diff --git a/CMakeLists.txt b/CMakeLists.txt
index 39dec8f..b3a59f7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,4 +22,6 @@ target_link_libraries(day_02 m)
 
 add_executable(day_03 day_3.c input_handler.c)
 
-add_executable(day_04 day_4.c input_handler.c)
\ No newline at end of file
+add_executable(day_04 day_4.c input_handler.c)
+
+add_executable(day_05 day_5.c input_handler.c map.c)
\ No newline at end of file
diff --git a/day_4.c b/day_4.c
index 1bc7827..aadca1e 100644
--- a/day_4.c
+++ b/day_4.c
@@ -120,6 +120,6 @@ CardList* load_cards(const char *file_name) {
         clist_add(list, card);
     }
 
-    free(buffer);
+    // free(buffer);
     return list;
 }
diff --git a/day_5.c b/day_5.c
new file mode 100644
index 0000000..df4a6a0
--- /dev/null
+++ b/day_5.c
@@ -0,0 +1,20 @@
+//
+// Created by lennart on 12/7/23.
+//
+
+#include <stdio.h>
+#include <assert.h>
+#include "map.h"
+
+int main() {
+    Map *map = map_create();
+
+    for(int i = 0; i < 10000; i++) {
+        map_put(map, i, i + 1);
+    }
+    for(int i = 1; i < 10000; i++) {
+        assert(map_get(map, i) == i + 1);
+    }
+
+    map_free(map);
+}
diff --git a/map.c b/map.c
new file mode 100644
index 0000000..bffa4bc
--- /dev/null
+++ b/map.c
@@ -0,0 +1,142 @@
+//
+// Created by lennart on 12/8/23.
+//
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <assert.h>
+#include "map.h"
+
+#include <stdlib.h>
+
+// Load factor. Targets how much space is used under ideal conditions
+#define LOAD_FACTOR 0.5
+
+typedef struct MapNode_struct {
+    struct MapNode_struct *next;
+    int key;
+    int value;
+} MapNode;
+
+struct _Map {
+    MapNode *nodes;
+    uint32_t size;
+    uint32_t capacity;
+};
+
+static uint32_t map_index(Map *map, int key);
+static void map_set_node(Map *map, MapNode node);
+static void map_ensure_capacity(Map *map);
+
+__inline Map* map_create() {
+    Map *map = malloc(sizeof (Map));
+    map->size = 0;
+    map->capacity = 1 << 4;
+    map->nodes = calloc(sizeof (MapNode), map->capacity);
+    return map;
+}
+
+void map_put(Map *map, int key, int value) {
+    map_ensure_capacity(map);
+    MapNode node;
+    node.key = key;
+    node.value = value;
+    node.next = NULL;
+    map_set_node(map, node);
+}
+
+__inline int map_get(Map *map, int key) {
+    MapNode node = map->nodes[map_index(map, key)];
+    int value = node.value;
+    MapNode *next = node.next;
+    while(__glibc_unlikely(next != NULL && (size_t) next != UINTPTR_MAX)) {
+        if(next->key == key) {
+            value = next->value;
+            break;
+        }
+        next = next->next;
+    }
+    return value;
+}
+
+bool map_has(Map *map, int key) {
+    MapNode node = map->nodes[map_index(map, key)];
+    bool value = node.next != NULL && node.key == key;
+    MapNode *next = node.next;
+    while(__glibc_unlikely(!value && next != NULL && (size_t) next != UINTPTR_MAX)) {
+        if(next->key == key) {
+            value = true;
+        }
+        next = next->next;
+    }
+    return value;
+}
+
+void map_free(Map *map) {
+    for(int i = 0; i < map->capacity; i++) {
+        MapNode *next = map->nodes[i].next;
+        if(next != NULL) {
+            while((size_t) next != UINTPTR_MAX) {
+                MapNode *after = next->next;
+                free(next);
+                next = after;
+            }
+        }
+    }
+    free(map->nodes);
+    free(map);
+}
+
+static __inline uint32_t map_index(Map *map, int key) {
+    return key & (map->capacity - 1);
+}
+
+__inline void map_set_node(Map *map, MapNode node) {
+    MapNode *current = &map->nodes[map_index(map, node.key)];
+    if(current->next == NULL || current->key == node.key) {
+        if(current->next == NULL) map->size++;
+        current->key = node.key;
+        current->value = node.value;
+        current->next = (MapNode*) UINTPTR_MAX;
+    } else {
+        while((size_t) current->next != UINTPTR_MAX) {
+            current = current->next;
+            if(current->key == node.key) break;
+        }
+        if(current->key == node.key) {
+            current->value = node.value;
+        } else {
+            MapNode *copy = malloc(sizeof (MapNode));
+            *copy = node;
+            copy->next = (MapNode*) UINTPTR_MAX;
+            current->next = copy;
+            map->size++;
+        }
+    }
+}
+
+__inline void map_ensure_capacity(Map *map) {
+    uint32_t capacity = map->capacity;
+    if(__glibc_unlikely(map->size >= capacity * LOAD_FACTOR)) {
+        map->capacity <<= 1;
+        map->size = 0; // Re-add all nodes
+        MapNode *nodes = map->nodes;
+        map->nodes = calloc(sizeof (MapNode), map->capacity);
+        for(int i = 0; i < capacity; i++) {
+            MapNode node = nodes[i];
+            MapNode *next = node.next;
+            map_set_node(map, node);
+            if(next != NULL) {
+                while((size_t) next != UINTPTR_MAX) {
+                    map_set_node(map, *next);
+                    MapNode *after = next->next;
+                    assert(after != NULL);
+                    free(next);
+                    next = after;
+                }
+            }
+        }
+        free(nodes);
+    }
+}
diff --git a/map.h b/map.h
new file mode 100644
index 0000000..1cddc9d
--- /dev/null
+++ b/map.h
@@ -0,0 +1,23 @@
+//
+// Created by lennart on 12/8/23.
+//
+
+#include <stdbool.h>
+
+#ifndef MAP_H
+
+struct _Map;
+typedef struct _Map Map;
+
+Map* map_create();
+
+void map_put(Map *map, int key, int value);
+
+int map_get(Map *map, int key);
+
+bool map_has(Map *map, int key);
+
+void map_free(Map *map);
+
+#define MAP_H
+#endif //MAP_H