Add knapsack exercise (#377)

* Add knapsack exercise

* Use association lists to represet items.

config.json

@@ -671,6 +671,17 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 5
+      },
+      {
+        "slug": "knapsack",
+        "name": "Knapsack",
+        "uuid": "41029de7-58a6-431b-9f82-6d4603604fb3",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 7,
+        "topics": [
+          "control_flow_loops"
+        ]
       }
     ]
   },

exercises/practice/knapsack/.docs/instructions.md

@@ -0,0 +1,35 @@
+# Instructions
+
+In this exercise, let's try to solve a classic problem.
+
+Bob is a thief.
+After months of careful planning, he finally manages to crack the security systems of a high-class apartment.
+
+In front of him are many items, each with a value (v) and weight (w).
+Bob, of course, wants to maximize the total value he can get; he would gladly take all of the items if he could.
+However, to his horror, he realizes that the knapsack he carries with him can only hold so much weight (W).
+
+Given a knapsack with a specific carrying capacity (W), help Bob determine the maximum value he can get from the items in the house.
+Note that Bob can take only one of each item.
+
+All values given will be strictly positive.
+Items will be represented as a list of items.
+Each item will have a weight and value.
+
+For example:
+
+```none
+Items: [
+  { "weight": 5, "value": 10 },
+  { "weight": 4, "value": 40 },
+  { "weight": 6, "value": 30 },
+  { "weight": 4, "value": 50 }
+]
+
+Knapsack Limit: 10
+```
+
+For the above, the first item has weight 5 and value 10, the second item has weight 4 and value 40, and so on.
+
+In this example, Bob should take the second and fourth item to maximize his value, which, in this case, is 90.
+He cannot get more than 90 as his knapsack has a weight limit of 10.

exercises/practice/knapsack/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "knapsack.el"
+    ],
+    "test": [
+      "knapsack-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "blurb": "Given a knapsack that can only carry a certain weight, determine which items to put in the knapsack in order to maximize their combined value.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Knapsack_problem"
+}

exercises/practice/knapsack/.meta/example.el

@@ -0,0 +1,22 @@
+;;; knapsack.el --- Knapsack (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl-lib)
+
+(defun maximum-value (maximum-weight items)
+  (let ((table (make-vector (1+ maximum-weight) 0)))
+    (dolist (item items)
+      (let ((weight (alist-get :weight item))
+            (value (alist-get :value item)))
+        (cl-loop for index from maximum-weight downto weight
+          for new-value = (+ value (aref table (- index weight)))
+          do (when (> new-value (aref table index))
+               (aset table index new-value)))))
+    (aref table maximum-weight)))
+
+(provide 'knapsack)
+;;; knapsack.el ends here
+

exercises/practice/knapsack/.meta/tests.toml

@@ -0,0 +1,36 @@
+# 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.
+
+[a4d7d2f0-ad8a-460c-86f3-88ba709d41a7]
+description = "no items"
+include = false
+
+[3993a824-c20e-493d-b3c9-ee8a7753ee59]
+description = "no items"
+reimplements = "a4d7d2f0-ad8a-460c-86f3-88ba709d41a7"
+
+[1d39e98c-6249-4a8b-912f-87cb12e506b0]
+description = "one item, too heavy"
+
+[833ea310-6323-44f2-9d27-a278740ffbd8]
+description = "five items (cannot be greedy by weight)"
+
+[277cdc52-f835-4c7d-872b-bff17bab2456]
+description = "five items (cannot be greedy by value)"
+
+[81d8e679-442b-4f7a-8a59-7278083916c9]
+description = "example knapsack"
+
+[f23a2449-d67c-4c26-bf3e-cde020f27ecc]
+description = "8 items"
+
+[7c682ae9-c385-4241-a197-d2fa02c81a11]
+description = "15 items"

exercises/practice/knapsack/knapsack-test.el

@@ -0,0 +1,73 @@
+;;; knapsack-test.el --- Tests for Knapsack (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(load-file "knapsack.el")
+(declare-function maximum-value "knapsack.el" (maximum-weight items))
+
+
+(ert-deftest no-items ()
+  (should (= 0 (maximum-value 100 '()))))
+
+
+(ert-deftest one-item-too-heavy ()
+  (should (= 0 (maximum-value 10 '(((:weight . 100) (:value . 1)))))))
+
+
+(ert-deftest five-items-cannot-be-greedy-by-weight ()
+  (should (= 21 (maximum-value 10 '(((:weight . 2) (:value . 5))
+                                    ((:weight . 2) (:value . 5))
+                                    ((:weight . 2) (:value . 5))
+                                    ((:weight . 2) (:value . 5))
+                                    ((:weight . 10) (:value . 21)))))))
+
+
+(ert-deftest five-items-cannot-be-greedy-by-value ()
+  (should (= 80 (maximum-value 10 '(((:weight . 2) (:value . 20))
+                                    ((:weight . 2) (:value . 20))
+                                    ((:weight . 2) (:value . 20))
+                                    ((:weight . 2) (:value . 20))
+                                    ((:weight . 10) (:value . 50)))))))
+
+
+(ert-deftest example-knapsack ()
+  (should (= 90 (maximum-value 10 '(((:weight . 5) (:value . 10))
+                                    ((:weight . 4) (:value . 40))
+                                    ((:weight . 6) (:value . 30))
+                                    ((:weight . 4) (:value . 50)))))))
+
+
+(ert-deftest 8-items ()
+  (should (= 900 (maximum-value 104 '(((:weight . 25) (:value . 350))
+                                      ((:weight . 35) (:value . 400))
+                                      ((:weight . 45) (:value . 450))
+                                      ((:weight . 5) (:value . 20))
+                                      ((:weight . 25) (:value . 70))
+                                      ((:weight . 3) (:value . 8))
+                                      ((:weight . 2) (:value . 5))
+                                      ((:weight . 2) (:value . 5)))))))
+
+
+(ert-deftest 15-items ()
+  (should (= 1458 (maximum-value 750 '(((:weight . 70) (:value . 135))
+                                       ((:weight . 73) (:value . 139))
+                                       ((:weight . 77) (:value . 149))
+                                       ((:weight . 80) (:value . 150))
+                                       ((:weight . 82) (:value . 156))
+                                       ((:weight . 87) (:value . 163))
+                                       ((:weight . 90) (:value . 173))
+                                       ((:weight . 94) (:value . 184))
+                                       ((:weight . 98) (:value . 192))
+                                       ((:weight . 106) (:value . 201))
+                                       ((:weight . 110) (:value . 210))
+                                       ((:weight . 113) (:value . 214))
+                                       ((:weight . 115) (:value . 221))
+                                       ((:weight . 118) (:value . 229))
+                                       ((:weight . 120) (:value . 240)))))))
+
+
+(provide 'knapsack-test)
+;;; knapsack-test.el ends here

exercises/practice/knapsack/knapsack.el

@@ -0,0 +1,14 @@
+;;; knapsack.el --- Knapsack (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(defun maximum-value (maximum-weight items)
+  (error "Delete this S-Expression and write your own implementation"))
+
+
+(provide 'knapsack)
+;;; knapsack.el ends here
+