Mailbox

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Mailbox, (Offset board representation ) a square-centric board representation where the encoding of every square resides in a separately addressable memory element, usually an element of an array for random access. The square number, or its file and rank, acts like an address to a post box, which might be empty or may contain one chess piece. As pointed out by Harm Geert Muller, not only the embedded 10x12 board, but various implementations are all mailbox, independently from elements in the array for padding that can act as a sentinel value.

=Implementations=
 * 8x8 Board
 * 10x12 Board
 * Vector Attacks
 * 0x88

=Pros & cons=

Pros

 * Easy, straightforward to understand and implement
 * Suitable with the same efficiency for any size of boards, from one can be fitted on 64-bit integers to much larger. Thus it is also easier to support multi-chess variants which boards’ sizes are quite different.
 * Suitable for almost all chess tasks and software where computing speed is not high requirements such as chess tools, GUI since it is much easier to develop and maintain.

Newcomers are suggested to implement their first chess engines using Mailbox thus they can get some basic knowledge and skills before starting more complicated chess projects.

Cons
In the view of developing chess engines:
 * Programming must use many loop and branch commands such as for, while, if (compare with Bitboards)
 * Hard to store patterns and match them
 * May have some inefficient high-frequency-use functions such as finding King locations, in-check
 * Not efficient to generate moves in stages since generating all moves, captures only, non-captures may take quite similar periods

=Speed= For basic tasks such as generating, making/unmaking moves, a "pure" mailbox maybe slower than Bitboards. However, when combining it with other methods such as Piece-Lists and optimize the code it could be as fast as Bitboards

=See also=
 * Array
 * Array of Nibbles
 * Mailbox in Minimax

=Publications=
 * Claude Shannon (1949). Programming a Computer for Playing Chess. pdf from The Computer History Museum
 * Dietrich Prinz (1952). Robot Chess. Research, Vol. 6, reprinted 1988 in Computer Chess Compendium
 * Alex Bell (1972). Games Playing with Computers. Allen & Unwin, ISBN-13: 978-0080212227
 * Dan Spracklen, Kathe Spracklen (1978). First Steps in Computer Chess Programming. BYTE, Vol. 3, No. 10, pdf from The Computer History Museum
 * David Levy (1979). Computer and Chess - How the monster thinks. Elektor, January 1979
 * Robert Hyatt (1981). Checkmate: The Cray-1 Plays Chess. Part 1. Cray Channels, Vol. 3, No. 1. pdf from The Computer History Museum

=Forum Posts=
 * Bitboards vs Mailbox vs 0X88 by Bruce Cleaver, CCC, June 02, 1998 » 0x88, Bitboards
 * 0x88 vs mailbox by Stuart Cracraft, CCC, September 16, 2004 » 0x88
 * Mailbox info by Anonymous, Winboard Forum, May 08, 2005
 * Board representation : 0x88 or 10x12 ? by Philippe, Winboard Forum, March 02, 2006 » 0x88
 * move generation with one dimensional "12 x 10" array by Andrew Short, CCC, August 22, 2008
 * mailbox & CPW by Harm Geert Muller, CCC, May 31, 2013
 * The mailbox trials by Harm Geert Muller, CCC, March 04, 2021

=External Links=
 * Board representation (chess) - Array based from Wikipedia
 * Chess board representations by Robert Hyatt
 * hgm-mailbox-trials/mailbox7b.c at main · maksimKorzh/hgm-mailbox-trials · GitHub hosted by Maksim Korzh
 * Mailbox (disambiguation) from Wikipedia

=References=

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