Add \underline support (#456)

![image](https://cloud.githubusercontent.com/assets/6820/14412284/e63218b6-ff13-11e5-912c-fc6e30928b93.png)

Fixes #209.

src/buildHTML.js

@@ -51,6 +51,7 @@ var groupToType = {
     op: "mop",
     katex: "mord",
     overline: "mord",
+    underline: "mord",
     rule: "mord",
     leftright: "minner",
     sqrt: "mord",
@@ -928,6 +929,32 @@ groupTypes.overline = function(group, options, prev) {
     return makeSpan(["overline", "mord"], [vlist], options.getColor());
 };
 
+groupTypes.underline = function(group, options, prev) {
+    // Underlines are handled in the TeXbook pg 443, Rule 10.
+
+    // Build the inner group.
+    var innerGroup = buildGroup(group.value.body, options);
+
+    var ruleWidth = fontMetrics.metrics.defaultRuleThickness /
+        options.style.sizeMultiplier;
+
+    // Create the line above the body
+    var line = makeSpan(
+        [options.style.reset(), Style.TEXT.cls(), "underline-line"]);
+    line.height = ruleWidth;
+    line.maxFontSize = 1.0;
+
+    // Generate the vlist, with the appropriate kerns
+    var vlist = buildCommon.makeVList([
+        {type: "kern", size: ruleWidth},
+        {type: "elem", elem: line},
+        {type: "kern", size: 3 * ruleWidth},
+        {type: "elem", elem: innerGroup},
+    ], "top", innerGroup.height, options);
+
+    return makeSpan(["underline", "mord"], [vlist], options.getColor());
+};
+
 groupTypes.sqrt = function(group, options, prev) {
     // Square roots are handled in the TeXbook pg. 443, Rule 11.
 

src/buildMathML.js

@@ -422,6 +422,20 @@ groupTypes.overline = function(group, options) {
     return node;
 };
 
+groupTypes.underline = function(group, options) {
+    var operator = new mathMLTree.MathNode(
+        "mo", [new mathMLTree.TextNode("\u203e")]);
+    operator.setAttribute("stretchy", "true");
+
+    var node = new mathMLTree.MathNode(
+        "munder",
+        [buildGroup(group.value.body, options),
+         operator]);
+    node.setAttribute("accentunder", "true");
+
+    return node;
+};
+
 groupTypes.rule = function(group) {
     // TODO(emily): Figure out if there's an actual way to draw black boxes
     // in MathML.

src/functions.js

@@ -170,6 +170,17 @@ defineFunction("\\overline", {
     };
 });
 
+// An underline
+defineFunction("\\underline", {
+    numArgs: 1,
+}, function(context, args) {
+    var body = args[0];
+    return {
+        type: "underline",
+        body: body,
+    };
+});
+
 // A box of the width and height
 defineFunction("\\rule", {
     numArgs: 2,

static/katex.less

@@ -352,25 +352,23 @@
         position: relative;
     }
 
-    .overline {
+    .overline .overline-line,
+    .underline .underline-line {
+        width: 100%;
 
-        .overline-line {
-            width: 100%;
+        &:before {
+            border-bottom-style: solid;
+            border-bottom-width: 1px;
+            content: "";
+            display: block;
+        }
 
-            &:before {
-                border-bottom-style: solid;
-                border-bottom-width: 1px;
-                content: "";
-                display: block;
-            }
-
-            &:after {
-                border-bottom-style: solid;
-                border-bottom-width: 0.04em;
-                content: "";
-                display: block;
-                margin-top: -1px;
-            }
+        &:after {
+            border-bottom-style: solid;
+            border-bottom-width: 0.04em;
+            content: "";
+            display: block;
+            margin-top: -1px;
         }
     }
 

test/screenshotter/ss_data.yaml

@@ -85,7 +85,7 @@ NullDelimiterInteraction: a \bigl. + 2 \quad \left. + a \right)
 OpLimits: |
     {\sin_2^2 \lim_2^2 \int_2^2 \sum_2^2}
     {\displaystyle \lim_2^2 \int_2^2 \intop_2^2 \sum_2^2}
-Overline: \overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{y}}
+OverUnderline: x\underline{x}\underline{\underline{x}}\underline{x_{x_{x_x}}}\underline{x^{x^{x^x}}}\overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{\underline{x}}\underline{\overline{x}}}
 Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x}
 PrimeSpacing: f'+f_2'+f^{f'}
 RlapBug: \frac{\rlap{x}}{2}