Example 1

gistfile1.rb

require "json"
require "rspec"

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|
  a["distance"] = miles_to_meters(a.fetch("distance"))
  a["calories"] = a.fetch("calories").to_i
  a["duration"] = a.fetch("duration").to_i
  a["captured_at"] = Time.at(a.fetch("captured_at")).to_date
  a
end

groups_of_activities = activity_data.group_by {|a| a["captured_at"]}
summaries = groups_of_activities.map do |date, activity_group|
  activity_group.reduce do |memo, obj|
    memo["distance"] += obj["distance"]
    memo["duration"] += obj["duration"]
    memo["calories"] += obj["calories"]
    memo["date"] = memo["captured_at"]
    memo
  end
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