satellite (#64)

Author: glennj

config.json

@@ -494,6 +494,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 6
+      },
+      {
+        "slug": "satellite",
+        "name": "Satellite",
+        "uuid": "689501f5-1404-4358-b1f2-aea7798352c9",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 7
       }
     ]
   },

exercises/practice/satellite/.busted

@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}

exercises/practice/satellite/.docs/instructions.md

@@ -0,0 +1,27 @@
+# Instructions
+
+Imagine you need to transmit a binary tree to a satellite approaching Alpha Centauri and you have limited bandwidth.
+Since the tree has no repeating items it can be uniquely represented by its [pre-order and in-order traversals][wiki].
+
+Write the software for the satellite to rebuild the tree from the traversals.
+
+A pre-order traversal reads the value of the current node before (hence "pre") reading the left subtree in pre-order.
+Afterwards the right subtree is read in pre-order.
+
+An in-order traversal reads the left subtree in-order then the current node and finally the right subtree in-order.
+So in order from left to right.
+
+For example the pre-order traversal of this tree is [a, i, x, f, r].
+The in-order traversal of this tree is [i, a, f, x, r]
+
+```text
+  a
+ / \
+i   x
+   / \
+  f   r
+```
+
+Note: the first item in the pre-order traversal is always the root.
+
+[wiki]: https://en.wikipedia.org/wiki/Tree_traversal

exercises/practice/satellite/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "satellite.moon"
+    ],
+    "test": [
+      "satellite_spec.moon"
+    ],
+    "example": [
+      ".meta/example.moon"
+    ]
+  },
+  "blurb": "Rebuild binary trees from pre-order and in-order traversals."
+}

exercises/practice/satellite/.meta/example.moon

@@ -0,0 +1,35 @@
+indexof = (list, elem) ->
+  for i, v in ipairs list
+    if v == elem
+      return i
+  return -1
+
+uniq = (list) ->
+  seen = {}
+  for elem in *list
+    return false if seen[elem]
+    seen[elem] = true
+  true
+
+
+do_tree = (preorder, inorder) ->
+  return {} if #preorder == 0
+
+  root = preorder[1]
+  idx = indexof inorder, root
+  assert idx > 0, "traversals must have the same elements"
+
+  return {
+    v: root,
+    l: do_tree {table.unpack preorder, 2, idx}, {table.unpack inorder, 1, idx - 1},
+    r: do_tree {table.unpack preorder, idx + 1, #preorder}, {table.unpack inorder, idx + 1, #inorder}
+  }
+
+
+{
+  tree: (preorder, inorder) ->
+    assert #preorder == #inorder, "traversals must have the same length"
+    assert uniq(preorder) and uniq(inorder), "traversals must contain unique items"
+
+    do_tree preorder, inorder
+}

exercises/practice/satellite/.meta/tests.toml

@@ -0,0 +1,37 @@
+# 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.
+
+[8df3fa26-811a-4165-9286-ff9ac0850d19]
+description = "Empty tree"
+
+[f945ccfc-05e3-47d7-825b-0270559d43ad]
+description = "Tree with one item"
+
+[a0121d5f-37b0-48dd-9c64-cba4c4464135]
+description = "Tree with many items"
+
+[6074041f-4891-4d81-a128-401050c2a3b0]
+description = "Reject traversals of different length"
+
+[27916ce4-45f3-4d8b-8528-496fedc157ca]
+description = "Reject inconsistent traversals of same length"
+
+[d86a3d72-76a9-43b5-9d3a-e64cb1216035]
+description = "Reject traversals with repeated items"
+
+[af31ae02-7e5b-4452-a990-bccb3fca9148]
+description = "A degenerate binary tree"
+
+[ee54463d-a719-4aae-ade4-190d30ce7320]
+description = "Another degenerate binary tree"
+
+[87123c08-c155-4486-90a4-e2f75b0f3e8f]
+description = "Tree with many more items"

exercises/practice/satellite/satellite.moon

@@ -0,0 +1,4 @@
+{
+  tree: (preorder, inorder) ->
+    error 'Implement me'
+}

exercises/practice/satellite/satellite_spec.moon

@@ -0,0 +1,96 @@
+Satellite = require 'satellite'
+
+describe 'satellite', ->
+  it "Empty tree", ->
+    result = Satellite.tree {}, {}
+    expected = {}
+    assert.are.same expected, result
+
+  pending "Tree with one item", ->
+    result = Satellite.tree {"a"}, {"a"}
+    expected = { v: "a", l: {}, r: {} }
+    assert.are.same expected, result
+
+  pending "Tree with many items", ->
+    result = Satellite.tree {"a", "i", "x", "f", "r"}, {"i", "a", "f", "x", "r"}
+    expected = {
+        v: "a",
+        l: { v: "i", l: {}, r: {} },
+        r: {
+          v: "x",
+          l: { v: "f", l: {}, r: {} },
+          r: { v: "r", l: {}, r: {} }
+        }
+      }
+    assert.are.same expected, result
+
+  pending "Reject traversals of different length", ->
+    f = -> Satellite.tree {"a", "b"}, {"b", "a", "r"}
+    assert.has.errors f, "traversals must have the same length"
+
+  pending "Reject inconsistent traversals of same length", ->
+    f = -> Satellite.tree {"x", "y", "z"}, {"a", "b", "c"}
+    assert.has.errors f, "traversals must have the same elements"
+
+  pending "Reject traversals with repeated items", ->
+    f = -> Satellite.tree {"a", "b", "a"}, {"b", "a", "a"}
+    assert.has.errors f, "traversals must contain unique items"
+
+  pending "A degenerate binary tree", ->
+    result = Satellite.tree {"a", "b", "c", "d"}, {"d", "c", "b", "a"}
+    expected = {
+        v: "a",
+        l: {
+          v: "b",
+          l: {
+            v: "c",
+            l: { v: "d", l: {}, r: {} },
+            r: {}
+          },
+          r: {}
+        },
+        r: {}
+      }
+    assert.are.same expected, result
+
+  pending "Another degenerate binary tree", ->
+    result = Satellite.tree {"a", "b", "c", "d"}, {"a", "b", "c", "d"}
+    expected = {
+        v: "a",
+        l: {},
+        r: {
+          v: "b",
+          l: {},
+          r: {
+            v: "c",
+            l: {},
+            r: { v: "d", l: {}, r: {} }
+          }
+        }
+      }
+    assert.are.same expected, result
+
+  pending "Tree with many more items", ->
+    result = Satellite.tree {"a", "b", "d", "g", "h", "c", "e", "f", "i"}, {"g", "d", "h", "b", "a", "e", "c", "i", "f"}
+    expected = {
+        v: "a",
+        l: {
+          v: "b",
+          l: {
+            v: "d",
+            l: { v: "g", l: {}, r: {} },
+            r: { v: "h", l: {}, r: {} }
+          },
+          r: {}
+        },
+        r: {
+          v: "c",
+          l: { v: "e", l: {}, r: {} },
+          r: {
+            v: "f",
+            l: { v: "i", l: {}, r: {} },
+            r: {}
+          }
+        }
+      }
+    assert.are.same expected, result