Add pov exercise

config.json

@@ -892,6 +892,14 @@
         "prerequisites": [],
         "difficulty": 8
       },
+      {
+        "slug": "pov",
+        "name": "POV",
+        "uuid": "6cbdabf6-4a22-43ec-b2e4-6611ea253237",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 8
+      },
       {
         "slug": "rest-api",
         "name": "REST API",

exercises/practice/pov/.docs/instructions.md

@@ -0,0 +1,41 @@
+# Instructions
+
+Reparent a tree on a selected node.
+
+A [tree][wiki-tree] is a special type of [graph][wiki-graph] where all nodes are connected but there are no cycles.
+That means, there is exactly one path to get from one node to another for any pair of nodes.
+
+This exercise is all about re-orientating a tree to see things from a different point of view.
+For example family trees are usually presented from the ancestor's perspective:
+
+```text
+    +------0------+
+    |      |      |
+  +-1-+  +-2-+  +-3-+
+  |   |  |   |  |   |
+  4   5  6   7  8   9
+```
+
+But there is no inherent direction in a tree.
+The same information can be presented from the perspective of any other node in the tree, by pulling it up to the root and dragging its relationships along with it.
+So the same tree from 6's perspective would look like:
+
+```text
+        6
+        |
+  +-----2-----+
+  |           |
+  7     +-----0-----+
+        |           |
+      +-1-+       +-3-+
+      |   |       |   |
+      4   5       8   9
+```
+
+This lets us more simply describe the paths between two nodes.
+So for example the path from 6-9 (which in the first tree goes up to the root and then down to a different leaf node) can be seen to follow the path 6-2-0-3-9.
+
+This exercise involves taking an input tree and re-orientating it from the point of view of one of the nodes.
+
+[wiki-graph]: https://en.wikipedia.org/wiki/Tree_(graph_theory)
+[wiki-tree]: https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)

exercises/practice/pov/.meta/Example.roc

@@ -0,0 +1,99 @@
+Pov :: {}.{
+	Tree := [Empty, Node({ label : Str, children : Set(Tree) })].{
+
+		## Are two trees equal?
+		is_eq : _ # enable the default is_eq implementation for the Tree type
+
+		## Return the tree from the point of view of the node with the given label.
+		## Return Err(NotFound) if no such node is found.
+		from_pov : Tree, Str -> Try(Tree, [NotFound, ..])
+		from_pov = |tree, from| {
+			root_path = tree |> nodes_to_root(from)?
+			match root_path.first() {
+				Err(ListWasEmpty) => {
+					crash "Unreachable: nodes_to_root cannot return Ok([])"
+				}
+				Ok(Empty) => {
+					crash "Unreachable: target cannot be Empty"
+				}
+				Ok(Node(target)) => {
+					parent = root_path.get(1) ?? Empty
+					match parent {
+						Empty => Ok(tree) # the target node is already the root of the tree
+						Node(parent_node) => {
+							tree_without_target = tree |> drop(from)
+							from_parent_pov = tree_without_target |> from_pov(parent_node.label)?
+							new_children = target.children.insert(from_parent_pov)
+							Node({ label: from, children: new_children }) |> Ok
+						}
+					}
+				}
+			}
+		}
+
+		## Find the list of nodes between the two given nodes and return their labels
+		## If either of these nodes don't exist, return Err(NotFound)
+		path_to : Tree, Str, Str -> Try(List(Str), [NotFound, ..])
+		path_to = |tree, from, to| {
+			from_tree = tree |> from_pov(to)?
+			from_tree
+				|> nodes_to_root(from)?
+				.map(
+					|child| {
+						match child {
+							Empty => {
+								crash "Unreachable: trees must never contain Empty children"
+							}
+							Node(node) => node.label
+						}
+					},
+				)
+				|> Ok
+		}
+	}
+}
+
+## Return all nodes on the path from the target node up to the root.
+## If the node is not found, Err(NotFound) is returned.
+nodes_to_root : Pov.Tree, Str -> Try(List(Pov.Tree), [NotFound, ..])
+nodes_to_root = |tree, label| {
+	match tree {
+		Empty => Err(NotFound)
+		Node(node) => {
+			if node.label == label
+				Ok([Node(node)])
+					else {
+						node.children
+							.to_list()
+							.fold_until(
+								Err(NotFound),
+								|state, child| {
+									match child |> nodes_to_root(label) {
+										Ok(subPath) => Break(Ok(subPath.append(Node(node))))
+										Err(NotFound) => Continue(state)
+									}
+								},
+							)
+					}
+		}
+	}
+}
+
+## Drop the node with the given label, if it exists
+drop : Pov.Tree, Str -> Pov.Tree
+drop = |tree, label| {
+	match tree {
+		Empty => Empty
+		Node(node) => {
+			if node.label == label {
+				Empty
+			} else {
+				filtered_children =
+					node.children
+						.map(|child| drop(child, label))
+						.drop_if(|child| child == Empty)
+				Node({ label: node.label, children: filtered_children })
+			}
+		}
+	}
+}

exercises/practice/pov/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ageron"
+  ],
+  "files": {
+    "solution": [
+      "Pov.roc"
+    ],
+    "test": [
+      "pov-test.roc"
+    ],
+    "example": [
+      ".meta/Example.roc"
+    ]
+  },
+  "blurb": "Reparent a graph on a selected node.",
+  "source": "Adaptation of exercise from 4clojure",
+  "source_url": "https://www.4clojure.com/"
+}

exercises/practice/pov/.meta/template.j2

@@ -0,0 +1,59 @@
+{%- import "generator_macros.j2" as macros with context -%}
+{{ macros.canonical_ref() }}
+{{ macros.header() }}
+
+import {{ exercise | to_pascal }} exposing [Tree]
+
+{% macro roc_tree(tree) -%}
+{%- set children = tree["children"] or [] -%}
+{%- if children == [] -%}
+Node({ label: {{ tree["label"] | to_roc }}, children: Set.empty() })
+{%- else %}
+Node({
+    label: {{ tree["label"] | to_roc }},
+    children: Set.from_list([
+{%- for child in (tree["children"]) %}
+      {{ roc_tree(child) }},
+{%- endfor %}
+    ])
+})
+{%- endif %}
+{%- endmacro %}
+
+
+{% for supercase in cases %}
+##
+## {{ supercase["description"] }}
+##
+
+{% for case in supercase["cases"] -%}
+# {{ case["description"] }}
+{% if case["property"] == "fromPov" %}
+expect {
+    tree = Tree.({{ roc_tree(case["input"]["tree"]) | indent(8) }})
+    result = tree.from_pov({{ case["input"]["from"] | to_roc }})
+    {%- if case["expected"] %}
+    expected = Ok({{ roc_tree(case["expected"]) | indent(8) }})
+    result == expected
+    {%- else %}
+    result == Err(NotFound)
+    {%- endif %}
+}
+{% elif case["property"] == "pathTo" %}
+expect {
+    tree = Tree.({{ roc_tree(case["input"]["tree"]) | indent(8) }})
+    result = tree.path_to({{ case["input"]["from"] | to_roc }}, {{ case["input"]["to"] | to_roc }})
+    {%- if case["expected"] %}
+    expected = Ok({{ case["expected"] | to_roc }})
+    result == expected
+    {%- else %}
+    result == Err(NotFound)
+    {%- endif %}
+}
+{% else %}
+# This test case is not supported yet: perhaps you can try implementing it?
+{% endif %}
+{% endfor %}
+{% endfor %}
+
+{{ macros.footer() }}

exercises/practice/pov/.meta/tests.toml

@@ -0,0 +1,55 @@
+# 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.
+
+[1b3cd134-49ad-4a7d-8376-7087b7e70792]
+description = "Reroot a tree so that its root is the specified node. -> Results in the same tree if the input tree is a singleton"
+
+[0778c745-0636-40de-9edd-25a8f40426f6]
+description = "Reroot a tree so that its root is the specified node. -> Can reroot a tree with a parent and one sibling"
+
+[fdfdef0a-4472-4248-8bcf-19cf33f9c06e]
+description = "Reroot a tree so that its root is the specified node. -> Can reroot a tree with a parent and many siblings"
+
+[cbcf52db-8667-43d8-a766-5d80cb41b4bb]
+description = "Reroot a tree so that its root is the specified node. -> Can reroot a tree with new root deeply nested in tree"
+
+[e27fa4fa-648d-44cd-90af-d64a13d95e06]
+description = "Reroot a tree so that its root is the specified node. -> Moves children of the new root to same level as former parent"
+
+[09236c7f-7c83-42cc-87a1-25afa60454a3]
+description = "Reroot a tree so that its root is the specified node. -> Can reroot a complex tree with cousins"
+
+[f41d5eeb-8973-448f-a3b0-cc1e019a4193]
+description = "Reroot a tree so that its root is the specified node. -> Errors if target does not exist in a singleton tree"
+
+[9dc0a8b3-df02-4267-9a41-693b6aff75e7]
+description = "Reroot a tree so that its root is the specified node. -> Errors if target does not exist in a large tree"
+
+[02d1f1d9-428d-4395-b026-2db35ffa8f0a]
+description = "Given two nodes, find the path between them -> Can find path to parent"
+
+[d0002674-fcfb-4cdc-9efa-bfc54e3c31b5]
+description = "Given two nodes, find the path between them -> Can find path to sibling"
+
+[c9877cd1-0a69-40d4-b362-725763a5c38f]
+description = "Given two nodes, find the path between them -> Can find path to cousin"
+
+[9fb17a82-2c14-4261-baa3-2f3f234ffa03]
+description = "Given two nodes, find the path between them -> Can find path not involving root"
+
+[5124ed49-7845-46ad-bc32-97d5ac7451b2]
+description = "Given two nodes, find the path between them -> Can find path from nodes other than x"
+
+[f52a183c-25cc-4c87-9fc9-0e7f81a5725c]
+description = "Given two nodes, find the path between them -> Errors if destination does not exist"
+
+[f4fe18b9-b4a2-4bd5-a694-e179155c2149]
+description = "Given two nodes, find the path between them -> Errors if source does not exist"

exercises/practice/pov/Pov.roc

@@ -0,0 +1,21 @@
+Pov :: {}.{
+	Tree := [Empty, Node({ label : Str, children : Set(Tree) })].{
+
+		## Are two trees equal?
+		is_eq : _ # enable the default is_eq implementation for the Tree type
+
+		## Return the tree from the point of view of the node with the given label.
+		## Return Err(NotFound) if no such node is found.
+		from_pov : Tree, Str -> Try(Tree, [NotFound, ..])
+		from_pov = |tree, from| {
+			crash "Please implement the 'from_pov' function"
+		}
+
+		## Return the labels of the nodes between the two given nodes
+		## If either of these nodes don't exist, return Err(NotFound)
+		path_to : Tree, Str, Str -> Try(List(Str), [NotFound, ..])
+		path_to = |tree, from, to| {
+			crash "Please implement the 'path_to' function"
+		}
+	}
+}