From d1ca9346d8e86b873c3cf5511105b0252e6c54ad Mon Sep 17 00:00:00 2001
From: Paul Morris <paulwmorris@gmail.com>
Date: Sat, 23 Apr 2016 16:04:56 -0400
Subject: [PATCH] Issue 4828/2: Add StaffSymbol.ledger-positions-function

Lets the user provide a (quoted) function that is used
to determine the list of ledger positions for a given
note head position.
---
 lily/staff-symbol.cc           | 16 ++++++++++++++--
 scm/define-grob-properties.scm |  3 +++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/lily/staff-symbol.cc b/lily/staff-symbol.cc
index 527a5c7baf..0697085e11 100644
--- a/lily/staff-symbol.cc
+++ b/lily/staff-symbol.cc
@@ -137,7 +137,7 @@ Staff_symbol::line_positions (Grob *me)
 vector<Real>
 Staff_symbol::ledger_positions (Grob *me, int pos, Item const *head)
 {
-  // allow the option to override ledger positions via note head grob
+  // allow override of ledger positions via note head grob...
   if (head)
     {
       SCM posns = head->get_property ("ledger-positions");
@@ -145,6 +145,17 @@ Staff_symbol::ledger_positions (Grob *me, int pos, Item const *head)
         return ly_scm2floatvector (posns);
     }
 
+  // ...or via custom ledger positions function
+  SCM lp_function = me->get_property ("ledger-positions-function");
+  if (scm_is_pair (lp_function))
+    {
+      SCM func = scm_eval (lp_function, scm_interaction_environment ());
+      if (ly_is_procedure (func))
+        return ly_scm2floatvector (scm_call_2 (func,
+                                               me->self_scm (),
+                                               scm_from_int (pos)));
+    }
+
   SCM ledger_positions = me->get_property ("ledger-positions");
   Real ledger_extra = robust_scm2double (me->get_property ("ledger-extra"), 0);
   vector<Real> line_positions = Staff_symbol::line_positions (me);
@@ -174,7 +185,7 @@ Staff_symbol::ledger_positions (Grob *me, int pos, Item const *head)
   Direction dir = (Direction)sign (pos - nearest_line);
 
   if (scm_is_pair (ledger_positions))
-    // custom ledger line positions
+    // custom ledger positions via StaffSymbol.ledger-positions
     {
       Real min_pos = HUGE_VAL;
       Real max_pos = -HUGE_VAL;
@@ -410,6 +421,7 @@ ADD_INTERFACE (Staff_symbol,
                "ledger-extra "
                "ledger-line-thickness "
                "ledger-positions "
+               "ledger-positions-function "
                "line-count "
                "line-positions "
                "staff-space "
diff --git a/scm/define-grob-properties.scm b/scm/define-grob-properties.scm
index dcf9705998..60b9d5b000 100644
--- a/scm/define-grob-properties.scm
+++ b/scm/define-grob-properties.scm
@@ -579,6 +579,9 @@ are added.")
 When set on a @code{StaffSymbol} grob it defines a repeating
 pattern of ledger lines and any parenthesized groups will always be
 shown together.")
+     (ledger-positions-function ,scheme? "A quoted Scheme procedure that
+takes a @code{StaffSymbol} grob and the vertical position of a note head
+as arguments and returns a list of ledger line positions.")
      (left-bound-info ,list? "An alist of properties for determining
 attachments of spanners to edges.")
      (left-padding ,ly:dimension? "The amount of space that is put
-- 
2.39.2