diff --git a/aoc/arena.c b/aoc/arena.c
index 86cbe5c..8848897 100644
--- a/aoc/arena.c
+++ b/aoc/arena.c
@@ -6,7 +6,7 @@ arena_init(void *mem, ptrdiff_t size) {
     Arena *arena = mem;
     arena->mem = arena + 1;
     arena->top = arena->mem;
-    arena->lim = arena->mem + size;
+    arena->lim = (char *) mem + size;
     return arena;
 }
 
@@ -15,9 +15,32 @@ align_forward(void *ptr, ptrdiff_t align) {
     return (void *)(((uintptr_t)ptr + align - 1) & ~(align - 1));
 }
 
+static void
+pretty_print_size(char buf[64], ptrdiff_t size) {
+    if (size < 1024) {
+        sprintf(buf, "%ld bytes", size);
+    }
+    else if (size < 1024 * 1024) {
+        sprintf(buf, "%ld KB", size / 1024);
+    }
+    else if (size < 1024 * 1024 * 1024) {
+        sprintf(buf, "%ld MB", size / (1024 * 1024));
+    }
+    else {
+        sprintf(buf, "%ld GB", size / (1024 * 1024 * 1024));
+    }
+}
+
 static void
 arena_oom(Arena *arena, ptrdiff_t size, ptrdiff_t align) {
-    loge("Out of memory in arena %p: requested size %ld, align %ld", arena, size, align);
+    char used[64];
+    pretty_print_size(used, (uintptr_t) arena->top - (uintptr_t) arena->mem);
+    char available[64];
+    pretty_print_size(available, (uintptr_t) arena->lim - (uintptr_t) arena->top);
+    char requested[64];
+    pretty_print_size(requested, size);
+    loge("Out of memory in arena %p: %s used, %s available, %s requested, %ld alignment",
+            arena, used, available, requested, align);
     BP();
 }
 
diff --git a/day-21/input.txt b/day-21/input.txt
new file mode 100644
index 0000000..d260332
--- /dev/null
+++ b/day-21/input.txt
@@ -0,0 +1,5 @@
+935A
+319A
+480A
+789A
+176A
diff --git a/day-21/main.c b/day-21/main.c
new file mode 100644
index 0000000..8ea841c
--- /dev/null
+++ b/day-21/main.c
@@ -0,0 +1,514 @@
+#include "aoc.h"
+#include <stdlib.h>
+
+typedef struct Vec2i {
+    i32 x, y;
+} Vec2i;
+
+typedef enum {
+    TABLE_NUMPAD = 0,
+    TABLE_DIRECTIONS = 1,
+} Table;
+
+typedef enum {
+    D_LEFT,
+    D_UP,
+    D_RIGHT,
+    D_DOWN,
+    D_A,
+
+    NUM_DIRECTION_KEYS,
+} DirectionKey;
+
+typedef enum {
+    N_0,
+    N_1,
+    N_2,
+    N_3,
+    N_4,
+    N_5,
+    N_6,
+    N_7,
+    N_8,
+    N_9,
+    N_A,
+
+    NUM_NUMPAD_KEYS,
+} NumpadKey;
+
+static const Vec2i TABLES[2][128] = {
+    [TABLE_NUMPAD] = {
+        [N_0] = { 1, 0 },
+        [N_A] = { 2, 0 },
+        [N_1] = { 0, 1 },
+        [N_2] = { 1, 1 },
+        [N_3] = { 2, 1 },
+        [N_4] = { 0, 2 },
+        [N_5] = { 1, 2 },
+        [N_6] = { 2, 2 },
+        [N_7] = { 0, 3 },
+        [N_8] = { 1, 3 },
+        [N_9] = { 2, 3 },
+    },
+    [TABLE_DIRECTIONS] = {
+        [D_A]     = { 2, 1 },
+        [D_UP]    = { 1, 1 },
+        [D_LEFT]  = { 0, 0 },
+        [D_DOWN]  = { 1, 0 },
+        [D_RIGHT] = { 2, 0 },
+    },
+};
+
+typedef DYNAMIC_ARRAY(str) Strings;
+
+static Strings NUMPAD_PATHS[NUM_NUMPAD_KEYS][NUM_NUMPAD_KEYS]; // PATHS_TABLE[a][b] gives a list of strings that each represent a path from a to b
+static Strings DIRECTION_PATHS[NUM_DIRECTION_KEYS][NUM_DIRECTION_KEYS];
+
+static DirectionKey
+parse_direction_key(u8 ch) {
+    switch (ch) {
+        case '<': return D_LEFT;
+        case '^': return D_UP;
+        case '>': return D_RIGHT;
+        case 'v': return D_DOWN;
+        case 'A': return D_A;
+        default: ASSERT(0);
+    }
+}
+
+static NumpadKey
+parse_numpad_key(u8 ch) {
+    if (ch >= '0' && ch <= '9') {
+        return N_0 + (ch - '0');
+    }
+    else if (ch == 'A') {
+        return N_A;
+    }
+    else {
+        NOT_REACHABLE();
+    }
+}
+
+typedef struct {
+    Arena *arena;
+    isize len, cap;
+    u8 *data;
+} StrBuf;
+
+static inline bool
+is_valid_position(Table table, i32 x, i32 y) {
+    if (table == TABLE_NUMPAD) {
+        if (x < 0 || x > 2) return false;
+        if (y < 0 || y > 3) return false;
+        if (x == 0 && y == 0) return false;
+        return true;
+    }
+    else if (table == TABLE_DIRECTIONS) {
+        if (x < 0 || x > 2) return false;
+        if (y < 0 || y > 1) return false;
+        if (x == 0 && y == 1) return false;
+        return true;
+    }
+    else {
+        NOT_REACHABLE();
+    }
+}
+
+static str
+sb_copy_str(StrBuf *buf, Arena *arena) {
+    str result = {0};
+    if (buf->len > 0) {
+        result.data = ARENA_ALLOC_ARRAY(arena, u8, buf->len);
+        result.len = buf->len;
+        memcpy(result.data, buf->data, buf->len);
+    }
+    return result;
+}
+
+static void
+sb_push_str(StrBuf *sb, str s) {
+    ASSERT(sb->arena);
+    if (s.len > 0) {
+        if (sb->len + s.len > sb->cap) {
+            isize new_cap = sb->cap + MIN(64, s.len);
+            u8 *new_data = ARENA_ALLOC_ARRAY(sb->arena, u8, new_cap);
+            ASSERT(new_data);
+            if (sb->len > 0) {
+                memcpy(new_data, sb->data, sb->len);
+            }
+            sb->cap = new_cap;
+            sb->data = new_data;
+        }
+        ASSERT(sb->len + s.len <= sb->cap);
+        memcpy(sb->data + sb->len, s.data, s.len);
+        sb->len += s.len;
+    }
+}
+
+static void
+enumerate_paths(Arena *arena, Table table, i32 x, i32 y, i32 dx, i32 dy, StrBuf *acc, Strings *strings) {
+    if (!is_valid_position(table, x, y)) return;
+
+    if (dx == 0 && dy == 0) {
+        *push(strings, arena) = sb_copy_str(acc, arena);
+        return;
+    }
+
+    if (dx > 0) {
+        sb_push_str(acc, STR(">"));
+        enumerate_paths(arena, table, x + 1, y, dx - 1, dy, acc, strings);
+        acc->len--;
+    }
+    else if (dx < 0) {
+        sb_push_str(acc, STR("<"));
+        enumerate_paths(arena, table, x - 1, y, dx + 1, dy, acc, strings);
+        acc->len--;
+    }
+
+    if (dy > 0) {
+        sb_push_str(acc, STR("^"));
+        enumerate_paths(arena, table, x, y + 1, dx, dy - 1, acc, strings);
+        acc->len--;
+    }
+    else if (dy < 0) {
+        sb_push_str(acc, STR("v"));
+        enumerate_paths(arena, table, x, y - 1, dx, dy + 1, acc, strings);
+        acc->len--;
+    }
+}
+
+static Strings
+enumerate_paths_by_key(Arena *arena, Table table, u8 start_key, u8 end_key) {
+    i32 start_x = TABLES[table][start_key].x;
+    i32 start_y = TABLES[table][start_key].y;
+    i32 dx = TABLES[table][end_key].x - start_x;
+    i32 dy = TABLES[table][end_key].y - start_y;
+    StrBuf acc = { .arena = arena };
+    Strings strings = {0};
+    enumerate_paths(arena, table, start_x, start_y, dx, dy, &acc, &strings);
+    return strings;
+}
+
+str sb_str(StrBuf *buf) {
+    return (str) { buf->data, buf->len };
+}
+
+static void
+build_sequences(Arena *arena, Table table, str code, i32 index,
+        u8 prev_key, StrBuf *current_path, Strings *result) 
+{
+    if (!current_path) {
+        current_path = ARENA_ALLOC(arena, StrBuf);
+        current_path->arena = arena;
+    }
+    if (index == code.len) {
+        *push(result, arena) = sb_copy_str(current_path, arena);
+        return;
+    }
+    else {
+        u8 key;
+        Strings *paths = NULL;
+        if (table == TABLE_NUMPAD) {
+            key = parse_numpad_key(code.data[index]);
+            ASSERT(prev_key < NUM_NUMPAD_KEYS);
+            ASSERT(key < NUM_NUMPAD_KEYS);
+            paths = &NUMPAD_PATHS[prev_key][key];
+        }
+        else if (table == TABLE_DIRECTIONS) {
+            key = parse_direction_key(code.data[index]);
+            ASSERT(prev_key < NUM_DIRECTION_KEYS);
+            ASSERT(key < NUM_DIRECTION_KEYS);
+            paths = &DIRECTION_PATHS[prev_key][key];
+        }
+        else {
+            NOT_REACHABLE();
+        }
+        ASSERT(paths);
+        for (isize i = 0; i < paths->len; i++) {
+            sb_push_str(current_path, paths->data[i]);
+            sb_push_str(current_path, STR("A"));
+            build_sequences(arena, table, code, index + 1, key, current_path, result);
+            current_path->len -= paths->data[i].len + 1;
+        }
+    }
+}
+
+static void
+init_tables(Arena *arena) {
+    for (isize i = 0; i < NUM_NUMPAD_KEYS; i++) {
+        for (isize j = 0; j < NUM_NUMPAD_KEYS; j++) {
+            NUMPAD_PATHS[i][j] = enumerate_paths_by_key(arena, TABLE_NUMPAD, i, j);
+        }
+    }
+
+    for (isize i = 0; i < NUM_DIRECTION_KEYS; i++) {
+        for (isize j = 0; j < NUM_DIRECTION_KEYS; j++) {
+            DIRECTION_PATHS[i][j] = enumerate_paths_by_key(arena, TABLE_DIRECTIONS, i, j);
+        }
+    }
+}
+
+typedef struct CacheEntry {
+    struct CacheEntry *next;
+    u64 hash;
+    str code;
+    i32 depth;
+    i64 value;
+} CacheEntry;
+
+#define CACHE_EXP 16
+#define CACHE_SIZE (1ull << CACHE_EXP)
+#define CACHE_MASK (CACHE_SIZE - 1)
+
+typedef struct Cache {
+    Arena *arena;
+
+    CacheEntry *entries[CACHE_SIZE];
+    isize count;
+} Cache;
+
+static u64
+hash_pair(str code, i32 depth) {
+    // FNV-1a
+    u64 hash = 14695981039346656037ull;
+    for (isize i = 0; i < code.len; i++) {
+        hash ^= code.data[i];
+        hash *= 1099511628211ull;
+    }
+    hash ^= depth;
+    hash *= 1099511628211ull;
+    return hash;
+}
+
+static void
+cache_init(Cache *cache, Arena *arena) {
+    cache->arena = arena;
+    memset(cache->entries, 0, sizeof(cache->entries));
+}
+
+static CacheEntry *
+cache_get(Cache *cache, str code, i32 depth) {
+    u64 hash = hash_pair(code, depth);
+    u64 index = hash & CACHE_MASK;
+    for (CacheEntry *e = cache->entries[index]; e; e = e->next) {
+        if (e->hash == hash && e->depth == depth && str_eq(e->code, code)) {
+            return e;
+        }
+    }
+    return NULL;
+}
+
+static str
+copy_str(Arena *destination, str s) {
+    str result = {0};
+    if (s.len > 0) {
+        result.data = ARENA_ALLOC_ARRAY(destination, u8, s.len);
+        result.len = s.len;
+        memcpy(result.data, s.data, s.len);
+    }
+    return result;
+}
+
+static void
+cache_print_stats(Cache *cache) {
+    isize total_size = 0;
+    for (isize i = 0; i < CACHE_SIZE; i++) {
+        for (CacheEntry *e = cache->entries[i]; e; e = e->next) {
+            printf("Code: " STR_FMT " Depth: %d Value: %ld\n", STR_ARG(e->code), e->depth, e->value);
+            total_size += e->code.len + sizeof(CacheEntry);
+        }
+    }
+    printf("Total size occupied by cache: %ld\n", total_size);
+}
+
+static void
+cache_put(Cache *cache, str code, i32 depth, i64 value) {
+    CacheEntry *entry = cache_get(cache, code, depth);
+    if (entry) {
+        entry->value = value;
+    }
+    else {
+        u64 hash = hash_pair(code, depth);
+        u64 index = hash & CACHE_MASK;
+        CacheEntry *entry = ARENA_ALLOC(cache->arena, CacheEntry);
+        entry->hash = hash;
+        entry->code = copy_str(cache->arena, code);
+        entry->depth = depth;
+        entry->value = value;
+        entry->next = cache->entries[index];
+        cache->entries[index] = entry;
+        cache->count++;
+    }
+}
+
+static i64
+find_shortest_dpad(Arena temp, str code, i32 depth, Cache *cache) {
+    ASSERT(code.len > 0);
+    CacheEntry *cache_entry = NULL;
+    i64 result = 0;
+    if (depth == 0) {
+        ASSERT(code.len > 0);
+        result = code.len;
+    }
+    else if ((cache_entry = cache_get(cache, code, depth))) {
+        ASSERT(cache_entry->value > 0);
+        result = cache_entry->value;
+    }
+    else {
+        str iter = code;
+        str sub;
+        while (iter.len > 0) {
+            {
+                str_find_result result = str_find_left(iter, STR("A"));
+                sub = str_sub(iter, 0, result.found ? result.offset + 1 : iter.len);
+            }
+            iter = str_sub(iter, sub.len, iter.len);
+            ASSERT(sub.data[sub.len - 1] == 'A');
+            Strings sequences = {0};
+            build_sequences(&temp, TABLE_DIRECTIONS, sub, 0, D_A, NULL, &sequences);
+            i64 shortest = INT64_MAX;
+            str shortest_str = STR("");
+            for (isize i = 0; i < sequences.len; i++) {
+                ASSERT(sequences.data[i].data);
+                i64 candidate = find_shortest_dpad(temp, sequences.data[i], depth - 1, cache);
+                ASSERT(candidate > 0);
+                shortest = MIN(shortest, candidate);
+            }
+            ASSERT(shortest > 0);
+            result += shortest;
+        }
+        cache_put(cache, code, depth, result);
+    }
+    ASSERT(result > 0);
+    return result;
+}
+
+static i64
+find_shortest(Arena temp, str code, i32 depth, Cache *cache) {
+    Strings sequences = {0};
+    build_sequences(&temp, TABLE_NUMPAD, code, 0, N_A, NULL, &sequences);
+    i64 shortest = INT64_MAX;
+    for (isize i = 0; i < sequences.len; i++) {
+        i64 candidate = find_shortest_dpad(temp, sequences.data[i], depth - 1, cache);
+        shortest = MIN(shortest, candidate);
+    }
+    return shortest;
+}
+
+static i64
+parse_numeric_prefix(Arena *arena, str s) {
+    isize end = 0;
+    while (end < s.len && s.data[end] >= '0' && s.data[end] <= '9') {
+        end++;
+    }
+    str prefix = { s.data, end };
+    return parse_i64(prefix, *arena);
+}
+
+static u8 NUMPAD[4][3] = {
+    { '\0', '0', 'A' },
+    { '1', '2', '3' },
+    { '4', '5', '6' },
+    { '7', '8', '9' },
+};
+
+static u8 DPAD[2][3] = {
+    { '<', 'v', '>' },
+    { '\0', '^', 'A' },
+};
+
+static str
+test(Arena *perm, Arena temp, str input, i32 depth) {
+    if (depth == 0) {
+        str result = copy_str(perm, input);
+        return result;
+    }
+    i32 x, y;
+    if (depth == 1) {
+        x = 2;
+        y = 0;
+    }
+    else {
+        x = 2;
+        y = 1;
+    }
+    StrBuf output = { .arena = &temp };
+    for (isize i = 0; i < input.len; i++) {
+        u8 ch = input.data[i];
+        switch (ch) {
+            case '>' : x++; break;
+            case '<' : x--; break;
+            case '^' : y++; break;
+            case 'v' : y--; break;
+            case 'A': 
+            {
+                u8 symbol;
+                if (depth == 1) {
+                    ASSERT(x >= 0);
+                    ASSERT(x < 3);
+                    ASSERT(y >= 0);
+                    ASSERT(y < 4);
+                    ASSERT(!(x == 0 && y == 0));
+                    symbol = NUMPAD[y][x];
+                }
+                else {
+                    ASSERT(x >= 0);
+                    ASSERT(x < 3);
+                    ASSERT(y >= 0);
+                    ASSERT(y < 2);
+                    ASSERT(!(x == 0 && y == 1));
+                    symbol = DPAD[y][x];
+                }
+                str s = { &symbol, 1 };
+                sb_push_str(&output, s);
+            } break;
+            default: NOT_REACHABLE();
+        }
+        if (depth == 1) {
+            ASSERT(x >= 0);
+            ASSERT(x < 3);
+            ASSERT(y >= 0);
+            ASSERT(y < 4);
+            ASSERT(!(x == 0 && y == 0));
+        }
+        else {
+            ASSERT(x >= 0);
+            ASSERT(x < 3);
+            ASSERT(y >= 0);
+            ASSERT(y < 2);
+            ASSERT(!(x == 0 && y == 1));
+        }
+    }
+    return test(perm, temp, sb_str(&output), depth - 1);
+}
+
+int main(int argc, char **argv) {
+    Arena *perm = make_arena(Megabytes(2));
+    Arena *temp = make_arena(Megabytes(1));
+    Tokens lines = read_lines(perm, argv[1]);
+
+    ASSERT(str_eq(test(perm, *temp, STR("<A^A>^^AvvvA"), 1), STR("029A")));
+    ASSERT(str_eq(test(perm, *temp, STR("v<<A>>^A<A>AvA<^AA>A<vAAA>^A"), 2), STR("029A")));
+    ASSERT(str_eq(test(perm, *temp, STR("<vA<AA>>^AvAA<^A>A<v<A>>^AvA^A<vA>^A<v<A>^A>AAvA^A<v<A>A>^AAAvA<^A>A"), 3), STR("029A")));
+
+    init_tables(perm);
+
+    Cache *cache = ARENA_ALLOC(perm, Cache);
+    cache_init(cache, perm);
+
+    i64 part_1 = 0;
+    i64 part_2 = 0;
+    for (isize i = 0; i < lines.len; i++) {
+        str line = str_trim(lines.tokens[i]);
+        /* printf(STR_FMT "\n", STR_ARG(line)); */
+        i64 prefix = parse_numeric_prefix(temp, line);
+        i64 shortest_part_1 = find_shortest(*temp, line, 1 + 2, cache);
+        i64 shortest_part_2 = find_shortest(*temp, line, 1 + 25, cache);
+        part_1 += shortest_part_1 * prefix;
+        part_2 += shortest_part_2 * prefix;
+    }
+    printf("%ld\n", part_1);
+    printf("%ld\n", part_2);
+    return 0;
+}
+
diff --git a/day-21/test.txt b/day-21/test.txt
new file mode 100644
index 0000000..4cf0c29
--- /dev/null
+++ b/day-21/test.txt
@@ -0,0 +1,5 @@
+029A
+980A
+179A
+456A
+379A