Fix bugs in d3.zip.

* The returned array should be truncated to the length of the shortest argument.
 * With no arguments it should return an empty array.

I've updated the unit tests to be more verbose to detect the array of
single-element arrays returned for a single argument.

d3.js

@@ -67,12 +67,13 @@ d3.max = function(array, f) {
   return a;
 };
 d3.zip = function() {
-  var n = arguments.length,
-      length = d3.max(arguments, function(d) { return d.length; }),
+  var n = arguments.length;
+  if (n === 0) return [];
+  var length = d3.min(arguments, function(d) { return d.length; }),
       results = new Array(length);
   for (var i = 0; i < length; i++) {
     results[i] = new Array(n);
-    for (var j = 0; j < length; j++) results[i][j] = arguments[j][i];
+    for (var j = 0; j < n; j++) results[i][j] = arguments[j][i];
   }
   return results;
 };

src/core/zip.js

@@ -1,10 +1,11 @@
 d3.zip = function() {
-  var n = arguments.length,
-      length = d3.max(arguments, function(d) { return d.length; }),
+  var n = arguments.length;
+  if (n === 0) return [];
+  var length = d3.min(arguments, function(d) { return d.length; }),
       results = new Array(length);
   for (var i = 0; i < length; i++) {
     results[i] = new Array(n);
-    for (var j = 0; j < length; j++) results[i][j] = arguments[j][i];
+    for (var j = 0; j < n; j++) results[i][j] = arguments[j][i];
   }
   return results;
 };

tests/test-zip.js

@@ -1,10 +1,28 @@
 require("./../lib/env-js/envjs/node");
 require("./../d3");
 
+function s(array) {
+  return "[" + array.map(function(d) {
+    return "[" + d.join(",") + "]";
+  }) + "]";
+}
+
 console.log("zip [1, 2] [3, 4]:");
-console.log("  " + d3.zip([1, 2], [3, 4]));
+console.log("  " + s(d3.zip([1, 2], [3, 4])));
 console.log("");
 
 console.log("zip [1, 2] [3, 4] [5, 6, 7]:");
-console.log("  " + d3.zip([1, 2], [3, 4], [5, 6, 7]));
+console.log("  " + s(d3.zip([1, 2], [3, 4], [5, 6, 7])));
+console.log("");
+
+console.log("zip [1, 2, 3, 4, 5] [2, 4, 6, 8, 10]:");
+console.log("  " + s(d3.zip([1, 2, 3, 4, 5], [2, 4, 6, 8, 10])));
+console.log("");
+
+console.log("zip [1, 2, 3, 4, 5]:");
+console.log("  " + s(d3.zip([1, 2, 3, 4, 5])));
+console.log("");
+
+console.log("zip (no arguments):");
+console.log("  " + s(d3.zip()));
 console.log("");

tests/test-zip.out

@@ -1,6 +1,15 @@
 zip [1, 2] [3, 4]:
-  1,3,2,4
+  [[1,3],[2,4]]
 
 zip [1, 2] [3, 4] [5, 6, 7]:
-  1,3,5,2,4,6,,,7
+  [[1,3,5],[2,4,6]]
+
+zip [1, 2, 3, 4, 5] [2, 4, 6, 8, 10]:
+  [[1,2],[2,4],[3,6],[4,8],[5,10]]
+
+zip [1, 2, 3, 4, 5]:
+  [[1],[2],[3],[4],[5]]
+
+zip (no arguments):
+  []