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r.sim: Keep the diffusion coefficient in a walker-local variable - #7965
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The diffusion coefficient of a walker's move was stored in a raster-sized grid cell only to be read back by the same walker a few lines later, so concurrent walkers in the same cell could read each other's value. Keep it in a local variable instead and drop the grid. The local variable is a float, the type of the grid, rather than a double, so that single-threaded results do not change: the depth, discharge, sediment flux and erosion/deposition maps of single-threaded runs on the nc_spm elevation are byte-identical to those before this change. With more threads, results can differ from before, because a walker no longer moves with a coefficient another walker wrote. Written with the help of Claude Code.
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…eo#7965) The diffusion coefficient of a walker's move was stored in a raster-sized grid cell only to be read back by the same walker a few lines later, so with nprocs > 1, concurrent walkers in the same cell could read each other's value. Keep it in a local variable instead and drop the grid, which also saves one float grid of memory. The local variable is a float, the type of the grid, rather than a double, so that single-threaded results do not change: the depth, discharge, sediment flux and erosion/deposition maps of single-threaded runs on the nc_spm elevation are byte-identical to those before this change. With more threads, results can differ from before, because a walker no longer moves with a coefficient another walker wrote. Written with the help of Claude Code.
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In the r.sim.water and r.sim.sediment walker loop, the diffusion coefficient of a walker's move was written to a raster-sized grid and read back by the same walker a few lines later. With nprocs > 1, a walker on another thread in the same cell could overwrite it in between. This makes it a local variable and drops the grid, which also saves one float grid of memory. Single-threaded results do not change.
Details (AI generated)
grids->difwas used only inmain_loop()insimlib/hydro.c, so it is removed fromGridsand from both allocation functions insimlib/input.c.The analysis and code were done with Claude Code (Opus) and reviewed by me.