Pagination using Range header

lib/pliny.rb

@@ -6,6 +6,7 @@
 require_relative "pliny/helpers/encode"
 require_relative "pliny/helpers/params"
 require_relative "pliny/log"
+require_relative "pliny/range_parser"
 require_relative "pliny/request_store"
 require_relative "pliny/router"
 require_relative "pliny/utils"

lib/pliny/range_parser.rb

@@ -0,0 +1,55 @@
+module Pliny
+  class RangeParser
+    attr_reader :range_header
+    attr_reader :start, :end, :parameters
+
+    RANGE_FORMAT_ERROR = 'Invalid `Range` header. Please use format like `objects 0-99; sort=name, order=desc`.'.freeze
+
+    def initialize(range_header)
+      @range_header = range_header
+
+      set_defaults
+      return if range_header.nil?
+      parse
+    end
+
+    private
+
+    def parse
+      parts = range_header.split(';')
+      raise_range_format_error if parts.size > 2
+      bounds_str, parameters_str = parts
+      parse_range_bounds(bounds_str)
+      parse_range_parameters(parameters_str)
+    end
+
+    def parse_range_bounds(bounds_str)
+      return if bounds_str.nil?
+      unit, bounds = bounds_str.split(/\s+/, 2)
+      raise_range_format_error unless unit.downcase == 'objects'
+      /(?<start_bound>\d*)-(?<end_bound>\d*)/ =~ bounds
+      @start = start_bound.to_i unless start_bound.empty?
+      @end   = end_bound.to_i   unless end_bound.empty?
+    end
+
+    def parse_range_parameters(parameters_str)
+      return if parameters_str.nil?
+      @parameters = Hash[
+        parameters_str.split(',')
+        .map    { |option| option.split('=') }
+        .select { |k, v| k && v }
+        .map    { |k, v| [k.strip.to_sym, v.strip] }
+      ]
+    end
+
+    def raise_range_format_error
+      fail Pliny::Errors::BadRequest, RANGE_FORMAT_ERROR
+    end
+
+    def set_defaults
+      @start = nil
+      @end = nil
+      @parameters = {}
+    end
+  end
+end

spec/range_parser_spec.rb

@@ -0,0 +1,75 @@
+require "spec_helper"
+
+describe Pliny::RangeParser do
+  subject(:parser) { described_class.new(range_header) }
+
+  context 'with an empty header' do
+    let(:range_header) { nil }
+
+    it 'parses' do
+      assert_nil parser.start
+      assert_nil parser.end
+      assert_equal({}, parser.parameters)
+    end
+  end
+
+  context 'with a bound range' do
+    let(:range_header) { 'objects 0-99' }
+
+    it 'parses a start and an end' do
+      assert_equal 0, parser.start
+      assert_equal 99, parser.end
+      assert_equal({}, parser.parameters)
+    end
+  end
+
+  context 'with an unbound start range' do
+    let(:range_header) { 'objects -99' }
+
+    it 'parses a start' do
+      assert_nil parser.start
+      assert_equal 99, parser.end
+      assert_equal({}, parser.parameters)
+    end
+  end
+
+  context 'with an unbound end range' do
+    let(:range_header) { 'objects 0-' }
+
+    it 'parses a start' do
+      assert_equal 0, parser.start
+      assert_nil parser.end
+      assert_equal({}, parser.parameters)
+    end
+  end
+
+  context 'with parameters' do
+    let(:range_header) { 'objects 0-99; a=b, c=d' }
+
+    it 'parses parameters' do
+      assert_equal({ a: 'b', c: 'd' }, parser.parameters)
+    end
+  end
+
+  context 'with multiple semicolons' do
+    let(:range_header) { 'objects 0-99; a=b; c=d' }
+    let(:message) { Pliny::RangeParser::RANGE_FORMAT_ERROR }
+
+    it 'raises a bad request' do
+      assert_raises Pliny::Errors::BadRequest, message do
+        parser
+      end
+    end
+  end
+
+  context 'with a non objects unit' do
+    let(:range_header) { 'ids 0-99' }
+    let(:message) { Pliny::RangeParser::RANGE_FORMAT_ERROR }
+
+    it 'raises a bad request' do
+      assert_raises Pliny::Errors::BadRequest, message do
+        parser
+      end
+    end
+  end
+end