fix(core): Move the zenkaku width (zw) unit into core

Where it belongs.

This touches on sile-typesetter#1245.

core/base-shaper.lua

@@ -59,14 +59,21 @@ SILE.shapers.base = pl.class({
       return shapespace(width and width.length or items[1].width)
     end,
 
-    measureChar = function (self, char)
+    measureChar = function (self, char, fallible)
       local options = SILE.font.loadDefaults({})
       options.tracking = SILE.settings:get("shaper.tracking")
       local items = self:shapeToken(char, options)
+      local error_func
+      if fallible == nil or fallible == false then -- default
+        error_func = SU.error
+      else
+        error_func = SU.warn
+      end
       if #items > 0 then
         return { height = items[1].height, width = items[1].width }
       else
-        SU.error("Unable to measure character", char)
+        error_func("Unable to measure character", char)
+        return nil
       end
     end,
 

core/measurement.lua

@@ -1,3 +1,4 @@
+
 local function _tonumber (amount)
   return SU.cast("number", amount)
 end

core/settings.lua

@@ -68,6 +68,13 @@ function settings:_init()
     help = "Skip to be added to left side of line"
   })
 
+  self:declare({
+    parameter = "document.zenkakuchar",
+    default = "あ",
+    type = "string",
+    help = "The character measured to determine the length of a zenkaku width (全角幅)"
+  })
+
   SILE.registerCommand("set", function(options, content)
     local parameter = SU.required(options, "parameter", "\\set command")
     local makedefault = SU.boolean(options.makedefault, false)

core/units.lua

@@ -201,4 +201,24 @@ units["en"] = {
   definition = "0.5em"
 }
 
+-- jlreq measures distances in units of 1em, but also assumes that an em is the
+-- width of a full-width character. In SILE terms it isn't: measuring an "m" in
+-- a 10pt Japanese font gets you 5 points. So we measure a full-width character
+-- and use that as a unit. We call it zw following ptex (zenkaku width)
+units["zw"] = {
+  relative = true,
+  definition = function (v)
+    local zenkaku = SILE.settings:get("document.zenkakuchar")
+    local measure_zenkaku = SILE.shaper:measureChar(zenkaku, true)
+    if measure_zenkaku ~= nil then
+      measure_zenkaku = measure_zenkaku.width
+    else
+      measure_zenkaku = 1
+      SU.warn("Zenkaku width (全角幅) unit zw is falling back to 1em == 1zw as we cannot measure " .. zenkaku ..
+              ". Either change this char to one suitable for your language, or load a font that has it.")
+    end
+    return v * measure_zenkaku
+  end
+}
+
 return units

languages/ja.lua

@@ -183,16 +183,5 @@ return {
       SILE.call("medskip")
     end)
 
-    -- jlreq measures distances in units of 1em, but also assumes that an em is the
-    -- width of a full-width character. In SILE terms it isn't: measuring an "m" in
-    -- a 10pt Japanese font gets you 5 points. So we measure a full-width character
-    -- and use that as a unit. We call it zw following ptex (zenkaku width)
-    SILE.units["zw"] = {
-      relative = true,
-      definition = function (v)
-        return v * SILE.shaper:measureChar("あ").width
-      end
-    }
-
   end
 }

packages/ruby/init.lua

@@ -119,7 +119,6 @@ The \autodoc:package{ruby} package provides the
 \autodoc:command[check=false]{\ruby[reading=<ruby text>]{<base text>}} command
 which sets a piece of ruby above or beside the base text. For example:
 
-% Unit zw throws errors if there is not way to shape あ
 \font:add-fallback[family=Noto Sans CJK JP]
 
 \set[parameter=ruby.height,value=12pt]