size_aware_cache.cr

class SizeAwareCache
  @@cache = {} of String => Hash(Int32, String)

  def read(key : String, size : Int32)
    if value = @@cache.dig?(key, size)
      # Hit! (both key and size)
      return value
    elsif results = @@cache[key]?
      # Key hit, size miss
      sorted_results = results.to_a.sort_by(&.first)
      # Return the first element greater than the requested size (or the largest, if none are greater)
      sorted_results.find { |tuple| return tuple[1] if tuple[0] > size || tuple == sorted_results.last }
    else
      # Miss
      return nil
    end
  end

  def write(key : String, value : String, size : Int32)
    if @@cache[key]?
      @@cache[key][size] = value
    else
      @@cache[key] = {size => value}
    end
  end
end

require "spec"

describe SizeAwareCache do
  describe "#read" do
    it "returns nil when key is not found" do
      image_cache = SizeAwareCache.new
      image_cache.read("foo", 100).should be_nil
    end

    it "returns an exact match" do
      image_cache = SizeAwareCache.new
      image_cache.write("user-1", "[email protected]", 100)
      image_cache.read("user-1", 100).should eq("[email protected]")
    end

    it "returns the first image greater than the requested size" do
      image_cache = SizeAwareCache.new
      image_cache.write("user-1", "[email protected]", 100)
      image_cache.write("user-1", "[email protected]", 200)
      image_cache.write("user-1", "[email protected]", 300)
      image_cache.read("user-1", 150).should eq("[email protected]")
    end

    it "returns the largest image below the requested size" do
      image_cache = SizeAwareCache.new
      image_cache.write("user-1", "[email protected]", 100)
      image_cache.write("user-1", "[email protected]", 200)
      image_cache.write("user-1", "[email protected]", 300)
      image_cache.read("user-1", 400).should eq("[email protected]")
    end
  end
end