Index: aoc2015.c
==================================================================
--- aoc2015.c
+++ aoc2015.c
@@ -4,26 +4,45 @@
 #include <stdlib.h>
 #include <string.h>
 #include "aocdailies.h"
 #include "aocutils.h"
 
-static void gridchange(bool *grid, int v,
+struct LampGrid {
+    unsigned lit:1;
+    unsigned bright:31;
+};
+
+static void gridchange(struct LampGrid *grid, int v,
                        unsigned r1, unsigned c1, unsigned r2, unsigned c2) {
     unsigned firstrow = (r1 < r2) ? r1 : r2;
     unsigned lastrow = (r1 < r2) ? r2 : r1;
     unsigned firstcol = (c1 < c2) ? c1 : c2;
     unsigned lastcol = (c1 < c2) ? c2 : c1;
     for (unsigned row = firstrow; row <= lastrow; row++) {
         for (unsigned col = firstcol; col <= lastcol; col++) {
-            grid[1000*row + col] = (v < 0) ? !grid[1000*row + col] : !!v;
+            size_t index = 1000*row + col;
+            switch (v) {
+                default: fprintf(stderr, "bad value detected\n");
+                         exit(EXIT_FAILURE);
+                         //  break; // never reached
+                case -1: grid[index].lit = 0;
+                         if (grid[index].bright > 0) grid[index].bright -= 1;
+                         break;
+                case 1: grid[index].lit = 1;
+                        grid[index].bright += 1;
+                        break;
+                case 2: grid[index].lit = 1 - grid[index].lit;
+                        grid[index].bright += 2;
+                        break;
+            }
         }
     }
 }
 
 void aoc201506(char *data, size_t len) {
     (void)len; // unused argument
-    bool *grid_ptr = calloc(1000 * 1000, 1);
+    struct LampGrid *grid_ptr = calloc(1000 * 1000, sizeof *grid_ptr);
     char *line = strtok(data, "\n");
     while (line) {
         unsigned x1, x2, y1, y2;
         char *err = line;
         while (!isdigit((unsigned char)*err)) err++;
@@ -30,22 +49,24 @@
         y1 = strtoul(err, &err, 10); err++;
         x1 = strtoul(err, &err, 10); while (!isdigit((unsigned char)*err)) err++;
         y2 = strtoul(err, &err, 10); err++;
         x2 = strtoul(err, &err, 10);
         if (line[6] == 'n') gridchange(grid_ptr, 1, y1, x1, y2, x2);
-        if (line[6] == 'f') gridchange(grid_ptr, 0, y1, x1, y2, x2);
-        if (line[6] == ' ') gridchange(grid_ptr, -1, y1, x1, y2, x2);
+        if (line[6] == 'f') gridchange(grid_ptr, -1, y1, x1, y2, x2);
+        if (line[6] == ' ') gridchange(grid_ptr, 2, y1, x1, y2, x2);
         line = strtok(NULL, "\n");
     }
-    unsigned lit = 0;
+    unsigned lit = 0, brightness = 0;
     for (unsigned row = 0; row < 1000; row++) {
         for (unsigned col = 0; col < 1000; col++) {
-            lit += grid_ptr[1000*row + col];
+            lit += grid_ptr[1000*row + col].lit;
+            brightness += grid_ptr[1000*row + col].bright;
         }
     }
     free(grid_ptr);
     printf("After following the instructions, there are %u lights lit.\n", lit);
+    printf("Total brightness is {%u}.\n", brightness);
 }
 
 void aoc201505(char *data, size_t len) {
     (void)len; // unused argument
     unsigned nice = 0, nice2 = 0;