diff --git a/src/holt59/aoc/2015/day1.py b/src/holt59/aoc/2015/day1.py
index 8ddf9bc..0c3e64b 100644
--- a/src/holt59/aoc/2015/day1.py
+++ b/src/holt59/aoc/2015/day1.py
@@ -1,10 +1,12 @@
-import sys
+from typing import Any, Iterator
 
-line = sys.stdin.read().strip()
-
-floor = 0
-floors = [(floor := floor + (1 if c == "(" else -1)) for c in line]
+from ..base import BaseSolver
 
 
-print(f"answer 1 is {floors[-1]}")
-print(f"answer 2 is {floors.index(-1)}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        floor = 0
+        floors = [(floor := floor + (1 if c == "(" else -1)) for c in input]
+
+        yield floors[-1]
+        yield floors.index(-1)
diff --git a/src/holt59/aoc/2015/day10.py b/src/holt59/aoc/2015/day10.py
index 55cdef2..f7569d4 100644
--- a/src/holt59/aoc/2015/day10.py
+++ b/src/holt59/aoc/2015/day10.py
@@ -1,7 +1,7 @@
 import itertools
-import sys
+from typing import Any, Iterator
 
-line = sys.stdin.read().strip()
+from ..base import BaseSolver
 
 # see http://www.se16.info/js/lands2.htm for the explanation of 'atoms' (or elements)
 #
@@ -9,7 +9,7 @@ line = sys.stdin.read().strip()
 # CodeGolf answer https://codegolf.stackexchange.com/a/8479/42148
 
 # fmt: off
-atoms = [
+ATOMS: list[tuple[str, tuple[int, ...]]] = [
     ("22", (0, )),  # 0
     ("13112221133211322112211213322112", (71, 90, 0, 19, 2, )),  # 1
     ("312211322212221121123222112", (1, )),  # 2
@@ -105,7 +105,7 @@ atoms = [
 ]
 # fmt: on
 
-starters = [
+STARTERS = [
     "1",
     "11",
     "21",
@@ -122,27 +122,26 @@ def look_and_say_length(s: str, n: int) -> int:
     if n == 0:
         return len(s)
 
-    if s in starters:
+    if s in STARTERS:
         return look_and_say_length(
             "".join(f"{len(list(g))}{k}" for k, g in itertools.groupby(s)), n - 1
         )
 
-    counts = {i: 0 for i in range(len(atoms))}
-    idx = next(i for i, (a, _) in enumerate(atoms) if s == a)
+    counts = {i: 0 for i in range(len(ATOMS))}
+    idx = next(i for i, (a, _) in enumerate(ATOMS) if s == a)
     counts[idx] = 1
 
     for _ in range(n):
-        c2 = {i: 0 for i in range(len(atoms))}
+        c2 = {i: 0 for i in range(len(ATOMS))}
         for i in counts:
-            for j in atoms[i][1]:
+            for j in ATOMS[i][1]:
                 c2[j] += counts[i]
         counts = c2
 
-    return sum(counts[i] * len(a[0]) for i, a in enumerate(atoms))
+    return sum(counts[i] * len(a[0]) for i, a in enumerate(ATOMS))
 
 
-answer_1 = look_and_say_length(line, 40)
-print(f"answer 1 is {answer_1}")
-
-answer_2 = look_and_say_length(line, 50)
-print(f"answer 2 is {answer_2}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any] | None:
+        yield look_and_say_length(input, 40)
+        yield look_and_say_length(input, 50)
diff --git a/src/holt59/aoc/2015/day11.py b/src/holt59/aoc/2015/day11.py
index 7d9ff1f..259ccf9 100644
--- a/src/holt59/aoc/2015/day11.py
+++ b/src/holt59/aoc/2015/day11.py
@@ -1,5 +1,7 @@
 import itertools
-import sys
+from typing import Any, Iterator
+
+from ..base import BaseSolver
 
 
 def is_valid(p: str) -> bool:
@@ -40,10 +42,8 @@ def find_next_password(p: str) -> str:
     return p
 
 
-line = sys.stdin.read().strip()
-
-answer_1 = find_next_password(line)
-print(f"answer 1 is {answer_1}")
-
-answer_2 = find_next_password(increment(answer_1))
-print(f"answer 2 is {answer_2}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        answer_1 = find_next_password(input)
+        yield answer_1
+        yield find_next_password(increment(answer_1))
diff --git a/src/holt59/aoc/2015/day12.py b/src/holt59/aoc/2015/day12.py
index bf1667d..9faabb6 100644
--- a/src/holt59/aoc/2015/day12.py
+++ b/src/holt59/aoc/2015/day12.py
@@ -1,6 +1,7 @@
 import json
-import sys
-from typing import TypeAlias
+from typing import Any, Iterator, TypeAlias
+
+from ..base import BaseSolver
 
 JsonObject: TypeAlias = dict[str, "JsonObject"] | list["JsonObject"] | int | str
 
@@ -18,10 +19,9 @@ def json_sum(value: JsonObject, ignore: str | None = None) -> int:
         return 0
 
 
-data: JsonObject = json.load(sys.stdin)
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        data: JsonObject = json.loads(input)
 
-answer_1 = json_sum(data)
-print(f"answer 1 is {answer_1}")
-
-answer_2 = json_sum(data, "red")
-print(f"answer 2 is {answer_2}")
+        yield json_sum(data)
+        yield json_sum(data, "red")
diff --git a/src/holt59/aoc/2015/day13.py b/src/holt59/aoc/2015/day13.py
index ef0d7a7..3113abd 100644
--- a/src/holt59/aoc/2015/day13.py
+++ b/src/holt59/aoc/2015/day13.py
@@ -1,10 +1,11 @@
 import itertools
-import sys
 from collections import defaultdict
-from typing import Literal, cast
+from typing import Any, Iterator, Literal, cast
 
 import parse  # type: ignore
 
+from ..base import BaseSolver
+
 
 def max_change_in_happiness(happiness: dict[str, dict[str, int]]) -> int:
     guests = list(happiness)
@@ -17,25 +18,23 @@ def max_change_in_happiness(happiness: dict[str, dict[str, int]]) -> int:
     )
 
 
-lines = sys.stdin.read().splitlines()
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-happiness: dict[str, dict[str, int]] = defaultdict(dict)
-for line in lines:
-    u1, gain_or_loose, hap, u2 = cast(
-        tuple[str, Literal["gain", "lose"], int, str],
-        parse.parse(  # type: ignore
-            "{} would {} {:d} happiness units by sitting next to {}.", line
-        ),
-    )
-    happiness[u1][u2] = hap if gain_or_loose == "gain" else -hap
+        happiness: dict[str, dict[str, int]] = defaultdict(dict)
+        for line in lines:
+            u1, gain_or_loose, hap, u2 = cast(
+                tuple[str, Literal["gain", "lose"], int, str],
+                parse.parse(  # type: ignore
+                    "{} would {} {:d} happiness units by sitting next to {}.", line
+                ),
+            )
+            happiness[u1][u2] = hap if gain_or_loose == "gain" else -hap
 
+        yield max_change_in_happiness(happiness)
+        for guest in list(happiness):
+            happiness["me"][guest] = 0
+            happiness[guest]["me"] = 0
 
-answer_1 = max_change_in_happiness(happiness)
-print(f"answer 1 is {answer_1}")
-
-for guest in list(happiness):
-    happiness["me"][guest] = 0
-    happiness[guest]["me"] = 0
-
-answer_2 = max_change_in_happiness(happiness)
-print(f"answer 2 is {answer_2}")
+        yield max_change_in_happiness(happiness)
diff --git a/src/holt59/aoc/2015/day14.py b/src/holt59/aoc/2015/day14.py
index 7f2b1d2..a17e521 100644
--- a/src/holt59/aoc/2015/day14.py
+++ b/src/holt59/aoc/2015/day14.py
@@ -1,9 +1,10 @@
-import sys
 from dataclasses import dataclass
-from typing import Literal, cast
+from typing import Any, Iterator, Literal, cast
 
 import parse  # type: ignore
 
+from ..base import BaseSolver
+
 
 @dataclass(frozen=True)
 class Reindeer:
@@ -13,50 +14,50 @@ class Reindeer:
     rest_time: int
 
 
-lines = sys.stdin.read().splitlines()
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-reindeers: list[Reindeer] = []
-for line in lines:
-    reindeer, speed, speed_time, rest_time = cast(
-        tuple[str, int, int, int],
-        parse.parse(  # type: ignore
-            "{} can fly {:d} km/s for {:d} seconds, "
-            "but then must rest for {:d} seconds.",
-            line,
-        ),
-    )
-    reindeers.append(
-        Reindeer(name=reindeer, speed=speed, fly_time=speed_time, rest_time=rest_time)
-    )
+        reindeers: list[Reindeer] = []
+        for line in lines:
+            reindeer, speed, speed_time, rest_time = cast(
+                tuple[str, int, int, int],
+                parse.parse(  # type: ignore
+                    "{} can fly {:d} km/s for {:d} seconds, "
+                    "but then must rest for {:d} seconds.",
+                    line,
+                ),
+            )
+            reindeers.append(
+                Reindeer(
+                    name=reindeer, speed=speed, fly_time=speed_time, rest_time=rest_time
+                )
+            )
 
-target = 1000 if len(reindeers) <= 2 else 2503
+        target = 1000 if len(reindeers) <= 2 else 2503
 
-states: dict[Reindeer, tuple[Literal["resting", "flying"], int]] = {
-    reindeer: ("resting", 0) for reindeer in reindeers
-}
-distances: dict[Reindeer, int] = {reindeer: 0 for reindeer in reindeers}
-points: dict[Reindeer, int] = {reindeer: 0 for reindeer in reindeers}
+        states: dict[Reindeer, tuple[Literal["resting", "flying"], int]] = {
+            reindeer: ("resting", 0) for reindeer in reindeers
+        }
+        distances: dict[Reindeer, int] = {reindeer: 0 for reindeer in reindeers}
+        points: dict[Reindeer, int] = {reindeer: 0 for reindeer in reindeers}
 
-for time in range(target):
-    for reindeer in reindeers:
-        if states[reindeer][0] == "flying":
-            distances[reindeer] += reindeer.speed
+        for time in self.progress.wrap(range(target)):
+            for reindeer in reindeers:
+                if states[reindeer][0] == "flying":
+                    distances[reindeer] += reindeer.speed
 
-    top_distance = max(distances.values())
-    for reindeer in reindeers:
-        if distances[reindeer] == top_distance:
-            points[reindeer] += 1
+            top_distance = max(distances.values())
+            for reindeer in reindeers:
+                if distances[reindeer] == top_distance:
+                    points[reindeer] += 1
 
-    for reindeer in reindeers:
-        if states[reindeer][1] == time:
-            if states[reindeer][0] == "resting":
-                states[reindeer] = ("flying", time + reindeer.fly_time)
-            else:
-                states[reindeer] = ("resting", time + reindeer.rest_time)
+            for reindeer in reindeers:
+                if states[reindeer][1] == time:
+                    if states[reindeer][0] == "resting":
+                        states[reindeer] = ("flying", time + reindeer.fly_time)
+                    else:
+                        states[reindeer] = ("resting", time + reindeer.rest_time)
 
-
-answer_1 = max(distances.values())
-print(f"answer 1 is {answer_1}")
-
-answer_2 = max(points.values()) - 1
-print(f"answer 2 is {answer_2}")
+        yield max(distances.values())
+        yield max(points.values()) - 1
diff --git a/src/holt59/aoc/2015/day15.py b/src/holt59/aoc/2015/day15.py
index 953dabf..9a894b0 100644
--- a/src/holt59/aoc/2015/day15.py
+++ b/src/holt59/aoc/2015/day15.py
@@ -1,9 +1,10 @@
 import math
-import sys
-from typing import Sequence, cast
+from typing import Any, Iterator, Sequence, cast
 
 import parse  # type: ignore
 
+from ..base import BaseSolver
+
 
 def score(ingredients: list[list[int]], teaspoons: Sequence[int]) -> int:
     return math.prod(
@@ -18,40 +19,38 @@ def score(ingredients: list[list[int]], teaspoons: Sequence[int]) -> int:
     )
 
 
-lines = sys.stdin.read().splitlines()
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-ingredients: list[list[int]] = []
-for line in lines:
-    _, *scores = cast(
-        tuple[str, int, int, int, int, int],
-        parse.parse(  # type: ignore
-            "{}: capacity {:d}, durability {:d}, flavor {:d}, "
-            "texture {:d}, calories {:d}",
-            line,
-        ),
-    )
-    ingredients.append(scores)
-
-total_teaspoons = 100
-calories: list[int] = []
-scores: list[int] = []
-
-for a in range(total_teaspoons + 1):
-    for b in range(total_teaspoons + 1 - a):
-        for c in range(total_teaspoons + 1 - a - b):
-            teaspoons = (a, b, c, total_teaspoons - a - b - c)
-
-            scores.append(score(ingredients, teaspoons))
-            calories.append(
-                sum(
-                    ingredient[-1] * teaspoon
-                    for ingredient, teaspoon in zip(ingredients, teaspoons)
-                )
+        ingredients: list[list[int]] = []
+        for line in lines:
+            _, *scores = cast(
+                tuple[str, int, int, int, int, int],
+                parse.parse(  # type: ignore
+                    "{}: capacity {:d}, durability {:d}, flavor {:d}, "
+                    "texture {:d}, calories {:d}",
+                    line,
+                ),
             )
+            ingredients.append(scores)
 
+        total_teaspoons = 100
+        calories: list[int] = []
+        scores: list[int] = []
 
-answer_1 = max(scores)
-print(f"answer 1 is {answer_1}")
+        for a in range(total_teaspoons + 1):
+            for b in range(total_teaspoons + 1 - a):
+                for c in range(total_teaspoons + 1 - a - b):
+                    teaspoons = (a, b, c, total_teaspoons - a - b - c)
 
-answer_2 = max(score for score, calory in zip(scores, calories) if calory == 500)
-print(f"answer 2 is {answer_2}")
+                    scores.append(score(ingredients, teaspoons))
+                    calories.append(
+                        sum(
+                            ingredient[-1] * teaspoon
+                            for ingredient, teaspoon in zip(ingredients, teaspoons)
+                        )
+                    )
+
+        yield max(scores)
+        yield max(score for score, calory in zip(scores, calories) if calory == 500)
diff --git a/src/holt59/aoc/2015/day16.py b/src/holt59/aoc/2015/day16.py
index 8a3b275..e335677 100644
--- a/src/holt59/aoc/2015/day16.py
+++ b/src/holt59/aoc/2015/day16.py
@@ -1,8 +1,9 @@
 import operator as op
 import re
-import sys
 from collections import defaultdict
-from typing import Callable
+from typing import Any, Callable, Iterator
+
+from ..base import BaseSolver
 
 MFCSAM: dict[str, int] = {
     "children": 3,
@@ -17,18 +18,10 @@ MFCSAM: dict[str, int] = {
     "perfumes": 1,
 }
 
-lines = sys.stdin.readlines()
 
-aunts: list[dict[str, int]] = [
-    {
-        match[1]: int(match[2])
-        for match in re.findall(R"((?P<compound>[^:, ]+): (?P<quantity>\d+))", line)
-    }
-    for line in lines
-]
-
-
-def match(operators: dict[str, Callable[[int, int], bool]]) -> int:
+def match(
+    aunts: list[dict[str, int]], operators: dict[str, Callable[[int, int], bool]]
+) -> int:
     return next(
         i
         for i, aunt in enumerate(aunts, start=1)
@@ -36,16 +29,29 @@ def match(operators: dict[str, Callable[[int, int], bool]]) -> int:
     )
 
 
-answer_1 = match(defaultdict(lambda: op.eq))
-print(f"answer 1 is {answer_1}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-answer_2 = match(
-    defaultdict(
-        lambda: op.eq,
-        trees=op.gt,
-        cats=op.gt,
-        pomeranians=op.lt,
-        goldfish=op.lt,
-    )
-)
-print(f"answer 2 is {answer_2}")
+        aunts: list[dict[str, int]] = [
+            {
+                match[1]: int(match[2])
+                for match in re.findall(
+                    R"((?P<compound>[^:, ]+): (?P<quantity>\d+))", line
+                )
+            }
+            for line in lines
+        ]
+
+        yield match(aunts, defaultdict(lambda: op.eq))
+
+        yield match(
+            aunts,
+            defaultdict(
+                lambda: op.eq,
+                trees=op.gt,
+                cats=op.gt,
+                pomeranians=op.lt,
+                goldfish=op.lt,
+            ),
+        )
diff --git a/src/holt59/aoc/2015/day17.py b/src/holt59/aoc/2015/day17.py
index e4704c3..c954f9b 100644
--- a/src/holt59/aoc/2015/day17.py
+++ b/src/holt59/aoc/2015/day17.py
@@ -1,5 +1,6 @@
-import sys
-from typing import Iterator
+from typing import Any, Iterator
+
+from ..base import BaseSolver
 
 
 def iter_combinations(value: int, containers: list[int]) -> Iterator[tuple[int, ...]]:
@@ -16,15 +17,18 @@ def iter_combinations(value: int, containers: list[int]) -> Iterator[tuple[int,
             yield (containers[i],) + combination
 
 
-containers = [int(c) for c in sys.stdin.read().split()]
-total = 25 if len(containers) <= 5 else 150
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        containers = [int(c) for c in input.split()]
+        total = 25 if len(containers) <= 5 else 150
 
-combinations = [combination for combination in iter_combinations(total, containers)]
+        combinations = [
+            combination for combination in iter_combinations(total, containers)
+        ]
 
-answer_1 = len(combinations)
-print(f"answer 1 is {answer_1}")
+        yield len(combinations)
 
-min_containers = min(len(combination) for combination in combinations)
-
-answer_2 = sum(1 for combination in combinations if len(combination) == min_containers)
-print(f"answer 2 is {answer_2}")
+        min_containers = min(len(combination) for combination in combinations)
+        yield sum(
+            1 for combination in combinations if len(combination) == min_containers
+        )
diff --git a/src/holt59/aoc/2015/day18.py b/src/holt59/aoc/2015/day18.py
index b6de077..847b1d7 100644
--- a/src/holt59/aoc/2015/day18.py
+++ b/src/holt59/aoc/2015/day18.py
@@ -1,66 +1,66 @@
 import itertools
-import sys
+from typing import Any, Iterator
 
 import numpy as np
 from numpy.typing import NDArray
 
-grid0 = np.array([[c == "#" for c in line] for line in sys.stdin.read().splitlines()])
+from ..base import BaseSolver
 
-# add an always off circle around
-grid0 = np.concatenate(
-    [
-        np.zeros((grid0.shape[0] + 2, 1), dtype=bool),
-        np.concatenate(
+
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        grid0 = np.array([[c == "#" for c in line] for line in input.splitlines()])
+
+        # add an always off circle around
+        grid0 = np.concatenate(
             [
-                np.zeros((1, grid0.shape[1]), dtype=bool),
-                grid0,
-                np.zeros((1, grid0.shape[1]), dtype=bool),
-            ]
-        ),
-        np.zeros((grid0.shape[0] + 2, 1), dtype=bool),
-    ],
-    axis=1,
-)
+                np.zeros((grid0.shape[0] + 2, 1), dtype=bool),
+                np.concatenate(
+                    [
+                        np.zeros((1, grid0.shape[1]), dtype=bool),
+                        grid0,
+                        np.zeros((1, grid0.shape[1]), dtype=bool),
+                    ]
+                ),
+                np.zeros((grid0.shape[0] + 2, 1), dtype=bool),
+            ],
+            axis=1,
+        )
 
-moves = list(itertools.product([-1, 0, 1], repeat=2))
-moves.remove((0, 0))
+        moves = list(itertools.product([-1, 0, 1], repeat=2))
+        moves.remove((0, 0))
 
-jjs, iis = np.meshgrid(
-    np.arange(1, grid0.shape[0] - 1, dtype=int),
-    np.arange(1, grid0.shape[1] - 1, dtype=int),
-)
-iis, jjs = iis.flatten(), jjs.flatten()
+        jjs, iis = np.meshgrid(
+            np.arange(1, grid0.shape[0] - 1, dtype=int),
+            np.arange(1, grid0.shape[1] - 1, dtype=int),
+        )
+        iis, jjs = iis.flatten(), jjs.flatten()
 
-ins = iis[:, None] + np.array(moves)[:, 0]
-jns = jjs[:, None] + np.array(moves)[:, 1]
+        ins = iis[:, None] + np.array(moves)[:, 0]
+        jns = jjs[:, None] + np.array(moves)[:, 1]
 
+        def game_of_life(grid: NDArray[np.bool_]) -> NDArray[np.bool_]:
+            neighbors_on = grid[ins, jns].sum(axis=1)
+            cells_on = grid[iis, jjs]
 
-def game_of_life(grid: NDArray[np.bool_]) -> NDArray[np.bool_]:
-    neighbors_on = grid[ins, jns].sum(axis=1)
-    cells_on = grid[iis, jjs]
+            grid = np.zeros_like(grid)
+            grid[iis, jjs] = (neighbors_on == 3) | (cells_on & (neighbors_on == 2))
 
-    grid = np.zeros_like(grid)
-    grid[iis, jjs] = (neighbors_on == 3) | (cells_on & (neighbors_on == 2))
+            return grid
 
-    return grid
+        grid = grid0
+        n_steps = 4 if len(grid) < 10 else 100
+        for _ in range(n_steps):
+            grid = game_of_life(grid)
 
+        yield grid.sum()
 
-grid = grid0
-n_steps = 4 if len(grid) < 10 else 100
-for _ in range(n_steps):
-    grid = game_of_life(grid)
+        n_steps = 5 if len(grid) < 10 else 100
+        grid = grid0
+        for _ in range(n_steps):
+            grid[[1, 1, -2, -2], [1, -2, 1, -2]] = True
+            grid = game_of_life(grid)
 
-answer_1 = grid.sum()
-print(f"answer 1 is {answer_1}")
+        grid[[1, 1, -2, -2], [1, -2, 1, -2]] = True
 
-
-n_steps = 5 if len(grid) < 10 else 100
-grid = grid0
-for _ in range(n_steps):
-    grid[[1, 1, -2, -2], [1, -2, 1, -2]] = True
-    grid = game_of_life(grid)
-
-grid[[1, 1, -2, -2], [1, -2, 1, -2]] = True
-
-answer_2 = sum(cell for line in grid for cell in line)
-print(f"answer 2 is {answer_2}")
+        yield sum(cell for line in grid for cell in line)
diff --git a/src/holt59/aoc/2015/day19.py b/src/holt59/aoc/2015/day19.py
index db57831..6bcfb41 100644
--- a/src/holt59/aoc/2015/day19.py
+++ b/src/holt59/aoc/2015/day19.py
@@ -1,56 +1,58 @@
-import sys
 from collections import defaultdict
+from typing import Any, Iterator
 
-replacements_s, molecule = sys.stdin.read().split("\n\n")
-
-REPLACEMENTS: dict[str, list[str]] = defaultdict(list)
-for replacement_s in replacements_s.splitlines():
-    p = replacement_s.split(" => ")
-    REPLACEMENTS[p[0]].append(p[1])
-molecule = molecule.strip()
-
-generated = [
-    molecule[:i] + replacement + molecule[i + len(symbol) :]
-    for symbol, replacements in REPLACEMENTS.items()
-    for replacement in replacements
-    for i in range(len(molecule))
-    if molecule[i:].startswith(symbol)
-]
-
-answer_1 = len(set(generated))
-print(f"answer 1 is {answer_1}")
-
-inversion: dict[str, str] = {
-    replacement: symbol
-    for symbol, replacements in REPLACEMENTS.items()
-    for replacement in replacements
-}
-
-# there is actually only one way to create the molecule, and we can greedily replace
-# tokens with their replacements, e.g., if H => OH then we can replace OH by H directly
-# without thinking
-
-count = 0
-while molecule != "e":
-    i = 0
-    m2 = ""
-    while i < len(molecule):
-        found = False
-        for replacement in inversion:
-            if molecule[i:].startswith(replacement):
-                m2 += inversion[replacement]
-                i += len(replacement)
-                count += 1
-                found = True
-                break
-
-        if not found:
-            m2 += molecule[i]
-            i += 1
-
-    # print(m2)
-    molecule = m2
+from ..base import BaseSolver
 
 
-answer_2 = count
-print(f"answer 2 is {count}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        replacements_s, molecule = input.split("\n\n")
+
+        REPLACEMENTS: dict[str, list[str]] = defaultdict(list)
+        for replacement_s in replacements_s.splitlines():
+            p = replacement_s.split(" => ")
+            REPLACEMENTS[p[0]].append(p[1])
+        molecule = molecule.strip()
+
+        generated = [
+            molecule[:i] + replacement + molecule[i + len(symbol) :]
+            for symbol, replacements in REPLACEMENTS.items()
+            for replacement in replacements
+            for i in range(len(molecule))
+            if molecule[i:].startswith(symbol)
+        ]
+
+        yield len(set(generated))
+
+        inversion: dict[str, str] = {
+            replacement: symbol
+            for symbol, replacements in REPLACEMENTS.items()
+            for replacement in replacements
+        }
+
+        # there is actually only one way to create the molecule, and we can greedily replace
+        # tokens with their replacements, e.g., if H => OH then we can replace OH by H directly
+        # without thinking
+
+        count = 0
+        while molecule != "e":
+            i = 0
+            m2 = ""
+            while i < len(molecule):
+                found = False
+                for replacement in inversion:
+                    if molecule[i:].startswith(replacement):
+                        m2 += inversion[replacement]
+                        i += len(replacement)
+                        count += 1
+                        found = True
+                        break
+
+                if not found:
+                    m2 += molecule[i]
+                    i += 1
+
+            # print(m2)
+            molecule = m2
+
+        yield count
diff --git a/src/holt59/aoc/2015/day2.py b/src/holt59/aoc/2015/day2.py
index d2b8ffe..b492305 100644
--- a/src/holt59/aoc/2015/day2.py
+++ b/src/holt59/aoc/2015/day2.py
@@ -1,20 +1,24 @@
-import sys
+from typing import Any, Iterator
 
 import numpy as np
 
-lines = sys.stdin.read().splitlines()
+from ..base import BaseSolver
 
-length, width, height = np.array(
-    [[int(c) for c in line.split("x")] for line in lines]
-).T
 
-lw, wh, hl = (length * width, width * height, height * length)
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        length, width, height = np.array(
+            [[int(c) for c in line.split("x")] for line in input.splitlines()]
+        ).T
 
-answer_1 = np.sum(2 * (lw + wh + hl) + np.min(np.stack([lw, wh, hl]), axis=0))
-print(f"answer 1 is {answer_1}")
+        lw, wh, hl = (length * width, width * height, height * length)
 
-answer_2 = np.sum(
-    length * width * height
-    + 2 * np.min(np.stack([length + width, length + height, height + width]), axis=0)
-)
-print(f"answer 2 is {answer_2}")
+        yield np.sum(2 * (lw + wh + hl) + np.min(np.stack([lw, wh, hl]), axis=0))
+
+        yield np.sum(
+            length * width * height
+            + 2
+            * np.min(
+                np.stack([length + width, length + height, height + width]), axis=0
+            )
+        )
diff --git a/src/holt59/aoc/2015/day20.py b/src/holt59/aoc/2015/day20.py
index 4eb3f71..3e97484 100644
--- a/src/holt59/aoc/2015/day20.py
+++ b/src/holt59/aoc/2015/day20.py
@@ -1,10 +1,10 @@
 import itertools
-import sys
+from typing import Any, Iterator
 
-target = int(sys.stdin.read())
+from ..base import BaseSolver
 
 
-def presents(n: int, elf: int, max: int = target) -> int:
+def presents(n: int, elf: int, max: int) -> int:
     count = 0
     k = 1
     while k * k < n:
@@ -21,8 +21,9 @@ def presents(n: int, elf: int, max: int = target) -> int:
     return count
 
 
-answer_1 = next(n for n in itertools.count(1) if presents(n, 10) >= target)
-print(f"answer 1 is {answer_1}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        target = int(input)
 
-answer_2 = next(n for n in itertools.count(1) if presents(n, 11, 50) >= target)
-print(f"answer 2 is {answer_2}")
+        yield next(n for n in itertools.count(1) if presents(n, 10, target) >= target)
+        yield next(n for n in itertools.count(1) if presents(n, 11, 50) >= target)
diff --git a/src/holt59/aoc/2015/day21.py b/src/holt59/aoc/2015/day21.py
index cf71059..39f021b 100644
--- a/src/holt59/aoc/2015/day21.py
+++ b/src/holt59/aoc/2015/day21.py
@@ -1,7 +1,8 @@
 import itertools
-import sys
 from math import ceil
-from typing import TypeAlias
+from typing import Any, Iterator, TypeAlias
+
+from ..base import BaseSolver
 
 Modifier: TypeAlias = tuple[str, int, int, int]
 
@@ -33,34 +34,31 @@ RINGS: list[Modifier] = [
 ]
 
 
-lines = sys.stdin.read().splitlines()
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-player_hp = 100
+        player_hp = 100
 
-boss_attack = int(lines[1].split(":")[1].strip())
-boss_armor = int(lines[2].split(":")[1].strip())
-boss_hp = int(lines[0].split(":")[1].strip())
+        boss_attack = int(lines[1].split(":")[1].strip())
+        boss_armor = int(lines[2].split(":")[1].strip())
+        boss_hp = int(lines[0].split(":")[1].strip())
 
+        min_cost, max_cost = 1_000_000, 0
+        for equipments in itertools.product(WEAPONS, ARMORS, RINGS, RINGS):
+            if equipments[-1][0] != "" and equipments[-2] == equipments[-1]:
+                continue
 
-min_cost, max_cost = 1_000_000, 0
-for equipments in itertools.product(WEAPONS, ARMORS, RINGS, RINGS):
-    if equipments[-1][0] != "" and equipments[-2] == equipments[-1]:
-        continue
+            cost, player_attack, player_armor = (
+                sum(equipment[1:][k] for equipment in equipments) for k in range(3)
+            )
 
-    cost, player_attack, player_armor = (
-        sum(equipment[1:][k] for equipment in equipments) for k in range(3)
-    )
+            if ceil(boss_hp / max(1, player_attack - boss_armor)) <= ceil(
+                player_hp / max(1, boss_attack - player_armor)
+            ):
+                min_cost = min(cost, min_cost)
+            else:
+                max_cost = max(cost, max_cost)
 
-    if ceil(boss_hp / max(1, player_attack - boss_armor)) <= ceil(
-        player_hp / max(1, boss_attack - player_armor)
-    ):
-        min_cost = min(cost, min_cost)
-    else:
-        max_cost = max(cost, max_cost)
-
-
-answer_1 = min_cost
-print(f"answer 1 is {answer_1}")
-
-answer_2 = max_cost
-print(f"answer 2 is {answer_2}")
+        yield min_cost
+        yield max_cost
diff --git a/src/holt59/aoc/2015/day22.py b/src/holt59/aoc/2015/day22.py
index 0c488ec..715cf0c 100644
--- a/src/holt59/aoc/2015/day22.py
+++ b/src/holt59/aoc/2015/day22.py
@@ -1,8 +1,9 @@
 from __future__ import annotations
 
 import heapq
-import sys
-from typing import Literal, TypeAlias, cast
+from typing import Any, Iterator, Literal, TypeAlias, cast
+
+from ..base import BaseSolver
 
 PlayerType: TypeAlias = Literal["player", "boss"]
 SpellType: TypeAlias = Literal["magic missile", "drain", "shield", "poison", "recharge"]
@@ -155,23 +156,28 @@ def play(
     return winning_node
 
 
-lines = sys.stdin.read().splitlines()
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-player_hp = 50
-player_mana = 500
-player_armor = 0
+        player_hp = 50
+        player_mana = 500
+        player_armor = 0
 
-boss_hp = int(lines[0].split(":")[1].strip())
-boss_attack = int(lines[1].split(":")[1].strip())
+        boss_hp = int(lines[0].split(":")[1].strip())
+        boss_attack = int(lines[1].split(":")[1].strip())
 
-answer_1 = sum(
-    c
-    for _, c in play(player_hp, player_mana, player_armor, boss_hp, boss_attack, False)
-)
-print(f"answer 1 is {answer_1}")
+        yield sum(
+            c
+            for _, c in play(
+                player_hp, player_mana, player_armor, boss_hp, boss_attack, False
+            )
+        )
 
-# 1242 (not working)
-answer_2 = sum(
-    c for _, c in play(player_hp, player_mana, player_armor, boss_hp, boss_attack, True)
-)
-print(f"answer 2 is {answer_2}")
+        # 1242 (not working)
+        yield sum(
+            c
+            for _, c in play(
+                player_hp, player_mana, player_armor, boss_hp, boss_attack, True
+            )
+        )
diff --git a/src/holt59/aoc/2015/day3.py b/src/holt59/aoc/2015/day3.py
index 0c376c1..73284ab 100644
--- a/src/holt59/aoc/2015/day3.py
+++ b/src/holt59/aoc/2015/day3.py
@@ -1,7 +1,7 @@
-import sys
 from collections import defaultdict
+from typing import Any, Iterator
 
-line = sys.stdin.read().strip()
+from ..base import BaseSolver
 
 
 def process(directions: str) -> dict[tuple[int, int], int]:
@@ -27,8 +27,7 @@ def process(directions: str) -> dict[tuple[int, int], int]:
     return counts
 
 
-answer_1 = len(process(line))
-print(f"answer 1 is {answer_1}")
-
-answer_2 = len(process(line[::2]) | process(line[1::2]))
-print(f"answer 2 is {answer_2}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        yield len(process(input))
+        yield len(process(input[::2]) | process(input[1::2]))
diff --git a/src/holt59/aoc/2015/day4.py b/src/holt59/aoc/2015/day4.py
index 4b8efa9..e23f233 100644
--- a/src/holt59/aoc/2015/day4.py
+++ b/src/holt59/aoc/2015/day4.py
@@ -1,16 +1,20 @@
 import hashlib
 import itertools
-import sys
+from typing import Any, Iterator
 
-line = sys.stdin.read().strip()
+from ..base import BaseSolver
 
-it = iter(itertools.count(1))
-answer_1 = next(
-    i for i in it if hashlib.md5(f"{line}{i}".encode()).hexdigest().startswith("00000")
-)
-print(f"answer 1 is {answer_1}")
 
-answer_2 = next(
-    i for i in it if hashlib.md5(f"{line}{i}".encode()).hexdigest().startswith("000000")
-)
-print(f"answer 2 is {answer_2}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        it = iter(itertools.count(1))
+        yield next(
+            i
+            for i in it
+            if hashlib.md5(f"{input}{i}".encode()).hexdigest().startswith("00000")
+        )
+        yield next(
+            i
+            for i in it
+            if hashlib.md5(f"{input}{i}".encode()).hexdigest().startswith("000000")
+        )
diff --git a/src/holt59/aoc/2015/day5.py b/src/holt59/aoc/2015/day5.py
index 52fe963..7e1509e 100644
--- a/src/holt59/aoc/2015/day5.py
+++ b/src/holt59/aoc/2015/day5.py
@@ -1,4 +1,6 @@
-import sys
+from typing import Any, Iterator
+
+from ..base import BaseSolver
 
 VOWELS = "aeiou"
 FORBIDDEN = {"ab", "cd", "pq", "xy"}
@@ -27,10 +29,8 @@ def is_nice_2(s: str) -> bool:
     return True
 
 
-lines = sys.stdin.read().splitlines()
-
-answer_1 = sum(map(is_nice_1, lines))
-print(f"answer 1 is {answer_1}")
-
-answer_2 = sum(map(is_nice_2, lines))
-print(f"answer 2 is {answer_2}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
+        yield sum(map(is_nice_1, lines))
+        yield sum(map(is_nice_2, lines))
diff --git a/src/holt59/aoc/2015/day6.py b/src/holt59/aoc/2015/day6.py
index 1b5c21a..f46c42d 100644
--- a/src/holt59/aoc/2015/day6.py
+++ b/src/holt59/aoc/2015/day6.py
@@ -1,33 +1,32 @@
-import sys
-from typing import Literal, cast
+from typing import Any, Iterator, Literal, cast
 
 import numpy as np
 import parse  # type: ignore
 
-lines = sys.stdin.read().splitlines()
+from ..base import BaseSolver
 
-lights_1 = np.zeros((1000, 1000), dtype=bool)
-lights_2 = np.zeros((1000, 1000), dtype=int)
-for line in lines:
-    action, sx, sy, ex, ey = cast(
-        tuple[Literal["turn on", "turn off", "toggle"], int, int, int, int],
-        parse.parse("{} {:d},{:d} through {:d},{:d}", line),  # type: ignore
-    )
-    ex, ey = ex + 1, ey + 1
 
-    match action:
-        case "turn on":
-            lights_1[sx:ex, sy:ey] = True
-            lights_2[sx:ex, sy:ey] += 1
-        case "turn off":
-            lights_1[sx:ex, sy:ey] = False
-            lights_2[sx:ex, sy:ey] = np.maximum(lights_2[sx:ex, sy:ey] - 1, 0)
-        case "toggle":
-            lights_1[sx:ex, sy:ey] = ~lights_1[sx:ex, sy:ey]
-            lights_2[sx:ex, sy:ey] += 2
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lights_1 = np.zeros((1000, 1000), dtype=bool)
+        lights_2 = np.zeros((1000, 1000), dtype=int)
+        for line in input.splitlines():
+            action, sx, sy, ex, ey = cast(
+                tuple[Literal["turn on", "turn off", "toggle"], int, int, int, int],
+                parse.parse("{} {:d},{:d} through {:d},{:d}", line),  # type: ignore
+            )
+            ex, ey = ex + 1, ey + 1
 
-answer_1 = lights_1.sum()
-print(f"answer 1 is {answer_1}")
+            match action:
+                case "turn on":
+                    lights_1[sx:ex, sy:ey] = True
+                    lights_2[sx:ex, sy:ey] += 1
+                case "turn off":
+                    lights_1[sx:ex, sy:ey] = False
+                    lights_2[sx:ex, sy:ey] = np.maximum(lights_2[sx:ex, sy:ey] - 1, 0)
+                case "toggle":
+                    lights_1[sx:ex, sy:ey] = ~lights_1[sx:ex, sy:ey]
+                    lights_2[sx:ex, sy:ey] += 2
 
-answer_2 = lights_2.sum()
-print(f"answer 2 is {answer_2}")
+        yield lights_1.sum()
+        yield lights_2.sum()
diff --git a/src/holt59/aoc/2015/day7.py b/src/holt59/aoc/2015/day7.py
index a6e3fe5..b45ce6a 100644
--- a/src/holt59/aoc/2015/day7.py
+++ b/src/holt59/aoc/2015/day7.py
@@ -1,11 +1,7 @@
-import logging
 import operator
-import os
-import sys
-from typing import Callable
+from typing import Any, Callable, Iterator
 
-VERBOSE = os.getenv("AOC_VERBOSE") == "True"
-logging.basicConfig(level=logging.INFO if VERBOSE else logging.WARNING)
+from ..base import BaseSolver
 
 OPERATORS = {
     "AND": operator.and_,
@@ -36,48 +32,6 @@ def value_of(key: str) -> tuple[str, Callable[[dict[str, int]], int]]:
         return key, lambda values: values[key]
 
 
-lines = sys.stdin.read().splitlines()
-
-signals: Signals = {}
-values: dict[str, int] = {"": 0}
-
-for line in lines:
-    command, signal = line.split(" -> ")
-
-    if command.startswith("NOT"):
-        name = command.split(" ")[1]
-        signals[signal] = (
-            (name, ""),
-            (lambda values, _n=name: values[_n], lambda _v: 0),
-            lambda a, _b: ~a,
-        )
-
-    elif not any(command.find(name) >= 0 for name in OPERATORS):
-        try:
-            values[signal] = int(command)
-        except ValueError:
-            signals[signal] = (
-                (command, ""),
-                (lambda values, _c=command: values[_c], lambda _v: 0),
-                lambda a, _b: a,
-            )
-
-    else:
-        op: Callable[[int, int], int] = zero_op
-        lhs_s, rhs_s = "", ""
-
-        for name in OPERATORS:
-            if command.find(name) >= 0:
-                op = OPERATORS[name]
-                lhs_s, rhs_s = command.split(f" {name} ")
-                break
-
-        lhs_s, lhs_fn = value_of(lhs_s)
-        rhs_s, rhs_fn = value_of(rhs_s)
-
-        signals[signal] = ((lhs_s, rhs_s), (lhs_fn, rhs_fn), op)
-
-
 def process(
     signals: Signals,
     values: dict[str, int],
@@ -91,11 +45,52 @@ def process(
     return values
 
 
-values_1 = process(signals.copy(), values.copy())
-logging.info("\n" + "\n".join(f"{k}: {values_1[k]}" for k in sorted(values_1)))
-answer_1 = values_1["a"]
-print(f"answer 1 is {answer_1}")
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any] | None:
+        lines = input.splitlines()
 
-values_2 = process(signals.copy(), values | {"b": values_1["a"]})
-answer_2 = values_2["a"]
-print(f"answer 2 is {answer_2}")
+        signals: Signals = {}
+        values: dict[str, int] = {"": 0}
+
+        for line in lines:
+            command, signal = line.split(" -> ")
+
+            if command.startswith("NOT"):
+                name = command.split(" ")[1]
+                signals[signal] = (
+                    (name, ""),
+                    (lambda values, _n=name: values[_n], lambda _v: 0),
+                    lambda a, _b: ~a,
+                )
+
+            elif not any(command.find(name) >= 0 for name in OPERATORS):
+                try:
+                    values[signal] = int(command)
+                except ValueError:
+                    signals[signal] = (
+                        (command, ""),
+                        (lambda values, _c=command: values[_c], lambda _v: 0),
+                        lambda a, _b: a,
+                    )
+
+            else:
+                op: Callable[[int, int], int] = zero_op
+                lhs_s, rhs_s = "", ""
+
+                for name in OPERATORS:
+                    if command.find(name) >= 0:
+                        op = OPERATORS[name]
+                        lhs_s, rhs_s = command.split(f" {name} ")
+                        break
+
+                lhs_s, lhs_fn = value_of(lhs_s)
+                rhs_s, rhs_fn = value_of(rhs_s)
+
+                signals[signal] = ((lhs_s, rhs_s), (lhs_fn, rhs_fn), op)
+
+        values_1 = process(signals.copy(), values.copy())
+        for k in sorted(values_1):
+            self.logger.info(f"{k}: {values_1[k]}")
+        yield values_1["a"]
+
+        yield process(signals.copy(), values | {"b": values_1["a"]})["a"]
diff --git a/src/holt59/aoc/2015/day8.py b/src/holt59/aoc/2015/day8.py
index be22c22..d6662ad 100644
--- a/src/holt59/aoc/2015/day8.py
+++ b/src/holt59/aoc/2015/day8.py
@@ -1,35 +1,32 @@
-import logging
-import os
-import sys
+from typing import Any, Iterator
 
-VERBOSE = os.getenv("AOC_VERBOSE") == "True"
-logging.basicConfig(level=logging.INFO if VERBOSE else logging.WARNING)
+from ..base import BaseSolver
 
 
-lines = sys.stdin.read().splitlines()
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-answer_1 = sum(
-    # left and right quotes (not in memory)
-    2
-    # each \\ adds one character in the literals (compared to memory)
-    + line.count(R"\\")
-    # each \" adds one character in the literals (compared to memory)
-    + line[1:-1].count(R"\"")
-    # each \xFF adds 3 characters in the literals (compared to memory), but we must not
-    # count A\\x (A != \), but we must count A\\\x (A != \) - in practice we should also
-    # avoid \\\\x, etc., but this does not occur in the examples and the actual input
-    + 3 * (line.count(R"\x") - line.count(R"\\x") + line.count(R"\\\x"))
-    for line in lines
-)
-print(f"answer 1 is {answer_1}")
+        yield sum(
+            # left and right quotes (not in memory)
+            2
+            # each \\ adds one character in the literals (compared to memory)
+            + line.count(R"\\")
+            # each \" adds one character in the literals (compared to memory)
+            + line[1:-1].count(R"\"")
+            # each \xFF adds 3 characters in the literals (compared to memory), but we must not
+            # count A\\x (A != \), but we must count A\\\x (A != \) - in practice we should also
+            # avoid \\\\x, etc., but this does not occur in the examples and the actual input
+            + 3 * (line.count(R"\x") - line.count(R"\\x") + line.count(R"\\\x"))
+            for line in lines
+        )
 
-answer_2 = sum(
-    # needs to wrap in quotes (2 characters)
-    2
-    # needs to escape every \ with an extra \
-    + line.count("\\")
-    # needs to escape every " with an extra \ (including the first and last ones)
-    + line.count('"')
-    for line in lines
-)
-print(f"answer 2 is {answer_2}")
+        yield sum(
+            # needs to wrap in quotes (2 characters)
+            2
+            # needs to escape every \ with an extra \
+            + line.count("\\")
+            # needs to escape every " with an extra \ (including the first and last ones)
+            + line.count('"')
+            for line in lines
+        )
diff --git a/src/holt59/aoc/2015/day9.py b/src/holt59/aoc/2015/day9.py
index f6e7176..e055964 100644
--- a/src/holt59/aoc/2015/day9.py
+++ b/src/holt59/aoc/2015/day9.py
@@ -1,27 +1,28 @@
 import itertools
-import sys
 from collections import defaultdict
-from typing import cast
+from typing import Any, Iterator, cast
 
 import parse  # type: ignore
 
-lines = sys.stdin.read().splitlines()
+from ..base import BaseSolver
 
-distances: dict[str, dict[str, int]] = defaultdict(dict)
-for line in lines:
-    origin, destination, length = cast(
-        tuple[str, str, int],
-        parse.parse("{} to {} = {:d}", line),  # type: ignore
-    )
-    distances[origin][destination] = distances[destination][origin] = length
 
-distance_of_routes = {
-    route: sum(distances[o][d] for o, d in zip(route[:-1], route[1:]))
-    for route in map(tuple, itertools.permutations(distances))
-}
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        lines = input.splitlines()
 
-answer_1 = min(distance_of_routes.values())
-print(f"answer 1 is {answer_1}")
+        distances: dict[str, dict[str, int]] = defaultdict(dict)
+        for line in lines:
+            origin, destination, length = cast(
+                tuple[str, str, int],
+                parse.parse("{} to {} = {:d}", line),  # type: ignore
+            )
+            distances[origin][destination] = distances[destination][origin] = length
 
-answer_2 = max(distance_of_routes.values())
-print(f"answer 2 is {answer_2}")
+        distance_of_routes = {
+            route: sum(distances[o][d] for o, d in zip(route[:-1], route[1:]))
+            for route in map(tuple, itertools.permutations(distances))
+        }
+
+        yield min(distance_of_routes.values())
+        yield max(distance_of_routes.values())
diff --git a/src/holt59/aoc/__main__.py b/src/holt59/aoc/__main__.py
index c0bad6e..1ff9c44 100644
--- a/src/holt59/aoc/__main__.py
+++ b/src/holt59/aoc/__main__.py
@@ -178,7 +178,14 @@ def main():
 
     start = datetime.now()
     last = start
-    for i_answer, answer in enumerate(solver.solve(data.strip())):
+
+    it = solver.solve(data.strip())
+
+    if it is None:
+        solver.logger.error(f"no implementation for {year} day {day}")
+        exit()
+
+    for i_answer, answer in enumerate(it):
         current = datetime.now()
 
         if api:
diff --git a/src/holt59/aoc/base.py b/src/holt59/aoc/base.py
index 97cb666..bc644b9 100644
--- a/src/holt59/aoc/base.py
+++ b/src/holt59/aoc/base.py
@@ -7,10 +7,12 @@ _T = TypeVar("_T")
 
 class ProgressHandler(Protocol):
     @overload
-    def wrap(self, values: Sequence[_T]) -> Iterator[_T]: ...
+    def wrap(self, values: Sequence[_T]) -> Iterator[_T]:
+        ...
 
     @overload
-    def wrap(self, values: Iterable[_T], total: int) -> Iterator[_T]: ...
+    def wrap(self, values: Iterable[_T], total: int) -> Iterator[_T]:
+        ...
 
 
 class BaseSolver:
@@ -31,4 +33,5 @@ class BaseSolver:
         self.outputs = outputs
 
     @abstractmethod
-    def solve(self, input: str) -> Iterator[Any]: ...
+    def solve(self, input: str) -> Iterator[Any] | None:
+        ...