require 'rubygems'
require 'digest'
require 'securerandom'

require "hashtree/version"

class HashTree

  attr_reader :index, :branches, :settings

  def initialize(settings = {})
    @settings = {
      max_branch_size: 16
    }.merge(settings)
    @index = {}
    @branches = {
      free: {}
    }
  end

  # Public API
  def [](key)
    begin
      id = @index[key]
      if id
        read(id)
      else
        nil
      end
    rescue
      nil
    end
  end

  def []=(key, value)
    begin
      id = @index[key]
      if id
        update(id, value)
      else
        create(key, value)
      end
    rescue
      nil
    end
  end

  def delete(key)
    begin
      id = @index[key]
      if id
        doc = read(id)
        destroy(id)
        @index.delete(key)
        return doc
      else
        nil
      end
    rescue
      nil
    end
  end

  def clear
    @index = {}
    @branches = {}
  end

  def count
    @index.count
  end

  def branch(key)
    branch_id = get_branch_id(key)
    @branches[branch_id] if branch_id
  end

  private
  def get_branch_id(key)
    id = @index[key]
    if id
      branch_id = id[0..15]
    else
      nil
    end
  end

  # Private API
  def read(id)
    branch_id, leaf_id = id[0..15], id[16..32]
    @branches[branch_id][leaf_id]
  end

  def update(id, value)
    branch_id, leaf_id = id[0..15], id[16..32]
    @branches[branch_id][leaf_id] = value
    return value
  end

  def create(key, value)
    free_branch_id = @branches[:free].keys.sample
    if free_branch_id
      leaf_id = Digest::MD5.hexdigest(SecureRandom.uuid)[0..15]
      @index[key] = free_branch_id + leaf_id
      branch = @branches[free_branch_id]
      branch[leaf_id] = value
      @branches[:free].delete(branch_id) if branch.size >= @settings[:max_branch_size]
    else
      id = Digest::MD5.hexdigest(SecureRandom.uuid)
      @index[key] = id
      branch_id, leaf_id = id[0..15], id[16..32]
      @branches[branch_id] = {}
      @branches[branch_id][leaf_id] = value
    end
    return value
  end

  def destroy(id)
    branch_id, leaf_id = id[0..15], id[16..32]
    branch = @branches[branch_id]
    destroyed_element = branch.delete(leaf_id)
    if branch.size == 0
      @branches.delete(branch_id)
      @branches[:free].delete(branch_id)
    elsif branch.size > 0 and branch.size < 16
      @branches[:free][branch_id] = nil
    end
  end

end