Add Knapsack exercise (#1644)

config.json

@@ -1555,6 +1555,18 @@
           "enumeration"
         ],
         "difficulty": 8
+      },
+      {
+        "slug": "knapsack",
+        "name": "Knapsack",
+        "uuid": "91b585ce-b476-4c0d-aa2b-c1487a6e466f",
+        "practices": [],
+        "prerequisites": [
+          "numbers",
+          "arrays",
+          "enumeration"
+        ],
+        "difficulty": 8
       }
     ]
   },

exercises/practice/knapsack/.docs/hints.md

@@ -0,0 +1,55 @@
+# Hints
+
+## A starting point: brute-force recursion
+
+If you're stuck, a good starting point is a brute-force recursive solution.
+You can see it sketched out in the first half of the article ["Demystifying the 0-1 knapsack problem: top solutions explained"](demystifying-the-knapsack-problem).
+
+## Dynamic programming: what is it?
+
+For a more efficient solution, you can improve your recursive solution using *dynamic programming*, which is introduced in the second half of [the above-mentioned article](demystifying-the-knapsack-problem).
+
+For a more general explainer, see the video ["5 Simple Steps for Solving Dynamic Programming Problems"](solving-dynamic-programming-problems)
+
+## Dynamic programming and the knapsack problem
+
+If you need a more visual walkthrough of how to apply dynamic programming to the knapsack problem, see the video ["0/1 Knapsack problem | Dynamic Programming"](0-1-knapsack-problem).
+
+Also worth mentioning is [this answer](intuition-of-dp-for-knapsack-problem) to a question on Reddit, *"What is the intuition behind Knapsack problem solution using dynamic programming?"*.
+Here is the answer in full:
+
+> The intuition behind the solution is basically like any dynamic programming solution: split the task into many sub-tasks, and save solutions to these sub-tasks for later use.
+>
+> In this case the sub task is to **"Try to fit x items into a knapsack of a smaller size"** instead of trying all possible variations in the whole thing right away.
+>
+> The idea here is that at any point you can ask, *"Does this item fit into the sack at all?"*
+> If not, you repeat by looking at a bigger portion of the sack until you reach the whole size of it.
+> If the item still doesn't fit, then it's simply not part of any solution.
+>
+> If it does fit, however, then there are two options.
+> Either the maximum value for that portion of the sack is achieved without the item, or with the item.
+> If the former is true then we can just take the previous solution because we already tried the previous items.
+> (For example, if we try item 4 and it doesn't increase our maximum then we can just use our previous solution for items 1-3.)
+> If the latter is true then we put item 4 in, which takes some value off of our capacity.
+> The remaining capacity gets filled with a previous solution.
+> How?
+> Well, we already tried smaller capacities beforehand, so there should be a solution for that smaller, in this case remaining, capacity.
+>
+> So the idea is to split the entire knapsack problem into smaller knapsack problems.
+> Instead of testing 10 items with capacity 50, you first try (after the trivial case of 0) 1 item and capacity 10, 20, 30, 40 and 50 (or however many sub tasks you want to create) and then take another item and start again at capacity 10.
+>
+> If you see item 1 fits into capacity 20+, then all these slots in the table now contain this value.
+> Then you look at item 2 from capacity 10-50 again.
+> Let's assume item 2 fits into capacity 20 as well.
+> Then now you check whether it is a new maximum or not, and if it is, then you update the table.
+> Now you look at capacity 30 for item 2.
+> You see that item 2 fits; this means 10 capacity would remain if you take it.
+> However there, as of now, was no item that fits into 10 capacity, thus the solution remains the same as before.
+> At 40 this changes: you now realize that even if you include item 2 there are 20 capacity remaining, thus you can fill that space with the previous solution, which was item 1.
+> Thus for 40 capacity, as of now, the optimal solution is to take item 1 and 2.
+> And so on.
+
+[demystifying-the-knapsack-problem]: https://www.educative.io/blog/0-1-knapsack-problem-dynamic-solution
+[solving-dynamic-programming-problems]: https://www.youtube.com/watch?v=aPQY__2H3tE
+[0-1-knapsack-problem]: https://www.youtube.com/watch?v=cJ21moQpofY
+[intuition-of-dp-for-knapsack-problem]: https://www.reddit.com/r/explainlikeimfive/comments/junw6n/comment/gces429

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 +1,17 @@
+{
+  "authors": ["fpsvogel"],
+  "files": {
+    "solution": [
+      "knapsack.rb"
+    ],
+    "test": [
+      "knapsack_test.rb"
+    ],
+    "example": [
+      ".meta/example.rb"
+    ]
+  },
+  "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.rb

@@ -0,0 +1,25 @@
+# This solution uses dynamic programming to memoize solutions to overlapping
+# subproblems, so that they don't need to be recomputed. It's essentially a
+# recursive solution that remembers best-so-far outputs of previous inputs. The
+# algorithm has a time complexity of O(n * W), where n is the number of items
+# and W is the knapsack's maximum weight.
+class Knapsack
+  def initialize(max_weight)
+    @max_weight = max_weight
+  end
+
+  def max_value(items)
+    # e.g. max_values[3] is the maximum value so far for a maximum weight of 3.
+    max_values = Array.new(@max_weight + 1, 0)
+
+    items.each do |item|
+      @max_weight.downto(item.weight) do |weight|
+        value_with_item = max_values[weight - item.weight] + item.value
+
+        max_values[weight] = [max_values[weight], value_with_item].max
+      end
+    end
+
+    max_values[@max_weight]
+  end
+end

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.rb

@@ -0,0 +1,7 @@
+=begin
+Write your code for the 'Knapsack' exercise in this file. Make the tests in
+`knapsack_test.rb` pass.
+
+To get started with TDD, see the `README.md` file in your
+`ruby/knapsack` directory.
+=end

exercises/practice/knapsack/knapsack_test.rb

@@ -0,0 +1,127 @@
+require 'minitest/autorun'
+require_relative 'knapsack'
+
+class KnapsackTest < Minitest::Test
+  Item = Data.define(:weight, :value)
+
+  def test_no_items
+    # skip
+    max_weight = 100
+    items = []
+    expected = 0
+    actual = Knapsack.new(max_weight).max_value(items)
+
+    assert_equal expected, actual,
+      "When there are no items, the resulting value must be 0."
+  end
+
+  def test_one_item_too_heavy
+    skip
+    max_weight = 10
+    items = [Item.new(weight: 100, value: 1)]
+    expected = 0
+    actual = Knapsack.new(max_weight).max_value(items)
+
+    assert_equal expected, actual,
+      "When there is one item that is too heavy, the resulting value must be 0."
+  end
+
+  def test_five_items_cannot_be_greedy_by_weight
+    skip
+    max_weight = 10
+    items = [
+      Item.new(weight: 2, value: 5),
+      Item.new(weight: 2, value: 5),
+      Item.new(weight: 2, value: 5),
+      Item.new(weight: 2, value: 5),
+      Item.new(weight: 10, value: 21)
+    ]
+    expected = 21
+    actual = Knapsack.new(max_weight).max_value(items)
+
+    assert_equal expected, actual,
+      "Do not prioritize the most valuable items per weight when that would " \
+      "result in a lower total value."
+  end
+
+  def test_five_items_cannot_be_greedy_by_value
+    skip
+    max_weight = 10
+    items = [
+      Item.new(weight: 2, value: 20),
+      Item.new(weight: 2, value: 20),
+      Item.new(weight: 2, value: 20),
+      Item.new(weight: 2, value: 20),
+      Item.new(weight: 10, value: 50)
+    ]
+    expected = 80
+    actual = Knapsack.new(max_weight).max_value(items)
+
+    assert_equal expected, actual,
+      "Do not prioritize the items with the highest value when that would " \
+      "result in a lower total value."
+  end
+
+  def test_example_knapsack
+    skip
+    max_weight = 10
+    items = [
+      Item.new(weight: 5, value: 10),
+      Item.new(weight: 4, value: 40),
+      Item.new(weight: 6, value: 30),
+      Item.new(weight: 4, value: 50)
+    ]
+    expected = 90
+    actual = Knapsack.new(max_weight).max_value(items)
+
+    assert_equal expected, actual,
+      "A small example knapsack must result in a value of 90."
+  end
+
+  def test_eight_items
+    skip
+    max_weight = 104
+    items = [
+      Item.new(weight: 25, value: 350),
+      Item.new(weight: 35, value: 400),
+      Item.new(weight: 45, value: 450),
+      Item.new(weight: 5, value: 20),
+      Item.new(weight: 25, value: 70),
+      Item.new(weight: 3, value: 8),
+      Item.new(weight: 2, value: 5),
+      Item.new(weight: 2, value: 5)
+    ]
+    expected = 900
+    actual = Knapsack.new(max_weight).max_value(items)
+
+    assert_equal expected, actual,
+      "A larger example knapsack with 8 items must result in a value of 900."
+  end
+
+  def test_fifteen_items
+    skip
+    max_weight = 750
+    items = [
+      Item.new(weight: 70, value: 135),
+      Item.new(weight: 73, value: 139),
+      Item.new(weight: 77, value: 149),
+      Item.new(weight: 80, value: 150),
+      Item.new(weight: 82, value: 156),
+      Item.new(weight: 87, value: 163),
+      Item.new(weight: 90, value: 173),
+      Item.new(weight: 94, value: 184),
+      Item.new(weight: 98, value: 192),
+      Item.new(weight: 106, value: 201),
+      Item.new(weight: 110, value: 210),
+      Item.new(weight: 113, value: 214),
+      Item.new(weight: 115, value: 221),
+      Item.new(weight: 118, value: 229),
+      Item.new(weight: 120, value: 240)
+    ]
+    expected = 1458
+    actual = Knapsack.new(max_weight).max_value(items)
+
+    assert_equal expected, actual,
+      "A very large example knapsack with 15 items must result in a value of 1458."
+  end
+end