commit 2c9de8cefdade730af1da8dcdf36f3e40df62e8e
Author: Dominic Grimm <dominic.grimm@gmail.com>
Date:   Sat Nov 20 13:14:36 2021 +0100

    init

diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..163eb75
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,9 @@
+root = true
+
+[*.cr]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
diff --git a/.github/workflows/crystal.yml b/.github/workflows/crystal.yml
new file mode 100644
index 0000000..ff8902c
--- /dev/null
+++ b/.github/workflows/crystal.yml
@@ -0,0 +1,19 @@
+name: Crystal CI
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: crystallang/crystal
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install dependencies
+      run: shards install
+    - name: Run tests
+      run: crystal spec -v
diff --git a/.gitignore b/.gitignore
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index 0000000..0bbd4a9
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,9 @@
+/docs/
+/lib/
+/bin/
+/.shards/
+*.dwarf
+
+# Libraries don't need dependency lock
+# Dependencies will be locked in applications that use them
+/shard.lock
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..37977e4
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2021 Dominic Grimm <dominic.grimm@gmail.com>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
diff --git a/README.md b/README.md
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--- /dev/null
+++ b/README.md
@@ -0,0 +1,56 @@
+# lifo
+
+[![GitHub release](https://img.shields.io/github/release/grimmigerFuchs/lifo.svg)](https://github.com/grimmigerFuchs/lifo/releases)
+
+A library for working with LIFO like data structures (stacks, queues).
+
+## Installation
+
+1. Add the dependency to your `shard.yml`:
+
+   ```yaml
+   dependencies:
+     stack:
+       github: grimmigerFuchs/lifo
+   ```
+
+2. Run `shards install`
+
+## Usage
+
+```crystal
+require "lifo"
+
+stack = Lifo::Stack(String).new(3)
+stack.push("Foo")
+stack.push("Bar")
+stack.push("Baz")
+
+p! stack.pop
+# stack.pop # => "Baz"
+p! stack.pop
+# stack.pop # => "Bar"
+p! stack.peek
+# stack.peek # => "Foo"
+
+queue = Lifo::Queue(String).new(3)
+queue.enqueue("John Doe")
+queue.enqueue("Jane Doe")
+
+p! queue.dequeue
+# queue.dequeue # => "John Doe"
+p! queue.peek
+# queue.peek # => "Jane Doe"
+```
+
+## Contributing
+
+1. Fork it (<https://github.com/grimmigerFuchs/lifo/fork>)
+2. Create your feature branch (`git checkout -b my-new-feature`)
+3. Commit your changes (`git commit -am 'Add some feature'`)
+4. Push to the branch (`git push origin my-new-feature`)
+5. Create a new Pull Request
+
+## Contributors
+
+- [Dominic Grimm](https://github.com/grimmigerFuchs) - creator and maintainer
diff --git a/shard.yml b/shard.yml
new file mode 100644
index 0000000..7eeb3e9
--- /dev/null
+++ b/shard.yml
@@ -0,0 +1,10 @@
+name: lifo
+description: A library for working with LIFO like data structures (stacks, queues).
+version: 0.1.0
+
+authors:
+  - Dominic Grimm <dominic.grimm@gmail.com>
+
+crystal: 1.2.2
+
+license: MIT
diff --git a/spec/spec_helper.cr b/spec/spec_helper.cr
new file mode 100644
index 0000000..202da46
--- /dev/null
+++ b/spec/spec_helper.cr
@@ -0,0 +1,2 @@
+require "spec"
+require "../src/lifo"
diff --git a/spec/stack/queue.cr b/spec/stack/queue.cr
new file mode 100644
index 0000000..c8c5248
--- /dev/null
+++ b/spec/stack/queue.cr
@@ -0,0 +1,104 @@
+describe Lifo::Queue do
+  describe "#initialize" do
+    it "sets capacity to -1 if none is given" do
+      queue = Lifo::Queue(Int32).new
+      queue.capacity.should eq -1
+    end
+  end
+
+  describe "#enqueue" do
+    it "enqueues to the queue" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue(1)
+      queue.peek.should eq(1)
+      queue.enqueue(3)
+      queue.peek.should eq(1)
+      queue.enqueue(5)
+      queue.peek.should eq(1)
+    end
+
+    it "returns queue size" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue(1)
+      queue.enqueue(3)
+      queue.size.should eq(2)
+    end
+
+    it "raises OverflowError if queue is full" do
+      queue = Lifo::Queue(Int32).new(0)
+
+      expect_raises(Lifo::OverflowError) do
+        queue.enqueue(1)
+      end
+    end
+
+    it "doesn't raise an error when queue is infinite" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue(1)
+    end
+  end
+
+  describe "#enqueue?" do
+    it "enqueues to the queue" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue?(1)
+      queue.peek.should eq(1)
+      queue.enqueue?(3)
+      queue.peek.should eq(1)
+      queue.enqueue?(5)
+      queue.peek.should eq(1)
+    end
+
+    it "returns queue size" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue?(1)
+      queue.enqueue?(3)
+      queue.size.should eq(2)
+    end
+
+    it "returns nil if error raised" do
+      queue = Lifo::Queue(Int32).new(0)
+      queue.enqueue?(1).should be_nil
+    end
+  end
+
+  describe "#dequeue" do
+    it "dequeues from the queue" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue(1)
+      queue.enqueue(3)
+      queue.peek.should eq(1)
+      queue.dequeue
+      queue.peek.should eq(3)
+      queue.dequeue
+    end
+
+    it "returns dequeued element" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue(1)
+      queue.enqueue(3)
+      queue.peek.should eq(1)
+      queue.dequeue.should eq(1)
+      queue.dequeue.should eq(3)
+    end
+
+    it "raises UnderflowError if queue is empty" do
+      queue = Lifo::Queue(Int32).new(0)
+      expect_raises(Lifo::UnderflowError) do
+        queue.dequeue
+      end
+    end
+  end
+
+  describe "#dequeue?" do
+    it "dequeues from the queue" do
+      queue = Lifo::Queue(Int32).new
+      queue.enqueue(1)
+      queue.enqueue(3)
+      queue.peek.should eq(1)
+      queue.dequeue?
+      queue.peek.should eq(3)
+      queue.dequeue?
+    end
+  end
+end
diff --git a/spec/stack/stack.cr b/spec/stack/stack.cr
new file mode 100644
index 0000000..6725e0a
--- /dev/null
+++ b/spec/stack/stack.cr
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+describe Lifo::Stack do
+  describe "#initialize" do
+    it "sets capacity to -1 if none is given" do
+      stack = Lifo::Stack(Int32).new
+      stack.capacity.should eq -1
+    end
+  end
+
+  describe "#push" do
+    it "pushes to the stack" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.peek.should eq(1)
+      stack.push(3)
+      stack.peek.should eq(3)
+    end
+
+    it "returns stack size" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.push(3)
+      stack.size.should eq(2)
+    end
+
+    it "raises OverflowError if stack is full" do
+      stack = Lifo::Stack(Int32).new(0)
+
+      expect_raises(Lifo::OverflowError) do
+        stack.push(1)
+      end
+    end
+
+    it "doesn't raise an error when stack is infinite" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+    end
+  end
+
+  describe "#push?" do
+    it "pushes to the stack" do
+      stack = Lifo::Stack(Int32).new
+      stack.push?(1)
+      stack.peek.should eq(1)
+      stack.push?(3)
+      stack.peek.should eq(3)
+    end
+
+    it "returns stack size" do
+      stack = Lifo::Stack(Int32).new
+      stack.push?(1)
+      stack.push?(3)
+      stack.size.should eq(2)
+    end
+
+    it "returns nil if error raised" do
+      stack = Lifo::Stack(Int32).new(0)
+      stack.push?(1).should be_nil
+    end
+  end
+
+  describe "#pop" do
+    it "pops from the stack" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.push(3)
+      stack.peek.should eq(3)
+      stack.pop
+      stack.peek.should eq(1)
+      stack.pop
+    end
+
+    it "returns popped element" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.push(3)
+      stack.pop.should eq(3)
+      stack.pop.should eq(1)
+    end
+
+    it "raises UnderflowError if stack is empty" do
+      stack = Lifo::Stack(Int32).new
+      expect_raises(Lifo::UnderflowError) do
+        stack.pop
+      end
+    end
+  end
+
+  describe "#pop?" do
+    it "pops from the stack" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.push(3)
+      stack.peek.should eq(3)
+      stack.pop?
+      stack.peek.should eq(1)
+      stack.pop?
+    end
+
+    it "returns popped element" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.push(3)
+      stack.peek.should eq(3)
+      stack.pop?.should eq(3)
+      stack.peek.should eq(1)
+      stack.pop?.should eq(1)
+    end
+
+    it "returns nil if stack is empty" do
+      stack = Lifo::Stack(Int32).new
+      stack.pop?.should be_nil
+    end
+  end
+
+  describe "#peek" do
+    it "returns top element" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(2)
+      stack.peek.should eq(2)
+    end
+
+    it "raises UnderflowError if stack is empty" do
+      stack = Lifo::Stack(Int32).new
+      expect_raises(Lifo::UnderflowError) do
+        stack.peek
+      end
+    end
+  end
+
+  describe "#peek?" do
+    it "returns top element" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.push(3)
+      stack.peek?.should eq(3)
+    end
+
+    it "returns nil if stack is empty" do
+      stack = Lifo::Stack(Int32).new
+      stack.peek?.should be_nil
+    end
+  end
+
+  describe "#size" do
+    it "returns stack size" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.push(3)
+      stack.size.should eq(2)
+    end
+  end
+
+  describe "#empty?" do
+    it "returns true if stack is empty" do
+      stack = Lifo::Stack(Int32).new
+      stack.empty?.should be_true
+    end
+
+    it "returns false if stack is not empty" do
+      stack = Lifo::Stack(Int32).new
+      stack.push(1)
+      stack.empty?.should be_false
+    end
+  end
+end
diff --git a/spec/stack_spec.cr b/spec/stack_spec.cr
new file mode 100644
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--- /dev/null
+++ b/spec/stack_spec.cr
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+require "./spec_helper"
+
+require "./stack/stack"
+require "./stack/queue"
diff --git a/src/lifo.cr b/src/lifo.cr
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index 0000000..56f142b
--- /dev/null
+++ b/src/lifo.cr
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+require "./lifo/version"
+require "./lifo/exception"
+require "./lifo/lifolike"
+require "./lifo/stack"
+require "./lifo/queue"
diff --git a/src/lifo/exception.cr b/src/lifo/exception.cr
new file mode 100644
index 0000000..3cb4ffc
--- /dev/null
+++ b/src/lifo/exception.cr
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+module Lifo
+  class OverflowError < Exception
+  end
+
+  class UnderflowError < Exception
+  end
+end
diff --git a/src/lifo/lifolike.cr b/src/lifo/lifolike.cr
new file mode 100644
index 0000000..e843d34
--- /dev/null
+++ b/src/lifo/lifolike.cr
@@ -0,0 +1,20 @@
+module Lifo
+  abstract class LifoLike(T)
+    protected property data = [] of T
+    getter capacity
+
+    def initialize(@capacity = -1)
+      if @capacity < -1
+        raise ArgumentError.new("capacity must be >= -1")
+      end
+    end
+
+    def size : Int32
+      @data.size
+    end
+
+    def empty? : Bool
+      size == 0
+    end
+  end
+end
diff --git a/src/lifo/queue.cr b/src/lifo/queue.cr
new file mode 100644
index 0000000..3a1ec66
--- /dev/null
+++ b/src/lifo/queue.cr
@@ -0,0 +1,60 @@
+module Lifo
+  class Queue(T) < LifoLike(T)
+    def initialize(capacity = -1)
+      super(capacity)
+    end
+
+    def enqueue(e : T) : Int32
+      if @capacity == -1 || size < @capacity
+        @data << e
+
+        size
+      else
+        raise OverflowError.new
+      end
+    end
+
+    def enqueue?(e : T) : Int32?
+      begin
+        enqueue(e)
+      rescue OverflowError
+        nil
+      end
+    end
+
+    def dequeue : T
+      begin
+        res = @data.first
+        @data.shift
+      rescue Enumerable::EmptyError
+        raise UnderflowError.new
+      end
+
+      res
+    end
+
+    def dequeue? : T?
+      begin
+        dequeue
+      rescue UnderflowError
+        nil
+      end
+    end
+
+    def peek : T
+      begin
+        @data.first
+      rescue IndexError
+        raise UnderflowError.new
+      end
+    end
+
+    def peek? : T?
+      begin
+        peek
+      rescue UnderflowError
+        nil
+      end
+    end
+  end
+end
diff --git a/src/lifo/stack.cr b/src/lifo/stack.cr
new file mode 100644
index 0000000..8c2a4e0
--- /dev/null
+++ b/src/lifo/stack.cr
@@ -0,0 +1,60 @@
+module Lifo
+  class Stack(T) < LifoLike(T)
+    def initialize(capacity = -1)
+      super(capacity)
+    end
+
+    def push(e : T) : Int32
+      if @capacity == -1 || size < @capacity
+        @data << e
+
+        size
+      else
+        raise OverflowError.new
+      end
+    end
+
+    def push?(e : T) : Int32?
+      begin
+        push(e)
+      rescue OverflowError
+        nil
+      end
+    end
+
+    def pop : T
+      begin
+        res = @data.last
+        @data.pop
+      rescue IndexError
+        raise UnderflowError.new
+      end
+
+      res
+    end
+
+    def pop? : T?
+      begin
+        pop
+      rescue UnderflowError
+        nil
+      end
+    end
+
+    def peek : T
+      begin
+        @data.last
+      rescue IndexError
+        raise UnderflowError.new
+      end
+    end
+
+    def peek? : T?
+      begin
+        peek
+      rescue UnderflowError
+        nil
+      end
+    end
+  end
+end
diff --git a/src/lifo/version.cr b/src/lifo/version.cr
new file mode 100644
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--- /dev/null
+++ b/src/lifo/version.cr
@@ -0,0 +1,3 @@
+module Lifo
+  VERSION = {{ `shards version`.stringify.chomp }}
+end