Add pov exercise

config.json

@@ -463,6 +463,14 @@
         "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 +1,67 @@
+module [fromPov, pathTo]
+
+Tree : [Empty, Node { label : Str, children : Set Tree }]
+
+## Return all nodes on the path from the target node up to the root.
+## If the node is not found, Err NotFound is returned.
+nodesToRoot : Tree, Str -> Result (List Tree) [NotFound]
+nodesToRoot = \tree, label ->
+    when tree is
+        Empty -> Err NotFound
+        Node node ->
+            if node.label == label then
+                Ok [Node node]
+            else
+                node.children
+                |> Set.walkUntil (Err NotFound) \state, child ->
+                    when child |> nodesToRoot label is
+                        Ok subPath -> subPath |> List.append (Node node) |> Ok |> Break
+                        Err NotFound -> state |> Continue
+
+## Drop the node with the given label, if it exists
+drop : Tree, Str -> Tree
+drop = \tree, label ->
+    when tree is
+        Empty -> Empty
+        Node node ->
+            if node.label == label then
+                Empty
+            else
+                filteredChildren =
+                    node.children
+                    |> Set.map \child -> drop child label
+                    |> Set.dropIf \child -> child == Empty
+                Node { label: node.label, children: filteredChildren }
+
+## Return the tree from the point of view of the node with the given label.
+## Return Err NotFound if no such node is found.
+fromPov : Tree, Str -> Result Tree [NotFound]
+fromPov = \tree, from ->
+    rootPath : List Tree
+    rootPath = tree |> nodesToRoot? from
+    when rootPath |> List.first is
+        Err ListWasEmpty -> crash "Unreachable: nodesToRoot cannot return Ok []"
+        Ok Empty -> crash "Unreachable: target cannot be Empty"
+        Ok (Node target) ->
+            parent = rootPath |> List.get 1 |> Result.withDefault Empty
+            when parent is
+                Empty -> Ok tree # the target node is already the root of the tree
+                Node parentNode ->
+                    treeWithoutTarget = tree |> drop from
+                    fromParentPov = treeWithoutTarget |> fromPov? parentNode.label
+                    newChildren = target.children |> Set.insert fromParentPov
+                    Node { label: from, children: newChildren } |> 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
+pathTo : Tree, Str, Str -> Result (List Str) [NotFound]
+pathTo = \tree, from, to ->
+    fromTree = tree |> fromPov? from
+    fromTree
+        |> nodesToRoot? to
+        |> List.map \child ->
+            when child is
+                Empty -> crash "Unreachable: trees must never contain Empty children"
+                Node node -> node.label
+        |> Ok
+

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 +1,57 @@
+{%- import "generator_macros.j2" as macros with context -%}
+{{ macros.canonical_ref() }}
+{{ macros.header() }}
+
+import {{ exercise | to_pascal }} exposing [fromPov, pathTo]
+
+Tree : [Empty, Node { label : Str, children : Set 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.fromList [
+{%- 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 = {{ roc_tree(case["input"]["tree"]) | indent(8) }}
+    result = tree |> {{ case["property"] | to_camel }} {{ case["input"]["from"] | to_roc }}
+    {%- if case["expected"] %}
+    expected = {{ roc_tree(case["expected"]) | indent(8) }} |> Ok
+    result == expected
+    {%- else %}
+    result == Err NotFound
+    {%- endif %}
+{% elif case["property"] == "pathTo" %}
+expect
+    tree = {{ roc_tree(case["input"]["tree"]) | indent(8) }}
+    result = tree |> {{ case["property"] | to_camel }} {{ 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 %}

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 +1,20 @@
+module [fromPov, pathTo, isEquivalentTo]
+
+Tree : [Empty, Node { label : Str, children : List Tree }]
+
+## Are tree1 and tree2 identical, ignoring the order of the children?
+isEquivalentTo : Tree, Tree -> Bool
+isEquivalentTo = \tree1, tree2 ->
+    crash "Please implement the 'isEquivalentTo' function"
+
+## Return the tree from the point of view of the node with the given label.
+## Return Err NotFound if no such node is found.
+fromPov : Tree, Str -> Result Tree [NotFound]
+fromPov = \tree, from ->
+    crash "Please implement the 'fromPov' function"
+
+## Return the labels of the nodes between the two given nodes
+## If either of these nodes don't exist, return Err NotFound
+pathTo : Tree, Str, Str -> Result (List Str) [NotFound]
+pathTo = \tree, from, to ->
+    crash "Please implement the 'pathTo' function"