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Add functions provided by the midend to tell a front end whether GUI
buttons for undo and redo should currently be greyed out.

[originally from svn r9139]

--- a/devel.but

+++ b/devel.but

@@ -3096,6 +3096,24 @@

 \cw{activate_timer()} to be called from within a call to this

 function.

+\H{midend-can-undo} \cw{midend_can_undo()}

+

+\c int midend_can_undo(midend *me);

+

+Returns \cw{TRUE} if the midend is currently in a state where the undo

+operation is meaningful (i.e. at least one position exists on the undo

+chain before the present one). Front ends may wish to use this to

+visually activate and deactivate an undo button.

+

+\H{midend-can-redo} \cw{midend_can_redo()}

+

+\c int midend_can_redo(midend *me);

+

+Returns \cw{TRUE} if the midend is currently in a state where the redo

+operation is meaningful (i.e. at least one position exists on the redo

+chain after the present one). Front ends may wish to use this to

+visually activate and deactivate a redo button.

+

 \H{midend-serialise} \cw{midend_serialise()}

 \c void midend_serialise(midend *me,

--- a/midend.c

+++ b/midend.c

@@ -428,6 +428,16 @@

     me->pressed_mouse_button = 0;

 }

+int midend_can_undo(midend *me)

+{

+    return (me->statepos > 1);

+}

+

+int midend_can_redo(midend *me)

+{

+    return (me->statepos < me->nstates);

+}

+

 static int midend_undo(midend *me)

 {

     if (me->statepos > 1) {

--- a/puzzles.h

+++ b/puzzles.h

@@ -253,6 +253,8 @@

 int midend_can_format_as_text_now(midend *me);

 char *midend_text_format(midend *me);

 char *midend_solve(midend *me);

+int midend_can_undo(midend *me);

+int midend_can_redo(midend *me);

 void midend_supersede_game_desc(midend *me, char *desc, char *privdesc);

 char *midend_rewrite_statusbar(midend *me, char *text);

 void midend_serialise(midend *me,

-- 

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