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arXiv 2107.07792cs.CGcs.CCcs.DS
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Fréchet距离下时间序列近似近邻搜索的紧界

Tight Bounds for Approximate Near Neighbor Searching for Time Series under the Fréchet Distance

Karl Bringmann, Anne Driemel, André Nusser, Ioannis Psarros

摘要:

我们研究了连续Fréchet距离下的$c$ -近似近邻问题:给定一组具有$m$顶点、半径$δ> 0$和参数$k \leq m$的$n$多边形曲线,我们希望将这些曲线预处理成一个数据结构,给定一个具有$k$顶点的查询曲线$q$,或者返回一个具有至多$c\cdot δ$到$q$的Fréchet距离的输入曲线,或返回不存在Fréchet距离最大为$δ$到$q$的输入曲线。我们专注于输入和查询是一维多边形曲线(也称为时间序列)的情况,并对这种情况进行了全面的分析。我们获得了新的上界,该上界提供了逼近因子、预处理时间和查询时间之间的不同权衡。我们的数据结构在几个方面改进了现有技术。我们表明,对于任何$0 < \varepsilon \leq 1$的逼近因子$(1+\varepsilon)$可以实现在相同的渐近时间范围内作为以前最好的结果为$(2+\varepsilon)$。此外,我们表明,$(2+\varepsilon)$的近似因子可以通过使用预处理时间和空间$O(nm)$,这是线性的输入大小,和查询时间在$O(\frac{1}{\varepsilon})^{k+2}$,其中以前最好的结果使用预处理时间在$n \cdot O(\frac{m}{\varepsilon k})^k$和查询时间在$O(1)^k$。我们补充我们的上界与匹配的条件下界的基础上的正交向量假设。有趣的是,我们的一些下界已经适用于$k$的任何超常数值。这是通过证明作为中间问题的正交向量问题的单侧稀疏版本的硬度来实现的,我们认为这是独立的兴趣。

英文摘要:

We study the $c$-approximate near neighbor problem under the continuous Fréchet distance: Given a set of $n$ polygonal curves with $m$ vertices, a radius $δ> 0$, and a parameter $k \leq m$, we want to preprocess the curves into a data structure that, given a query curve $q$ with $k$ vertices, either returns an input curve with Fréchet distance at most $c\cdot δ$ to $q$, or returns that there exists no input curve with Fréchet distance at most $δ$ to $q$. We focus on the case where the input and the queries are one-dimensional polygonal curves -- also called time series -- and we give a comprehensive analysis for this case. We obtain new upper bounds that provide different tradeoffs between approximation factor, preprocessing time, and query time. Our data structures improve upon the state of the art in several ways. We show that for any $0 < \varepsilon \leq 1$ an approximation factor of $(1+\varepsilon)$ can be achieved within the same asymptotic time bounds as the previously best result for $(2+\varepsilon)$. Moreover, we show that an approximation factor of $(2+\varepsilon)$ can be obtained by using preprocessing time and space $O(nm)$, which is linear in the input size, and query time in $O(\frac{1}{\varepsilon})^{k+2}$, where the previously best result used preprocessing time in $n \cdot O(\frac{m}{\varepsilon k})^k$ and query time in $O(1)^k$. We complement our upper bounds with matching conditional lower bounds based on the Orthogonal Vectors Hypothesis. Interestingly, some of our lower bounds already hold for any super-constant value of $k$. This is achieved by proving hardness of a one-sided sparse version of the Orthogonal Vectors problem as an intermediate problem, which we believe to be of independent interest.

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