commit a8363ae1689c43250d785ef97a492c0775a0b915
Author: Danila Fedorin <danila.fedorin@gmail.com>
Date:   Sat Dec 5 15:32:50 2020 -0800

    Write initial version of the library.

diff --git a/.gitignore b/.gitignore
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+++ b/.gitignore
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+/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
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+++ b/LICENSE
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+The MIT License (MIT)
+
+Copyright (c) 2020 your-name-here
+
+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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+++ b/README.md
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+# advent
+Collection of common code that may help in solving advent of code
+puzzles.
+
+## Installation
+
+1. Add the dependency to your `shard.yml`:
+
+   ```yaml
+   dependencies:
+     advent:
+       git: https://dev.danilafe.com/Advent-of-Code/advent
+   ```
+
+2. Run `shards install`
+
+## Usage
+
+```crystal
+require "advent"
+```
diff --git a/shard.yml b/shard.yml
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+++ b/shard.yml
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+name: advent
+version: 0.1.0
+
+authors:
+  - Danila Fedorin <danila.fedorin@gmail.com>
+
+crystal: 0.35.1
+
+license: MIT
diff --git a/spec/advent_spec.cr b/spec/advent_spec.cr
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+++ b/spec/advent_spec.cr
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+require "./spec_helper"
diff --git a/spec/graph_spec.cr b/spec/graph_spec.cr
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+++ b/spec/graph_spec.cr
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+require "./spec_helper"
+
+describe Graph do
+  describe "#find_path" do
+    it "reports no paths in a graph with no edges" do
+      gr = Graph(String).new
+      elems = ["a", "b", "c", "d"]
+      elems.each &->gr.add_node(String)
+      elems.each do |elem|
+        elems.each do |other|
+          next if elem == other
+          gr.find_path(elem, other).should be_nil
+        end
+      end
+    end
+
+    it "computes the shortest path" do
+      gr = Graph(String).new
+      [{"a", "b", 1}, {"a", "c", 1}, {"c", "b", 1}, {"b", "d", 1}].each do |t|
+        from, to, cost = t
+        gr.add_edge(from, to, cost)
+      end
+      gr.find_path("a", "d").should eq({["a", "b", "d"], 2})
+    end
+  end
+end
diff --git a/spec/heap_spec.cr b/spec/heap_spec.cr
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+++ b/spec/heap_spec.cr
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+require "./spec_helper"
+
+describe Array do
+  describe "#heapify" do
+    it "preserves the elements in the array" do
+      a = Array(Int32).new(20) { rand(10) }
+      a.sort.should eq(a.clone.heapify!.sort)
+    end
+
+    it "creates an array with proper ordering" do
+      a = Array(Int32).new(20) { rand(10) }
+      a.heapify!
+      a.is_heap?.should be_true
+    end
+
+    it "maintains heap property while popping" do
+      a = Array(Int32).new(20) { rand(10) }
+      a.heapify!
+      20.times do |i|
+        a.heap_pop
+        a.is_heap?.should be_true
+      end
+    end
+
+    it "maintains heap property while pushing" do
+      a = [] of Int32
+      20.times do 
+        a.heap_push(rand 10)
+        a.is_heap?.should be_true
+      end
+    end
+
+    it "pops numbers in descending order" do
+      a = Array(Int32).new(20) { rand(10) }
+      a.heapify!
+      last = Int32::MAX
+      20.times do
+        popped = a.heap_pop
+        popped.should be <= last
+        last = popped
+      end
+    end
+  end
+end
diff --git a/spec/knapsack_spec.cr b/spec/knapsack_spec.cr
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+++ b/spec/knapsack_spec.cr
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+describe Array do
+  describe "#knapsack" do
+    it "works with costs of one" do
+      ans = [1,2,3,4,5,6].shuffle.knapsack(3) do |i|
+        {1, i}
+      end
+      ans[1].sort!
+      ans.should eq({15, [4,5,6]})
+    end
+
+    it "works in the non-greedy case" do
+      ans = [{2, 2}, {2, 2}, {3, 3}].shuffle.knapsack(4) do |i|
+        i
+      end
+      ans.should eq({4, [{2,2},{2,2}]})
+    end
+  end
+end
diff --git a/spec/spec_helper.cr b/spec/spec_helper.cr
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+++ b/spec/spec_helper.cr
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+require "spec"
+require "../src/advent"
diff --git a/src/advent.cr b/src/advent.cr
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--- /dev/null
+++ b/src/advent.cr
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+require "./advent/*"
diff --git a/src/advent/graph.cr b/src/advent/graph.cr
new file mode 100644
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--- /dev/null
+++ b/src/advent/graph.cr
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+class Graph(A)
+  def initialize
+    @edges = {} of A => Set({A, Int32})
+  end
+
+  def add_node(n)
+    @edges[n] = Set({A, Int32}).new
+  end
+
+  def add_edge(f, t, c = 1)
+    @edges[f] ||= Set({A, Int32}).new
+    @edges[f] << {t, c}
+  end
+
+  def add_biedge(f, t, c = 1)
+    add_edge(f, t, c)
+    add_edge(t, f, c)
+  end
+
+  def find_path(f, t)
+    visited = Set(A).new
+    candidates = Set { f }
+    distances = {f => 0}
+    prev = {} of A => A
+
+    while !candidates.empty?
+      candidate = candidates.min_by { |c| distances[c] }
+      break if candidate == t
+      visited << candidate
+      candidates.delete candidate
+      dist = distances[candidate]
+
+      @edges.fetch(candidate, Set({A, Int32}).new).each do |e|
+        node, cost = e
+        new_dist = dist + cost
+        candidates << node unless visited.includes? node
+        next if (old_dist = distances[node]?) && old_dist < new_dist
+        distances[node] = new_dist 
+        prev[node] = candidate
+      end
+    end
+
+    backtrack = t
+    path = [t] of A
+    while backtrack != f
+      return nil unless prev_bt = prev[backtrack]?
+      path << prev_bt
+      backtrack = prev_bt
+    end
+    {path.reverse!, distances[t]}
+  end
+end
diff --git a/src/advent/heap.cr b/src/advent/heap.cr
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+++ b/src/advent/heap.cr
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+class Array(T)
+  def bubble_up(i, &cmp)
+    return if i >= size
+    while i != 0
+      j = (i-1)//2
+      break if yield self[i], self[j]
+      self[i], self[j] = self[j], self[i]
+      i = j
+    end
+  end
+
+  def percalate_down(i, &cmp)
+    while i*2+1 < size
+      j1, j2 = i*2+1, i*2+2
+      v1 = self[j1]
+      v2 = self[j2]?
+      if v2 && (yield v1, v2) && (yield self[i], v2)
+        self[j2], self[i] = self[i], v2
+        i = j2
+      elsif yield self[i], v1
+        self[j1], self[i] = self[i], v1
+        i = j1
+      else
+        break
+      end
+    end
+  end
+
+  def heapify!(&cmp : T,T -> Bool)
+    size.times do |i|
+      bubble_up(i, &cmp)
+    end
+    self
+  end
+
+  def heapify!
+    heapify! do |i,j|
+      i < j
+    end
+  end
+
+  def heap_push(v, &cmp : T,T -> Bool)
+    self << v
+    bubble_up(size - 1, &cmp)
+  end
+
+  def heap_push(v)
+    heap_push(v) do |i,j|
+      i < j
+    end
+  end
+
+  def heap_pop(&cmp : T,T -> Bool)
+    self[0], self[size-1] = self[size-1], self[0]
+    v = pop
+    percalate_down(0, &cmp)
+    v
+  end
+
+  def heap_pop
+    heap_pop do |i, j|
+      i < j
+    end
+  end
+
+  def is_heap?(&cmp : T,T -> Bool)
+    (size-1).times do |i|
+      i = size - i - 1
+      vi = self[i]
+      vp = self[(i-1)//2]
+      return false unless (yield self[i], self[(i-1)//2]) || vi == vp
+    end
+    return true
+  end
+
+  def is_heap?
+    is_heap? do |i,j|
+      i < j
+    end
+  end
+
+end
diff --git a/src/advent/knapsack.cr b/src/advent/knapsack.cr
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+++ b/src/advent/knapsack.cr
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+class Array(T)
+  def knapsack(budget, &cv : T -> {Int32,Int32})
+    cost_values = map &cv
+
+    memo = {} of {Int32, Int32} => Int32
+    bt = {} of {Int32, Int32} => Bool
+    compute = uninitialized Int32, Int32 -> Int32
+    compute = ->(size : Int32, budget : Int32) {
+      if m = memo[{size, budget}]?
+        return m 
+      end
+      return memo[{size, budget}] = 0 if size == 0
+
+      cost, value = cost_values[size-1]
+      no_val = compute.call(size-1, budget)
+      yes_val = (budget < cost) ? 0 : compute.call(size-1, budget - cost) + value  
+
+      if yes_val > no_val
+        bt[{size, budget}] = true
+        return yes_val
+      else
+        bt[{size, budget}] = false
+        return no_val
+      end
+    }
+
+    value = compute.call(size, budget)
+    i = size
+    items = [] of T
+    while i != 0
+      if bt[{i, budget}]
+        items << self[i-1]
+        budget -= cost_values[i-1][0]
+      end
+      i -= 1
+    end
+    {value, items}
+  end
+end