diff --git a/app/pool.py b/app/pool.py
index 0f89a46..1ddd2c9 100644
--- a/app/pool.py
+++ b/app/pool.py
@@ -1 +1,120 @@
 """Question pool validation."""
+
+from __future__ import annotations
+
+import json
+from typing import Any
+
+from app.scoring import SCORE_FNS
+
+OPTION_KEYS = {"A", "B", "C", "D"}
+
+
+class PoolValidationError(ValueError):
+    pass
+
+
+def parse_pool_json(pool_json: str | dict[str, Any]) -> dict[str, Any]:
+    if isinstance(pool_json, str):
+        try:
+            data = json.loads(pool_json)
+        except json.JSONDecodeError as exc:
+            raise PoolValidationError(f"Invalid JSON: {exc.msg}") from exc
+    else:
+        data = pool_json
+    return validate_pool(data)
+
+
+def validate_pool(data: dict[str, Any]) -> dict[str, Any]:
+    if not isinstance(data, dict):
+        raise PoolValidationError("Pool must be a JSON object")
+    title = data.get("title")
+    if not isinstance(title, str) or not title.strip():
+        raise PoolValidationError("Pool title is required")
+    score_fn = data.get("score_fn", "linear_decay")
+    if score_fn not in SCORE_FNS:
+        raise PoolValidationError(f"Unknown score function: {score_fn}")
+    time_limit_default = _positive_int(data.get("time_limit_default", 60), "time_limit_default")
+    questions = data.get("questions")
+    if not isinstance(questions, list) or not questions:
+        raise PoolValidationError("Pool must include at least one question")
+
+    normalized_questions: list[dict[str, Any]] = []
+    for index, question in enumerate(questions):
+        if not isinstance(question, dict):
+            raise PoolValidationError(f"Question {index} must be an object")
+        normalized_questions.append(_validate_question(question, index, time_limit_default))
+
+    return {
+        "title": title.strip(),
+        "score_fn": score_fn,
+        "time_limit_default": time_limit_default,
+        "questions": normalized_questions,
+    }
+
+
+def question_count(pool: dict[str, Any]) -> int:
+    return len(pool["questions"])
+
+
+def get_question(pool: dict[str, Any], question_idx: int) -> dict[str, Any]:
+    try:
+        return pool["questions"][question_idx]
+    except IndexError as exc:
+        raise PoolValidationError("Question index out of range") from exc
+
+
+def question_time_limit(pool: dict[str, Any], question_idx: int) -> int:
+    question = get_question(pool, question_idx)
+    return int(question.get("time_limit") or pool["time_limit_default"])
+
+
+def public_question_payload(pool: dict[str, Any], question_idx: int) -> dict[str, Any]:
+    question = get_question(pool, question_idx)
+    return {
+        "question_idx": question_idx,
+        "text": question["text"],
+        "options": question["options"],
+        "time_limit": question_time_limit(pool, question_idx),
+    }
+
+
+def _validate_question(question: dict[str, Any], index: int, default_limit: int) -> dict[str, Any]:
+    qid = question.get("id")
+    if not isinstance(qid, str) or not qid.strip():
+        raise PoolValidationError(f"Question {index} id is required")
+    text = question.get("text")
+    if not isinstance(text, str) or not text.strip():
+        raise PoolValidationError(f"Question {index} text is required")
+    options = question.get("options")
+    if not isinstance(options, dict) or set(options) != OPTION_KEYS:
+        raise PoolValidationError(f"Question {index} options must be exactly A, B, C, D")
+    for key, value in options.items():
+        if not isinstance(value, str) or not value.strip():
+            raise PoolValidationError(f"Question {index} option {key} is required")
+    correct = question.get("correct")
+    if correct not in OPTION_KEYS:
+        raise PoolValidationError(f"Question {index} correct must be one of A, B, C, D")
+
+    normalized = {
+        "id": qid.strip(),
+        "text": text.strip(),
+        "options": {key: options[key].strip() for key in sorted(OPTION_KEYS)},
+        "correct": correct,
+    }
+    if "time_limit" in question and question["time_limit"] is not None:
+        normalized["time_limit"] = _positive_int(question["time_limit"], f"Question {index} time_limit")
+    else:
+        normalized["time_limit"] = default_limit
+    explanation = question.get("explanation")
+    if explanation is not None:
+        if not isinstance(explanation, str):
+            raise PoolValidationError(f"Question {index} explanation must be text")
+        normalized["explanation"] = explanation.strip()
+    return normalized
+
+
+def _positive_int(value: Any, label: str) -> int:
+    if isinstance(value, bool) or not isinstance(value, int) or value <= 0:
+        raise PoolValidationError(f"{label} must be a positive integer")
+    return value
diff --git a/app/scoring.py b/app/scoring.py
index f4be998..eadac44 100644
--- a/app/scoring.py
+++ b/app/scoring.py
@@ -1 +1,40 @@
 """Score functions."""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+
+ScoreFn = Callable[[bool, int, int], int]
+SCORE_FNS: dict[str, ScoreFn] = {}
+
+
+def register(name: str) -> Callable[[ScoreFn], ScoreFn]:
+    def decorator(func: ScoreFn) -> ScoreFn:
+        SCORE_FNS[name] = func
+        return func
+
+    return decorator
+
+
+@register("linear_decay")
+def linear_decay(correct: bool, elapsed_ms: int, time_limit_ms: int) -> int:
+    if not correct:
+        return 0
+    elapsed_ms = max(0, min(elapsed_ms, time_limit_ms))
+    return round(1000 * (1 - 0.5 * elapsed_ms / time_limit_ms))
+
+
+@register("flat")
+def flat(correct: bool, elapsed_ms: int, time_limit_ms: int) -> int:
+    return 1000 if correct else 0
+
+
+@register("exponential_decay")
+def exponential_decay(correct: bool, elapsed_ms: int, time_limit_ms: int) -> int:
+    if not correct:
+        return 0
+    import math
+
+    elapsed_ms = max(0, min(elapsed_ms, time_limit_ms))
+    decay = math.exp(-2 * elapsed_ms / time_limit_ms)
+    return round(1000 * (0.5 + 0.5 * decay))