New puzzle! Or rather, new-ish, because this one has been lying around

in the 'unfinished' directory for a while, and has now been finished
up thanks to James Harvey putting in some effort and galvanising me to
put in the rest. This is 'Pearl', an implementation of Nikoli's 'Masyu'.

The code in Loopy that generates a random loop along grid edges to use
as the puzzle solution has been abstracted out into loopgen.[ch] so
that Pearl can use it for its puzzle solutions too. I've also
introduced a new utility module called 'tdq' (for 'to-do queue').

[originally from svn r9379]

puzzles.but

@@ -2908,6 +2908,71 @@ These parameters are available from the \q{Custom...} option on the
 
 \dd Size of grid in squares.
 
+\C{pearl} \i{Pearl}
+
+\cfg{winhelp-topic}{games.pearl}
+
+You have a grid of squares. Your job is to draw lines between the
+centres of horizontally or vertically adjacent squares, so that the
+lines form a single closed loop. In the resulting grid, some of the
+squares that the loop passes through will contain corners, and some
+will be straight horizontal or vertical lines. (And some squares can
+be completely empty \dash the loop doesn't have to pass through every
+square.)
+
+Some of the squares contain black and white circles, which are clues
+that the loop must satisfy.
+
+A black circle in a square indicates that that square is a corner, but
+neither of the squares adjacent to it in the loop is also a corner.
+
+A while circle indicates that the square is a straight edge, but \e{at
+least one} of the squares adjacent to it in the loop is a corner.
+
+(In both cases, the clue only constrains the two squares adjacent
+\e{in the loop}, that is, the squares that the loop passes into after
+leaving the clue square. The squares that are only adjacent \e{in the
+grid} are not constrained.)
+
+Credit for this puzzle goes to \i{Nikoli}, who call it \q{Masyu}.
+\k{nikoli-pearl}.
+
+Thanks to James Harvey for assistance with the implementation.
+
+\B{nikoli-pearl}
+\W{http://www.nikoli.co.jp/en/puzzles/masyu/}\cw{http://www.nikoli.co.jp/en/puzzles/masyu/}
+
+\H{pearl-controls} \I{controls, for Pearl}Pearl controls
+
+Click with the left button on a grid edge to draw a segment of the
+loop through that edge, or to remove a segment once it is drawn.
+
+Drag with the left button through a series of squares to draw more
+than one segment of the loop in one go. Alternatively, drag over an
+existing part of the loop to undraw it, or to undraw part of it and
+then go in a different direction.
+
+Click with the right button on a grid edge to mark it with a cross,
+indicating that you are sure the loop does not go through that edge.
+(For instance, if you have decided which of the squares adjacent to a
+white clue has to be a corner, but don't yet know which way the corner
+turns, you might mark the one way it \e{can't} go with a cross.)
+
+(All the actions described in \k{common-actions} are also available.)
+
+\H{pearl-parameters} \I{parameters, for Pearl}Pearl parameters
+
+These parameters are available from the \q{Custom...} option on the
+\q{Type} menu.
+
+\dt \e{Width}, \e{Height}
+
+\dd Size of grid in squares.
+
+\dt \e{Difficulty}
+
+\dd Controls the difficulty of the generated puzzle.
+
 \A{licence} \I{MIT licence}\ii{Licence}
 
 This software is \i{copyright} 2004-2010 Simon Tatham.