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+# Interface to Google Drive in Racket
+
+## Fred Martin
+### April 22, 2017
+
+# Overview
+This set of code provides an interface to searching through one's Google Drive account. 
+Its most important feature is that it provides a *folder-delimited search*.
+
+The essential model of files in Google Drive is that they are in one big “pile.” So you can't directly find a file in a
+given folder.
+
+This code recursively collects all folders found within a given folder, and then 
+construct a search query that includes a list of all the subfolders (flattened into a single list).
+
+This then allows you to perform a folder-delimited search.
+
+**Authorship note:** All of the code described here was written by myself.
+
+# Libraries Used
+The code uses four libraries:
+
+```
+(require net/url)
+(require (planet ryanc/webapi:1:=1/oauth2))
+(require json)
+(require net/uri-codec)
+```
+
+* The ```net/url``` library provides the ability to make REST-style https queries to the Google Drive API.
+* Ryan Culpepper's ```webapi``` library is used to provide the ```oauth2``` interface required for authentication.
+* The ```json``` library is used to parse the replies from the Google Drive API.
+* The ```net/uri-codec``` library is used to format parameters provided in API calls into an ASCII encoding used by Google Drive.
+
+# Key Code Excerpts
+
+Here is a discussion of the most essential procedures, including a description of how they embody ideas from 
+UMass Lowell's COMP.3010 Organization of Programming languages course.
+
+Five examples are shown and they are individually numbered. 
+
+## 1. Initialization using a Global Object
+
+The following code creates a global object, ```drive-client``` that is used in each of the subsequent API calls:
+
+```
+(define drive-client
+  (oauth2-client
+   #:id "548798434144-6s8abp8aiqh99bthfptv1cc4qotlllj6.apps.googleusercontent.com"
+   #:secret "<email me for secret if you want to use my API>"))
+ ```
+ 
+ While using global objects is not a central theme in the course, it's necessary to show this code to understand
+ the later examples.
+ 
+## 2. Selectors and Predicates using Procedural Abstraction
+
+A set of procedures was created to operate on the core ```drive-file``` object. Drive-files may be either
+actual file objects or folder objects. In Racket, they are represented as a hash table.
+
+```folder?``` accepts a ```drive-file```, inspects its ```mimeType```, and returns ```#t``` or ```#f```:
+
+```
+(define (folder? drive-file)
+  (string=? (hash-ref drive-file 'mimeType "nope") "application/vnd.google-apps.folder"))
+```
+
+Another object produced by the Google Drive API is a list of drive-file objects ("```drive#fileList```"). 
+When converted by the JSON library,
+this list appears as hash map. 
+
+```get-files``` retrieves a list of the files themselves, and ```get-id``` retrieves the unique ID
+associated with a ```drive#fileList``` object:
+
+```
+(define (get-files obj)
+  (hash-ref obj 'files))
+
+(define (get-id obj)
+  (hash-ref obj 'id))
+```
+## 3. Using Recursion to Accumulate Results
+
+The low-level routine for interacting with Google Drive is named ```list-children```. This accepts an ID of a 
+folder object, and optionally, a token for which page of results to produce.
+
+A lot of the work here has to do with pagination. Because it's a web interface, one can only obtain a page of
+results at a time. So it's necessary to step through each page. When a page is returned, it includes a token
+for getting the next page. The ```list-children``` just gets one page:
+
+```
+(define (list-children folder-id . next-page-token)
+  (read-json
+   (get-pure-port
+    (string->url (string-append "https://www.googleapis.com/drive/v3/files?"
+                                "q='" folder-id "'+in+parents"
+                                "&key=" (send drive-client get-id)
+                                (if (= 1 (length next-page-token))
+                                    (string-append "&pageToken=" (car next-page-token))
+                                    "")
+;                                "&pageSize=5"
+                                ))
+    token)))
+```
+The interesting routine is ```list-all-children```. This routine is directly invoked by the user.
+It optionally accepts a page token; when it's used at top level this parameter will be null.
+
+The routine uses ```let*``` to retrieve one page of results (using the above ```list-children``` procedure)
+and also possibly obtain a token for the next page.
+
+If there is a need to get more pages, the routine uses ```append``` to pre-pend the current results with 
+a recursive call to get the next page (and possibly more pages).
+
+Ultimately, when there are no more pages to be had, the routine terminates and returns the current page. 
+
+This then generates a recursive process from the recursive definition.
+
+```
+(define (list-all-children folder-id . next-page-token)
+  (let* ((this-page (if (= 0 (length next-page-token))
+                      (list-children folder-id)
+                      (list-children folder-id (car next-page-token))))
+         (page-token (hash-ref this-page 'nextPageToken #f)))
+    (if page-token
+        (append (get-files this-page)
+              (list-all-children folder-id page-token))
+        (get-files this-page))))
+```
+
+## 4. Filtering a List of File Objects for Only Those of Folder Type
+
+The ```list-all-children``` procedure creates a list of all objects contained within a given folder.
+These objects include the files themselves and other folders.
+
+The ```filter``` abstraction is then used with the ```folder?``` predicate to make a list of subfolders
+contained in a given folder:
+
+```
+(define (list-folders folder-id)
+  (filter folder? (list-all-children folder-id)))
+```
+
+## 5. Recursive Descent on a Folder Hierarchy
+
+These procedures are used together in ```list-all-folders```, which accepts a folder ID and recursively
+obtains the folders at the current level and then recursively calls itself to descend completely into the folder
+hierarchy.
+
+```map``` and ```flatten``` are used to accomplish the recursive descent:
+
+```
+(define (list-all-folders folder-id)
+  (let ((this-level (list-folders folder-id)))
+    (begin
+      (display (length this-level)) (display "... ")
+      (append this-level
+              (flatten (map list-all-folders (map get-id this-level)))))))
+```