/// <summary>
/// Scrapes all book data from a page within this category. For each category, books are collected from 9 total URLs. 1 URL for page 1 and 8 URLs for pages 2 thru 5. Even
/// though more URLs are requested for pages 2 thru 5, the total bandwidth consumed is still lower because each ajax page is very small compared to a normal page.
/// December 2016 Update: Ajax URLs used for pages 2 thru 5 now display all 20 items - Amazon is no longer using the 'isAboveTheFold' query string to divide items into
/// sub pages for the ajax pages, except in the Japan domain.
/// </summary>
/// <param name="qPage">The page number used in the query string.</param>
/// <param name="qAboveFold">Pages 2 through 5 are retrieved via Amazon's ajax urls. But they are divided into two sub pages per page (only applies to Japan domain). This query string
/// indicates which sub page is being retrieved.</param>
/// <returns>An enumerable list of sub categories found on the page. Subcategories are only checked on page 1. For the other pages, null is returned.</returns>
public async Task <IEnumerable <Category> > RetrieveCategoryData(int qPage, int?qAboveFold = null)
{
try
{
string url;
if (qPage == 1)
{
url = string.Format("{0}?pg=1", _URL); // page 1 we get full page so we can get the sub categories
}
else if (qAboveFold == null)
{
url = string.Format("{0}?_encoding=UTF8&pg={1}&ajax=1", _URL, qPage); // ajax page
}
else
{
url = string.Format("{0}?_encoding=UTF8&pg={1}&ajax=1&isAboveTheFold={2}", _URL, qPage, qAboveFold); // ajax page
}
HtmlDocument doc = null;
bool loaded = false;
int attempts = 0;
HtmlNodeCollection itemLinks = null;
while (attempts < 5 && loaded == false)
{
try
{
attempts++;
using (GZipWebClient gZipWebClient = new GZipWebClient())
{
using (Stream stream = await gZipWebClient.OpenReadTaskAsync(url))
{
// get encoding. todo: consider setting the encoding based on the url, instead of doing all this work
Encoding encoding = null;
try
{
var responseHeaders = gZipWebClient.ResponseHeaders.GetValues("Content-Type");
var encodingString = responseHeaders[0].Split('=').LastOrDefault();
if (!string.IsNullOrWhiteSpace(encodingString))
{
encoding = Encoding.GetEncoding(encodingString);
}
}
catch {}
if (encoding == null)
{
encoding = Encoding.GetEncoding("ISO-8859-1");
}
doc = new HtmlDocument();
doc.Load(stream, encoding);
}
}
// delete nodes with extra links we do not want
// they are links from the "More to Explore" section of the page found in the div with id='zg_col2'
if (qPage == 1)
{
var removeNode = doc.DocumentNode.SelectSingleNode("//div[@id='zg_col2']");
if (removeNode != null)
{
removeNode.Remove();
}
else
{
var removeNodes = doc.DocumentNode.SelectNodes("//div[@class='zg_more']"); // alternative method in case 'zg_col2' is not found
foreach (HtmlNode node in removeNodes)
{
node.Remove();
}
}
}
itemLinks = doc.DocumentNode.SelectNodes(XPathItemLinks); // determine all the books on the page by checking for this html
if (itemLinks == null)
{
// no books found... did we land on a Captcha page?
var titleNode = doc.DocumentNode.SelectSingleNode("//title");
if (titleNode != null && (titleNode.InnerText.ToUpper().Contains("AMAZON CAPTCHA") || titleNode.InnerText.ToUpper().Contains("BOT CHECK")))
{
throw new Exception("No books found. Landed on Captcha");
}
else
{
// we only throw an exception if the page is 1
// because on other pages it is possible that there really are no books because not all sub categories have 100 books
if (qPage == 1)
{
throw new Exception("No books found on page");
}
else
{
return(null); // do not attempt to retry or show error if the page greater than 1
}
}
}
loaded = true; // if we get here with no exception, then it was loaded successfully
}
catch (Exception ex)
{
#if DEBUG
if (attempts == 1)
{
Logger.Log(string.Format("Error downloading page. Attemping to retry... URL: {0}", url), ex);
}
#endif
if (attempts == 5 && loaded == false)
{
throw new Exception("Attempts exceeded 5", ex);
}
}
}
int rank = 1;
if (qPage > 1) // rank starting number will be different for other pages
{
rank = ((qPage - 1) * 20) + 1;
if (qAboveFold == 0)
{
rank += 3; // there are only 3 items on the first ajax page, hopefully that is true for every category
// consider using the rank number field on the html page
}
}
int tempBooks = 0;
foreach (HtmlNode node in itemLinks)
{
string price = "N/A";
HtmlNode priceNode = node.SelectSingleNode(XPathPrice);
if (priceNode != null)
{
price = priceNode.InnerText;
}
else
{
// no price displayed, check availability
HtmlNode availNode = node.SelectSingleNode(XPathAvailability);
if (availNode != null)
{
// translate Japanese text
if (availNode.InnerText == OutOfStockJPN)
{
price = OutOfStock;
}
else if (availNode.InnerText == CurrentlyUnavailableJPN)
{
price = CurrentlyUnavailable;
}
else
{
price = availNode.InnerText;
}
}
}
string link = node.GetAttributeValue("href", "").Trim();
string[] split = link.Split(new string[] { "/ref=" }, StringSplitOptions.None)[0].Split('/');
string ISBN = split[split.Length - 1]; // parse the link to get the ISBN
string title = "";
var fullTitleNode = node.SelectSingleNode(".//span[@title]");
if (fullTitleNode != null)
{
title = fullTitleNode.GetAttributeValue("title", "").Trim();
}
else
{
title = node.InnerText.Trim();
}
// get author if possible
string author = "N/A";
var authorNode = node.SelectSingleNode(XPathAuthor);
if (authorNode != null)
{
author = authorNode.InnerText;
}
Books[rank - 1] = new Book(title, author, ISBN, price);
tempBooks++;
rank++;
}
Counter.IncrementBooksAdded(tempBooks);
/*
* // this code checks for missing books. Amazon will sometimes incorrectly deliver a page in which there will be a rank number but no item.
* List<HtmlNode> rankDivs = root.Descendants("span").Where(n => n.GetAttributeValue("class", "").Equals("zg_rankNumber")).ToList<HtmlNode>();
* if(rankDivs.Count != tempBooks.Count)
* {
* System.Diagnostics.Debug.WriteLine("Missing book(s) at " + url);
* }
*/
if (qPage == 1) // check for subcategories if page is 1
{
HtmlNode lastUlElement = doc.GetElementbyId("zg_browseRoot").Descendants("ul").Last();
bool hasSubCategories = !lastUlElement.Descendants().Any(n => n.GetAttributeValue("class", "").Equals("zg_selected"));
if (hasSubCategories)
{
IEnumerable <HtmlNode> aElements = lastUlElement.Descendants().Where(n => n.OriginalName == "a");
return
(from aElement in aElements
select new Category(string.Format("{0} > {1}", _name, aElement.InnerText), aElement.GetAttributeValue("href", "").Trim()));
}
}
}
catch (Exception ex)
{
Logger.Log(string.Format("Failed to process page {0} of URL: {1}", qPage, _URL), ex);
}
return(null);
}