From 73364778d21a8d1959eeb27f2bf127c4d9e8aba2 Mon Sep 17 00:00:00 2001
From: David Kastrup <dak@gnu.org>
Date: Wed, 1 Oct 2014 16:11:10 +0200
Subject: [PATCH] Issue 630/1: Implement Grob_property_info::matched_pop

This can be passed a token returned from Grob_property_info::push and
will then only revert the matching push or nothing at all.  This is used
to limit the fallout from internal property reversions that might not
match the original override because of an intervening property push/pop.
---
 lily/context-property.cc        | 44 ++++++++++++++++++++++++++++-----
 lily/include/grob-properties.hh |  3 ++-
 2 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/lily/context-property.cc b/lily/context-property.cc
index 45b5d496c1..70370e3800 100644
--- a/lily/context-property.cc
+++ b/lily/context-property.cc
@@ -190,18 +190,21 @@ Grob_property_info::create ()
   alist defined in a parent context. BASED-ON should always be a tail
   of ALIST.
 
-  Push or pop (depending on value of VAL) a single entry from a
+  Push a single entry from a
   translator property list by name of PROP.  GROB_PROPERTY_PATH
   indicates nested alists, eg. '(beamed-stem-lengths details)
+
+  Return value can be passed to matched_pop and will only cancel the
+  same override then.
 */
-void
+SCM
 Grob_property_info::push (SCM grob_property_path, SCM new_value)
 {
   /*
     Don't mess with MIDI.
   */
   if (!create ())
-    return;
+    return SCM_EOL;
 
   SCM symbol = scm_car (grob_property_path);
   SCM rest = scm_cdr (grob_property_path);
@@ -209,10 +212,12 @@ Grob_property_info::push (SCM grob_property_path, SCM new_value)
     {
       // poor man's typechecking
       if (typecheck_grob (symbol, nested_create_alist (rest, new_value))) {
-        props_->alist_ = scm_acons (grob_property_path, new_value, props_->alist_);
+        SCM cell = scm_cons (grob_property_path, new_value);
+        props_->alist_ = scm_cons (cell, props_->alist_);
         props_->nested_++;
+        return cell;
       }
-      return;
+      return SCM_EOL;
     }
 
   /* it's tempting to replace the head of the list if it's the same
@@ -221,7 +226,34 @@ Grob_property_info::push (SCM grob_property_path, SCM new_value)
   */
 
   if (typecheck_grob (symbol, new_value))
-    props_->alist_ = scm_acons (symbol, new_value, props_->alist_);
+    {
+      SCM cell = scm_cons (symbol, new_value);
+      props_->alist_ = scm_cons (cell, props_->alist_);
+      return cell;
+    }
+  return SCM_EOL;
+}
+
+void
+Grob_property_info::matched_pop (SCM cell)
+{
+  if (!scm_is_pair (cell))
+    return;
+  if (!check ())
+    return;
+  SCM current_alist = props_->alist_;
+  SCM daddy = props_->based_on_;
+  for (SCM p = current_alist; !scm_is_eq (p, daddy); p = scm_cdr (p))
+    {
+      if (scm_is_eq (scm_car (p), cell))
+        {
+          if (scm_is_pair (scm_car (cell)))
+            props_->nested_--;
+          props_->alist_ = partial_list_copy (current_alist, p, scm_cdr (p));
+          return;
+        }
+    }
+  return;
 }
 
 /*
diff --git a/lily/include/grob-properties.hh b/lily/include/grob-properties.hh
index 37569442c6..157613debb 100644
--- a/lily/include/grob-properties.hh
+++ b/lily/include/grob-properties.hh
@@ -47,7 +47,8 @@ public:
   bool check ();
   bool create ();
   SCM updated ();
-  void push (SCM path, SCM value);
+  SCM push (SCM path, SCM value);
+  void matched_pop (SCM);
   void pop (SCM path);
   void pushpop (SCM path, SCM value)
   {
-- 
2.39.5