From 4f549143f04c4606870451c54d227c746f5eccd8 Mon Sep 17 00:00:00 2001
From: Huidae Cho <grass4u@gmail.com>
Date: Tue, 17 Oct 2023 12:42:28 -0600
Subject: [PATCH] r.flowaccumulate: Handle null values properly

---
 src/raster/r.flowaccumulation/accumulate.c |  7 +++----
 src/raster/r.flowaccumulation/global.h     |  8 ++++++++
 src/raster/r.flowaccumulation/main.c       | 11 ++++++++---
 3 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/src/raster/r.flowaccumulation/accumulate.c b/src/raster/r.flowaccumulation/accumulate.c
index fc0c9ca114..e36105a910 100644
--- a/src/raster/r.flowaccumulation/accumulate.c
+++ b/src/raster/r.flowaccumulation/accumulate.c
@@ -1,7 +1,6 @@
 #include <stdlib.h>
 #include "global.h"
 
-#define DIR_NULL        0x80000000
 #define ACCUM(row, col) accum_map->cells[(size_t)(row)*ncols + (col)]
 #define FIND_UP(row, col)                                                     \
     ((row > 0 ? (col > 0 && DIR(row - 1, col - 1) == SE ? NW : 0) |           \
@@ -41,7 +40,7 @@ void accumulate(struct raster_map *dir_map, struct raster_map *accum_map)
 #pragma omp parallel for schedule(dynamic) private(col)
     for (row = 0; row < nrows; row++) {
         for (col = 0; col < ncols; col++)
-            if (DIR(row, col) != DIR_NULL)
+            if (DIR(row, col) != cell_null)
                 UP(row, col) = FIND_UP(row, col);
     }
 #endif
@@ -51,7 +50,7 @@ void accumulate(struct raster_map *dir_map, struct raster_map *accum_map)
         for (col = 0; col < ncols; col++)
             /* if the current cell is not null and has no upstream cells, start
              * tracing down */
-            if (DIR(row, col) != DIR_NULL && !UP(row, col))
+            if (DIR(row, col) != cell_null && !UP(row, col))
                 trace_down(dir_map, accum_map, row, col, 1);
     }
 
@@ -103,7 +102,7 @@ static void trace_down(struct raster_map *dir_map, struct raster_map *accum_map,
     /* if the downstream cell is null or any upstream cells of the downstream
      * cell have never been visited, stop tracing down */
     if (row < 0 || row >= nrows || col < 0 || col >= ncols ||
-        DIR(row, col) == DIR_NULL || !(up = UP(row, col)) ||
+        DIR(row, col) == cell_null || !(up = UP(row, col)) ||
         !(accum_up = sum_up(accum_map, row, col, up)))
         return;
 
diff --git a/src/raster/r.flowaccumulation/global.h b/src/raster/r.flowaccumulation/global.h
index 7c4539664e..cf7b52f229 100644
--- a/src/raster/r.flowaccumulation/global.h
+++ b/src/raster/r.flowaccumulation/global.h
@@ -32,4 +32,12 @@ long long timeval_diff(struct timeval *, struct timeval *, struct timeval *);
 /* accumulate.c */
 void accumulate(struct raster_map *, struct raster_map *);
 
+#ifdef _MAIN_C_
+#define GLOBAL
+#else
+#define GLOBAL extern
+#endif
+
+GLOBAL CELL cell_null;
+
 #endif
diff --git a/src/raster/r.flowaccumulation/main.c b/src/raster/r.flowaccumulation/main.c
index 7c7931dbb3..eee5ae329a 100644
--- a/src/raster/r.flowaccumulation/main.c
+++ b/src/raster/r.flowaccumulation/main.c
@@ -177,6 +177,8 @@ int main(int argc, char *argv[])
     nrows = Rast_window_rows();
     ncols = Rast_window_cols();
 
+    Rast_set_c_null_value(&cell_null, 1);
+
     dir_map = G_malloc(sizeof *dir_map);
     dir_map->nrows = nrows;
     dir_map->ncols = ncols;
@@ -186,15 +188,18 @@ int main(int argc, char *argv[])
         Rast_get_c_row(dir_fd, dir_map->cells + ncols * row, row);
         if (dir_format == DIR_DEG) {
             for (col = 0; col < ncols; col++)
-                DIR(row, col) = pow(2, abs(DIR(row, col) / 45.));
+                if (DIR(row, col) != cell_null)
+                    DIR(row, col) = pow(2, abs(DIR(row, col) / 45.));
         }
         else if (dir_format == DIR_DEG45) {
             for (col = 0; col < ncols; col++)
-                DIR(row, col) = pow(2, 8 - abs(DIR(row, col)));
+                if (DIR(row, col) != cell_null)
+                    DIR(row, col) = pow(2, 8 - abs(DIR(row, col)));
         }
         else {
             for (col = 0; col < ncols; col++)
-                DIR(row, col) = abs(DIR(row, col));
+                if (DIR(row, col) != cell_null)
+                    DIR(row, col) = abs(DIR(row, col));
         }
     }
     G_percent(1, 1, 1);