From a6feaecef45645b1112c7ce911090d48aceb8848 Mon Sep 17 00:00:00 2001
From: Ian Jackson <ian@liberator.relativity.greenend.org.uk>
Date: Sat, 6 Nov 2010 12:57:40 +0000
Subject: [PATCH] slopecalc handles non-initial @ correctly

---
 layout/slopecalc | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/layout/slopecalc b/layout/slopecalc
index a89dd5e..fd42c16 100755
--- a/layout/slopecalc
+++ b/layout/slopecalc
@@ -17,8 +17,8 @@
 #  the slope _has_ changed a default distance is supplied
 #
 # the first use of @... to specify an absolute location
-#  acts only to fix the zero point of the absolute position scale
-#  and does not actually specify the location of the current point
+#  acts only to fix the zero point of the absolute position scale;
+#  the (relative to start) location of the current point must already be known
 
 use strict qw(vars refs);
 
@@ -30,8 +30,8 @@ our @cp= ({ L => 0 });
 # $cp[]{H}    height
 # $cp[]{L}    absolute dist
 # $cp[]{S}    slope
-
 our $absoffset;
+
 our $numre= '(?:\\d{1,6}(?:\\.\\d*)?|\\.\\d+)';
 
 our $progname= $0;
@@ -147,10 +147,16 @@ sub parse_args () {
 	    arg_item(S, $1 * 0.01);
 	} elsif (m/^\@([-+]?$numre)$/o) {
 	    if (defined $absoffset) {
-		arg_item(L, $1 + $absoffset);
-	    } else {
-		$absoffset= -$1;
-	    }
+               arg_item(L, $1 + $absoffset);
+           } else {
+	       if (!@cp) {
+		   $absoffset= -$1;
+	       } elsif (defined $cp[$#cp]{L}) {
+		   $absoffset= $cp[$#cp]{L} - $1;
+	       } else {
+		   arg_error("point \#$#cp location still unknown at \`$_'");
+	       }
+           }
 	} elsif (m/^\+($numre)$/o) {
 	    complete_current_point("new point started because of +...");
 	    arg_item(L, $cp[$#cp]{L} + $1);
-- 
2.30.2