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Uses correct coefficients for local optimality in CH alternatives

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The alpha constant is for the local optimality T-Test threshold.

Before we used epsilon for the T-Test threshold, but the epsilon
constant is meant to be used for the stretch test(s) only.

This changeset fixes the local optimality T-Test and uses the
epsilon constant for the two stretch tests:
- We test the stretch for the total route against epsilon and
- We test the detour against the epsilon now, too

We can discuss if the second stretch test should actually use
epsilon, too, or a adapted value of it - but definitly not alpha.
This commit is contained in:
Daniel J. Hofmann 2017-05-31 13:46:47 +02:00 committed by Michael Krasnyk
parent 9158f69ea0
commit de1d5f199f
2 changed files with 5 additions and 6 deletions
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4
features/testbot/alternative_loop.feature
View File

@ -55,8 +55,8 @@ Feature: Alternative route
When I route I should get
| from | to | route | alternative |
| b | c | bc,bc | |
#| c | b | bc,bc | | # alternative path depends on phantom snapping order
| b | c | bc,bc | ab,ae,ef,fd,cd,cd |
#| c | b | bc,bc | cd,fd,ef,ae,ab,ab | # alternative path depends on phantom snapping order
| 1 | c | ab,bc,bc | ab,ae,ef,fd,cd,cd |
#| c | 1 | bc,ab | cd,fd,ef,ae,ab | # alternative path depends on phantom snapping order
| 2 | c | bc,bc | |

7
src/engine/routing_algorithms/alternative_path.cpp
View File

@ -24,7 +24,7 @@ namespace ch
namespace
{
const double constexpr VIAPATH_ALPHA = 0.10;
const double constexpr VIAPATH_ALPHA = 0.25; // alternative is local optimum on 25% sub-paths
const double constexpr VIAPATH_EPSILON = 0.15; // alternative at most 15% longer
const double constexpr VIAPATH_GAMMA = 0.75; // alternative shares at most 75% with the shortest.
@ -398,8 +398,7 @@ bool viaNodeCandidatePassesTTest(
{
return false;
}
const EdgeWeight T_threshold =
static_cast<EdgeWeight>(VIAPATH_EPSILON * weight_of_shortest_path);
const EdgeWeight T_threshold = static_cast<EdgeWeight>(VIAPATH_ALPHA * weight_of_shortest_path);
EdgeWeight unpacked_until_weight = 0;
std::stack<SearchSpaceEdge> unpack_stack;
@ -732,7 +731,7 @@ alternativePathSearch(SearchEngineData<Algorithm> &engine_working_data,
(approximated_sharing <= upper_bound_to_shortest_path_weight * VIAPATH_GAMMA);
const bool stretch_passes =
(approximated_weight - approximated_sharing) <
((1. + VIAPATH_ALPHA) * (upper_bound_to_shortest_path_weight - approximated_sharing));
((1. + VIAPATH_EPSILON) * (upper_bound_to_shortest_path_weight - approximated_sharing));
if (weight_passes && sharing_passes && stretch_passes)
{