class Neighbor::SparseVector

def from_array(arr)
  arr = arr.to_a
  @dimensions = arr.size
  @indices = []
  @values = []
  arr.each_with_index do |v, i|
    if v != 0
      @indices << i
      @values << v.to_f
    end
  end
end

def from_hash(data, dimensions)
  elements = data.select { |_, v| v != 0 }.sort
  @dimensions = dimensions.to_i
  @indices = elements.map { |v| v[0].to_i }
  @values = elements.map { |v| v[1].to_f }
end

def from_parts(dimensions, indices, values)
  vec = allocate
  vec.instance_variable_set(:@dimensions, dimensions)
  vec.instance_variable_set(:@indices, indices)
  vec.instance_variable_set(:@values, values)
  vec
end

def from_text(string)
  elements, dimensions = string.split("/", 2)
  indices = []
  values = []
  elements[1..-2].split(",").each do |e|
    index, value = e.split(":", 2)
    indices << index.to_i - 1
    values << value.to_f
  end
  from_parts(dimensions.to_i, indices, values)
end

def initialize(value, dimensions = NO_DEFAULT)
  if value.is_a?(Hash)
    if dimensions == NO_DEFAULT
      raise ArgumentError, "missing dimensions"
    end
    from_hash(value, dimensions)
  else
    unless dimensions == NO_DEFAULT
      raise ArgumentError, "extra argument"
    end
    from_array(value)
  end
end

def to_a
  arr = Array.new(dimensions, 0.0)
  @indices.zip(@values) do |i, v|
    arr[i] = v
  end
  arr
end

def to_s
  "{#{@indices.zip(@values).map { |i, v| "#{i.to_i + 1}:#{v.to_f}" }.join(",")}}/#{@dimensions.to_i}"
end