read_time.rb

# Calculates reading time based on the formula used by Medium
# https://medium.com/the-story/read-time-and-you-bc2048ab620c

# Installation: drop this into your _plugins folder
# Usage: {{ page.content | read_time }}
# Note: this requires img tags to be in the form of <img ... />. If you're
# using this on a post listing page, make sure to markdownify the post content
# first.
module ReadTimeFilter
    def read_time(input)
        words_per_minute = 275
        # number of seconds per image to start at, default is 12s
        img_time_max = 12
        # time per image will decrease by 1 for every image, to a minimum
        # of this time, default is 3s
        img_time_min = 3

        strings = input.split(/<img.* \/>/)

        seconds = (strings.join(" ").split.size.to_f / (words_per_minute / 60))

        # number of images minus one for correct number of iterations
        (strings.size - 2).times do |i|
            t = (img_time_max - i)
            image_time = t > img_time_min ? t : img_time_min
            seconds = seconds + image_time
        end

        minutes = (seconds / 60).ceil

        "#{minutes} min read"
    end
end

Liquid::Template.register_filter(ReadTimeFilter)