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.h

@@ -347,6 +347,43 @@ void edsf_merge(int *dsf, int v1, int v2, int inverse);
 void dsf_merge(int *dsf, int v1, int v2);
 void dsf_init(int *dsf, int len);
 
+/*
+ * tdq.c
+ */
+
+/*
+ * Data structure implementing a 'to-do queue', a simple
+ * de-duplicating to-do list mechanism.
+ *
+ * Specification: a tdq is a queue which can hold integers from 0 to
+ * n-1, where n was some constant specified at tdq creation time. No
+ * integer may appear in the queue's current contents more than once;
+ * an attempt to add an already-present integer again will do nothing,
+ * so that that integer is removed from the queue at the position
+ * where it was _first_ inserted. The add and remove operations take
+ * constant time.
+ *
+ * The idea is that you might use this in applications like solvers:
+ * keep a tdq listing the indices of grid squares that you currently
+ * need to process in some way. Whenever you modify a square in a way
+ * that will require you to re-scan its neighbours, add them to the
+ * list with tdq_add; meanwhile you're constantly taking elements off
+ * the list when you need another square to process. In solvers where
+ * deductions are mostly localised, this should prevent repeated
+ * O(N^2) loops over the whole grid looking for something to do. (But
+ * if only _most_ of the deductions are localised, then you should
+ * respond to an empty to-do list by re-adding everything using
+ * tdq_fill, so _then_ you rescan the whole grid looking for newly
+ * enabled non-local deductions. Only if you've done that and emptied
+ * the list again finding nothing new to do are you actually done.)
+ */
+typedef struct tdq tdq;
+tdq *tdq_new(int n);
+void tdq_free(tdq *tdq);
+void tdq_add(tdq *tdq, int k);
+int tdq_remove(tdq *tdq);        /* returns -1 if nothing available */
+void tdq_fill(tdq *tdq);         /* add everything to the tdq at once */
+
 /*
  * laydomino.c
  */