From 9d61b4ef87dbbff2ff43f6b6af3b7299e496f0e7 Mon Sep 17 00:00:00 2001
From: Martin Mares <mj@ucw.cz>
Date: Fri, 8 Jun 2018 23:39:45 +0200
Subject: [PATCH] Versioning, compatibility level and penalties around lists

---
 test-ref.tex |  1 +
 test.tex     |  1 +
 ucw-algo.tex |  3 +--
 ucwmac.tex   | 27 +++++++++++++++++++++++++++
 4 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/test-ref.tex b/test-ref.tex
index 95915ab..4c05c66 100644
--- a/test-ref.tex
+++ b/test-ref.tex
@@ -1,4 +1,5 @@
 \input ucwmac.tex
+\ucwcompat\ucwmaxcompat
 \ucwmodule{ofs}
 \ucwmodule{ref}
 \ucwmodule{pdfmeta}
diff --git a/test.tex b/test.tex
index 42ffd77..26ecbd8 100644
--- a/test.tex
+++ b/test.tex
@@ -1,4 +1,5 @@
 \input ucwmac.tex
+\ucwcompat\ucwmaxcompat
 \input ucw-ofs.tex
 \input ucw-verb.tex
 \input ucw-algo.tex
diff --git a/ucw-algo.tex b/ucw-algo.tex
index e356ef4..420e31f 100644
--- a/ucw-algo.tex
+++ b/ucw-algo.tex
@@ -53,7 +53,6 @@
 \def\algostep#1{\hbox to \algolabelwidth{\hss #1~~}}
 
 \def\algoinit{
-	\goodbreak
 	\begingroup
 	\algospacing
 	\inititemize
@@ -68,7 +67,7 @@
 \def\proc#1{\typedalgo{\localestr{alg:proc}}{#1}}
 \def\headlessalgo{\algoinit}
 
-\def\endalgo{\goodbreak\endlist\endgroup}
+\def\endalgo{\endlist\endgroup}
 \let\endproc=\endalgo
 
 \def\algoitem{%
diff --git a/ucwmac.tex b/ucwmac.tex
index 87c0b46..6c7a5de 100644
--- a/ucwmac.tex
+++ b/ucwmac.tex
@@ -231,12 +231,17 @@
 \newbox\itembox
 \newcount\itemcount
 
+% Penalties (changed at compatibility level 1)
+\newcount\preitemizepenalty	\preitemizepenalty=0
+\newcount\postitemizepenalty	\postitemizepenalty=0
+
 \def\preitemize{
 	\ifitems
 		\vskip\prenesteditemizeskip
 		\advance\leftskip by \nesteditemindent
 		\advance\rightskip by \nesteditemnarrow
 	\else
+		\ifnum\preitemizepenalty=0\else\penalty\preitemizepenalty\fi
 		\vskip\preitemizeskip
 		\advance\leftskip by \itemindent
 		\advance\rightskip by \itemnarrow
@@ -248,6 +253,7 @@
 	\ifitems
 		\vskip\postnesteditemizeskip
 	\else
+		\ifnum\postitemizepenalty=0\else\penalty\postitemizepenalty\fi
 		\vskip\postitemizeskip
 	\fi
 }
@@ -357,6 +363,27 @@
 	\expandafter\let\csname ucwmod:#1\endcsname=\relax
 }
 
+% Compatibility levels
+% We try to be backwards compatible as much as we can, so all changes in behavior
+% (except for addition of new control sequences) are versioned. By default, ucwmac
+% starts in compatibility level 0, which should produce the same results as historic
+% versions of ucwmac. Use \ucwcompat{level} to upgrade to a given level, or if you
+% do not care about compatibility, \ucwcompat\ucwmaxcompat.
+
+\chardef\ucwcurrentcompat=0	% Currently active compatibility level
+\chardef\ucwmaxcompat=1		% Maximum supported compatibility level
+
+\def\ucwcompat#1{
+	\ucwcurrentcompat=#1
+	\ifcase #1
+		% Level 0 (default): old ucwmac
+	\or	% Level 1
+		\preitemizepenalty=-500
+		\postitemizepenalty=-500
+	\else\errmessage{Unsupported compatibility level #1 requested.}
+	\fi
+}
+
 %%% Epilog %%%
 
 % Let's hide all internal macros
-- 
2.39.2