improve bwhist reporting of the median calculation (was wrong)

src/util/bwstat.c

@@ -76,7 +76,7 @@ main(int argc, char **argv)
     double d;
     unsigned char *bp;
     long num_pixels = 0L ;
-    long sum = 0L , partial_sum = 0L ;
+    long sum = 0L, cumulative_pixels = 0L, median_rank;
     int max = -1, min = 256 , mode = 0 , median = 0 ;
     double mean, var = 0.0 , skew = 0.0 ;
     FILE *fp;
@@ -86,7 +86,7 @@ main(int argc, char **argv)
     setmode(fileno(stdin), O_BINARY);
     setmode(fileno(stdout), O_BINARY);
 
-    if (BU_STR_EQUAL(argv[1], "-h") || BU_STR_EQUAL(argv[1], "-?"))
+    if (argc > 1 && (BU_STR_EQUAL(argv[1], "-h") || BU_STR_EQUAL(argv[1], "-?")))
 	bu_exit(1, "%s", Usage);
 
     /* check for verbose flag */
@@ -120,6 +120,16 @@ main(int argc, char **argv)
 	for (i = 0; i < n; i++)
 	    bin[ *bp++ ]++;
     }
+    if (ferror(fp)) {
+	bu_exit(1, "bwstat: error reading input\n");
+    }
+    if (fp != stdin) {
+	fclose(fp);
+    }
+    if (num_pixels == 0) {
+	bu_exit(1, "bwstat: input contains no pixels\n");
+    }
+    median_rank = num_pixels / 2 + num_pixels % 2;
 
     /*
      * Find sum, min, max, mode. (sum and mode initialized to 0;
@@ -143,13 +153,14 @@ main(int argc, char **argv)
 
     /*
      * Now do a second pass to compute median,
-     * variance and skew. (median and partial_sum initialized to 0;
+     * variance and skew. (median and cumulative_pixels initialized to 0;
      * var and skew initialized to 0.0)
      */
     for (i = 0; i < 256; i++) {
-	if (partial_sum < sum/2.0) {
-	    partial_sum += i * bin[i];
-	    median = i;
+	if (cumulative_pixels < median_rank) {
+	    cumulative_pixels += bin[i];
+	    if (cumulative_pixels >= median_rank)
+		median = i;
 	}
 	d = (double)i - mean;
 	var += bin[i] * d * d;

src/util/tests/CMakeLists.txt

@@ -19,9 +19,17 @@ brlcad_add_test(
     -P "${CMAKE_CURRENT_SOURCE_DIR}/test_pixembed_edge_replication.cmake"
 )
 
+brlcad_add_test(
+  NAME util_bwstat_statistics
+  COMMAND
+    "${CMAKE_COMMAND}" "-DBWSTAT=$<TARGET_FILE:bwstat>" "-DTEST_DIR=${CMAKE_CURRENT_BINARY_DIR}"
+    -P "${CMAKE_CURRENT_SOURCE_DIR}/test_bwstat_statistics.cmake"
+)
+
 cmakefiles(
   CMakeLists.txt
   test_bwcrop_single_axis.cmake
+  test_bwstat_statistics.cmake
   test_pixdiff_bytes.cmake
   test_pixembed_edge_replication.cmake
 )

src/util/tests/test_bwstat_statistics.cmake

@@ -0,0 +1,39 @@
+if(NOT DEFINED BWSTAT OR NOT DEFINED TEST_DIR)
+  message(FATAL_ERROR "BWSTAT and TEST_DIR are required")
+endif()
+
+set(input_file "${TEST_DIR}/bwstat-input.bw")
+set(empty_file "${TEST_DIR}/bwstat-empty.bw")
+file(REMOVE "${input_file}" "${empty_file}")
+
+string(ASCII 1 1 255 input_data)
+file(WRITE "${input_file}" "${input_data}")
+file(WRITE "${empty_file}" "")
+
+execute_process(
+  COMMAND "${BWSTAT}" "${input_file}"
+  RESULT_VARIABLE bwstat_result
+  OUTPUT_VARIABLE bwstat_output
+  ERROR_VARIABLE bwstat_error
+)
+if(NOT bwstat_result EQUAL 0)
+  message(FATAL_ERROR "bwstat failed (${bwstat_result}): ${bwstat_error}")
+endif()
+if(NOT bwstat_output MATCHES "Median  +1")
+  message(FATAL_ERROR "bwstat reported the wrong median:\n${bwstat_output}")
+endif()
+
+execute_process(
+  COMMAND "${BWSTAT}" "${empty_file}"
+  RESULT_VARIABLE bwstat_result
+  OUTPUT_VARIABLE bwstat_output
+  ERROR_VARIABLE bwstat_error
+)
+if(bwstat_result EQUAL 0)
+  message(FATAL_ERROR "bwstat accepted empty input:\n${bwstat_output}")
+endif()
+if(NOT bwstat_error MATCHES "input contains no pixels")
+  message(FATAL_ERROR "bwstat did not explain the empty input error:\n${bwstat_error}")
+endif()
+
+file(REMOVE "${input_file}" "${empty_file}")