Index: aoc2015.c
==================================================================
--- aoc2015.c
+++ aoc2015.c
@@ -1,11 +1,52 @@
+#include <ctype.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include "aocdailies.h"
 #include "aocutils.h"
+
+static void gridchange(bool *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;
+        }
+    }
+}
+
+void aoc201506(char *data, size_t len) {
+    (void)len; // unused argument
+    bool *grid_ptr = calloc(1000 * 1000, 1);
+    char *line = strtok(data, "\n");
+    while (line) {
+        unsigned x1, x2, y1, y2;
+        char *err = line;
+        while (!isdigit((unsigned char)*err)) err++;
+        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);
+        line = strtok(NULL, "\n");
+    }
+    unsigned lit = 0;
+    for (unsigned row = 0; row < 1000; row++) {
+        for (unsigned col = 0; col < 1000; col++) {
+            lit += grid_ptr[1000*row + col];
+        }
+    }
+    free(grid_ptr);
+    printf("After following the instructions, there are %u lights lit.\n", lit);
+}
 
 void aoc201505(char *data, size_t len) {
     (void)len; // unused argument
     unsigned nice = 0, nice2 = 0;
     char *line = strtok(data, "\n"), *curr;

Index: aocdailies.c
==================================================================
--- aocdailies.c
+++ aocdailies.c
@@ -35,12 +35,12 @@
         case 201511: p = aoc201511; break;
         case 201510: p = aoc201510; break;
         case 201509: p = aoc201509; break;
         case 201508: p = aoc201508; break;
         case 201507: p = aoc201507; break;
-        case 201506: p = aoc201506; break;
         #endif
+        case 201506: p = aoc201506; break;
         case 201505: p = aoc201505; break;
         case 201504: p = aoc201504; break;
         case 201503: p = aoc201503; break;
         case 201502: p = aoc201502; break;
         case 201501: p = aoc201501; break;

Index: aocdailies.h
==================================================================
--- aocdailies.h
+++ aocdailies.h
@@ -34,12 +34,12 @@
 aocfunc aoc201511;
 aocfunc aoc201510;
 aocfunc aoc201509;
 aocfunc aoc201508;
 aocfunc aoc201507;
-aocfunc aoc201506;
 #endif
+aocfunc aoc201506;
 aocfunc aoc201505;
 aocfunc aoc201504;
 aocfunc aoc201503;
 aocfunc aoc201502;
 aocfunc aoc201501;