chop.rb

def chop(target, array, range_slice = 0)
  return chop(target, array, 0..(array.size - 1)) if range_slice == 0
  first = range_slice.first
  last = range_slice.last
  size = last - first + 1
  if size >= 2
    midpoint = (last - first) / 2 + first
    if array[midpoint] == target
      midpoint
    elsif array[midpoint] > target
      chop(target, array, left_slice(range_slice))
    else
      chop(target, array, right_slice(range_slice))
    end
  elsif last == first
    array[first] == target ? first : -1
  else
    -1
  end
end

def left_slice(range)
  size = range.last - range.first + 1
  if size % 2 == 0
    upper_limit = size / 2 - 1 + range.first
  else
    upper_limit = size / 2 - 1 + range.first
  end
  range.first..upper_limit
end

def right_slice(range)
  size = range.last - range.first + 1
  lower_limit = size / 2 + range.first
  lower_limit..range.last
end

chop_spec.rb

require 'chop'

describe "chop" do
  context "empty array" do
    it "fails to find the target specified" do
      expect(chop 3, []).to eq(-1)
    end
  end

  context "single element array" do
    it "fails if target not in array" do
      expect(chop 3, [1]).to eq(-1)
    end

    it "returns target index if target found in array" do
      expect(chop 1, [1]).to eq(0)
    end
  end

  context "three element array" do
    it "returns target index if target is found" do
      expect(chop 1, [1, 3, 5]).to eq(0)
      expect(chop 3, [1, 3, 5]).to eq(1)
      expect(chop 5, [1, 3, 5]).to eq(2)
    end

    it "fails when the target is not found" do
      expect(chop 0, [1, 3, 5]).to eq(-1)
      expect(chop 2, [1, 3, 5]).to eq(-1)
      expect(chop 4, [1, 3, 5]).to eq(-1)
      expect(chop 6, [1, 3, 5]).to eq(-1)
    end
  end


  context "four element array" do
    it "returns target index if target is found" do
      expect(chop 1, [1, 3, 5, 7]).to eq(0)
      expect(chop 3, [1, 3, 5, 7]).to eq(1)
      expect(chop 5, [1, 3, 5, 7]).to eq(2)
      expect(chop 7, [1, 3, 5, 7]).to eq(3)
    end

    it "fails when the target is not found" do
      expect(chop 0, [1, 3, 5, 7]).to eq(-1)
      expect(chop 2, [1, 3, 5, 7]).to eq(-1)
      expect(chop 4, [1, 3, 5, 7]).to eq(-1)
      expect(chop 6, [1, 3, 5, 7]).to eq(-1)
      expect(chop 8, [1, 3, 5, 7]).to eq(-1)
    end
  end
end

describe "left and right array slices" do
  describe "#left_slice" do
    context "returns the range for the left side of an array" do
      specify "for even sized arrays" do
        expect(left_slice 0..1).to eq(0..0)
        expect(left_slice 1..2).to eq(1..1)
        expect(left_slice 1..4).to eq(1..2)
        expect(left_slice 4..9).to eq(4..6)
      end

      specify "for odd sized arrays" do
        expect(left_slice 1..3).to eq(1..1)
        expect(left_slice 1..5).to eq(1..2)
        expect(left_slice 3..7).to eq(3..4)
      end
    end
  end

  describe "#right_slice" do
    context "returns the right side of an array" do
      specify "for even sized arrays" do
        expect(right_slice 0..1).to eq(1..1)
        expect(right_slice 1..2).to eq(2..2)
        expect(right_slice 1..4).to eq(3..4)
      end

      specify "for odd sized arrays" do
        expect(right_slice 1..3).to eq(2..3)
        expect(right_slice 1..5).to eq(3..5)
      end
    end
  end
end

Gemfile

source "http://rubygems.org"

gem "rspec"

Gemfile.lock

GEM
  remote: http://rubygems.org/
  specs:
    diff-lcs (1.1.3)
    rspec (2.11.0)
      rspec-core (~> 2.11.0)
      rspec-expectations (~> 2.11.0)
      rspec-mocks (~> 2.11.0)
    rspec-core (2.11.1)
    rspec-expectations (2.11.2)
      diff-lcs (~> 1.1.3)
    rspec-mocks (2.11.2)

PLATFORMS
  ruby

DEPENDENCIES
  rspec