# Intel® Threading Building Blocks.
Shortpath sample

This directory contains a simple example that solves the single source shortest path problem.
It is parameterized by N, a number of nodes, and a start and end node in [0..N). A graph is generated with N nodes and some random number of connections between those nodes. A parallel algorithm based on A* is used to find the shortest path.
This algorithm varies from serial A* in that it needs to add nodes back to the open set when the g estimate (shortest path from start to the node) is improved, even if the node has already been "visited". This is because nodes are added and removed from the open-set in parallel, resulting in some less optimal paths being explored. The open-set is implemented with the concurrent_priority_queue.
Note that since we re-visit nodes, the f estimate (on which the priority queue is sorted) is not technically needed, so we could use this same parallel algorithm with just a concurrent_queue. However, keeping the f estimate and using `concurrent_priority_queue` results in much better performance.
Silent mode prints run time only, regular mode prints the shortest path length, and verbose mode prints out the shortest path.
The generated graph follows a pattern in which the closer two pairs of node ids are together, the fewer hops there are in a typical path between those nodes. So, for example, the path between 5 and 7 likely has few hops whereas 14 to 78 has more and 0 to 9999 has even more, etc.

System Requirements

For the most up to date system requirements, see the release notes.

Files

shortpath.cpp
  Driver.
Makefile
  Makefile for building the example.

Directories

msvs
  Contains Microsoft* Visual Studio* workspace for building and running the example with the Intel® C++ Compiler (Windows* systems only).
xcode
  Contains macOS* Xcode* workspace for building and running the example (macOS* systems only).

For information about the minimum supported version of IDE, see release notes.

Build instructions

General build directions can be found here.

Usage

`shortpath -h`
  Prints the help for command line options
`shortpath [#threads=value] [verbose] [silent] [N=value] [start=value] [end=value] [#threads]`
  `#threads` is the number of threads to use; a range of the form `low[:high]` where `low` and optional `high` are non-negative integers, or `'auto'` for the TBB default.
  `verbose` print full path to screen
  `silent` limits output to timing info; overrides verbose
  `N` number of nodes in graph
  `start` node to start path at
  `end` node to end path at
To run a short version of this example, e.g., for use with Intel® Parallel Inspector:
  Build a debug version of the example (see the build instructions).
  Run it with a small problem size and the desired number of threads, e.g., `shortpath 4 N=20 start=0 end=19`.

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