From fa333cfc70c116ce981dcda7dc047f26e77c95e5 Mon Sep 17 00:00:00 2001
From: Glenn Jackman <glenn.jackman@gmail.com>
Date: Wed, 18 Mar 2026 18:03:44 -0400
Subject: [PATCH] Add flatten-array

---
 config.json                                   |  8 +++
 exercises/practice/flatten-array/.busted      |  5 ++
 .../.docs/instructions.append.md              |  9 +++
 .../flatten-array/.docs/instructions.md       | 16 +++++
 .../flatten-array/.docs/introduction.md       |  7 +++
 .../practice/flatten-array/.meta/config.json  | 19 ++++++
 .../practice/flatten-array/.meta/example.moon | 15 +++++
 .../flatten-array/.meta/spec_generator.moon   | 34 ++++++++++
 .../practice/flatten-array/.meta/tests.toml   | 63 +++++++++++++++++++
 .../practice/flatten-array/flatten_array.moon |  4 ++
 .../flatten-array/flatten_array_spec.moon     | 57 +++++++++++++++++
 11 files changed, 237 insertions(+)
 create mode 100644 exercises/practice/flatten-array/.busted
 create mode 100644 exercises/practice/flatten-array/.docs/instructions.append.md
 create mode 100644 exercises/practice/flatten-array/.docs/instructions.md
 create mode 100644 exercises/practice/flatten-array/.docs/introduction.md
 create mode 100644 exercises/practice/flatten-array/.meta/config.json
 create mode 100644 exercises/practice/flatten-array/.meta/example.moon
 create mode 100644 exercises/practice/flatten-array/.meta/spec_generator.moon
 create mode 100644 exercises/practice/flatten-array/.meta/tests.toml
 create mode 100644 exercises/practice/flatten-array/flatten_array.moon
 create mode 100644 exercises/practice/flatten-array/flatten_array_spec.moon

diff --git a/config.json b/config.json
index be183d8..36b61f5 100644
--- a/config.json
+++ b/config.json
@@ -394,6 +394,14 @@
         "prerequisites": [],
         "difficulty": 4
       },
+      {
+        "slug": "flatten-array",
+        "name": "Flatten Array",
+        "uuid": "9f9b0efb-3bd0-4dc5-8df7-1edaf68ea046",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "isbn-verifier",
         "name": "ISBN Verifier",
diff --git a/exercises/practice/flatten-array/.busted b/exercises/practice/flatten-array/.busted
new file mode 100644
index 0000000..86b84e7
--- /dev/null
+++ b/exercises/practice/flatten-array/.busted
@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}
diff --git a/exercises/practice/flatten-array/.docs/instructions.append.md b/exercises/practice/flatten-array/.docs/instructions.append.md
new file mode 100644
index 0000000..4c45e1b
--- /dev/null
+++ b/exercises/practice/flatten-array/.docs/instructions.append.md
@@ -0,0 +1,9 @@
+# dummy
+
+## Moonscript-specific instructions
+
+Lua, and Moonscript, cannot store the value `nil` in a table.
+Lua's concept of `nil` is the _absence_ of a value.
+Lua uses the assignment of `nil` to a table key to delete that key from the table.
+
+In this exercise, you'll exclude the **string** `"null"` from the flattened array.
diff --git a/exercises/practice/flatten-array/.docs/instructions.md b/exercises/practice/flatten-array/.docs/instructions.md
new file mode 100644
index 0000000..b5b8271
--- /dev/null
+++ b/exercises/practice/flatten-array/.docs/instructions.md
@@ -0,0 +1,16 @@
+# Instructions
+
+Take a nested array of any depth and return a fully flattened array.
+
+Note that some language tracks may include null-like values in the input array, and the way these values are represented varies by track.
+Such values should be excluded from the flattened array.
+
+Additionally, the input may be of a different data type and contain different types, depending on the track.
+
+Check the test suite for details.
+
+## Example
+
+input: `[1, [2, 6, null], [[null, [4]], 5]]`
+
+output: `[1, 2, 6, 4, 5]`
diff --git a/exercises/practice/flatten-array/.docs/introduction.md b/exercises/practice/flatten-array/.docs/introduction.md
new file mode 100644
index 0000000..a314857
--- /dev/null
+++ b/exercises/practice/flatten-array/.docs/introduction.md
@@ -0,0 +1,7 @@
+# Introduction
+
+A shipment of emergency supplies has arrived, but there's a problem.
+To protect from damage, the items — flashlights, first-aid kits, blankets — are packed inside boxes, and some of those boxes are nested several layers deep inside other boxes!
+
+To be prepared for an emergency, everything must be easily accessible in one box.
+Can you unpack all the supplies and place them into a single box, so they're ready when needed most?
diff --git a/exercises/practice/flatten-array/.meta/config.json b/exercises/practice/flatten-array/.meta/config.json
new file mode 100644
index 0000000..274f470
--- /dev/null
+++ b/exercises/practice/flatten-array/.meta/config.json
@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "flatten_array.moon"
+    ],
+    "test": [
+      "flatten_array_spec.moon"
+    ],
+    "example": [
+      ".meta/example.moon"
+    ]
+  },
+  "blurb": "Take a nested list and return a single list with all values except nil/null.",
+  "source": "Interview Question",
+  "source_url": "https://reference.wolfram.com/language/ref/Flatten.html"
+}
diff --git a/exercises/practice/flatten-array/.meta/example.moon b/exercises/practice/flatten-array/.meta/example.moon
new file mode 100644
index 0000000..3925045
--- /dev/null
+++ b/exercises/practice/flatten-array/.meta/example.moon
@@ -0,0 +1,15 @@
+{
+  flatten: (input) ->
+    result = {}
+
+    -- a recursive closure
+    _flatten = (list) ->
+      for elem in *list
+        if type(elem) == 'table'
+          _flatten elem
+        elseif elem != 'null'
+          table.insert result, elem
+
+    _flatten input
+    result
+}
diff --git a/exercises/practice/flatten-array/.meta/spec_generator.moon b/exercises/practice/flatten-array/.meta/spec_generator.moon
new file mode 100644
index 0000000..ba9ff78
--- /dev/null
+++ b/exercises/practice/flatten-array/.meta/spec_generator.moon
@@ -0,0 +1,34 @@
+json = (require 'dkjson').use_lpeg!
+
+int_list = (list) -> "{#{table.concat list, ', '}}"
+
+-- Lua can't store the literal `nil` in a table.  nil, being the
+-- absence of a value, is used to delete a key in a table.  And
+-- `ipairs` stops at the first index gap.  Seems to me, the
+-- easiest way to render an arbitrary list of lists is to
+-- stringify it back into JSON, and then munge the string.
+-- Unfortunately, we seem to lose _trailing_ nulls this way.
+
+nested_lists = (list, desc) ->
+  a = json.encode list
+  a = a\gsub '%[', '{'
+  a = a\gsub '%]', '}'
+  a = a\gsub ',', ', '
+  a = a\gsub 'null', '"null"'
+  
+  if desc == "null values are omitted from the final result"
+    a\gsub '}$', ', "null"}'
+  else
+    a
+
+{
+  module_imports: {'flatten'},
+
+  generate_test: (case, level) ->
+    lines = {
+      "input = #{nested_lists case.input.array, case.description}",
+      "expected = #{int_list case.expected}",
+      "assert.are.same expected, flatten input"
+    }
+    table.concat [indent line, level for line in *lines], '\n'
+}
diff --git a/exercises/practice/flatten-array/.meta/tests.toml b/exercises/practice/flatten-array/.meta/tests.toml
new file mode 100644
index 0000000..44acf17
--- /dev/null
+++ b/exercises/practice/flatten-array/.meta/tests.toml
@@ -0,0 +1,63 @@
+# 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.
+
+[8c71dabd-da60-422d-a290-4a571471fb14]
+description = "empty"
+
+[d268b919-963c-442d-9f07-82b93f1b518c]
+description = "no nesting"
+
+[3f15bede-c856-479e-bb71-1684b20c6a30]
+description = "flattens a nested array"
+
+[c84440cc-bb3a-48a6-862c-94cf23f2815d]
+description = "flattens array with just integers present"
+
+[d3d99d39-6be5-44f5-a31d-6037d92ba34f]
+description = "5 level nesting"
+
+[d572bdba-c127-43ed-bdcd-6222ac83d9f7]
+description = "6 level nesting"
+
+[0705a8e5-dc86-4cec-8909-150c5e54fa9c]
+description = "null values are omitted from the final result"
+
+[c6cf26de-8ccd-4410-84bd-b9efd88fd2bc]
+description = "consecutive null values at the front of the list are omitted from the final result"
+include = false
+
+[bc72da10-5f55-4ada-baf3-50e4da02ec8e]
+description = "consecutive null values at the front of the array are omitted from the final result"
+reimplements = "c6cf26de-8ccd-4410-84bd-b9efd88fd2bc"
+
+[382c5242-587e-4577-b8ce-a5fb51e385a1]
+description = "consecutive null values in the middle of the list are omitted from the final result"
+include = false
+
+[6991836d-0d9b-4703-80a0-3f1f23eb5981]
+description = "consecutive null values in the middle of the array are omitted from the final result"
+reimplements = "382c5242-587e-4577-b8ce-a5fb51e385a1"
+
+[ef1d4790-1b1e-4939-a179-51ace0829dbd]
+description = "6 level nest list with null values"
+include = false
+
+[dc90a09c-5376-449c-a7b3-c2d20d540069]
+description = "6 level nested array with null values"
+reimplements = "ef1d4790-1b1e-4939-a179-51ace0829dbd"
+
+[85721643-705a-4150-93ab-7ae398e2942d]
+description = "all values in nested list are null"
+include = false
+
+[51f5d9af-8f7f-4fb5-a156-69e8282cb275]
+description = "all values in nested array are null"
+reimplements = "85721643-705a-4150-93ab-7ae398e2942d"
diff --git a/exercises/practice/flatten-array/flatten_array.moon b/exercises/practice/flatten-array/flatten_array.moon
new file mode 100644
index 0000000..8e128f3
--- /dev/null
+++ b/exercises/practice/flatten-array/flatten_array.moon
@@ -0,0 +1,4 @@
+{
+  flatten: (input) ->
+    error 'implement me'
+}
diff --git a/exercises/practice/flatten-array/flatten_array_spec.moon b/exercises/practice/flatten-array/flatten_array_spec.moon
new file mode 100644
index 0000000..77aca82
--- /dev/null
+++ b/exercises/practice/flatten-array/flatten_array_spec.moon
@@ -0,0 +1,57 @@
+import flatten from require 'flatten_array'
+
+describe 'flatten-array', ->
+  it 'empty', ->
+    input = {}
+    expected = {}
+    assert.are.same expected, flatten input
+
+  pending 'no nesting', ->
+    input = {0, 1, 2}
+    expected = {0, 1, 2}
+    assert.are.same expected, flatten input
+
+  pending 'flattens a nested array', ->
+    input = {{{}}}
+    expected = {}
+    assert.are.same expected, flatten input
+
+  pending 'flattens array with just integers present', ->
+    input = {1, {2, 3, 4, 5, 6, 7}, 8}
+    expected = {1, 2, 3, 4, 5, 6, 7, 8}
+    assert.are.same expected, flatten input
+
+  pending '5 level nesting', ->
+    input = {0, 2, {{2, 3}, 8, 100, 4, {{{50}}}}, -2}
+    expected = {0, 2, 2, 3, 8, 100, 4, 50, -2}
+    assert.are.same expected, flatten input
+
+  pending '6 level nesting', ->
+    input = {1, {2, {{3}}, {4, {{5}}}, 6, 7}, 8}
+    expected = {1, 2, 3, 4, 5, 6, 7, 8}
+    assert.are.same expected, flatten input
+
+  pending 'null values are omitted from the final result', ->
+    input = {1, 2, "null"}
+    expected = {1, 2}
+    assert.are.same expected, flatten input
+
+  pending 'consecutive null values at the front of the array are omitted from the final result', ->
+    input = {"null", "null", 3}
+    expected = {3}
+    assert.are.same expected, flatten input
+
+  pending 'consecutive null values in the middle of the array are omitted from the final result', ->
+    input = {1, "null", "null", 4}
+    expected = {1, 4}
+    assert.are.same expected, flatten input
+
+  pending '6 level nested array with null values', ->
+    input = {0, 2, {{2, 3}, 8, {{100}}, "null", {{}}}, -2}
+    expected = {0, 2, 2, 3, 8, 100, -2}
+    assert.are.same expected, flatten input
+
+  pending 'all values in nested array are null', ->
+    input = {"null", {{{}}}, "null", "null", {{}}}
+    expected = {}
+    assert.are.same expected, flatten input