// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "chrome/browser/net/referrer.h"
#include <limits.h>
#include "base/compiler_specific.h"
#include "base/logging.h"
#include "base/message_loop/message_loop.h"
#include "base/values.h"
#include "chrome/browser/net/predictor.h"
namespace chrome_browser_net {
//------------------------------------------------------------------------------
// Smoothing parameter for updating subresource_use_rate_.
// We always combine our old expected value, weighted by some factor W (we use
// kWeightingForOldConnectsExpectedValue), with the new expected value Enew.
// The new "expected value" is the number of actual connections made due to the
// current navigations.
// That means that IF we end up needing to connect, we should apply the formula:
// Eupdated = Eold * W + Enew * (1 - W)
// If we visit the containing url, but don't end up needing a connection, then
// Enew == 0, so we use the formula:
// Eupdated = Eold * W
// To achieve the above updating algorithm, we end up doing the multiplication
// by W every time we contemplate doing a preconnection (i.e., when we navigate
// to the containing URL, and consider doing a preconnection), and then IFF we
// learn that we really needed a connection to the subresource, we complete the
// above algorithm by adding the (1 - W) for each connection we make.
// We weight the new expected value by a factor which is in the range of 0.0 to
// 1.0.
static const double kWeightingForOldConnectsExpectedValue = 0.66;
// To estimate the expected value of the number of connections that we'll need
// when a referrer is navigated to, we start with the following low initial
// value.
// Each time we do indeed (again) need the subresource, this value will get
// increased.
// Each time we navigate to the refererrer but never end up needing this
// subresource, the value will decrease.
// Very conservative is 0.0, which will mean that we have to wait for a while
// before doing much speculative acvtivity. We do persist results, so we'll
// save the asymptotic (correct?) learned answer in the long run.
// Some browsers blindly make 2 connections all the time, so we'll use that as
// a starting point.
static const double kInitialConnectsExpectedValue = 2.0;
Referrer::Referrer() : use_count_(1) {}
void Referrer::SuggestHost(const GURL& url) {
// Limit how large our list can get, in case we make mistakes about what
// hostnames are in sub-resources (example: Some advertisments have a link to
// the ad agency, and then provide a "surprising" redirect to the advertised
// entity, which then (mistakenly) appears to be a subresource on the page
// hosting the ad).
// TODO(jar): Do experiments to optimize the max count of suggestions.
static const size_t kMaxSuggestions = 10;
if (!url.has_host()) // TODO(jar): Is this really needed????
return;
DCHECK(url == url.GetWithEmptyPath());
SubresourceMap::iterator it = find(url);
if (it != end()) {
it->second.SubresourceIsNeeded();
return;
}
if (kMaxSuggestions <= size()) {
DeleteLeastUseful();
DCHECK(kMaxSuggestions > size());
}
(*this)[url].SubresourceIsNeeded();
}
void Referrer::DeleteLeastUseful() {
// Find the item with the lowest value. Most important is preconnection_rate,
// and least is lifetime (age).
GURL least_useful_url;
double lowest_rate_seen = 0.0;
// We use longs for durations because we will use multiplication on them.
int64 least_useful_lifetime = 0; // Duration in milliseconds.
const base::Time kNow(base::Time::Now()); // Avoid multiple calls.
for (SubresourceMap::iterator it = begin(); it != end(); ++it) {
int64 lifetime = (kNow - it->second.birth_time()).InMilliseconds();
double rate = it->second.subresource_use_rate();
if (least_useful_url.has_host()) {
if (rate > lowest_rate_seen)
continue;
if (lifetime <= least_useful_lifetime)
continue;
}
least_useful_url = it->first;
lowest_rate_seen = rate;
least_useful_lifetime = lifetime;
}
if (least_useful_url.has_host())
erase(least_useful_url);
}
bool Referrer::Trim(double reduce_rate, double threshold) {
std::vector<GURL> discarded_urls;
for (SubresourceMap::iterator it = begin(); it != end(); ++it) {
if (!it->second.Trim(reduce_rate, threshold))
discarded_urls.push_back(it->first);
}
for (size_t i = 0; i < discarded_urls.size(); ++i)
erase(discarded_urls[i]);
return size() > 0;
}
bool ReferrerValue::Trim(double reduce_rate, double threshold) {
subresource_use_rate_ *= reduce_rate;
return subresource_use_rate_ > threshold;
}
void Referrer::Deserialize(const base::Value& value) {
if (value.GetType() != base::Value::TYPE_LIST)
return;
const base::ListValue* subresource_list(
static_cast<const base::ListValue*>(&value));
size_t index = 0; // Bounds checking is done by subresource_list->Get*().
while (true) {
std::string url_spec;
if (!subresource_list->GetString(index++, &url_spec))
return;
double rate;
if (!subresource_list->GetDouble(index++, &rate))
return;
GURL url(url_spec);
// TODO(jar): We could be more direct, and change birth date or similar to
// show that this is a resurrected value we're adding in. I'm not yet sure
// of how best to optimize the learning and pruning (Trim) algorithm at this
// level, so for now, we just suggest subresources, which leaves them all
// with the same birth date (typically start of process).
SuggestHost(url);
(*this)[url].SetSubresourceUseRate(rate);
}
}
base::Value* Referrer::Serialize() const {
base::ListValue* subresource_list(new base::ListValue);
for (const_iterator it = begin(); it != end(); ++it) {
base::StringValue* url_spec(new base::StringValue(it->first.spec()));
base::FundamentalValue* rate(new base::FundamentalValue(
it->second.subresource_use_rate()));
subresource_list->Append(url_spec);
subresource_list->Append(rate);
}
return subresource_list;
}
//------------------------------------------------------------------------------
ReferrerValue::ReferrerValue()
: birth_time_(base::Time::Now()),
navigation_count_(0),
preconnection_count_(0),
preresolution_count_(0),
subresource_use_rate_(kInitialConnectsExpectedValue) {
}
void ReferrerValue::SubresourceIsNeeded() {
DCHECK_GE(kWeightingForOldConnectsExpectedValue, 0);
DCHECK_LE(kWeightingForOldConnectsExpectedValue, 1.0);
++navigation_count_;
subresource_use_rate_ += 1 - kWeightingForOldConnectsExpectedValue;
}
void ReferrerValue::ReferrerWasObserved() {
subresource_use_rate_ *= kWeightingForOldConnectsExpectedValue;
// Note: the use rate is temporarilly possibly incorect, as we need to find
// out if we really end up connecting. This will happen in a few hundred
// milliseconds (when content arrives, etc.).
// Value of subresource_use_rate_ should be sampled before this call.
}
} // namespace chrome_browser_net