Add strain exxercise

config.json

@@ -175,6 +175,14 @@
           "math"
         ]
       },
+      {
+        "slug": "strain",
+        "name": "Strain",
+        "uuid": "1b205a7a-7602-4e96-941c-3e56b6aecb3a",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 2
+      },
       {
         "slug": "triangle",
         "name": "Triangle",

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.

exercises/practice/strain/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "BNAndras"
+  ],
+  "files": {
+    "solution": [
+      "strain.coffee"
+    ],
+    "test": [
+      "strain.spec.coffee"
+    ],
+    "example": [
+      ".meta/example.coffee"
+    ]
+  },
+  "blurb": "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.",
+  "source": "Conversation with James Edward Gray II",
+  "source_url": "http://graysoftinc.com/"
+}

exercises/practice/strain/.meta/example.coffee

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+class Strain
+  keep: (lst, predicate) -> 
+    x for x in lst when predicate(x)
+
+  discard: (lst, predicate) ->
+    x for x in lst when !predicate(x)
+
+module.exports = Strain

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"

exercises/practice/strain/strain.coffee

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+class Strain
+  keep: (lst, predicate) -> 
+
+  discard: (lst, predicate) ->
+
+module.exports = Strain

exercises/practice/strain/strain.spec.coffee

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+Strain = require './strain'
+
+describe 'Strain', ->
+  alwaysTrue = (_) -> true
+  alwaysFalse = (_) -> false
+  isEven = (n) -> n % 2 == 0
+  isOdd = (n) -> n % 2 == 1
+  containsFive = (lst) -> 5 in lst
+  startsWithZ = (str) -> str[0] == 'z'
+
+  strain = new Strain()
+
+  it 'keep on empty list returns empty list', ->
+    data = []
+    result = strain.keep(data, alwaysTrue)
+    expected = []
+    expect(result).toEqual expected
+
+  it 'keeps everything', ->
+    data = [1, 3, 5]
+    result = strain.keep(data, alwaysTrue)
+    expected = [1, 3, 5]
+    expect(result).toEqual expected
+
+  it 'keeps nothing', ->
+    data = [1, 3, 5]
+    result = strain.keep(data, alwaysFalse)
+    expected = []
+    expect(result).toEqual expected
+
+  it 'keeps first and last', ->
+    data = [1, 2, 3]
+    result = strain.keep(data, isOdd)
+    expected = [1, 3]
+    expect(result).toEqual expected
+
+  it 'keeps neither first nor last', ->
+    data = [1, 2, 3]
+    result = strain.keep(data, isEven)
+    expected = [2]
+    expect(result).toEqual expected
+
+  it 'keeps strings', ->
+    data = ["apple", "zebra", "banana", "zombies", "cherimoya", "zealot"]
+    result = strain.keep(data, startsWithZ)
+    expected = ["zebra", "zombies", "zealot"]
+    expect(result).toEqual expected
+
+  it 'keeps lists', ->
+    data = [
+        [1, 2, 3],
+        [5, 5, 5],
+        [5, 1, 2],
+        [2, 1, 2],
+        [1, 5, 2],
+        [2, 2, 1],
+        [1, 2, 5]
+    ]
+    result = strain.keep(data, containsFive)
+    expected = [
+        [5, 5, 5],
+        [5, 1, 2],
+        [1, 5, 2],
+        [1, 2, 5]
+    ]
+    expect(result).toEqual expected
+
+  it 'discard on empty list returns empty list', ->
+    data = []
+    result = strain.discard(data, alwaysTrue)
+    expected = []
+    expect(result).toEqual expected
+
+  it 'discards everything', ->
+    data = [1, 3, 5]
+    result = strain.discard(data, alwaysTrue)
+    expected = []
+    expect(result).toEqual expected
+
+  it 'discards nothing', ->
+    data = [1, 3, 5]
+    result = strain.discard(data, alwaysFalse)
+    expected = [1, 3, 5]
+    expect(result).toEqual expected
+
+  it 'discards first and last', ->
+    data = [1, 2, 3]
+    result = strain.discard(data, isOdd)
+    expected = [2]
+    expect(result).toEqual expected
+
+  it 'discards neither first or last', ->
+    data = [1, 2, 3]
+    result = strain.discard(data, isEven)
+    expected = [1, 3]
+    expect(result).toEqual expected
+
+  it 'discards strings', ->
+    data = ["apple", "zebra", "banana", "zombies", "cherimoya", "zealot"]
+    result = strain.discard(data, startsWithZ)
+    expected = ["apple", "banana", "cherimoya"]
+    expect(result).toEqual expected
+
+  it 'discards lists', ->
+    data = [
+        [1, 2, 3],
+        [5, 5, 5],
+        [5, 1, 2],
+        [2, 1, 2],
+        [1, 5, 2],
+        [2, 2, 1],
+        [1, 2, 5]
+    ]
+    result = strain.discard(data, containsFive)
+    expected = [
+        [1, 2, 3],
+        [2, 1, 2],
+        [2, 2, 1]
+    ]
+    expect(result).toEqual expected