Add parallel letter exercise

config.json

@@ -1337,6 +1337,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "parallel-letter-frequency",
+        "name": "Parallel Letter Frequency",
+        "uuid": "f893d23c-c432-48fd-ae87-0561451cb180",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "dominoes",
         "name": "Dominoes",

exercises/practice/parallel-letter-frequency/.docs/instructions.append.md

@@ -0,0 +1,11 @@
+# Instruction Appendix
+
+For building a concurrent/parallel program in Crystal, [might this resource prove useful][resource].
+
+The online environment for this exercise is a bit different from most other exercises.
+Instead of compiling Crystal into a single-threaded binary, this exercises uses Crystal exprimental multi-threading capabilities.
+As such, the execution might behave differently than expected in some environments.
+
+For local development, it is recommended that you have Crystal version 1.19.0 or later installed, and add `-Dpreview_mt` flag when compiling or running your code to enable multi-threading support.
+
+[resource]: https://crystal-lang.org/reference/latest/guides/concurrency.html

exercises/practice/parallel-letter-frequency/.docs/instructions.md

@@ -0,0 +1,7 @@
+# Instructions
+
+Count the frequency of letters in texts using parallel computation.
+
+Parallelism is about doing things in parallel that can also be done sequentially.
+A common example is counting the frequency of letters.
+Employ parallelism to calculate the total frequency of each letter in a list of texts.

exercises/practice/parallel-letter-frequency/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "meatball133"
+  ],
+  "files": {
+    "solution": [
+      "src/parallel_letter_frequency.cr"
+    ],
+    "test": [
+      "spec/parallel_letter_frequency_spec.cr"
+    ],
+    "example": [
+      ".meta/src/example.cr"
+    ]
+  },
+  "blurb": "Count the frequency of letters in texts using parallel computation."
+}

exercises/practice/parallel-letter-frequency/.meta/src/example.cr

@@ -0,0 +1,29 @@
+class ParallelLetterFrequency
+  def self.calculate_frequencies(input : Array(String)) : Hash(String, Int32)
+    frequencies = Hash(String, Int32).new(0)
+    channel = Channel(Hash(String, Int32)).new
+
+    input.each do |text|
+      spawn do
+        local = Hash(String, Int32).new(0)
+        processed = 0
+
+        text.each_char do |char|
+          next unless char.letter?
+          local[char.downcase.to_s] += 1
+
+          # Crystal fibers are cooperatively scheduled; yielding occasionally
+          # prevents one long-running fiber from starving the others.
+        end
+        channel.send(local)
+      end
+    end
+    input.size.times do |x|
+      hash = channel.receive
+      frequencies.merge!(hash) do |key, oldval, newval|
+        oldval + newval
+      end
+    end
+    frequencies
+  end
+end

exercises/practice/parallel-letter-frequency/.meta/test_template.ecr

@@ -0,0 +1,12 @@
+require "spec"
+require "../src/*"
+
+describe "<%-= to_capitalized(@json["exercise"].to_s) %>" do
+<%- @json["cases"].as_a.each do |cases| %>
+    <%= status()%> "<%-= cases["description"] %>" do
+        input = [<%= cases["input"]["texts"].as_a.map { |text| text.to_s.inspect }.join(", ") %>] <%= to_s_deep(cases["input"]["texts"]) == "[]" ? "of String" : ""%>
+        expected = <%= to_s_deep(cases["expected"]) == "{}" ? "{} of String => Int32" : to_s_deep(cases["expected"]) %>
+        <%= to_capitalized(@json["exercise"].to_s) %>.<%= cases["property"].to_s.underscore %>(input).should eq(expected)
+    end
+<% end %>
+end

exercises/practice/parallel-letter-frequency/.meta/tests.toml

@@ -0,0 +1,49 @@
+# 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.
+
+[c054d642-c1fa-4234-8007-9339f2337886]
+description = "no texts"
+
+[818031be-49dc-4675-b2f9-c4047f638a2a]
+description = "one text with one letter"
+
+[c0b81d1b-940d-4cea-9f49-8445c69c17ae]
+description = "one text with multiple letters"
+
+[708ff1e0-f14a-43fd-adb5-e76750dcf108]
+description = "two texts with one letter"
+
+[1b5c28bb-4619-4c9d-8db9-a4bb9c3bdca0]
+description = "two texts with multiple letters"
+
+[6366e2b8-b84c-4334-a047-03a00a656d63]
+description = "ignore letter casing"
+
+[92ebcbb0-9181-4421-a784-f6f5aa79f75b]
+description = "ignore whitespace"
+
+[bc5f4203-00ce-4acc-a5fa-f7b865376fd9]
+description = "ignore punctuation"
+
+[68032b8b-346b-4389-a380-e397618f6831]
+description = "ignore numbers"
+
+[aa9f97ac-3961-4af1-88e7-6efed1bfddfd]
+description = "Unicode letters"
+
+[7b1da046-701b-41fc-813e-dcfb5ee51813]
+description = "combination of lower- and uppercase letters, punctuation and white space"
+
+[4727f020-df62-4dcf-99b2-a6e58319cb4f]
+description = "large texts"
+
+[adf8e57b-8e54-4483-b6b8-8b32c115884c]
+description = "many small texts"

exercises/practice/parallel-letter-frequency/src/parallel_letter_frequency.cr

@@ -0,0 +1,5 @@
+class ParallelLetterFrequency
+  def self.calculate_frequencies(input : Array(String)) : Hash(String, Int32)
+    # Write your code for the 'ParallelLetterFrequency' exercise in this file.
+  end
+end