【codewar-6kyu】PaginationHelper

##题目描述

Description:

For this exercise you will be strengthening your page-fu mastery. You will complete the PaginationHelper class, which is a utility class helpful for querying paging information related to an array.

The class is designed to take in an array of values and an integer indicating how many items will be allowed per each page. The types of values contained within the collection/array are not relevant.

The following are some examples of how this class is used:

helper = PaginationHelper(['a','b','c','d','e','f'], 4)
helper.page_count # should == 2
helper.item_count # should == 6
helper.page_item_count(0)  # should == 4
helper.page_item_count(1) # last page - should == 2
helper.page_item_count(2) # should == -1 since the page is invalid

# page_ndex takes an item index and returns the page that it belongs on
helper.page_index(5) # should == 1 (zero based index)
helper.page_index(2) # should == 0
helper.page_index(20) # should == -1
helper.page_index(-10) # should == -1 because negative indexes are invalid

##思路分析

进入6kyu 之后突然就吃力了些，感觉难起来的并不是编码，而是逻辑的完备性。

细心些，考虑各种非常规情况，超出处理阈值的情况，一定要——滴水不漏。

##代码解析

Python

# TODO: complete this class

class PaginationHelper:

  # The constructor takes in an array of items and a integer indicating
  # how many items fit within a single page
  def __init__(self, collection, items_per_page):
    self.collection = collection
    self.items_per_page = items_per_page
      
  
  # returns the number of items within the entire collection
  def item_count(self):
    return len(self.collection)
  
  # returns the number of pages
  def page_count(self):
    if len(self.collection) % self.items_per_page == 0:
      return len(self.collection) / self.items_per_page
    else:
      return len(self.collection) / self.items_per_page + 1
    
      
  
  # returns the number of items on the current page. page_index is zero based
  # this method should return -1 for page_index values that are out of range
  def page_item_count(self,page_index):
    if page_index >= self.page_count():
      return -1
    elif page_index == self.page_count() - 1:
      return len(self.collection) % self.items_per_page or self.items_per_page
    else:
      return self.items_per_page
        
      
  
  # determines what page an item is on. Zero based indexes.
  # this method should return -1 for item_index values that are out of range
  def page_index(self,item_index):
    if item_index >= len(self.collection) or item_index < 0:
      return -1
    else:
      return item_index / self.items_per_page