[letterspace] fix handling of interword space

Fix https://github.com/lualatex/luaotfload/issues/297

Interword glue hasn’t been considered yet. This again adapts the
relevant logic from Context to our letterspacing method. The code is
deliberately simplistic and will most likely not address all
constellations of a glue preceding a character.

1 files changed, 71 insertions, 23 deletions

diff --git a/src/luaotfload-letterspace.lua b/src/luaotfload-letterspace.lua
index ab81881..5fa25f9 100644
--- a/src/luaotfload-letterspace.lua
+++ b/src/luaotfload-letterspace.lua
@@ -6,6 +6,9 @@ if not modules then modules = { } end modules ['letterspace'] = {
     license   = "see context related readme files"
 }
 
+--- This code diverged quite a bit from its origin in Context. Please
+--- do *not* report bugs on the Context list.
+
 local log                = luaotfload.log
 local logreport          = log.report
 
@@ -39,7 +42,6 @@ local new_node           = nodedirect.new
 
 local nodepool           = nodedirect.pool
 local new_kern           = nodepool.kern
-local new_glue           = nodepool.glue
 
 local nodecodes          = nodes.nodecodes
 
@@ -47,6 +49,7 @@ local glyph_code         = nodecodes.glyph
 local kern_code          = nodecodes.kern
 local disc_code          = nodecodes.disc
 local math_code          = nodecodes.math
+local glue_code          = nodecodes.glue
 
 local fonthashes         = fonts.hashes
 local chardata           = fonthashes.characters
@@ -81,11 +84,17 @@ local kerncodes = bothways { [0] = "fontkern"
                            , [1] = "userkern"
                            , [2] = "accentkern"
                            }
+local skipcodes = bothways {  [0] = "userskip"
+                           , [13] = "spaceskip"
+                           , [14] = "xspaceskip"
+                           }
 
-kerncodes.kerning    = kerncodes.fontkern --- idiosyncrasy
-local kerning_code   = kerncodes.kerning
-local userkern_code  = kerncodes.userkern
-
+kerncodes.kerning       = kerncodes.fontkern --- idiosyncrasy
+local kerning_code      = kerncodes.kerning
+local userkern_code     = kerncodes.userkern
+local userskip_code     = skipcodes.userskip
+local spaceskip_code    = skipcodes.spaceskip
+local xspaceskip_code   = skipcodes.xspaceskip
 
 -----------------------------------------------------------------------
 --- node-res
@@ -93,19 +102,30 @@ local userkern_code  = kerncodes.userkern
 
 local glue_spec   = new_node "glue_spec"
 
-nodepool.glue = function (width, stretch, shrink,
-                          stretch_order, shrink_order)
+local new_gluespec = function (width,
+                               stretch,       shrink,
+                               stretch_order, shrink_order)
+  local spec = copy_node(glue_spec)
+  if width         then setfield(spec, "width"        , width        )  end
+  if stretch       then setfield(spec, "stretch"      , stretch      )  end
+  if shrink        then setfield(spec, "shrink"       , shrink       )  end
+  if stretch_order then setfield(spec, "stretch_order", stretch_order)  end
+  if shrink_order  then setfield(spec, "shrink_order" , shrink_order )  end
+  return spec
+end
+
+local new_glue = function (width, stretch, shrink,
+                           stretch_order, shrink_order)
   local n = new_node "glue"
-  if not width then
+  if not width then return n end
     -- no spec
-  elseif width == false or tonumber(width) then
-    local s = copy_node(glue_spec)
-    if width         then setfield(s, "width"        , width        )  end
-    if stretch       then setfield(s, "stretch"      , stretch      )  end
-    if shrink        then setfield(s, "shrink"       , shrink       )  end
-    if stretch_order then setfield(s, "stretch_order", stretch_order)  end
-    if shrink_order  then setfield(s, "shrink_order" , shrink_order )  end
-    setfield(n, "spec", s)
+  if width == false then
+    local width = tonumber(width)
+    if width then
+      setfield(n, "spec",
+               new_gluespec(width, stretch, shrink,
+                            stretch_order, shrink_order))
+    end
   else
     -- shared
     setfield(n, "spec", copy_node(width))
@@ -198,6 +218,22 @@ local kern_injector = function (fillup, kern)
   return new_kern(kern)
 end
 
+local kernable_skip = function (n)
+  local st = getsubtype (n)
+  return st == userskip_code
+      or st == spaceskip_code
+      or st == xspaceskip_code
+end
+
+local function spec_injector (fillup, width, stretch, shrink)
+  if fillup then
+    local spec = new_gluespec(width, 2 * stretch, 2 * shrink)
+    setfield(spec, "stretch_order", 1)
+    return spec
+  end
+  return new_gluespec(width,stretch,shrink)
+end
+
 --[[doc--
 
     Caveat lector.
@@ -221,13 +257,11 @@ kerncharacters = function (head)
 
   local identifiers   = fonthashes.identifiers
   local kernfactors   = kernfactors
-
   local firstkern     = true
 
   while start do
     local id = getid(start)
     if id == glyph_code then
-
       --- 1) look up kern factor (slow, but cached rudimentarily)
       local krn
       local fontid = getfont(start)
@@ -304,6 +338,21 @@ kerncharacters = function (head)
         if not pid then
           -- nothing
 
+        elseif pid == glue_code and kernable_skip(prev) then
+          local spec = getfield(prev, "spec")
+          local wd   = getfield(spec, "width")
+          if wd > 0 then
+            --- formula taken from Context
+            ---      existing_width extended by four times the
+            ---      width times the font’s kernfactor
+            local newwd     = wd + --[[two en to a quad]] 4 * wd * krn
+            local stretched = (getfield(spec,"stretch") * newwd) / wd
+            local shrunk    = (getfield(spec,"shrink")  * newwd) / wd
+            setfield(prev, "spec",
+                     spec_injector(fillup, newwd, stretched, shrunk))
+            done = true
+          end
+
         elseif pid == kern_code then
           local prev_subtype = getsubtype(prev)
           if prev_subtype == kerning_code   --- context does this by means of an
@@ -412,11 +461,10 @@ kerncharacters = function (head)
               krn = quaddata[lastfont]*krn -- here
             end
             setfield(disc, "replace", kern_injector(false, krn))
-          end
-
-        end
-      end
-    end
+          end --[[if replace and prv and nxt]]
+        end --[[if not pid]]
+      end --[[if prev]]
+    end --[[if id == glyph_code]]
 
     ::nextnode::
     if start then