std::tuple<double, ValueFunction> IncrementalPruning::operator()(const M & model) {
// Initialize "global" variables
S = model.getS();
A = model.getA();
O = model.getO();
auto v = makeValueFunction(S); // TODO: May take user input
unsigned timestep = 0;
Pruner prune(S);
Projecter projecter(model);
const bool useTolerance = checkDifferentSmall(tolerance_, 0.0);
double variation = tolerance_ * 2; // Make it bigger
while ( timestep < horizon_ && ( !useTolerance || variation > tolerance_ ) ) {
++timestep;
// Compute all possible outcomes, from our previous results.
// This means that for each action-observation pair, we are going
// to obtain the same number of possible outcomes as the number
// of entries in our initial vector w.
auto projs = projecter(v[timestep-1]);
size_t finalWSize = 0;
// In this method we split the work by action, which will then
// be joined again at the end of the loop.
for ( size_t a = 0; a < A; ++a ) {
// We prune each outcome separately to be sure
// we do not replicate work later.
for ( size_t o = 0; o < O; ++o ) {
const auto begin = std::begin(projs[a][o]);
const auto end = std::end (projs[a][o]);
projs[a][o].erase(prune(begin, end, unwrap), end);
}
// Here we reduce at the minimum the cross-summing, by alternating
// merges. We pick matches like a reverse binary tree, so that
// we always pick lists that have been merged the least.
//
// Example for O==7:
//
// 0 <- 1 2 <- 3 4 <- 5 6
// 0 ------> 2 4 ------> 6
// 2 <---------------- 6
//
// In particular, the variables are:
//
// - oddOld: Whether our starting step has an odd number of elements.
// If so, we skip the last one.
// - front: The id of the element at the "front" of our current pass.
// note that since passes can be backwards this can be high.
// - back: Opposite of front, which excludes the last element if we
// have odd elements.
// - stepsize: The space between each "first" of each new merge.
// - diff: The space between each "first" and its match to merge.
// - elements: The number of elements we have left to merge.
bool oddOld = O % 2;
int i, front = 0, back = O - oddOld, stepsize = 2, diff = 1, elements = O;
while ( elements > 1 ) {
for ( i = front; i != back; i += stepsize ) {
projs[a][i] = crossSum(projs[a][i], projs[a][i + diff], a, stepsize > 0);
const auto begin = std::begin(projs[a][i]);
const auto end = std::end (projs[a][i]);
projs[a][i].erase(prune(begin, end, unwrap), end);
--elements;
}
const bool oddNew = elements % 2;
const int tmp = back;
back = front - ( oddNew ? 0 : stepsize );
front = tmp - ( oddOld ? 0 : stepsize );
stepsize *= -2;
diff *= -2;
oddOld = oddNew;
}
// Put the result where we can find it
if (front != 0)
projs[a][0] = std::move(projs[a][front]);
finalWSize += projs[a][0].size();
}
VList w;
w.reserve(finalWSize);
// Here we don't have to do fancy merging since no cross-summing is involved
for ( size_t a = 0; a < A; ++a )
w.insert(std::end(w), std::make_move_iterator(std::begin(projs[a][0])), std::make_move_iterator(std::end(projs[a][0])));
// We have them all, and we prune one final time to be sure we have
// computed the parsimonious set of value functions.
const auto begin = std::begin(w);
const auto end = std::end (w);
w.erase(prune(begin, end, unwrap), end);
v.emplace_back(std::move(w));
// Check convergence
if ( useTolerance )
variation = weakBoundDistance(v[timestep-1], v[timestep]);
}
return std::make_tuple(useTolerance ? variation : 0.0, v);
}
