Spliner

gem install spliner

require 'spliner'

# Initialize a spline interpolation with x range 0.0..2.0
my_spline = Spliner::Spliner.new [0.0, 1.0, 2.0], [0.0, 1.0, 2.0]

# Interpolate for a single value
y1 = my_spline[0.5]

# Perform interpolation on 11 values ranging from 0..2.0
y_values = my_spline[(0.0..2.0).step(0.1)]

# You may prefer to use the shortcut class method
y2 = Spliner::Spliner[[0.0, 1.0, 2.0], [0.0, 1.0, 0.5], 0.5]

# perform extrapolation outside key points using linear Y = aX + b
ex_spline = Spliner::Spliner.new [0.0, 1.0, 2.0], [0.0, 1.0, 2.0], :extrapolate => '10%'
xx = ex_spline[2.1] # returns 0.4124999999999999

# perform extrapolation outside key points using linear Y = aX + b
ex_spline = Spliner::Spliner.new [0.0, 1.0, 2.0], [0.0, 1.0, 2.0], :extrapolate => '10%', :emethod => :hold)
xx = ex_spline[2.1] # returns 0.5

# Alternative intialization using Hash
ar_spline = Spliner::Spliner.new({1.0 => 0.0, 2.0 => 3.0, 3.0 => 1.0})

# When duplicate X values are encountered, two or more discontinuous curves are used
two_spline = Spliner::Spliner.new [1.0, 2.0, 2.0, 3.0], [0.0, 3.0, 0.0, 1.0]
puts two_spline.sections # prints 2

(The MIT License)

Copyright (C) 2012 Tallak Tveide

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 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.