From 99a2e8a73238e83847f0a1b697736a7a6d0492c3 Mon Sep 17 00:00:00 2001
From: BNAndras <20251272+BNAndras@users.noreply.github.com>
Date: Sun, 8 Mar 2026 21:48:48 -0700
Subject: [PATCH] add `strain`

---
 config.json                                   |   8 ++
 exercises/practice/strain/.busted             |   5 +
 .../practice/strain/.docs/instructions.md     |  29 +++++
 exercises/practice/strain/.meta/config.json   |  19 +++
 exercises/practice/strain/.meta/example.moon  |   7 ++
 .../practice/strain/.meta/spec_generator.moon |  52 +++++++++
 exercises/practice/strain/.meta/tests.toml    |  52 +++++++++
 exercises/practice/strain/strain.moon         |   7 ++
 exercises/practice/strain/strain_spec.moon    | 108 ++++++++++++++++++
 9 files changed, 287 insertions(+)
 create mode 100644 exercises/practice/strain/.busted
 create mode 100644 exercises/practice/strain/.docs/instructions.md
 create mode 100644 exercises/practice/strain/.meta/config.json
 create mode 100644 exercises/practice/strain/.meta/example.moon
 create mode 100644 exercises/practice/strain/.meta/spec_generator.moon
 create mode 100644 exercises/practice/strain/.meta/tests.toml
 create mode 100644 exercises/practice/strain/strain.moon
 create mode 100644 exercises/practice/strain/strain_spec.moon

diff --git a/config.json b/config.json
index 6050599..cead9b2 100644
--- a/config.json
+++ b/config.json
@@ -178,6 +178,14 @@
         "prerequisites": [],
         "difficulty": 2
       },
+      {
+        "slug": "strain",
+        "name": "Strain",
+        "uuid": "9fd2db0a-e806-451c-9bea-3d895dccd6f6",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 2
+      },
       {
         "slug": "sum-of-multiples",
         "name": "Sum of Multiples",
diff --git a/exercises/practice/strain/.busted b/exercises/practice/strain/.busted
new file mode 100644
index 0000000..86b84e7
--- /dev/null
+++ b/exercises/practice/strain/.busted
@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}
diff --git a/exercises/practice/strain/.docs/instructions.md b/exercises/practice/strain/.docs/instructions.md
new file mode 100644
index 0000000..3469ae6
--- /dev/null
+++ b/exercises/practice/strain/.docs/instructions.md
@@ -0,0 +1,29 @@
+# Instructions
+
+Implement the `keep` and `discard` operation on collections.
+Given a collection and a predicate on the collection's elements, `keep` returns a new collection containing those elements where the predicate is true, while `discard` returns a new collection containing those elements where the predicate is false.
+
+For example, given the collection of numbers:
+
+- 1, 2, 3, 4, 5
+
+And the predicate:
+
+- is the number even?
+
+Then your keep operation should produce:
+
+- 2, 4
+
+While your discard operation should produce:
+
+- 1, 3, 5
+
+Note that the union of keep and discard is all the elements.
+
+The functions may be called `keep` and `discard`, or they may need different names in order to not clash with existing functions or concepts in your language.
+
+## Restrictions
+
+Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library!
+Solve this one yourself using other basic tools instead.
diff --git a/exercises/practice/strain/.meta/config.json b/exercises/practice/strain/.meta/config.json
new file mode 100644
index 0000000..0bc45c6
--- /dev/null
+++ b/exercises/practice/strain/.meta/config.json
@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "BNAndras"
+  ],
+  "files": {
+    "solution": [
+      "strain.moon"
+    ],
+    "test": [
+      "strain_spec.moon"
+    ],
+    "example": [
+      ".meta/example.moon"
+    ]
+  },
+  "blurb": "Implement the `keep` and `discard` operation on collections.",
+  "source": "Conversation with James Edward Gray II",
+  "source_url": "http://graysoftinc.com/"
+}
diff --git a/exercises/practice/strain/.meta/example.moon b/exercises/practice/strain/.meta/example.moon
new file mode 100644
index 0000000..6ea65a3
--- /dev/null
+++ b/exercises/practice/strain/.meta/example.moon
@@ -0,0 +1,7 @@
+{
+  keep: (list, predicate) ->
+    [item for item in *list when predicate(item)]
+
+  discard: (list, predicate) ->
+    [item for item in *list when not predicate(item)]
+}
diff --git a/exercises/practice/strain/.meta/spec_generator.moon b/exercises/practice/strain/.meta/spec_generator.moon
new file mode 100644
index 0000000..ed6b3c1
--- /dev/null
+++ b/exercises/practice/strain/.meta/spec_generator.moon
@@ -0,0 +1,52 @@
+format_predicate = (pred) ->
+  p = pred\gsub "fn%(x%) %-> ", "(x) -> "
+  p = p\gsub "contains%(x, 5%)", "contains x, 5"
+  p = p\gsub "starts_with%(x, 'z'%)", "starts_with x, 'z'"
+  p
+
+format_list = (list) ->
+  if #list == 0
+    return "{}"
+  elseif type(list[1]) == 'string'
+    return "{" .. table.concat([quote(elem) for elem in *list], ', ') .. "}"
+  else
+    return "{" .. table.concat(list, ', ') .. "}"
+
+format_value = (val, level) ->
+  if #val == 0
+    '{}'
+  elseif type(val[1]) == 'table'
+    rows = [indent format_list(row), level + 1 for row in *val]
+    table.insert rows, 1, '{'
+    table.insert rows, indent('}', level)
+    table.concat rows, '\n'
+  else
+    format_list(val)
+
+{
+  module_name: 'Strain'
+
+  test_helpers: [[
+  local starts_with, contains
+
+  starts_with = (str, prefix) ->
+    str\sub(1, #prefix) == prefix
+
+  contains = (list, element) ->
+    for item in *list
+      if item == element
+        return true
+    false
+]]
+
+  generate_test: (case, level) ->
+
+
+    lines = {
+      "result = Strain.#{case.property} #{format_value(case.input.list, level)}, #{format_predicate case.input.predicate}"
+      "expected = #{format_value(case.expected, level)}"
+      "assert.are.same expected, result"
+    }
+
+    table.concat [indent line, level for line in *lines], '\n'
+}
diff --git a/exercises/practice/strain/.meta/tests.toml b/exercises/practice/strain/.meta/tests.toml
new file mode 100644
index 0000000..3a617b4
--- /dev/null
+++ b/exercises/practice/strain/.meta/tests.toml
@@ -0,0 +1,52 @@
+# 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.
+
+[26af8c32-ba6a-4eb3-aa0a-ebd8f136e003]
+description = "keep on empty list returns empty list"
+
+[f535cb4d-e99b-472a-bd52-9fa0ffccf454]
+description = "keeps everything"
+
+[950b8e8e-f628-42a8-85e2-9b30f09cde38]
+description = "keeps nothing"
+
+[92694259-6e76-470c-af87-156bdf75018a]
+description = "keeps first and last"
+
+[938f7867-bfc7-449e-a21b-7b00cbb56994]
+description = "keeps neither first nor last"
+
+[8908e351-4437-4d2b-a0f7-770811e48816]
+description = "keeps strings"
+
+[2728036b-102a-4f1e-a3ef-eac6160d876a]
+description = "keeps lists"
+
+[ef16beb9-8d84-451a-996a-14e80607fce6]
+description = "discard on empty list returns empty list"
+
+[2f42f9bc-8e06-4afe-a222-051b5d8cd12a]
+description = "discards everything"
+
+[ca990fdd-08c2-4f95-aa50-e0f5e1d6802b]
+description = "discards nothing"
+
+[71595dae-d283-48ca-a52b-45fa96819d2f]
+description = "discards first and last"
+
+[ae141f79-f86d-4567-b407-919eaca0f3dd]
+description = "discards neither first nor last"
+
+[daf25b36-a59f-4f29-bcfe-302eb4e43609]
+description = "discards strings"
+
+[a38d03f9-95ad-4459-80d1-48e937e4acaf]
+description = "discards lists"
diff --git a/exercises/practice/strain/strain.moon b/exercises/practice/strain/strain.moon
new file mode 100644
index 0000000..e575fbc
--- /dev/null
+++ b/exercises/practice/strain/strain.moon
@@ -0,0 +1,7 @@
+{
+  keep: (list, predicate) ->
+    error 'Implement me'
+  
+  discard: (list, predicate) ->
+    error 'Implement me'
+}
diff --git a/exercises/practice/strain/strain_spec.moon b/exercises/practice/strain/strain_spec.moon
new file mode 100644
index 0000000..8c64114
--- /dev/null
+++ b/exercises/practice/strain/strain_spec.moon
@@ -0,0 +1,108 @@
+Strain = require 'strain'
+
+describe 'strain', ->
+  local starts_with, contains
+
+  starts_with = (str, prefix) ->
+    str\sub(1, #prefix) == prefix
+
+  contains = (list, element) ->
+    for item in *list
+      if item == element
+        return true
+    false
+
+  it 'keep on empty list returns empty list', ->
+    result = Strain.keep {}, (x) -> true
+    expected = {}
+    assert.are.same expected, result
+
+  pending 'keeps everything', ->
+    result = Strain.keep {1, 3, 5}, (x) -> true
+    expected = {1, 3, 5}
+    assert.are.same expected, result
+
+  pending 'keeps nothing', ->
+    result = Strain.keep {1, 3, 5}, (x) -> false
+    expected = {}
+    assert.are.same expected, result
+
+  pending 'keeps first and last', ->
+    result = Strain.keep {1, 2, 3}, (x) -> x % 2 == 1
+    expected = {1, 3}
+    assert.are.same expected, result
+
+  pending 'keeps neither first nor last', ->
+    result = Strain.keep {1, 2, 3}, (x) -> x % 2 == 0
+    expected = {2}
+    assert.are.same expected, result
+
+  pending 'keeps strings', ->
+    result = Strain.keep {'apple', 'zebra', 'banana', 'zombies', 'cherimoya', 'zealot'}, (x) -> starts_with x, 'z'
+    expected = {'zebra', 'zombies', 'zealot'}
+    assert.are.same expected, result
+
+  pending 'keeps lists', ->
+    result = Strain.keep {
+      {1, 2, 3}
+      {5, 5, 5}
+      {5, 1, 2}
+      {2, 1, 2}
+      {1, 5, 2}
+      {2, 2, 1}
+      {1, 2, 5}
+    }, (x) -> contains x, 5
+    expected = {
+      {5, 5, 5}
+      {5, 1, 2}
+      {1, 5, 2}
+      {1, 2, 5}
+    }
+    assert.are.same expected, result
+
+  pending 'discard on empty list returns empty list', ->
+    result = Strain.discard {}, (x) -> true
+    expected = {}
+    assert.are.same expected, result
+
+  pending 'discards everything', ->
+    result = Strain.discard {1, 3, 5}, (x) -> true
+    expected = {}
+    assert.are.same expected, result
+
+  pending 'discards nothing', ->
+    result = Strain.discard {1, 3, 5}, (x) -> false
+    expected = {1, 3, 5}
+    assert.are.same expected, result
+
+  pending 'discards first and last', ->
+    result = Strain.discard {1, 2, 3}, (x) -> x % 2 == 1
+    expected = {2}
+    assert.are.same expected, result
+
+  pending 'discards neither first nor last', ->
+    result = Strain.discard {1, 2, 3}, (x) -> x % 2 == 0
+    expected = {1, 3}
+    assert.are.same expected, result
+
+  pending 'discards strings', ->
+    result = Strain.discard {'apple', 'zebra', 'banana', 'zombies', 'cherimoya', 'zealot'}, (x) -> starts_with x, 'z'
+    expected = {'apple', 'banana', 'cherimoya'}
+    assert.are.same expected, result
+
+  pending 'discards lists', ->
+    result = Strain.discard {
+      {1, 2, 3}
+      {5, 5, 5}
+      {5, 1, 2}
+      {2, 1, 2}
+      {1, 5, 2}
+      {2, 2, 1}
+      {1, 2, 5}
+    }, (x) -> contains x, 5
+    expected = {
+      {1, 2, 3}
+      {2, 1, 2}
+      {2, 2, 1}
+    }
+    assert.are.same expected, result