Index: aoc2015.c
==================================================================
--- aoc2015.c
+++ aoc2015.c
@@ -1,8 +1,9 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include "aocdailies.h"
 #include "aocutils.h"
 
 struct House {
     int row, col;
@@ -13,10 +14,11 @@
 #else
 #  error MAX_HOUSES already defined
 #endif
 void aoc201503(char *data, size_t len) {
     (void)len; // unused argument
+    char *data2 = data; // save it
     struct House house[MAX_HOUSES] = {0};
     unsigned nhouse = 1; // house at [0, 0] visited
     int x = 0, y = 0;
     while (*data) {
         switch (*data) {
@@ -43,10 +45,44 @@
             house[nhouse].col = y;
             nhouse += 1;
         }
     }
     printf("Santa delivered at least 1 present to %u houses.\n", nhouse);
+
+    memset(house, 0, sizeof house);
+    nhouse = 1;
+    struct House p[2] = {0};
+    int who = 1; // 0: santa; 1: santa bot
+    // go around from the beginning
+    while (*data2) {
+        who = 1 - who; // santa; bot; santa; bot; ..., ..., ...
+        switch (*data2) {
+            default: break;
+            case '^': p[who].row++; break;
+            case 'v': p[who].row--; break;
+            case '>': p[who].col++; break;
+            case '<': p[who].col--; break;
+        }
+        data2++;
+        bool visited = false;
+        for (size_t k = 0; k < nhouse; k++) {
+            if ((house[k].row == p[who].row) && (house[k].col == p[who].col)) {
+                visited = true;
+                break;
+            }
+        }
+        if (!visited) {
+            if (nhouse == MAX_HOUSES) {
+                fprintf(stderr, "Need more houses in aoc201503()\n");
+                exit(EXIT_FAILURE);
+            }
+            house[nhouse].row = p[who].row;
+            house[nhouse].col = p[who].col;
+            nhouse += 1;
+        }
+    }
+    printf("Santa and santa bot delivered at least 1 present to %u houses.\n", nhouse);
 }
 #undef MAX_HOUSES
 
 void aoc201502(char *data, size_t len) {
     (void)len; // unused argument