA JMZ.F scanA, start ; scan 1 location every 2 cycles
Having two scan pointers is one disadvantage of this loop. When the scan triggers it is neccessary to either determine which pointer holds the location to be attacked, or else attack both.
An effective variation is to change the JMP instruction to DJN, to colour core against other scanners. Use something similar to DJN.F scanB,<-500 to create a sequential decoy, or alternatively decrement through your scan pointer with DJN.F scanB,@ptrB.
scanB ADD incB, ptrB ptrB SNE start+hop, start ; scan 2 locations every 3 cycles JMP scanB ; ... incB DAT step, step
It appears at first glace, if the SNE detects something through it's a-field, the scan loop will not drop through into the attack phase. Stepping through the code however, reveals how some simple arithmetic 'copies' the value from the a-field of ptrC to the b-field. The scan loop can then drop through with the b-field of ptrC pointing to whatever has been found.
scanC ADD incC, ptrC ptrC SNE.X start, start ; scan 3 locations every 4 cycles ADD.X incC, ptrC JMZ.F scanC, @ptrC ; ... incC DAT 2*step, step
scanD ADD incD, ptrD ptrD SNE start+hop, start ; scan 4 locations every 5 cycles ADD incD, ptrD SNE *ptrD, @ptrD JMP scanD ; ... incD DAT step, step
* Permanent - a.k.a. one-shots. Switch to a core-clear with one or more SPL passes though core, beginning with the location found by the scan. After this, the clear wipes with DAT. Simple and effective. See Scanny Boy for an example. * Fixed Length - switch to a fixed length attack. Afterwards, switch back to scanning. The two most common types of fixed length attacks are SPL carpets and special bombs (stun and incendiary for example). Take a look at Rave and The Marsupial Lion, which use these techniques. * Variable Length - switch to a linear SPL wipe beginning at the location where the scan detected something, continuing until it reaches empty core. After this, switch back to scanning. This technique proves the most effective against paper/imps. Check He Scans Alone for a good example. * Ongoing - a linear SPL wipe runs parallel to the scan. When something is found by the scan, the pointer for the SPL wipe is adjusted to point to the scanned location. However, the most common implementation of this technique introduces an extra instruction to the scan loop, reducing the speed. Blur 2 is a clear example of an ongoing attack.The scanner needs somehow to ensure it doesn't attack itself. One way to achieve this is to check the scan pointer isn't within a range which points to our scanner before attacking. Other alternatives are to use a scan pattern which avoids detecting the scanner, or using some means of hiding the scanner from itself (reflections).
* When our scanner detects itself either once, or a number of times, enter the end-game. For example Mini HSA switches after 16 self-scans. * After a number of iterations of the scan loop, drop through to the end game. In Blur 2, there is a counter which is decremented by the scan loop. When this reaches zero, control will pass into the end-game. * After a number of iterations of the attack, drop through to the end game. In Dandelion III, there is a counter which is decremented if the scanner attacks something. When this reaches zero, control will pass into the end-game. * Switch when a pre-determined location is altered by the attack. See myZizzor for an example, which has a small decoy the scanner detects and attacks, thus altering the checked location.
* Clear - switch to a core-clear, which wipes core with DAT. Optionally, the clear may wipe with SPL prior to the DAT wipe. The most often seen type clear is the d-clear, with spiral clears also being a popular choice. Scanners containing a spiral clear include Eggbeater, Memories, SETI and Win! * DAT Switch - the scan continues, bombing with DAT instead of it's previous attack. Many of the HSA-style warriors work in this way, including Mini HSA. Another example is the scanner from Enough is Enough!
Below are some references to a few instructive examples of scanners. These illustrate the methods discussed above. Generally, those which function in a similar manner are grouped together, with the most readable example in the group listed first:
Blur 2 by Anton Marsden (CW36) - implements an ongoing attack from the scan loop, followed up by a d-clear end-game. After reading Blur 2, continue with Hazy Lazy, Silver Talon, Stalker, Eggbeater (CW46) and the scanner from Enough is Enough! (CW79).
HAL 9000 by Justin Kao (CW50) - an alternative means of implementing an ongoing attack, or scan directed clear.
Mini HSA from Electric Head by Anton Marsden (CW59) - provides an excellent introduction to the workings of HSA. Scanning continues into the end-game, with the SPL wipe becoming a DAT wipe. Also, check He Scans Alone, Mischief (CW80) and Razor.
Beholder's Eye v1.7 by W. Mintardjo - an easy to read spl/jmp bomber. Continue reading with Iron Gate (CW19), Harmony II and Memories (CW19).
myZizzor by Magnus Paulsson - performs a variable length attack, followed by a SPL / DAT clear.
Origin of Storms by John Metcalf (CW86) - a variable length attack realized in the scan loop, followed by the ever-present d-clear.
Rave 4.1 by Stefan Strack - a good example of a .66c scan with a fixed length SPL carpet. Also, you should take a look at SETI. Next study Win! and Recon 2, two warriors based on the same concept, employing an abundance of effective techniques.
Scanny Boy by David van Dam (CW29) - drops straight through from the scan loop into the end-game's SPL / DAT clear and provides a good example of a one-shot. See also Tiny BiShot 2.0, The Bloodhound, G2 and Phantasm 50.