Add download function

Author: BruceSherwood

GlowScriptOffline/glowscript_libraries/RScompiler.3.2.min.js

@@ -43,7 +43,7 @@
       </table>
       <div></div>
       <h1 class="Heading-1"><font color="#0000a0">File operations</font></h1>
-        <p class="Normal">In Web VPython, you can import an external JavaScript library or read a file on the user's local computer. <em>The following information does not apply to VPython 7, where one can use the standard Python methods for importing modules and reading or writing files.</em></p>
+        <p class="Normal">In Web VPython, you can import an external JavaScript library, read a file on the user's local computer, or download a file containing text generated by your program. <em>The following information does not apply to VPython 7, where one can use the standard Python methods for importing modules and reading or writing files.</em></p>
         <p class="Normal"></p>
         <h1 class="Heading-1"><font color="#0000a0">get_library: import a library</font></h1>
         <p class="Normal">To import a JavaScript library, do this:</p>
@@ -71,9 +71,13 @@ <h1 class="Heading-1"><font color="#0000a0">read_local_file: user chooses a file
         <p class="Normal">If you say <strong>read_local_file()</strong>, the button is appended to the bottom of the window, corresponding to the jQuery location $('body').</p>
         <div></div>
         <div class="Normal"></div>
-        <h1 class="Heading-1"><font color="#0000a0">Writing a file</font></h1>
-        <p class="Normal">Due to security issues, it is not possible to write directly to the user's computer storage from a browser. Currently the only way to write data to a local file is to print the data, select the print area (ctrl-A), copy (ctrl-C),  paste into a local text editor, and save the file.</p>
-<p class="Normal">&nbsp;</p>
+        <h1 class="Heading-1"><font color="#0000a0">d</font><font color="#0000a0">ownload: writing a file</font></h1>
+        <p class="Normal">Due to security issues, it is not possible to write directly from a browser to an arbitrary file in the user's computer storage. However, a browser can write to the user's download folder. In Web VPython you can write a string to the download folder using the download function:</p>
+        <p class="program">download(filename, data) <br />
+        </p>
+        <p class="Normal">Both the filename and the data must be strings. A useful way to create the data string is to use Python f-strings; see <a href="text_output.html"><strong>Text Output</strong></a>. Note that repeated execution with the same file name results in the later downloaded files having (1), (2), (3)..added .to the file name.</p>
+        <p class="Normal">&nbsp; </p>
+        <p class="Normal">&nbsp;</p>
     <!-- InstanceEndEditable -->		
     </div>
 </div>

GlowScriptOffline/glowscript_libraries/compiler.3.2.min.js

@@ -772,7 +772,7 @@ where compile() was called, in untrusted/run.js.
 		"vec", "vector", "rate", "sleep", "update", "color", "paths", "shapes", "canvas",
 		"vertex", "triangle", "quad", "label", "distant_light", "local_light", "attach_trail", "attach_arrow",
 		"sqrt", "pi", "sin", "cos", "tan", "asin", "acos", "atan", "atan2", "exp", "log", "pow",
-		"factorial", "combin", "button", "radio", "checkbox", "slider", "checkbox", "text",
+		"factorial", "combin", "button", "radio", "checkbox", "slider", "checkbox", "text", "download",
 		"radians", "degrees", "get_library", "read_local_file"]
 
 		if (options.lang == 'vpython') { 

GlowScriptOffline/glowscript_libraries/glow.3.2.min.js

@@ -62,6 +62,16 @@
         return info
     }
 
+    function download(filename, data){ // send data file to Download directory
+        if (filename.constructor !== String) throw new Error('A download file name must be a string.')
+        if (data.constructor !== String) throw new Error('The download data must be a string.')
+        let a = document.createElement("a")
+        a.setAttribute('href', 'data:text/text;charset=utf-8,' + encodeURI(data));
+        a.setAttribute('download', filename);
+        a.click()
+        a.remove()
+    }
+    
     // http://my.opera.com/edvakf/blog/how-to-overcome-a-minimum-time-interval-in-javascript
     // This function is supposed to return almost instantly if there is nothing pending to do
     /*
@@ -789,6 +799,7 @@
         msclock: msclock,
         get_library: get_library,
         read_local_file: read_local_file,
+        download: download,
         parse_html,
         display_2D,
         pytype