add linked list exercise (#432)

* add linked list exercise

Author: kmarker1101

config.json

@@ -926,6 +926,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 6
+      },
+      {
+        "slug": "linked-list",
+        "name": "Linked List",
+        "uuid": "ede3b6fb-b399-44a8-8aee-7bdcabec408e",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

exercises/practice/linked-list/.docs/instructions.md

@@ -0,0 +1,26 @@
+# Instructions
+
+Your team has decided to use a doubly linked list to represent each train route in the schedule.
+Each station along the train's route will be represented by a node in the linked list.
+
+You don't need to worry about arrival and departure times at the stations.
+Each station will simply be represented by a number.
+
+Routes can be extended, adding stations to the beginning or end of a route.
+They can also be shortened by removing stations from the beginning or the end of a route.
+
+Sometimes a station gets closed down, and in that case the station needs to be removed from the route, even if it is not at the beginning or end of the route.
+
+The size of a route is measured not by how far the train travels, but by how many stations it stops at.
+
+~~~~exercism/note
+The linked list is a fundamental data structure in computer science, often used in the implementation of other data structures.
+As the name suggests, it is a list of nodes that are linked together.
+It is a list of "nodes", where each node links to its neighbor or neighbors.
+In a **singly linked list** each node links only to the node that follows it.
+In a **doubly linked list** each node links to both the node that comes before, as well as the node that comes after.
+
+If you want to dig deeper into linked lists, check out [this article][intro-linked-list] that explains it using nice drawings.
+
+[intro-linked-list]: https://medium.com/basecs/whats-a-linked-list-anyway-part-1-d8b7e6508b9d
+~~~~

exercises/practice/linked-list/.docs/introduction.md

@@ -0,0 +1,6 @@
+# Introduction
+
+You are working on a project to develop a train scheduling system for a busy railway network.
+
+You've been asked to develop a prototype for the train routes in the scheduling system.
+Each route consists of a sequence of train stations that a given train stops at.

exercises/practice/linked-list/.meta/config.json

@@ -0,0 +1,18 @@
+{
+  "authors": [
+    "kmarker1101"
+  ],
+  "files": {
+    "solution": [
+      "linked-list.el"
+    ],
+    "test": [
+      "linked-list-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "blurb": "Implement a doubly linked list.",
+  "source": "Classic computer science topic"
+}

exercises/practice/linked-list/.meta/example.el

@@ -0,0 +1,88 @@
+;;; linked-list.el --- Linked List (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(require 'cl-lib)
+
+
+(cl-defstruct node
+  value prev next)
+
+
+(cl-defstruct dll
+  head tail)
+
+
+(defun dll-create ()
+  (make-dll :head nil :tail nil))
+
+
+(defun dll-push (list value)
+  (let ((new-node (make-node :value value :prev (dll-tail list) :next nil)))
+    (if (dll-tail list)
+        (setf (node-next (dll-tail list)) new-node)
+      (setf (dll-head list) new-node))
+    (setf (dll-tail list) new-node)))
+
+
+(defun dll-pop (list)
+  (when (dll-tail list)
+    (let ((value (node-value (dll-tail list)))
+          (new-tail (node-prev (dll-tail list))))
+      (if new-tail
+          (setf (node-next new-tail) nil)
+        (setf (dll-head list) nil))
+      (setf (dll-tail list) new-tail)
+      value)))
+
+
+(defun dll-unshift (list value)
+  (let ((new-node (make-node :value value :prev nil :next (dll-head list))))
+    (if (dll-head list)
+        (setf (node-prev (dll-head list)) new-node)
+      (setf (dll-tail list) new-node))
+    (setf (dll-head list) new-node)))
+
+
+(defun dll-shift (list)
+  (when (dll-head list)
+    (let ((value (node-value (dll-head list)))
+          (new-head (node-next (dll-head list))))
+      (if new-head
+          (setf (node-prev new-head) nil)
+        (setf (dll-tail list) nil))
+      (setf (dll-head list) new-head)
+      value)))
+
+
+(defun dll-count (list)
+  (let ((count 0)
+        (current (dll-head list)))
+    (while current
+      (setq count (1+ count))
+      (setq current (node-next current)))
+    count))
+
+
+(defun dll-delete (list value)
+  (let ((current (dll-head list)))
+    (while current
+      (if (equal (node-value current) value)
+          (progn
+            (let ((prev-node (node-prev current))
+                  (next-node (node-next current)))
+              (if prev-node
+                  (setf (node-next prev-node) next-node)
+                (setf (dll-head list) next-node))
+              (if next-node
+                  (setf (node-prev next-node) prev-node)
+                (setf (dll-tail list) prev-node)))
+            (setq current nil))
+        (setq current (node-next current))))))
+
+
+(provide 'linked-list)
+;;; linked-list.el ends here

exercises/practice/linked-list/.meta/tests.toml

@@ -0,0 +1,67 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[7f7e3987-b954-41b8-8084-99beca08752c]
+description = "pop gets element from the list"
+
+[c3f67e5d-cfa2-4c3e-a18f-7ce999c3c885]
+description = "push/pop respectively add/remove at the end of the list"
+
+[00ea24ce-4f5c-4432-abb4-cc6e85462657]
+description = "shift gets an element from the list"
+
+[37962ee0-3324-4a29-b588-5a4c861e6564]
+description = "shift gets first element from the list"
+
+[30a3586b-e9dc-43fb-9a73-2770cec2c718]
+description = "unshift adds element at start of the list"
+
+[042f71e4-a8a7-4cf0-8953-7e4f3a21c42d]
+description = "pop, push, shift, and unshift can be used in any order"
+
+[88f65c0c-4532-4093-8295-2384fb2f37df]
+description = "count an empty list"
+
+[fc055689-5cbe-4cd9-b994-02e2abbb40a5]
+description = "count a list with items"
+
+[8272cef5-130d-40ea-b7f6-5ffd0790d650]
+description = "count is correct after mutation"
+
+[229b8f7a-bd8a-4798-b64f-0dc0bb356d95]
+description = "popping to empty doesn't break the list"
+
+[4e1948b4-514e-424b-a3cf-a1ebbfa2d1ad]
+description = "shifting to empty doesn't break the list"
+
+[e8f7c600-d597-4f79-949d-8ad8bae895a6]
+description = "deletes the only element"
+
+[fd65e422-51f3-45c0-9fd0-c33da638f89b]
+description = "deletes the element with the specified value from the list"
+
+[59db191a-b17f-4ab7-9c5c-60711ec1d013]
+description = "deletes the element with the specified value from the list, re-assigns tail"
+
+[58242222-5d39-415b-951d-8128247f8993]
+description = "deletes the element with the specified value from the list, re-assigns head"
+
+[ee3729ee-3405-4bd2-9bad-de0d4aa5d647]
+description = "deletes the first of two elements"
+
+[47e3b3b4-b82c-4c23-8c1a-ceb9b17cb9fb]
+description = "deletes the second of two elements"
+
+[7b420958-f285-4922-b8f9-10d9dcab5179]
+description = "delete does not modify the list if the element is not found"
+
+[7e04828f-6082-44e3-a059-201c63252a76]
+description = "deletes only the first occurrence"