feat(math): Support square roots

Author: Omikhleia

packages/math/base-elements.lua

@@ -1334,6 +1334,85 @@ end
 
 function elements.table.output (_) end
 
+elements.sqrt = pl.class(elements.mbox)
+elements.sqrt._type = "Sqrt"
+
+function elements.sqrt:__tostring ()
+   return self._type .. "(" .. tostring(self.radicand) .. ")"
+end
+
+function elements.sqrt:_init (radicand)
+   elements.mbox._init(self)
+   self.radicand = radicand
+   table.insert(self.children, radicand)
+   self.relX = SILE.types.length(0) -- x position relative to its parent box
+   self.relY = SILE.types.length(0) -- y position relative to its parent box
+end
+
+function elements.sqrt:styleChildren ()
+   self.radicand.mode = self.mode
+end
+
+function elements.sqrt:shape ()
+   local mathMetrics = self:getMathMetrics()
+   local scaleDown = self:getScaleDown()
+   local constants = mathMetrics.constants
+
+   self.radicalRuleThickness = constants.radicalRuleThickness * scaleDown
+   if self.mode == mathMode.display or self.mode == mathMode.displayCramped then
+      self.radicalVerticalGap = constants.radicalDisplayStyleVerticalGap * scaleDown
+   else
+      self.radicalVerticalGap = constants.radicalVerticalGap * scaleDown
+   end
+   self.extraAscender = constants.radicalExtraAscender * scaleDown
+
+   -- HACK: More or less ad hoc values, see output method for more details
+   self.symbolWidth = SILE.shaper:measureChar("√").width
+   self.symbolHeight = SILE.types.length("1.1ex"):tonumber() * scaleDown
+
+   self.width = self.radicand.width + SILE.types.length(self.symbolWidth)
+   self.height = self.radicand.height + self.radicalVerticalGap + self.extraAscender
+   self.depth = self.radicand.depth
+   self.radicand:shape()
+   self.radicand.relX = self.symbolWidth
+end
+
+local function _r (number)
+   -- Lua 5.3+ formats floats as 1.0 and integers as 1
+   -- Also some PDF readers do not like double precision.
+   return math.floor(number) == number and math.floor(number) or tonumber(string.format("%.5f", number))
+end
+
+function elements.sqrt:output (x, y, line)
+   -- HACK FIXME:
+   -- OpenType might say we need to assemble the radical sign from parts.
+   -- Frankly, it's much easier to just draw it as a graphic :-)
+   -- Hence, here we use a PDF graphic operators to draw a nice radical sign.
+   -- Some values here are ad hoc, but they look good.
+   local h = self.height:tonumber()
+   local d = self.depth:tonumber()
+   local sw = self.symbolWidth
+   local dh = h - self.symbolHeight
+   local symbol = {
+      _r(self.radicalRuleThickness), "w", -- line width
+      2, "j", -- round line joins
+      _r(sw), _r(self.extraAscender), "m",
+      _r(sw * 0.4), _r(h + d), "l",
+      _r(sw * 0.15), _r(dh), "l",
+      0, _r(dh + 0.5), "l",
+      "S"
+   }
+   local svg = table.concat(symbol, " ")
+   SILE.outputter:drawSVG(svg, x, y, sw, h, 1)
+   -- And now we just need to draw the bar over the radicand
+   SILE.outputter:drawRule(
+      self.symbolWidth + scaleWidth(x, line),
+      y.length - scaleWidth(self.radicand.height, line) - self.radicalVerticalGap - self.radicalRuleThickness / 2,
+      scaleWidth(self.radicand.width, line),
+      self.radicalRuleThickness
+   )
+end
+
 elements.mathMode = mathMode
 elements.atomType = atomType
 elements.scriptType = scriptType

packages/math/texlike.lua

@@ -476,6 +476,7 @@ compileToMathML(
    convertTexlike(nil, {
       [==[
   \def{frac}{\mfrac{#1}{#2}}
+  \def{sqrt}{\msqrt{#1}}
   \def{bi}{\mi[mathvariant=bold-italic]{#1}}
   \def{dsi}{\mi[mathvariant=double-struck]{#1}}
 

packages/math/typesetter.lua

@@ -125,6 +125,10 @@ function ConvertMathML (_, content)
          SU.error("Wrong number of children in mfrac: " .. #children)
       end
       return b.fraction(children[1], children[2])
+   elseif content.command == "msqrt" then
+      local children = convertChildren(content)
+      -- "The <msqrt> element generates an anonymous <mrow> box called the msqrt base
+      return b.sqrt(b.stackbox("H", children))
    elseif content.command == "mtable" or content.command == "table" then
       local children = convertChildren(content)
       return b.table(children, content.options)