diff --git a/examples/aoc2024/day23/part1.jou b/examples/aoc2024/day23/part1.jou
index f80e7cc9..86296be0 100644
--- a/examples/aoc2024/day23/part1.jou
+++ b/examples/aoc2024/day23/part1.jou
@@ -5,7 +5,6 @@ import "stdlib/io.jou"
 
 class Computer:
     name: byte[3]
-    group_id: int  # computers connected together have same group id
     connections: void*[100]  # TODO: should be Computer*[100], can't be due to compiler bug
 
     def connect(self, other: Computer*) -> None:
@@ -34,23 +33,12 @@ def find_or_create_computer(computers: Computer*, ncomputers: int*, name: byte*)
     c = &computers[*ncomputers]
     assert strlen(name) == 2
     strcpy(c->name, name)
-    c->group_id = *ncomputers  # unique initially
     memset(c->connections, 0, sizeof(c->connections))
 
     ++*ncomputers
     return c
 
 
-def merge_groups(computers: Computer*, ncomputers: int, id1: int, id2: int) -> None:
-    printf("Merge %d,%d\n", id1, id2)
-    if id1 == id2:
-        return
-
-    for i = 0; i < ncomputers; i++:
-        if computers[i].group_id == id1:
-            computers[i].group_id = id2
-
-
 def main() -> int:
     computers: Computer* = malloc(sizeof(computers[0]) * 5000)
     assert computers != NULL
diff --git a/examples/aoc2024/day23/part2.jou b/examples/aoc2024/day23/part2.jou
new file mode 100644
index 00000000..391906e4
--- /dev/null
+++ b/examples/aoc2024/day23/part2.jou
@@ -0,0 +1,252 @@
+import "stdlib/str.jou"
+import "stdlib/mem.jou"
+import "stdlib/io.jou"
+
+
+def add_computer_if_not_exist(names: byte[3]*, ncomputers: int*, name: byte*) -> None:
+    for i = 0; i < *ncomputers; i++:
+        if strcmp(names[i], name) == 0:
+            return
+
+    assert strlen(name) == 2
+    strcpy(names[(*ncomputers)++], name)
+
+
+def find_computer(names: byte[3]*, ncomputers: int, name: byte*) -> int:
+    for i = 0; i < ncomputers; i++:
+        if strcmp(names[i], name) == 0:
+            return i
+    assert False
+
+
+# If conn_matrix[i][j] is True, then computers i and j are connected
+def create_conn_matrix(ncomputers: int) -> bool**:
+    conn_matrix: bool** = malloc(sizeof(conn_matrix[0]) * ncomputers)
+    assert conn_matrix != NULL
+
+    for i = 0; i < ncomputers; i++:
+        conn_matrix[i] = calloc(sizeof(conn_matrix[i][0]), ncomputers)
+        assert conn_matrix[i] != NULL
+
+    return conn_matrix
+
+
+def sort_names(names: byte[3]*, len: int) -> None:
+    # very dumb algorithm
+    while True:
+        did_something = False
+        for i = 1; i < len; i++:
+            if strcmp(names[i-1], names[i]) > 0:
+                memswap(&names[i-1], &names[i], sizeof(names[0]))
+                did_something = True
+        if not did_something:
+            break
+
+
+def sort_group(group: int*) -> None:
+    # very dumb algorithm
+    while True:
+        did_something = False
+        for i = 1; group[i] != -1; i++:
+            if group[i-1] > group[i]:
+                memswap(&group[i-1], &group[i], sizeof(group[0]))
+                did_something = True
+        if not did_something:
+            break
+
+
+def group_contains(group: int*, value: int) -> bool:
+    for i = 0; group[i] != -1; i++:
+        if group[i] == value:
+            return True
+    return False
+
+
+def groups_are_equal(g1: int*, g2: int*) -> bool:
+    for i = 0; g1[i] != -1 or g2[i] != -1; i++:
+        if g1[i] != g2[i]:
+            return False
+    return True
+
+
+# Merges two groups of size n if:
+#   - they have n-1 same elements in common
+#   - the two non-common elements (one from each group) are connected
+#
+# Assumes groups are sorted.
+def merge_two_groups(group1: int*, group2: int*, conn_matrix: bool**) -> int*:
+    n = 0
+    while group1[n] != -1 and group2[n] != -1:
+        n++
+    assert group1[n] == -1
+    assert group2[n] == -1
+
+    for i = 1; i < n; i++:
+        assert group1[i-1] < group1[i]
+        assert group2[i-1] < group2[i]
+
+    # Find the elements that is only in group 1 or 2. If there are multiple, don't merge.
+    p1 = group1
+    p2 = group2
+    only1 = -1
+    only2 = -1
+
+    while *p1 != -1 or *p2 != -1:
+        if *p2 == -1 or (*p1 != -1 and *p2 != -1 and *p1 < *p2):
+            # Group 1 left behind, it has element which is not in group 2
+            if only1 != -1:
+                # Not the first one, refuse to merge
+                return NULL
+            only1 = *p1++
+        elif *p1 == -1 or (*p1 != -1 and *p2 != -1 and *p1 > *p2):
+            # Group 2 element missing from group 1
+            if only2 != -1:
+                # Not the first one, refuse to merge
+                return NULL
+            only2 = *p2++
+        else:
+            # Skip common element
+            assert *p1 != -1
+            assert *p2 != -1
+            assert *p1 == *p2
+            p1++
+            p2++
+
+    assert p1 == &group1[n]
+    assert p2 == &group2[n]
+
+    # They shouldn't have everything in common, then they would be the same group
+    assert only1 != -1
+    assert only2 != -1
+
+    if not conn_matrix[only1][only2]:
+        return NULL
+
+    result: int* = malloc(sizeof(result[0]) * (n+2))
+    assert result != NULL
+    memcpy(result, group1, sizeof(result[0]) * n)
+    result[n] = only2
+    result[n+1] = -1
+    return result
+
+
+# verbose parameter is useful when you're bored and you run this on the real input.
+# It is very slow... :)
+def merge_all_mergeable_pairs(conn_matrix: bool**, old_groups: int**) -> int**:
+    n = 0
+    while old_groups[n] != NULL:
+        n++
+    printf("Merging %d groups together", n)
+    fflush(stdout)
+
+    result: int** = malloc(sizeof(result[0]))
+    assert result != NULL
+    result_len = 0
+    result_alloc = 1
+
+    for g1 = old_groups; *g1 != NULL; g1++:
+        for g2 = &g1[1]; *g2 != NULL; g2++:
+            m = merge_two_groups(*g1, *g2, conn_matrix)
+            if m == NULL:
+                continue
+
+            sort_group(m)
+
+            already_exists = False
+            for i = 0; i < result_len; i++:
+                if groups_are_equal(result[i], m):
+                    already_exists = True
+                    break
+
+            if already_exists:
+                free(m)
+            else:
+                if result_len == result_alloc:
+                    result_alloc *= 2
+                    result = realloc(result, sizeof(result[0]) * (result_alloc + 1))
+                    assert result != NULL
+
+                result[result_len++] = m
+
+                # Print a dot every 100 successful merges.
+                # Helpful with actual input because it is really slow :)
+                if result_len % 100 == 0:
+                    printf(".")
+                    fflush(stdout)
+
+    printf("\n")
+    fflush(stdout)
+
+    result[result_len] = NULL
+    return result
+
+
+def main() -> int:
+    names: byte[3]* = malloc(sizeof(names[0]) * 5000)
+    assert names != NULL
+    ncomputers = 0
+
+    f = fopen("input", "r")
+    assert f != NULL
+
+    name1: byte[3]
+    name2: byte[3]
+    while fscanf(f, "%2s-%2s\n", name1, name2) == 2:
+        assert ncomputers < 5000
+        add_computer_if_not_exist(names, &ncomputers, name1)
+        assert ncomputers < 5000
+        add_computer_if_not_exist(names, &ncomputers, name2)
+
+    sort_names(names, ncomputers)
+
+    rewind(f)
+    conn_matrix = create_conn_matrix(ncomputers)
+    while fscanf(f, "%2s-%2s\n", name1, name2) == 2:
+        i1 = find_computer(names, ncomputers, name1)
+        i2 = find_computer(names, ncomputers, name2)
+        conn_matrix[i1][i2] = True
+        conn_matrix[i2][i1] = True
+
+    fclose(f)
+
+    # Create one group for each computer.
+    groups: int** = malloc(sizeof(groups[0]) * (ncomputers + 1))
+    assert groups != NULL
+    for i = 0; i < ncomputers; i++:
+        groups[i] = malloc(sizeof(groups[i][0]) * (ncomputers + 1))
+        groups[i][0] = i
+        groups[i][1] = -1
+    groups[ncomputers] = NULL
+
+    # Merge until there are at least two groups.
+    while groups[0] != NULL and groups[1] != NULL:
+        # Output: Merging 16 groups together
+        # Output: Merging 32 groups together
+        # Output: Merging 12 groups together
+        groups2 = merge_all_mergeable_pairs(conn_matrix, groups)
+
+        for i = 0; groups[i] != NULL; i++:
+            free(groups[i])
+        free(groups)
+        groups = groups2
+
+    # There should be one group remaining
+    assert groups[0] != NULL
+
+    # Output: co-de-ka-ta
+    for i = 0; groups[0][i] != -1; i++:
+        if i != 0:
+            printf("-")
+        printf("%s", names[groups[0][i]])
+    printf("\n")
+
+    free(groups[0])
+    free(groups)
+
+    for i = 0; i < ncomputers; i++:
+        free(conn_matrix[i])
+    free(conn_matrix)
+
+    free(names)
+
+    return 0