gecko-dev/content/media/TimeVarying.h

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/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-*/
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
* You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef MOZILLA_TIMEVARYING_H_
#define MOZILLA_TIMEVARYING_H_
#include "nsTArray.h"
namespace mozilla {
/**
* This is just for CTOR/DTOR tracking. We can't put this in
* TimeVarying directly because the different template instances have
* different sizes and that confuses things.
*/
class TimeVaryingBase {
protected:
TimeVaryingBase()
{
MOZ_COUNT_CTOR(TimeVaryingBase);
}
~TimeVaryingBase()
{
MOZ_COUNT_DTOR(TimeVaryingBase);
}
};
/**
* Objects of this class represent values that can change over time ---
* a mathematical function of time.
* Time is the type of time values, T is the value that changes over time.
* There are a finite set of "change times"; at each change time, the function
* instantly changes to a new value. ReservedChanges should be set to the
* expected number of change events that the object is likely to contain.
* This value should be 0 for all consumers unless you know that a higher value
* would be a benefit.
* There is also a "current time" which must always advance (not go backward).
* The function is constant for all times less than the current time.
* When the current time is advanced, the value of the function at the new
* current time replaces the values for all previous times.
*
* The implementation records a mCurrent (the value at the current time)
* and an array of "change times" (greater than the current time) and the
* new value for each change time. This is a simple but dumb implementation.
* We maintain the invariant that each change entry in the array must have
* a different value to the value in the previous change entry (or, for
* the first change entry, mCurrent).
*/
template <typename Time, typename T, uint32_t ReservedChanges>
class TimeVarying : public TimeVaryingBase {
public:
TimeVarying(const T& aInitial) : mCurrent(aInitial) {}
/**
* This constructor can only be called if mCurrent has a no-argument
* constructor.
*/
TimeVarying() : mCurrent() {}
/**
* Sets the value for all times >= aTime to aValue.
*/
void SetAtAndAfter(Time aTime, const T& aValue)
{
for (int32_t i = mChanges.Length() - 1; i >= 0; --i) {
NS_ASSERTION(i == int32_t(mChanges.Length() - 1),
"Always considering last element of array");
if (aTime > mChanges[i].mTime) {
if (mChanges[i].mValue != aValue) {
mChanges.AppendElement(Entry(aTime, aValue));
}
return;
}
if (aTime == mChanges[i].mTime) {
if ((i > 0 ? mChanges[i - 1].mValue : mCurrent) == aValue) {
mChanges.RemoveElementAt(i);
return;
}
mChanges[i].mValue = aValue;
return;
}
mChanges.RemoveElementAt(i);
}
if (mCurrent == aValue) {
return;
}
mChanges.InsertElementAt(0, Entry(aTime, aValue));
}
/**
* Returns the final value of the function. If aTime is non-null,
* sets aTime to the time at which the function changes to that final value.
* If there are no changes after the current time, returns INT64_MIN in aTime.
*/
const T& GetLast(Time* aTime = nullptr) const
{
if (mChanges.IsEmpty()) {
if (aTime) {
*aTime = INT64_MIN;
}
return mCurrent;
}
if (aTime) {
*aTime = mChanges[mChanges.Length() - 1].mTime;
}
return mChanges[mChanges.Length() - 1].mValue;
}
/**
* Returns the value of the function just before time aTime.
*/
const T& GetBefore(Time aTime) const
{
if (mChanges.IsEmpty() || aTime <= mChanges[0].mTime) {
return mCurrent;
}
int32_t changesLength = mChanges.Length();
if (mChanges[changesLength - 1].mTime < aTime) {
return mChanges[changesLength - 1].mValue;
}
for (uint32_t i = 1; ; ++i) {
if (aTime <= mChanges[i].mTime) {
NS_ASSERTION(mChanges[i].mValue != mChanges[i - 1].mValue,
"Only changed values appear in array");
return mChanges[i - 1].mValue;
}
}
}
/**
* Returns the value of the function at time aTime.
* If aEnd is non-null, sets *aEnd to the time at which the function will
* change from the returned value to a new value, or INT64_MAX if that
* never happens.
* If aStart is non-null, sets *aStart to the time at which the function
* changed to the returned value, or INT64_MIN if that happened at or
* before the current time.
*
* Currently uses a linear search, but could use a binary search.
*/
const T& GetAt(Time aTime, Time* aEnd = nullptr, Time* aStart = nullptr) const
{
if (mChanges.IsEmpty() || aTime < mChanges[0].mTime) {
if (aStart) {
*aStart = INT64_MIN;
}
if (aEnd) {
*aEnd = mChanges.IsEmpty() ? INT64_MAX : mChanges[0].mTime;
}
return mCurrent;
}
int32_t changesLength = mChanges.Length();
if (mChanges[changesLength - 1].mTime <= aTime) {
if (aEnd) {
*aEnd = INT64_MAX;
}
if (aStart) {
*aStart = mChanges[changesLength - 1].mTime;
}
return mChanges[changesLength - 1].mValue;
}
for (uint32_t i = 1; ; ++i) {
if (aTime < mChanges[i].mTime) {
if (aEnd) {
*aEnd = mChanges[i].mTime;
}
if (aStart) {
*aStart = mChanges[i - 1].mTime;
}
NS_ASSERTION(mChanges[i].mValue != mChanges[i - 1].mValue,
"Only changed values appear in array");
return mChanges[i - 1].mValue;
}
}
}
/**
* Advance the current time to aTime.
*/
void AdvanceCurrentTime(Time aTime)
{
for (uint32_t i = 0; i < mChanges.Length(); ++i) {
if (aTime < mChanges[i].mTime) {
mChanges.RemoveElementsAt(0, i);
return;
}
mCurrent = mChanges[i].mValue;
}
mChanges.Clear();
}
/**
* Make all currently pending changes happen aDelta later than their
* current change times.
*/
void InsertTimeAtStart(Time aDelta)
{
for (uint32_t i = 0; i < mChanges.Length(); ++i) {
mChanges[i].mTime += aDelta;
}
}
/**
* Replace the values of this function at aTimeOffset and later with the
* values of aOther taken from zero, so if aOther is V at time T >= 0
* then this function will be V at time T + aTimeOffset. aOther's current
* time must be >= 0.
*/
void Append(const TimeVarying& aOther, Time aTimeOffset)
{
NS_ASSERTION(aOther.mChanges.IsEmpty() || aOther.mChanges[0].mTime >= 0,
"Negative time not allowed here");
NS_ASSERTION(&aOther != this, "Can't self-append");
SetAtAndAfter(aTimeOffset, aOther.mCurrent);
for (uint32_t i = 0; i < aOther.mChanges.Length(); ++i) {
const Entry& e = aOther.mChanges[i];
SetAtAndAfter(aTimeOffset + e.mTime, e.mValue);
}
}
private:
struct Entry {
Entry(Time aTime, const T& aValue) : mTime(aTime), mValue(aValue) {}
// The time at which the value changes to mValue
Time mTime;
T mValue;
};
nsAutoTArray<Entry, ReservedChanges> mChanges;
T mCurrent;
};
}
#endif /* MOZILLA_TIMEVARYING_H_ */