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Documents authored by Gajjala, Rishikesh

Document
DOI: 10.4230/LIPIcs.FSTTCS.2024.17
Two Results on LPT: A Near-Linear Time Algorithm and Parcel Delivery Using Drones

Authors: L. Sunil Chandran, Rishikesh Gajjala, Shravan Mehra, and Saladi Rahul

Published in: LIPIcs, Volume 323, 44th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2024)

Abstract
The focus of this paper is to increase our understanding of the Longest Processing Time First (LPT) heuristic. LPT is a classical heuristic for the fundamental problem of uniform machine scheduling. For different machine speeds, LPT was first considered by Gonzalez et al. (SIAM J. Comput. 6(1):155–166, 1977). Since then, extensive work has been done to improve the approximation factor of the LPT heuristic. However, all known implementations of the LPT heuristic take O(mn) time, where m is the number of machines and n is the number of jobs. In this work, we come up with the first near-linear time implementation for LPT. Specifically, the running time is O((n+m)(log²m + log n)). Somewhat surprisingly, the result is obtained by mapping the problem to dynamic maintenance of lower envelope of lines, which has been well studied in the computational geometry community. Our second contribution is to analyze the performance of LPT for the Drones Warehouse Problem (DWP), which is a natural generalization of the uniform machine scheduling problem motivated by drone-based parcel delivery from a warehouse. In this problem, a warehouse has multiple drones and wants to deliver parcels to several customers. Each drone picks a parcel from the warehouse, delivers it, and returns to the warehouse (where it can also get charged). The speeds and battery lives of the drones could be different, and due to the limited battery life, each drone has a bounded range in which it can deliver parcels. The goal is to assign parcels to the drones so that the time taken to deliver all the parcels is minimized. We prove that the natural approach of solving this problem via the LPT heuristic has an approximation factor of ϕ, where ϕ ≈ 1.62 is the golden ratio.
Cite as

L. Sunil Chandran, Rishikesh Gajjala, Shravan Mehra, and Saladi Rahul. Two Results on LPT: A Near-Linear Time Algorithm and Parcel Delivery Using Drones. In 44th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 323, pp. 17:1-17:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{chandran_et_al:LIPIcs.FSTTCS.2024.17,
  author =	{Chandran, L. Sunil and Gajjala, Rishikesh and Mehra, Shravan and Rahul, Saladi},
  title =	{{Two Results on LPT: A Near-Linear Time Algorithm and Parcel Delivery Using Drones}},
  booktitle =	{44th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2024)},
  pages =	{17:1--17:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-355-3},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{323},
  editor =	{Barman, Siddharth and Lasota, S{\l}awomir},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2024.17},
  URN =		{urn:nbn:de:0030-drops-222060},
  doi =		{10.4230/LIPIcs.FSTTCS.2024.17},
  annote =	{Keywords: Scheduling, Approximation algorithms, Fine-grained complexity}
}
Document
DOI: 10.4230/LIPIcs.MFCS.2024.41
Krenn-Gu Conjecture for Sparse Graphs

Authors: L. Sunil Chandran, Rishikesh Gajjala, and Abraham M. Illickan

Published in: LIPIcs, Volume 306, 49th International Symposium on Mathematical Foundations of Computer Science (MFCS 2024)

Abstract
Greenberger–Horne–Zeilinger (GHZ) states are quantum states involving at least three entangled particles. They are of fundamental interest in quantum information theory, and the construction of such states of high dimension has various applications in quantum communication and cryptography. Krenn, Gu and Zeilinger discovered a correspondence between a large class of quantum optical experiments which produce GHZ states and edge-weighted edge-coloured multi-graphs with some special properties called the GHZ graphs. On such GHZ graphs, a graph parameter called dimension can be defined, which is the same as the dimension of the GHZ state produced by the corresponding experiment. Krenn and Gu conjectured that the dimension of any GHZ graph with more than 4 vertices is at most 2. An affirmative resolution of the Krenn-Gu conjecture has implications for quantum resource theory. Moreover, this would save huge computational resources used for finding experiments which lead to higher dimensional GHZ states. On the other hand, the construction of a GHZ graph on a large number of vertices with a high dimension would lead to breakthrough results. In this paper, we study the existence of GHZ graphs from the perspective of the Krenn-Gu conjecture and show that the conjecture is true for graphs of vertex connectivity at most 2 and for cubic graphs. We also show that the minimal counterexample to the conjecture should be 4-connected. Such information could be of great help in the search for GHZ graphs using existing tools like PyTheus. While the impact of the work is in quantum physics, the techniques in this paper are purely combinatorial, and no background in quantum physics is required to understand them.
Cite as

L. Sunil Chandran, Rishikesh Gajjala, and Abraham M. Illickan. Krenn-Gu Conjecture for Sparse Graphs. In 49th International Symposium on Mathematical Foundations of Computer Science (MFCS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 306, pp. 41:1-41:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{chandran_et_al:LIPIcs.MFCS.2024.41,
  author =	{Chandran, L. Sunil and Gajjala, Rishikesh and Illickan, Abraham M.},
  title =	{{Krenn-Gu Conjecture for Sparse Graphs}},
  booktitle =	{49th International Symposium on Mathematical Foundations of Computer Science (MFCS 2024)},
  pages =	{41:1--41:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-335-5},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{306},
  editor =	{Kr\'{a}lovi\v{c}, Rastislav and Ku\v{c}era, Anton{\'\i}n},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2024.41},
  URN =		{urn:nbn:de:0030-drops-205978},
  doi =		{10.4230/LIPIcs.MFCS.2024.41},
  annote =	{Keywords: Graph colourings, Perfect matchings, Quantum Physics}
}
Document
DOI: 10.4230/LIPIcs.FSTTCS.2021.8
Generalizations of Length Limited Huffman Coding for Hierarchical Memory Settings

Authors: Shashwat Banchhor, Rishikesh Gajjala, Yogish Sabharwal, and Sandeep Sen

Published in: LIPIcs, Volume 213, 41st IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2021)

Abstract
In this paper, we study the problem of designing prefix-free encoding schemes having minimum average code length that can be decoded efficiently under a decode cost model that captures memory hierarchy induced cost functions. We also study a special case of this problem that is closely related to the length limited Huffman coding (LLHC) problem; we call this the soft-length limited Huffman coding problem. In this version, there is a penalty associated with each of the n characters of the alphabet whose encodings exceed a specified bound D(≤ n) where the penalty increases linearly with the length of the encoding beyond D. The goal of the problem is to find a prefix-free encoding having minimum average code length and total penalty within a pre-specified bound P. This generalizes the LLHC problem. We present an algorithm to solve this problem that runs in time O(nD). We study a further generalization in which the penalty function and the objective function can both be arbitrary monotonically non-decreasing functions of the codeword length. We provide dynamic programming based exact and PTAS algorithms for this setting.
Cite as

Shashwat Banchhor, Rishikesh Gajjala, Yogish Sabharwal, and Sandeep Sen. Generalizations of Length Limited Huffman Coding for Hierarchical Memory Settings. In 41st IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 213, pp. 8:1-8:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{banchhor_et_al:LIPIcs.FSTTCS.2021.8,
  author =	{Banchhor, Shashwat and Gajjala, Rishikesh and Sabharwal, Yogish and Sen, Sandeep},
  title =	{{Generalizations of Length Limited Huffman Coding for Hierarchical Memory Settings}},
  booktitle =	{41st IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2021)},
  pages =	{8:1--8:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-215-0},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{213},
  editor =	{Boja\'{n}czyk, Miko{\l}aj and Chekuri, Chandra},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2021.8},
  URN =		{urn:nbn:de:0030-drops-155193},
  doi =		{10.4230/LIPIcs.FSTTCS.2021.8},
  annote =	{Keywords: Approximation algorithms, Hierarchical memory, Prefix free codes}
}
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