Class: TrendLineCalculator

Inherits:
Object
  • Object
show all
Defined in:
lib/jirametrics/trend_line_calculator.rb

Constant Summary collapse

MINIMUM_POINTS =

Three points minimum. Two will always fit a straight line perfectly, so the result looks authoritative while carrying no evidence of a trend at all; drawing it was misleading.

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Instance Method Summary collapse

Constructor Details

#initialize(points) ⇒ TrendLineCalculator

Returns a new instance of TrendLineCalculator.



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# File 'lib/jirametrics/trend_line_calculator.rb', line 10

def initialize points
  @valid = points.size >= MINIMUM_POINTS
  return unless valid?

  sum_of_x = points.sum { |x, _y| x }
  sum_of_y = points.sum { |_x, y| y }
  sum_of_xy = points.sum { |x, y| x * y }
  sum_of_x2 = points.sum { |x, _y| x * x }
  n = points.size.to_f

  @slope = ((n * sum_of_xy) - (sum_of_x * sum_of_y)) / ((n * sum_of_x2) - (sum_of_x * sum_of_x))
  @offset = (sum_of_y - (@slope * sum_of_x)) / n

  @valid = false if vertical?
end

Instance Method Details

#calc_y(x:) ⇒ Object 



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# File 'lib/jirametrics/trend_line_calculator.rb', line 30

def calc_y x:
  ((x * @slope) + @offset).to_i
end

#chart_datapoints(range:, max_y:, min_y: 0) ⇒ Object

If the trend line can't be calculated then return an empty array. Otherwise, return an array with two (x,y) points, with which you can draw the trend line.



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# File 'lib/jirametrics/trend_line_calculator.rb', line 42

def chart_datapoints range:, max_y:, min_y: 0
  raise 'max_y is nil' if max_y.nil?
  return [] unless valid?

  data_points = []
  x_start = range.begin
  y_start = calc_y(x: range.begin).to_i
  x_end = range.end
  y_end = calc_y(x: range.end).to_i

  if y_start < min_y
    x_start = line_crosses_at y: 0
    y_start = 0
  end

  if y_start > max_y
    x_start = line_crosses_at y: max_y
    y_start = max_y
  end

  if y_end < min_y
    x_end = line_crosses_at y: 0
    y_end = 0
  end

  if y_end > max_y
    x_end = line_crosses_at y: max_y
    y_end = max_y
  end

  data_points << { x: x_start, y: y_start }
  data_points << { x: x_end, y: y_end }

  data_points
end

#horizontal? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/jirametrics/trend_line_calculator.rb', line 27

def horizontal? = @slope.zero?

#line_crosses_at(y:) ⇒ Object 



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# File 'lib/jirametrics/trend_line_calculator.rb', line 34

def line_crosses_at y:
  raise "line will never cross #{y}. Trend is perfectly horizontal" if horizontal?

  ((y.to_f - @offset) / @slope).to_i
end

#valid? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/jirametrics/trend_line_calculator.rb', line 26

def valid? = @valid

#vertical? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/jirametrics/trend_line_calculator.rb', line 28

def vertical? = @slope.nan? || @slope.infinite?