/*
 * program to place dips on wire-wrap board.
 * based on PLAC by D.G.Schweikert.
 *
 * There are two parts to this:
 * placA does an initial placement and placB improves it.
 *
 * A chip has a given number of pins and occupies a rectangular
 * shaped area of the circuit board.   Each chip has a package type.
 * A board is an array of sockets whose positions on the board are fixed.
 * Each socket serves a rectangular area of the circuit board.
 * and for each socket there is a list of the
 * package types that can be placed in that socket.
 *
 * Placement rules allow for any chip to be placed in any socket
 * providing that the package type is permitted for that socket.
 * if the area covered by the chip overlaps beyond the edge of the
 * area served by the socket then the neighbouring sockets are blocked.
 * only one chip is permitted in each socket.
 *
 * the initial placement performed by placA is constructive in that it
 * only places un-placed cells and does not move any cell that is already
 * placed.   Plac2 exchanges the positions of chips already placed.
 * Only Plac1 is able to move a chip into a position that blocks
 * sockets not already blocked by a chip in that positon.
 *
 * Plac1 choses an un-placed chip and places it in an unoccupied socket.
 * the chip chosen is the one 'most connected' to already placed chips
 * (see TM75-2255-4) and the socket is the one that minimises total wire
 * length.
 *
 * Plac2 choses pairs of chips whose placements can be exchanged and
 * which thereby reduce total wire length.
 *
 * It is assumed that a given chip appears at most once in any net.
 *
 */