Core War - Strategy

Strategy

Warriors are commonly divided into a number of broad categories, although actual warriors may often combine the behavior of two or more of these. Three of the common strategies (replicator, scanner and bomber) are also known as paper, scissors and stone, since their performance against each other approximates that of their namesakes in the well known playground game.

• A bomber (or rock) blindly copies a "bomb" at regular intervals in the core, hoping to hit the enemy. The bomb is often a DAT instruction, although other instructions, or even multi-instruction bombs, may be used. A bomber can be small and fast, and they gain an extra edge over scanning opponents since the bombs also serve as convenient distractions. Bombers are often combined with imp spirals (see below) to gain extra resiliency against replicators. The second published warrior in Core War history, Dwarf by A. K. Dewdney, was a bomber.
• A replicator (or paper) makes repeated copies of itself and executes them in parallel, eventually filling the entire core with copies of its code. Replicators are hard to kill, but often have difficulty killing their opponents. Replicators therefore tend to score a lot of ties, particularly against other replicators. The earliest example of a replicator is Mice by Chip Wendell.
  • A silk is a special type of very rapid replicator, named after Silk Warrior by Juha Pohjalainen. Most modern replicators are of this type. Silk replicators use parallel execution to copy their entire code with one instruction, and begin execution of the copy before it is finished.
• A scanner (or scissor) is designed to beat replicators, usually by bombing memory with SPL 0 instructions. This causes the enemy to create a huge number of processes which do nothing but create more processes. This slows down useful processes. When the enemy becomes so slow that it is unable to do anything useful, the memory is bombed with DAT instructions.
  • A scanner does not attack blindly, but tries to locate its enemy before launching a targeted attack. This makes it more effective against hard-to-kill opponents like replicators, but also leaves it vulnerable to decoys. Scanners are also generally more complex, and therefore larger and more fragile, than other types of warriors. He Scans Alone by P. Kline is an example of a strong scanner.
  • A vampire or pit-trapper tries to make its opponent's processes jump into a piece of its own code called a "pit". Vampires can be based on either bombers or scanners. A major weakness of vampires is that they can be easily attacked indirectly, since they must by necessity scatter pointers to their code all over the core. Their attacks are also slow, as it takes an extra round for the processes to reach the pit. myVamp5.4 by Paulsson is a good example of a vampire.
  • A one-shot is a very simple scanner that only scans the core until it finds the first target, and then permanently switches to an attack strategy, usually a core clear. Myrmidon by Roy van Rijn is a simple, yet effective oneshot.
• An imp (named after the first ever published warrior, Imp by A. K. Dewdney) is a trivial one-instruction mobile warrior that continually copies its sole instruction just ahead of its instruction pointer. Imps are hard to kill but next to useless for offense. Their use lies in the fact that they can easily be spawned in large numbers, and may survive even if the rest of the warrior is killed.
  • An imp ring or imp spiral consists of imps spaced at equal intervals around the core and executing alternately. The imps at each arm of the ring/spiral copy their instruction to the next arm, where it is immediately executed again. Rings and spirals are even harder to kill than simple imps, and they even have a (small) chance of killing warriors not protected against them. The number of arms in an imp ring or spiral must be relatively prime with the size of the core.
• A quickscanner attempts to catch its opponent early by using a very fast unrolled scanning loop. Quickscanning is an early-game strategy, and always requires some other strategy as a backup. Adding a quickscanning component to a warrior can improve its score against long warriors—such as other quickscanners. However, the unrolled scan can only target a limited number of locations, and is unlikely to catch a small opponent.
• A core clear is a simple warrior that sequentially overwrites every instruction in the core, sometimes even including itself. Core clears are not very common as stand-alone warriors, but they are often used as an end-game strategy by bombers and scanners.
• A bomb-dodger is a specialized strategy against bombers. It scans the core until it locates a bomb thrown by the bomber, and then copies its code there, assuming that the bomber will not attack the same spot again any time soon.