Support {smallmatrix}, {subarray}, and \substack (#1969)

* Support {smallmatrix}, {subarray}, and \substack

* Fix lint errors

* Update docs. Screenshots step 1.

* Screenshots step 2

* Screenshots step 3

* Pick up review comments

* Fixed arraycolsep for {smallmatrix}

* Fixed lint error

* Updated screenshots

src/environments/array.js

@@ -1,5 +1,6 @@
 // @flow
 import buildCommon from "../buildCommon";
+import Style from "../Style";
 import defineEnvironment from "../defineEnvironment";
 import defineFunction from "../defineFunction";
 import mathMLTree from "../mathMLTree";
@@ -26,7 +27,7 @@ export type AlignSpec = { type: "separator", separator: string } | {
 };
 
 // Type to indicate column separation in MathML
-export type ColSeparationType = "align" | "alignat";
+export type ColSeparationType = "align" | "alignat" | "small";
 
 function getHLines(parser: Parser): boolean[] {
     // Return an array. The array length = number of hlines.
@@ -175,7 +176,16 @@ const htmlBuilder: HtmlBuilder<"array"> = function(group, options) {
 
     // Horizontal spacing
     const pt = 1 / options.fontMetrics().ptPerEm;
-    const arraycolsep = 5 * pt; // \arraycolsep in article.cls
+    let arraycolsep = 5 * pt; // default value, i.e. \arraycolsep in article.cls
+    if (group.colSeparationType && group.colSeparationType === "small") {
+        // We're in a {smallmatrix}. Default column space is \thickspace,
+        // i.e. 5/18em = 0.2778em, per amsmath.dtx for {smallmatrix}.
+        // But that needs adjustment because LaTeX applies \scriptstyle to the
+        // entire array, including the colspace, but this function applies
+        // \scriptstyle only inside each element.
+        const localMultiplier = options.havingStyle(Style.SCRIPT).sizeMultiplier;
+        arraycolsep = 0.2778 * (localMultiplier / options.sizeMultiplier);
+    }
 
     // Vertical spacing
     const baselineskip = 12 * pt; // see size10.clo
@@ -379,7 +389,7 @@ const alignMap = {
 };
 
 const mathmlBuilder: MathMLBuilder<"array"> = function(group, options) {
-    const table = new mathMLTree.MathNode(
+    let table = new mathMLTree.MathNode(
         "mtable", group.body.map(function(row) {
             return new mathMLTree.MathNode(
                 "mtr", row.map(function(cell) {
@@ -401,7 +411,9 @@ const mathmlBuilder: MathMLBuilder<"array"> = function(group, options) {
 
     // The 0.16 and 0.09 values are found emprically. They produce an array
     // similar to LaTeX and in which content does not interfere with \hines.
-    const gap = 0.16 + group.arraystretch - 1 + (group.addJot ? 0.09 : 0);
+    const gap = (group.arraystretch === 0.5)
+        ? 0.1  // {smallmatrix}, {subarray}
+        : 0.16 + group.arraystretch - 1 + (group.addJot ? 0.09 : 0);
     table.setAttribute("rowspacing", gap + "em");
 
     // MathML table lines go only between cells.
@@ -463,6 +475,8 @@ const mathmlBuilder: MathMLBuilder<"array"> = function(group, options) {
         table.setAttribute("columnspacing", spacing.trim());
     } else if (group.colSeparationType === "alignat") {
         table.setAttribute("columnspacing", "0em");
+    } else if (group.colSeparationType === "small") {
+        table.setAttribute("columnspacing", "0.2778em");
     } else {
         table.setAttribute("columnspacing", "1em");
     }
@@ -484,13 +498,18 @@ const mathmlBuilder: MathMLBuilder<"array"> = function(group, options) {
         table.setAttribute("rowlines", rowLines.trim());
     }
 
-    if (menclose === "") {
-        return table;
-    } else {
-        const wrapper =  new mathMLTree.MathNode("menclose", [table]);
-        wrapper.setAttribute("notation", menclose.trim());
-        return wrapper;
+    if (menclose !== "") {
+        table = new mathMLTree.MathNode("menclose", [table]);
+        table.setAttribute("notation", menclose.trim());
     }
+
+    if (group.arraystretch && group.arraystretch < 1) {
+        // A small array. Wrap in scriptstyle so row gap is not too large.
+        table = new mathMLTree.MathNode("mstyle", [table]);
+        table.setAttribute("scriptlevel", "1");
+    }
+
+    return table;
 };
 
 // Convenience function for aligned and alignedat environments.
@@ -656,6 +675,63 @@ defineEnvironment({
     mathmlBuilder,
 });
 
+defineEnvironment({
+    type: "array",
+    names: ["smallmatrix"],
+    props: {
+        numArgs: 0,
+    },
+    handler(context) {
+        const payload = {arraystretch: 0.5};
+        const res = parseArray(context.parser, payload, "script");
+        res.colSeparationType = "small";
+        return res;
+    },
+    htmlBuilder,
+    mathmlBuilder,
+});
+
+defineEnvironment({
+    type: "array",
+    names: ["subarray"],
+    props: {
+        numArgs: 1,
+    },
+    handler(context, args) {
+        // Parsing of {subarray} is similar to {array}
+        const symNode = checkSymbolNodeType(args[0]);
+        const colalign: AnyParseNode[] =
+            symNode ? [args[0]] : assertNodeType(args[0], "ordgroup").body;
+        const cols = colalign.map(function(nde) {
+            const node = assertSymbolNodeType(nde);
+            const ca = node.text;
+            // {subarray} only recognizes "l" & "c"
+            if ("lc".indexOf(ca) !== -1) {
+                return {
+                    type: "align",
+                    align: ca,
+                };
+            }
+            throw new ParseError("Unknown column alignment: " + ca, nde);
+        });
+        if (cols.length > 1) {
+            throw new ParseError("{subarray} can contain only one column");
+        }
+        let res = {
+            cols,
+            hskipBeforeAndAfter: false,
+            arraystretch: 0.5,
+        };
+        res = parseArray(context.parser, res, "script");
+        if (res.body[0].length > 1) {
+            throw new ParseError("{subarray} can contain only one column");
+        }
+        return res;
+    },
+    htmlBuilder,
+    mathmlBuilder,
+});
+
 // A cases environment (in amsmath.sty) is almost equivalent to
 // \def\arraystretch{1.2}%
 // \left\{\begin{array}{@{}l@{\quad}l@{}} … \end{array}\right.

src/macros.js

@@ -426,6 +426,9 @@ defineMacro("\\varPhi", "\\mathit{\\Phi}");
 defineMacro("\\varPsi", "\\mathit{\\Psi}");
 defineMacro("\\varOmega", "\\mathit{\\Omega}");
 
+//\newcommand{\substack}[1]{\subarray{c}#1\endsubarray}
+defineMacro("\\substack", "\\begin{subarray}{c}#1\\end{subarray}");
+
 // \renewcommand{\colon}{\nobreak\mskip2mu\mathpunct{}\nonscript
 // \mkern-\thinmuskip{:}\mskip6muplus1mu\relax}
 defineMacro("\\colon", "\\nobreak\\mskip2mu\\mathpunct{}" +