Implement cases environment

See issue #278.  Although the official definition makes use of @{…}
notation, we use custom spacing instead, since that seems easier for now.

src/buildHTML.js

@@ -511,7 +511,9 @@ var groupTypes = {
 
         // Vertical spacing
         var baselineskip = 12 * pt; // see size10.clo
-        var arraystretch = 1; // factor, see lttab.dtx
+        // Default \arraystretch from lttab.dtx
+        // TODO(gagern): may get redefined once we have user-defined macros
+        var arraystretch = utils.deflt(group.value.arraystretch, 1);
         var arrayskip = arraystretch * baselineskip;
         var arstrutHeight = 0.7 * arrayskip; // \strutbox in ltfsstrc.dtx and
         var arstrutDepth = 0.3 * arrayskip;  // \@arstrutbox in lttab.dtx
@@ -565,14 +567,19 @@ var groupTypes = {
             body[r] = outrow;
         }
         var offset = totalHeight / 2 + fontMetrics.metrics.axisHeight;
-        var colalign = group.value.colalign || [];
+        var coldescriptions = group.value.cols || [];
         var cols = [];
         var colsep;
         for (c = 0; c < nc; ++c) {
+            var coldescr = coldescriptions[c] || {};
+            var sepwidth;
             if (c > 0 || group.value.hskipBeforeAndAfter) {
-                colsep = makeSpan(["arraycolsep"], []);
-                colsep.style.width = arraycolsep + "em";
-                cols.push(colsep);
+                sepwidth = utils.deflt(coldescr.pregap, arraycolsep);
+                if (sepwidth !== 0) {
+                    colsep = makeSpan(["arraycolsep"], []);
+                    colsep.style.width = sepwidth + "em";
+                    cols.push(colsep);
+                }
             }
             var col = [];
             for (r = 0; r < nr; ++r) {
@@ -588,13 +595,16 @@ var groupTypes = {
             }
             col = buildCommon.makeVList(col, "individualShift", null, options);
             col = makeSpan(
-                ["col-align-" + (colalign[c] || "c")],
+                ["col-align-" + (coldescr.align || "c")],
                 [col]);
             cols.push(col);
             if (c < nc - 1 || group.value.hskipBeforeAndAfter) {
-                colsep = makeSpan(["arraycolsep"], []);
-                colsep.style.width = arraycolsep + "em";
-                cols.push(colsep);
+                sepwidth = utils.deflt(coldescr.postgap, arraycolsep);
+                if (sepwidth !== 0) {
+                    colsep = makeSpan(["arraycolsep"], []);
+                    colsep.style.width = sepwidth + "em";
+                    cols.push(colsep);
+                }
             }
         }
         body = makeSpan(["mtable"], cols);

src/environments.js

@@ -1,3 +1,4 @@
+var fontMetrics = require("./fontMetrics");
 var parseData = require("./parseData");
 var ParseError = require("./ParseError");
 
@@ -66,10 +67,12 @@ var environmentDefinitions = [
             var parser = this;
             // Currently only supports alignment, no separators like | yet.
             colalign = colalign.value.map ? colalign.value : [colalign];
-            colalign = colalign.map(function(node) {
+            var cols = colalign.map(function(node) {
                 var ca = node.value;
                 if ("lcr".indexOf(ca) !== -1) {
-                    return ca;
+                    return {
+                        align: ca
+                    };
                 }
                 throw new ParseError(
                     "Unknown column alignment: " + node.value,
@@ -77,7 +80,7 @@ var environmentDefinitions = [
             });
             var res = {
                 type: "array",
-                colalign: colalign,
+                cols: cols,
                 hskipBeforeAndAfter: true // \@preamble in lttab.dtx
             };
             res = parseArray(parser, pos, mode, res);
@@ -111,8 +114,36 @@ var environmentDefinitions = [
             }
             return res;
         }
-    }
+    },
 
+    // A cases environment (in amsmath.sty) is almost equivalent to
+    // \def\arraystretch{1.2}%
+    // \left\{\begin{array}{@{}l@{\quad}l@{}} … \end{array}\right.
+    {
+        names: ["cases"],
+        handler: function(pos, mode, envName) {
+            var res = {
+                type: "array",
+                arraystretch: 1.2,
+                cols: [{
+                    align: "l",
+                    pregap: 0,
+                    postgap: fontMetrics.metrics.quad
+                }, {
+                    align: "l",
+                    pregap: 0,
+                    postgap: 0
+                }]
+            };
+            res = parseArray(this, pos, mode, res);
+            res.result = new ParseNode("leftright", {
+                body: [res.result],
+                left: "\\{",
+                right: "."
+            }, mode);
+            return res;
+        }
+    }
 ];
 
 module.exports = (function() {

src/utils.js

@@ -31,6 +31,13 @@ var contains = function(list, elem) {
     return indexOf(list, elem) !== -1;
 };
 
+/**
+ * Provide a default value if a setting is undefined
+ */
+var deflt = function(setting, defaultIfUndefined) {
+    return setting === undefined ? defaultIfUndefined : setting;
+};
+
 // hyphenate and escape adapted from Facebook's React under Apache 2 license
 
 var uppercase = /([A-Z])/g;
@@ -89,6 +96,7 @@ function clearNode(node) {
 
 module.exports = {
     contains: contains,
+    deflt: deflt,
     escape: escape,
     hyphenate: hyphenate,
     indexOf: indexOf,