Cut the dijkstra_algorithm.py time in half with various tricks

[Mortal]

Implement a number of performance optimizations that altogether cut down the running time to about half, as tested with a 2905 * 4029 raster with 1 start point and 1 end point.

• Don't use queue.PriorityQueue, as it is a threadsafe queue with unnecessary locking - use heapq.heappush() and heappop() instead.

• Don't use function calls - instead, inline all functions. Almost all the functions in the old code were only used in one place.

• Instead of dicts and sets, use lists of lists, which have faster lookup.

• Reduce the number of calls to feedback.setProgress().

[Gooong]

Hi @Mortal, thanks for the improvement. Will review and test this PR and release soon!

[biship]

@Mortal

QGIS version: 3.40.2-Bratislava
QGIS code revision: 14826ca1
Qt version: 5.15.13
Python version: 3.12.8
GDAL version: 3.9.3
GEOS version: 3.13.0-CAPI-1.19.0
PROJ version: Rel. 9.5.0, September 15th, 2024
PDAL version: 2.8.1 (git-version: a06325)
Algorithm started at: 2025-01-21T10:06:15
Algorithm 'Least Cost Path' starting…
Input parameters:
{ 'BOOLEAN_FIND_LEAST_PATH_TO_ALL_ENDS' : False, 'BOOLEAN_OUTPUT_LINEAR_REFERENCE' : False, 'INPUT_COST_RASTER' : 'D:/Downloads/qgis/slope.tif', 'INPUT_END_LAYER' : 'end.shp', 'INPUT_RASTER_BAND' : 1, 'INPUT_START_LAYER' : 'start.shp', 'OUTPUT' : 'TEMPORARY_OUTPUT' }

The size of cost raster is: 6401 * 5378
Searching least cost path...
Traceback (most recent call last):
File "C:\Users/nick/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\leastcostpath\least_cost_path_algorithm.py", line 246, in processAlgorithm
result = dijkstra(start_tuple, end_tuples, matrix, find_nearest, feedback)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users/nick/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\leastcostpath\dijkstra_algorithm.py", line 90, in dijkstra
its += 1
^^^
UnboundLocalError: cannot access local variable 'its' where it is not associated with a value

Execution failed after 8.32 seconds

Loading resulting layers
Algorithm 'Least Cost Path' finished

[biship]

Commenting out line 90 (its), crashes QGIS. The PR doesn't work for me.

Python Stack Trace
Windows fatal exception: access violation

Current thread 0x000089e8 (most recent call first):
File "C:\Users/nick/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\leastcostpath\least_cost_path_algorithm.py", line 452 in block2matrix
matrix = [[None if block.isNoData(i, j) else block.value(i, j) for j in range(block.width())]
File "C:\Users/nick/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\leastcostpath\least_cost_path_algorithm.py", line 238 in processAlgorithm
matrix, contains_negative = MinCostPathHelper.block2matrix(block)

Thread 0x00007d7c (most recent call first):
File "C:\PROGRA1/QGIS/apps/qgis/./python/plugins\processing\ProcessingPlugin.py", line 509 in executeAlgorithm
dlg.exec()
File "C:\PROGRA1/QGIS/apps/qgis/./python/plugins\processing\gui\ProcessingToolbox.py", line 272 in executeAlgorithm
self.executeWithGui.emit(alg.id(), self, self.in_place_mode, False)

Stack Trace

PyInit_sip :
PyInit_sip :
PyInit_sip :
pdal::PointView::layout :
PyLong_FromString :
PyObject_Vectorcall :
PyObject_Vectorcall :
PyEval_EvalFrameDefault :
PyFunction_Vectorcall :
PyArg_CheckPositional :
PyObject_Call :
PyObject_Call :
PyInit_sip :
PyInit_sip :
pdal::StreamPointTable::reset :
pdal::PointView::layout :
QgsProcessingAlgorithm::runPrepared :
QgsProcessingAlgRunnerTask::run :
pdal::PointView::layout :
QgsTask::start :
QThreadPoolPrivate::reset :
QThread::start :
BaseThreadInitThunk :
RtlUserThreadStart :

QGIS Info
QGIS Version: 3.40.2-Bratislava
QGIS code revision: 14826ca1
Compiled against Qt: 5.15.13
Running against Qt: 5.15.13
Compiled against GDAL: 3.9.3
Running against GDAL: 3.9.3

System Info
CPU Type: x86_64
Kernel Type: winnt
Kernel Version: 10.0.26100