diff --git a/Day15/Day15.py b/Day15/Day15.py
new file mode 100644
index 0000000..eddc05a
--- /dev/null
+++ b/Day15/Day15.py
@@ -0,0 +1,242 @@
+from pprint import pprint
+from collections import deque
+import fileinput
+import time
+
+
+class Elf:
+    power = 3
+    locations = set()
+    deaths = 0
+
+    def __init__(self):
+        self.health = 200
+        self.targeted = False
+
+    def __repr__(self):
+        return "E"
+
+
+class Goblin:
+    enemy = Elf
+    power = 3
+    locations = set()
+    deaths = 0
+
+    def __init__(self):
+        self.health = 200
+        self.targeted = False
+
+    def __repr__(self):
+        return "G"
+
+
+Elf.enemy = Goblin
+
+
+class Wall:
+    enemy = None
+
+    def __init__(self):
+        self.targeted = False
+
+    def __repr__(self):
+        return "#"
+
+
+class Empty:
+    enemy = None
+
+    def __init__(self):
+        self.targeted = False
+
+    def __repr__(self):
+        return "." if not self.targeted else "\033[33m?\033[0m"
+
+
+NEIGHBORS = [(0, -1), (-1, 0), (1, 0), (0, 1)]
+
+
+def read():
+    field = []
+    Elf.deaths = 0
+    Goblin.deaths = 0
+    Elf.locations.clear()
+    Goblin.locations.clear()
+    for y, line in enumerate(fileinput.input()):
+        row = []
+
+        for x, char in enumerate(line.rstrip()):
+            if char == "E":
+                row.append(Elf())
+                Elf.locations.add((x, y))
+            elif char == "G":
+                row.append(Goblin())
+                Goblin.locations.add((x, y))
+            elif char == "#":
+                row.append(Wall())
+            elif char == ".":
+                row.append(Empty())
+            else:
+                raise ValueError(char)
+
+        field.append(row)
+
+    return field
+
+
+def distance(a, b):
+    (ax, ay), (bx, by) = a, b
+    return abs(ax - bx) + abs(ay - by)
+
+
+def find_closest(field, pos, targets):
+    width = len(field[0])
+    height = len(field)
+
+    queue = deque([[pos]])
+    seen = {pos}
+    paths = []
+
+    while queue:
+        path = queue.popleft()
+        x, y = path[-1]
+
+        if (x, y) in targets:
+            paths.append(path[1:])
+
+        for dx, dy in NEIGHBORS:
+            x_, y_ = x + dx, y + dy
+            if 0 <= x_ < width and 0 <= y_ < height \
+               and type(field[y_][x_]) is Empty \
+               and (x_, y_) not in seen:
+                queue.append(path + [(x_, y_)])
+                seen.add((x_, y_))
+
+    if not paths or [] in paths:
+        return None
+
+    return min(paths, key=lambda path: (len(path), path[-1][1], path[-1][0], path[0][1], path[0][0]))
+
+
+
+def move(field, x, y, value):
+    for row in field:
+        for col in row:
+            col.targeted = False
+
+    targets = []
+    for tx, ty in value.enemy.locations:
+        targets.extend([(tx + dx, ty + dy) for dx, dy in NEIGHBORS])
+
+    if not targets:
+        raise StopIteration
+
+    for tx, ty in targets:
+        field[ty][tx].targeted = True
+
+    closest = find_closest(field, (x, y), targets)
+    # print_field(field, highlight=(x, y), path=closest)
+    # time.sleep(0.3)
+
+    if closest:
+        tx, ty = closest[0]
+        if type(field[ty][tx]) is Empty:
+            # print(f"{field[y][x]}@{x},{y} moves to {tx},{ty}")
+            type(value).locations.discard((x, y))
+            type(value).locations.add((tx, ty))
+            field[ty][tx] = field[y][x]
+            field[y][x] = Empty()
+            return tx, ty
+    return x, y
+
+
+def attack(field, x, y, value):
+    width = len(field[0])
+    height = len(field)
+
+    neighbors = [(field[y + y_][x + x_], (x + x_, y + y_))
+                    for x_, y_ in NEIGHBORS
+                    if 0 <= y + y_ < width and 0 <= x + x_ < width and \
+                    type(field[y + y_][x + x_]) is value.enemy]
+    if not neighbors:
+        return
+
+    target, (tx, ty) = \
+        min(neighbors, key=lambda x: (x[0].health, x[1][1], x[1][0]))
+    # print(f"{value}@{x},{y}:{value.health} attacks "
+    #         f"{target}@{tx},{ty}:{target.health}->{target.health - value.power}")
+    target.health -= value.power
+
+    if target.health <= 0:
+        # print(f"{target}@{tx,ty} dies :(")
+        type(target).locations.discard((tx, ty))
+        type(target).deaths += 1
+        field[ty][tx] = Empty()
+
+
+def print_field(field, steps=None, highlight=None, path=None):
+    if steps is not None:
+        print(f"round={steps}")
+    for y, row in enumerate(field):
+        units = []
+        for x, value in enumerate(row):
+            if highlight == (x, y):
+                print(end="\033[31;1m")
+            if path and (x, y) in path:
+                print(end="\033[44;1m")
+
+            print(value, end="\033[0m")
+
+            if type(value) in (Goblin, Elf):
+                units.append(value)
+
+        etc = sorted(units, key=lambda unit: (str(unit), unit.health))
+        print(" ", ", ".join(f"{x}({x.health})" for x in etc))
+    print()
+
+
+def step(field, steps):
+    # print_field(field, steps)
+    seen_units = set()
+    for y, row in enumerate(field):
+        for x, value in enumerate(row):
+            if type(value) not in (Elf, Goblin) or value in seen_units:
+                continue
+            seen_units.add(value)
+
+            tx, ty = move(field, x, y, value)
+            attack(field, tx, ty, value)
+
+
+def main():
+    field = read()
+
+    steps = 0
+    while True:
+        try:
+            step(field, steps)
+        except StopIteration:
+            total_health = 0
+            for row in field:
+                for value in row:
+                    if type(value) in (Elf, Goblin) and value.health > 0:
+                        total_health += value.health
+
+            print("Rounds:", steps)
+            print("Health:", total_health)
+            print("Elf Deaths:", Elf.deaths)
+            print(total_health * steps)
+            break
+
+        steps += 1
+
+
+while True:
+    main()
+    if Elf.deaths == 0:
+        print("yay")
+        break
+
+    Elf.power += 1
+    print("oh no :(, trying", Elf.power)
diff --git a/Day16/Day16A.c b/Day16/Day16A.c
new file mode 100644
index 0000000..e8d9911
--- /dev/null
+++ b/Day16/Day16A.c
@@ -0,0 +1,154 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <inttypes.h>
+
+typedef struct {
+    uint8_t before[4];
+    uint8_t opcode;
+    uint8_t a, b, c;
+    uint8_t after[4];
+} Sample;
+
+int sample_read(Sample *sample, FILE *file) {
+    int read = 0;
+    read +=
+        fscanf(file,
+               "Before: [%" SCNu8 ", %" SCNu8 ", %" SCNu8 ", %" SCNu8 "]\n",
+               &sample->before[0], &sample->before[1], &sample->before[2],
+               &sample->before[3]);
+
+    read += fscanf(file, "%" SCNu8 " %" SCNu8 " %" SCNu8 " %" SCNu8 "\n",
+                   &sample->opcode, &sample->a, &sample->b, &sample->c);
+
+    read +=
+        fscanf(file,
+               "After: [%" SCNu8 ", %" SCNu8 ", %" SCNu8 ", %" SCNu8 "]\n\n",
+               &sample->after[0], &sample->after[1], &sample->after[2],
+               &sample->after[3]);
+
+    return read == 12;
+}
+
+void sample_print(const Sample sample) {
+    printf(
+        "{\n"
+        "  before: [%d, %d, %d, %d],\n"
+        "  opcode: %d, a: %d, b: %d, c: %d,\n"
+        "  after: [%d, %d, %d, %d]\n"
+        "}\n",
+        sample.before[0], sample.before[1], sample.before[2], sample.before[3],
+        sample.opcode, sample.a, sample.b, sample.c,
+        sample.after[0], sample.after[1], sample.after[2], sample.after[3]
+    );
+}
+
+int sample_possible_opcodes(const Sample s) {
+    int possible_opcodes = 0;
+
+    // Addition
+    if (s.after[s.c] == s.before[s.a] + s.before[s.b]) {
+        printf("addr, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == s.before[s.a] + s.b) {
+        printf("addi, ");
+        possible_opcodes += 1;
+    }
+
+    // Multiplication
+    if (s.after[s.c] == s.before[s.a] * s.before[s.b]) {
+        printf("mulr, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == s.before[s.a] * s.b) {
+        printf("muli, ");
+        possible_opcodes += 1;
+    }
+
+    // Bitwise AND
+    if (s.after[s.c] == (s.before[s.a] & s.before[s.b])) {
+        printf("banr, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == (s.before[s.a] & s.b)) {
+        printf("bani, ");
+        possible_opcodes += 1;
+    }
+
+    // Bitwise OR
+    if (s.after[s.c] == (s.before[s.a] | s.before[s.b])) {
+        printf("borr, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == (s.before[s.a] | s.b)) {
+        printf("bori, ");
+        possible_opcodes += 1;
+    }
+
+    // Assignment
+    if (s.after[s.c] == s.before[s.a]) {
+        printf("setr, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == s.a) {
+        printf("seti, ");
+        possible_opcodes += 1;
+    }
+
+    // Greater-than testing
+    if (s.after[s.c] == (s.a > s.before[s.b])) {
+        printf("gtir, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == (s.before[s.a] > s.b)) {
+        printf("gtri, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == (s.before[s.a] > s.before[s.b])) {
+        printf("gtrr, ");
+        possible_opcodes += 1;
+    }
+
+    // Equality testing
+    if (s.after[s.c] == (s.a == s.before[s.b])) {
+        printf("eqir, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == (s.before[s.a] == s.b)) {
+        printf("eqri, ");
+        possible_opcodes += 1;
+    }
+    if (s.after[s.c] == (s.before[s.a] == s.before[s.b])) {
+        printf("eqrr, ");
+        possible_opcodes += 1;
+    }
+
+    return possible_opcodes;
+}
+
+int main(int argc, char **argv) {
+    if (argc != 2) {
+        printf("Usage: %s [input]\n", argv[0]);
+        return EXIT_FAILURE;
+    }
+
+    FILE *file = fopen(argv[1], "r");
+    Sample sample = {0};
+
+    int gt3 = 0, samples = 0;
+    while (sample_read(&sample, file)) {
+        sample_print(sample);
+        int possible_opcodes = sample_possible_opcodes(sample);
+        printf("%d possible opcodes.\n\n", possible_opcodes);
+        if (possible_opcodes >= 3) {
+            gt3 += 1;
+        }
+        samples += 1;
+    }
+    printf("%d samples\n", samples);
+    printf("%d\n", gt3);
+
+    fclose(file);
+    return EXIT_SUCCESS;
+}
diff --git a/Day16/Day16B.c b/Day16/Day16B.c
new file mode 100644
index 0000000..3ff0515
--- /dev/null
+++ b/Day16/Day16B.c
@@ -0,0 +1,194 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <inttypes.h>
+#include <assert.h>
+
+typedef struct {
+    uint8_t opcode;
+    uint32_t a, b, c;
+} Instruction;
+
+typedef struct {
+    uint32_t before[4];
+    Instruction instruction;
+    uint32_t after[4];
+} Sample;
+
+enum {
+    ADDR, ADDI, MULR, MULI,
+    BANR, BANI, BORR, BORI,
+    SETR, SETI, GTIR, GTRI,
+    GTRR, EQIR, EQRI, EQRR,
+};
+
+const char *names[] = {
+    "addr", "addi", "mulr", "muli",
+    "banr", "bani", "borr", "bori",
+    "setr", "seti", "gtir", "gtri",
+    "gtrr", "eqir", "eqri", "eqrr",
+};
+
+uint16_t table[16] = {
+    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
+    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
+    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
+    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
+};
+
+int opcodes[16];
+
+int sample_read(Sample *s, FILE *file) {
+    int read = 0;
+
+    read =
+        fscanf(file, "Before: [%u, %u, %u, %u]\n",
+               &s->before[0], &s->before[1], &s->before[2], &s->before[3]);
+    if (read != 4) return 0;
+
+    Instruction *i = &s->instruction;
+    read = fscanf(file, "%" SCNu8 " %u %u %u\n",
+                   &i->opcode, &i->a, &i->b, &i->c);
+    if (read != 4) return 0;
+
+    read =
+        fscanf(file, "After: [%u, %u, %u, %u]\n\n",
+               &s->after[0], &s->after[1], &s->after[2], &s->after[3]);
+    if (read != 4) return 0;
+
+    return 1;
+}
+
+uint16_t sample_possible_opcodes(const Sample s) {
+    uint16_t possible_opcodes = 0;
+
+    Instruction i = s.instruction;
+    // Addition
+    if (s.after[i.c] == s.before[i.a] + s.before[i.b])    possible_opcodes |= 1 << ADDR;
+    if (s.after[i.c] == s.before[i.a] + i.b)              possible_opcodes |= 1 << ADDI;
+    // Multiplication
+    if (s.after[i.c] == s.before[i.a] * s.before[i.b])    possible_opcodes |= 1 << MULR;
+    if (s.after[i.c] == s.before[i.a] * i.b)              possible_opcodes |= 1 << MULI;
+    // Bitwise AND
+    if (s.after[i.c] == (s.before[i.a] & s.before[i.b]))  possible_opcodes |= 1 << BANR;
+    if (s.after[i.c] == (s.before[i.a] & i.b))            possible_opcodes |= 1 << BANI;
+    // Bitwise OR
+    if (s.after[i.c] == (s.before[i.a] | s.before[i.b]))  possible_opcodes |= 1 << BORR;
+    if (s.after[i.c] == (s.before[i.a] | i.b))            possible_opcodes |= 1 << BORI;
+    // Assignment
+    if (s.after[i.c] == s.before[i.a])                    possible_opcodes |= 1 << SETR;
+    if (s.after[i.c] == i.a)                              possible_opcodes |= 1 << SETI;
+    // Greater-than testing
+    if (s.after[i.c] == (i.a > s.before[i.b]))            possible_opcodes |= 1 << GTIR;
+    if (s.after[i.c] == (s.before[i.a] > i.b))            possible_opcodes |= 1 << GTRI;
+    if (s.after[i.c] == (s.before[i.a] > s.before[i.b]))  possible_opcodes |= 1 << GTRR;
+    // Equality testing
+    if (s.after[i.c] == (i.a == s.before[i.b]))           possible_opcodes |= 1 << EQIR;
+    if (s.after[i.c] == (s.before[i.a] == i.b))           possible_opcodes |= 1 << EQRI;
+    if (s.after[i.c] == (s.before[i.a] == s.before[i.b])) possible_opcodes |= 1 << EQRR;
+
+    return possible_opcodes;
+}
+
+int bits_set(uint16_t n) {
+    int bit = -1;
+    for (int i = 0; i < 16; i++) {
+        if ((n >> i) & 1) {
+            if (bit != -1) return -1;
+            bit = i;
+        }
+    }
+    return bit;
+}
+
+void find_opcodes() {
+    int unknown = 16;
+    while (unknown > 0) {
+        for (int opcode = 0; opcode < 16; opcode++) {
+            int bit = bits_set(table[opcode]);
+            if (bit == -1) continue;
+
+            // Set every row in this column to 0
+            for (int j = 0; j < 16; j++)
+                table[j] &= ~(1 << bit);
+
+            opcodes[opcode] = bit;
+            unknown -= 1;
+        }
+    }
+
+    for (int i = 0; i < 16; i++) {
+        printf("%d: %s\n", i, names[opcodes[i]]);
+    }
+}
+
+int instruction_read(Instruction *i, FILE *file) {
+    return fscanf(file, "%" SCNu8 " %u %u %u\n",
+                  &i->opcode, &i->a, &i->b, &i->c) == 4;
+}
+
+void instruction_execute(const Instruction *i, uint32_t *r) {
+    switch (opcodes[i->opcode]) {
+    // Addition
+    case ADDR: r[i->c] = r[i->a] + r[i->b];    break;
+    case ADDI: r[i->c] = r[i->a] + i->b;       break;
+
+    // Multiplication
+    case MULR: r[i->c] = r[i->a] * r[i->b];    break;
+    case MULI: r[i->c] = r[i->a] * i->b;       break;
+
+    // Bitwise AND
+    case BANR: r[i->c] = (r[i->a] & r[i->b]);  break;
+    case BANI: r[i->c] = (r[i->a] & i->b);     break;
+
+    // Bitwise OR
+    case BORR: r[i->c] = (r[i->a] | r[i->b]);  break;
+    case BORI: r[i->c] = (r[i->a] | i->b);     break;
+
+    // Assignment
+    case SETR: r[i->c] = r[i->a];              break;
+    case SETI: r[i->c] = i->a;                 break;
+
+    // Greater-than testing
+    case GTIR: r[i->c] = (i->a > r[i->b]);     break;
+    case GTRI: r[i->c] = (r[i->a] > i->b);     break;
+    case GTRR: r[i->c] = (r[i->a] > r[i->b]);  break;
+
+    // Equality testing
+    case EQIR: r[i->c] = (i->a == r[i->b]);    break;
+    case EQRI: r[i->c] = (r[i->a] == i->b);    break;
+    case EQRR: r[i->c] = (r[i->a] == r[i->b]); break;
+
+    default: assert(0);
+    }
+}
+
+int main(int argc, char **argv) {
+    if (argc != 2) {
+        printf("Usage: %s [input]\n", argv[0]);
+        return EXIT_FAILURE;
+    }
+
+    FILE *file = fopen(argv[1], "r");
+    Sample sample = {0};
+
+    while (sample_read(&sample, file)) {
+        uint16_t possible_opcodes = sample_possible_opcodes(sample);
+        table[sample.instruction.opcode] &= possible_opcodes;
+    }
+    find_opcodes();
+
+    uint32_t registers[4] = {0};
+    Instruction instruction = {0};
+    while (instruction_read(&instruction, file)) {
+        const char *name = names[opcodes[instruction.opcode]];
+        printf("opcode: %2d (%s), a: %d, b: %d, c: %d\n", instruction.opcode,
+               name, instruction.a, instruction.b, instruction.c);
+        instruction_execute(&instruction, registers);
+        printf("registers: %d %d %d %d\n", registers[0], registers[1],
+               registers[2], registers[3]);
+    }
+
+    fclose(file);
+    return EXIT_SUCCESS;
+}