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Documents authored by Paraskevopoulos, Andreas

Document
DOI: 10.4230/OASIcs.ATMOS.2024.9
Online Vehicle Routing with Pickups and Deliveries Under Time-Dependent Travel-Time Constraints

Authors: Spyros Kontogiannis, Andreas Paraskevopoulos, and Christos Zaroliagis

Published in: OASIcs, Volume 123, 24th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2024)

Abstract
The Vehicle Routing Problem with pickups, deliveries and spatiotemporal service constraints (VRP_PDSTC) is a quite challenging algorithmic problem that can be dealt with in either an offline or an online fashion. In this work, we focus on a generalization, called VRP_PDSTCtd, in which the travel-time metric is time-dependent: the traversal-time per road segment (represented as a directed arc) is determined by some function of the departure-time from its tail towards its head. Time-dependence makes things much more complicated, even for the simpler problem of computing earliest-arrival-time paths which is a crucial subroutine to be solved (numerous times) by VRP_PDSTCtd schedulers. We propose two online schedulers of requests to workers, one which is a time-dependent variant of the classical Plain-Insertion heuristic, and an extension of it trying to digest some sort of forecasts for future demands for service. We enrich these two online schedulers with two additional heuristics, one targeting for distance-balanced assignments of work loads to the workers and another that makes local-search-improvements to the produced solutions. We conduct a careful experimental evaluation of the proposed algorithms on a real-world instance, with or without these heuristics, and compare their quality with human-curated assignments provided by professional experts (human operators at actual pickup-and-delivery control centers), and also with feasible solutions constructed from a relaxed MILP formulation of VRP_PDSTCtd, which is also introduced in this paper. Our findings are quite encouraging, demonstrating that the proposed algorithms produce solutions which (i) are significant improvements over the human-curated assignments, and (ii) have overall quality pretty close to that of the (extremely time-consuming) solutions provided by an exact solver for the MILP formulation.
Cite as

Spyros Kontogiannis, Andreas Paraskevopoulos, and Christos Zaroliagis. Online Vehicle Routing with Pickups and Deliveries Under Time-Dependent Travel-Time Constraints. In 24th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2024). Open Access Series in Informatics (OASIcs), Volume 123, pp. 9:1-9:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{kontogiannis_et_al:OASIcs.ATMOS.2024.9,
  author =	{Kontogiannis, Spyros and Paraskevopoulos, Andreas and Zaroliagis, Christos},
  title =	{{Online Vehicle Routing with Pickups and Deliveries Under Time-Dependent Travel-Time Constraints}},
  booktitle =	{24th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2024)},
  pages =	{9:1--9:20},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-350-8},
  ISSN =	{2190-6807},
  year =	{2024},
  volume =	{123},
  editor =	{Bouman, Paul C. and Kontogiannis, Spyros C.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2024.9},
  URN =		{urn:nbn:de:0030-drops-211972},
  doi =		{10.4230/OASIcs.ATMOS.2024.9},
  annote =	{Keywords: transport optimization heuristics, vehicle routing with pickups and deliveries, time-dependent travel-times}
}
Document
DOI: 10.4230/OASIcs.ATMOS.2022.12
REX: A Realistic Time-Dependent Model for Multimodal Public Transport

Authors: Spyros Kontogiannis, Paraskevi-Maria-Malevi Machaira, Andreas Paraskevopoulos, and Christos Zaroliagis

Published in: OASIcs, Volume 106, 22nd Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2022)

Abstract
We present the non-FIFO time-dependent graph model with REalistic vehicle eXchange times (REX) for schedule-based multimodal public transport, along with a novel query algorithm called TRIP-based LAbel-correction propagation (TRIPLA) algorithm that efficiently solves the realistic earliest-arrival routing problem. The REX model possesses all strong features of previous time-dependent graph models without suffering from their deficiencies. It handles non-negligible exchanges from one vehicle to another, as well as supports non-FIFO instances which are typical in public transport, without compromising space efficiency. We conduct a thorough experimental evaluation with real-world data which demonstrates that TRIPLA significantly outperforms all state-of-the-art query algorithms for multimodal earliest-arrival routing in schedule-based public transport.
Cite as

Spyros Kontogiannis, Paraskevi-Maria-Malevi Machaira, Andreas Paraskevopoulos, and Christos Zaroliagis. REX: A Realistic Time-Dependent Model for Multimodal Public Transport. In 22nd Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2022). Open Access Series in Informatics (OASIcs), Volume 106, pp. 12:1-12:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{kontogiannis_et_al:OASIcs.ATMOS.2022.12,
  author =	{Kontogiannis, Spyros and Machaira, Paraskevi-Maria-Malevi and Paraskevopoulos, Andreas and Zaroliagis, Christos},
  title =	{{REX: A Realistic Time-Dependent Model for Multimodal Public Transport}},
  booktitle =	{22nd Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2022)},
  pages =	{12:1--12:16},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-259-4},
  ISSN =	{2190-6807},
  year =	{2022},
  volume =	{106},
  editor =	{D'Emidio, Mattia and Lindner, Niels},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2022.12},
  URN =		{urn:nbn:de:0030-drops-171164},
  doi =		{10.4230/OASIcs.ATMOS.2022.12},
  annote =	{Keywords: multimodal journey planning, REX model, TRIPLA query algorithm, schedule-based timetables}
}
Document
DOI: 10.4230/OASIcs.ATMOS.2020.8
Time-Dependent Alternative Route Planning

Authors: Spyros Kontogiannis, Andreas Paraskevopoulos, and Christos D. Zaroliagis

Published in: OASIcs, Volume 85, 20th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2020)

Abstract
We present a new method for computing a set of alternative origin-to-destination routes in road networks with an underlying time-dependent metric. The resulting set is aggregated in the form of a time-dependent alternative graph and is characterized by minimum route overlap, small stretch factor, small size and low complexity. To our knowledge, this is the first work that deals with the time-dependent setting in the framework of alternative routes. Based on preprocessed minimum travel-time information between a small set of nodes and all other nodes in the graph, our algorithm carries out a collection phase for candidate alternative routes, followed by a pruning phase that cautiously discards uninteresting or low-quality routes from the candidate set. Our experimental evaluation on real time-dependent road networks demonstrates that the new algorithm performs much better (by one or two orders of magnitude) than existing baseline approaches. In particular, the entire alternative graph can be computed in less than 0.384sec for the road network of Germany, and in less than 1.24sec for that of Europe. Our approach provides also "quick-and-dirty" results of decent quality, in about 1/300 of the above mentioned query times for continental-size instances.
Cite as

Spyros Kontogiannis, Andreas Paraskevopoulos, and Christos D. Zaroliagis. Time-Dependent Alternative Route Planning. In 20th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2020). Open Access Series in Informatics (OASIcs), Volume 85, pp. 8:1-8:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{kontogiannis_et_al:OASIcs.ATMOS.2020.8,
  author =	{Kontogiannis, Spyros and Paraskevopoulos, Andreas and Zaroliagis, Christos D.},
  title =	{{Time-Dependent Alternative Route Planning}},
  booktitle =	{20th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2020)},
  pages =	{8:1--8:14},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-170-2},
  ISSN =	{2190-6807},
  year =	{2020},
  volume =	{85},
  editor =	{Huisman, Dennis and Zaroliagis, Christos D.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2020.8},
  URN =		{urn:nbn:de:0030-drops-131441},
  doi =		{10.4230/OASIcs.ATMOS.2020.8},
  annote =	{Keywords: time-dependent shortest path, alternative routes, travel-time oracle, plateau and penalty methods}
}
Document
DOI: 10.4230/OASIcs.ATMOS.2019.9
Exploiting Amorphous Data Parallelism to Speed-Up Massive Time-Dependent Shortest-Path Computations

Authors: Spyros Kontogiannis, Anastasios Papadopoulos, Andreas Paraskevopoulos, and Christos Zaroliagis

Published in: OASIcs, Volume 75, 19th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2019)

Abstract
We aim at exploiting parallelism in shared-memory multiprocessing systems, in order to speed up the execution time with as small redundancy in work as possible, for an elementary task that comes up frequently as a subroutine in the daily maintenance of large-scale time-dependent graphs representing real-world relationships or technological networks: the many-to-all time-dependent shortest paths (MATDSP) problem. MATDSP requires the computation of one time-dependent shortest-path tree (TDSPT) per origin-vertex and departure-time, from an arbitrary collection of pairs of origins and departure-times, towards all reachable destinations in the graph. Our goal is to explore the potential and highlight the limitations of amorphous data parallelism, when dealing with MATDSP in multicore computing environments with a given amount of processing elements and a shared memory to exploit. Apart from speeding-up execution time, consumption of resources (and energy) is also critical. Therefore, we aim at limiting the work overhead for solving a MATDSP instance, as measured by the overall number of arc relaxations in shortest-path computations, while trying to minimize the overall execution time. Towards this direction, we provide several algorithmic engineering interventions for solving MATDSP concerning: (i) the compact representation of the instance; (ii) the choice and the improvement of the time-dependent single-source shortest path algorithm that is used as a subroutine; (iii) the way according to which the overall work is allocated to the processing elements; (iv) the adoption of the amorphous data parallelism rationale, in order to avoid costly synchronization among the processing elements while doing their own part of the work. Our experimental evaluations, both on real-world and on synthetic benchmark instances of time-dependent road networks, provide insight how one should organize heavy MATDSP computations, depending on the application scenario. This insight is in some cases rather unexpected. For instance, it is not always the case that pure data parallelism (among otherwise totally independent processors) is the best choice for minimizing execution times. In certain cases it may be worthwhile to limit the level of data parallelism in favor of algorithmic parallelism, in order to achieve more efficient MATDSP computations.
Cite as

Spyros Kontogiannis, Anastasios Papadopoulos, Andreas Paraskevopoulos, and Christos Zaroliagis. Exploiting Amorphous Data Parallelism to Speed-Up Massive Time-Dependent Shortest-Path Computations. In 19th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2019). Open Access Series in Informatics (OASIcs), Volume 75, pp. 9:1-9:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{kontogiannis_et_al:OASIcs.ATMOS.2019.9,
  author =	{Kontogiannis, Spyros and Papadopoulos, Anastasios and Paraskevopoulos, Andreas and Zaroliagis, Christos},
  title =	{{Exploiting Amorphous Data Parallelism to Speed-Up Massive Time-Dependent Shortest-Path Computations}},
  booktitle =	{19th Symposium on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2019)},
  pages =	{9:1--9:18},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-128-3},
  ISSN =	{2190-6807},
  year =	{2019},
  volume =	{75},
  editor =	{Cacchiani, Valentina and Marchetti-Spaccamela, Alberto},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2019.9},
  URN =		{urn:nbn:de:0030-drops-114210},
  doi =		{10.4230/OASIcs.ATMOS.2019.9},
  annote =	{Keywords: amorphous data parallelism, delta-stepping algorithm, travel-time oracle, many-to-all shortest paths, time-dependent road networks}
}
Document
DOI: 10.4230/OASIcs.ATMOS.2017.4
Improved Oracles for Time-Dependent Road Networks

Authors: Spyros Kontogiannis, Georgia Papastavrou, Andreas Paraskevopoulos, Dorothea Wagner, and Christos Zaroliagis

Published in: OASIcs, Volume 59, 17th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2017)

Abstract
A novel landmark-based oracle (CFLAT) is presented, which provides earliest-arrival-time route plans in time-dependent road networks. To our knowledge, this is the first oracle that preprocesses combinatorial structures (collections of time-stamped min-travel-time-path trees) rather than travel-time functions. The preprocessed data structure is exploited by a new query algorithm (CFCA) which also computes (and pays for it) the actual connecting path that preserves the theoretical approximation guarantees. To make it practical and tackle the main burden of landmark-based oracles (the large preprocessing requirements), CFLAT is extensively engineered. A thorough experimental evaluation on two real-world benchmark instances shows that CFLAT achieves a significant improvement on preprocessing, approximation guarantees and query-times, in comparison to previous landmark-based oracles. It also achieves competitive query-time performance compared to state-of-art speedup heuristics for time-dependent road networks, whose query-times in most cases do not account for path construction.
Cite as

Spyros Kontogiannis, Georgia Papastavrou, Andreas Paraskevopoulos, Dorothea Wagner, and Christos Zaroliagis. Improved Oracles for Time-Dependent Road Networks. In 17th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2017). Open Access Series in Informatics (OASIcs), Volume 59, pp. 4:1-4:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{kontogiannis_et_al:OASIcs.ATMOS.2017.4,
  author =	{Kontogiannis, Spyros and Papastavrou, Georgia and Paraskevopoulos, Andreas and Wagner, Dorothea and Zaroliagis, Christos},
  title =	{{Improved Oracles for Time-Dependent Road Networks}},
  booktitle =	{17th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2017)},
  pages =	{4:1--4:17},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-042-2},
  ISSN =	{2190-6807},
  year =	{2017},
  volume =	{59},
  editor =	{D'Angelo, Gianlorenzo and Dollevoet, Twan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2017.4},
  URN =		{urn:nbn:de:0030-drops-78954},
  doi =		{10.4230/OASIcs.ATMOS.2017.4},
  annote =	{Keywords: Time-dependent shortest paths, FIFO property, Distance oracles}
}
Document
DOI: 10.4230/OASIcs.ATMOS.2014.46
Engineering Graph-Based Models for Dynamic Timetable Information Systems

Authors: Alessio Cionini, Gianlorenzo D'Angelo, Mattia D'Emidio, Daniele Frigioni, Kalliopi Giannakopoulou, Andreas Paraskevopoulos, and Christos Zaroliagis

Published in: OASIcs, Volume 42, 14th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (2014)

Abstract
Many efforts have been done in the last years to model public transport timetables in order to find optimal routes. The proposed models can be classified into two types: those representing the timetable as an array, and those representing it as a graph. The array-based models have been shown to be very effective in terms of query time, while the graph-based models usually answer queries by computing shortest paths, and hence they are suitable to be used in combination with speed-up techniques developed for road networks. In this paper, we focus on the dynamic behavior of graph-based models considering the case where transportation systems are subject to delays with respect to the given timetable. We make three contributions: (i) we give a simplified and optimized update routine for the well-known time-expanded model along with an engineered query algorithm; (ii) we propose a new graph-based model tailored for handling dynamic updates; (iii) we assess the effectiveness of the proposed models and algorithms by an experimental study, which shows that both models require negligible update time and a query time which is comparable to that required by some array-based models.
Cite as

Alessio Cionini, Gianlorenzo D'Angelo, Mattia D'Emidio, Daniele Frigioni, Kalliopi Giannakopoulou, Andreas Paraskevopoulos, and Christos Zaroliagis. Engineering Graph-Based Models for Dynamic Timetable Information Systems. In 14th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems. Open Access Series in Informatics (OASIcs), Volume 42, pp. 46-61, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2014)

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@InProceedings{cionini_et_al:OASIcs.ATMOS.2014.46,
  author =	{Cionini, Alessio and D'Angelo, Gianlorenzo and D'Emidio, Mattia and Frigioni, Daniele and Giannakopoulou, Kalliopi and Paraskevopoulos, Andreas and Zaroliagis, Christos},
  title =	{{Engineering Graph-Based Models for Dynamic Timetable Information Systems}},
  booktitle =	{14th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems},
  pages =	{46--61},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-939897-75-0},
  ISSN =	{2190-6807},
  year =	{2014},
  volume =	{42},
  editor =	{Funke, Stefan and Mihal\'{a}k, Mat\'{u}s},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2014.46},
  URN =		{urn:nbn:de:0030-drops-47522},
  doi =		{10.4230/OASIcs.ATMOS.2014.46},
  annote =	{Keywords: Timetabling, dynamic updates, queries, shortest paths}
}
Document
DOI: 10.4230/OASIcs.ATMOS.2013.108
Improved Alternative Route Planning

Authors: Andreas Paraskevopoulos and Christos Zaroliagis

Published in: OASIcs, Volume 33, 13th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (2013)

Abstract
We present improved methods for computing a set of alternative source-to-destination routes in road networks in the form of an alternative graph. The resulting alternative graphs are characterized by minimum path overlap, small stretch factor, as well as low size and complexity. Our approach improves upon a previous one by introducing a new pruning stage preceding any other heuristic method and by introducing a new filtering and fine-tuning of two existing methods. Our accompanying experimental study shows that the entire alternative graph can be computed pretty fast even in continental size networks.
Cite as

Andreas Paraskevopoulos and Christos Zaroliagis. Improved Alternative Route Planning. In 13th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems. Open Access Series in Informatics (OASIcs), Volume 33, pp. 108-122, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2013)

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@InProceedings{paraskevopoulos_et_al:OASIcs.ATMOS.2013.108,
  author =	{Paraskevopoulos, Andreas and Zaroliagis, Christos},
  title =	{{Improved Alternative Route Planning}},
  booktitle =	{13th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems},
  pages =	{108--122},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-939897-58-3},
  ISSN =	{2190-6807},
  year =	{2013},
  volume =	{33},
  editor =	{Frigioni, Daniele and Stiller, Sebastian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.ATMOS.2013.108},
  URN =		{urn:nbn:de:0030-drops-42485},
  doi =		{10.4230/OASIcs.ATMOS.2013.108},
  annote =	{Keywords: Alternative route, stretch factor, shortest path, non-overlapping path, penalty, plateau}
}
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