Example 1, updated

gistfile1.rb

require "json"
require "rspec"
require "date"

json_data = <<-JSON
{"activities": [
  {
      "captured_at": 1392366463,
      "distance": "1.12",
      "duration": 12,
      "calories": 100
  },
  {
      "captured_at": 1392355727,  
      "distance": "4.20",
      "duration": 80,
      "calories": 400
  },
  {
      "captured_at": 1392280163,
      "distance": "2.32",
      "duration": 20,
      "calories": 2200
  }
]}
JSON

def miles_to_meters(miles)
  (miles.to_f/0.00062137).to_i
end

activity_data = JSON.parse(json_data).fetch('activities')
activity_data.map! do |a|
  {
    distance: miles_to_meters(a.fetch("distance")),
    calories: a.fetch("calories").to_i,
    duration: a.fetch("duration").to_i,
    date: Time.at(a.fetch("captured_at")).to_date
  }
end

groups_of_activities = activity_data.group_by {|a| a[:date]}
summaries = groups_of_activities.map do |date, activity_group|
  {
    distance: activity_group.map{|a| a[:distance]}.reduce(&:+),
    duration: activity_group.map{|a| a[:duration]}.reduce(&:+),
    calories: activity_group.map{|a| a[:calories]}.reduce(&:+),
    date: date
  }
end

summary = summaries.first
describe "Summary" do
  it "summarizes distance" do
    expect(summary[:distance]).to eq(8561)
    expect(summary[:duration]).to eq(92)
    expect(summary[:calories]).to eq(500)
    expect(summary[:date]).to eq(Date.parse("2014-02-14"))
  end
end