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[hilbert_space] Simplify algorithm of space_partition::merge_subspaces() #961

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[hilbert_space] Simplify algorithm of space_partition::merge_subspaces() #961

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52 changes: 18 additions & 34 deletions c++/triqs/hilbert_space/space_partition.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -146,42 +146,26 @@ namespace triqs {
tmp_state(i) = amplitude_t(0.);
}

// 'Zigzag' traversal algorithm
while (!Cd_connections.empty()) {

// Take one C^+ - connection
// C^+|lower_subspace> = |upper_subspace>
idx_t lower_subspace, upper_subspace;
std::tie(lower_subspace, upper_subspace) = *std::begin(Cd_connections);

// - Reveals all subspaces reachable from lower_subspace by application of
// a 'zigzag' product C^+ C C^+ C C^+ ... of any length.
// - Removes all visited connections from Cd_connections/C_connections.
// - Merges lower_subspace with all subspaces generated from lower_subspace by application of (C C^+)^(2*n).
// - Merges upper_subspace with all subspaces generated from upper_subspace by application of (C^+ C)^(2*n).
std::function<void(idx_t, bool)> zigzag_traversal = [this, lower_subspace, upper_subspace, &Cd_connections, &C_connections,
&zigzag_traversal](idx_t i_subspace, // find all connections starting from i_subspace
bool upwards // if true, C^+ connection, otherwise C connection
) {
std::multimap<idx_t, idx_t>::iterator it;
while ((it = (upwards ? Cd_connections : C_connections).find(i_subspace)) != (upwards ? Cd_connections : C_connections).end()) {

auto f_subspace = it->second;
(upwards ? Cd_connections : C_connections).erase(it);

if (upwards)
subspaces.link(f_subspace, upper_subspace);
else
subspaces.link(f_subspace, lower_subspace);

// Recursively apply to all found f_subspace's with a 'flipped' direction
zigzag_traversal(f_subspace, !upwards);
// Merge all 'out' subspaces corresponding to the same 'in' subspace
// in 'conn'.
auto merge_conn_targets = [this](auto const& conn) {
if(conn.empty()) return;

auto conn_it = conn.cbegin();
idx_t i_subspace = conn_it->first;
idx_t f_subspace = conn_it->second;
++conn_it;
for(; conn_it != conn.cend(); ++conn_it) {
if(conn_it->first == i_subspace) {
subspaces.link(f_subspace, conn_it->second);
} else {
std::tie(i_subspace, f_subspace) = *conn_it;
}
};
}
};

// Apply to all C^+ connections starting from lower_subspace
zigzag_traversal(lower_subspace, true);
}
merge_conn_targets(Cd_connections);
merge_conn_targets(C_connections);

_update_index();

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