2nd/10_Wiki/Topics/Other/Search-Space.md

---
id: wiki-2026-0508-search-space
title: Search Space
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [State Space, Solution Space, Hypothesis Space]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [algorithms, search, optimization, ai, planning]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: python
  framework: algorithms
---

# Search Space

## 매 한 줄
> **"매 search space = 매 algorithm 의 매 explore-able 모든 candidate 의 set"**. 매 problem 을 매 (state, transition, goal) tuple 로 modeling 시 의 전체 reachable state set. 매 search 의 효율 = 매 1) space 의 size 줄이기 + 2) 매 promising region 의 priorit ize.

## 매 핵심

### 매 정의 components
- **State**: 매 partial / full solution candidate.
- **Initial state**: 매 search 시작 점.
- **Successor function**: 매 state → 매 reachable next states.
- **Goal test**: 매 state 가 매 valid solution 인지.
- **Path cost**: 매 path 의 매 quality metric.

### 매 size scaling
- **Combinatorial explosion**: 매 N-queens 의 N=8 → 매 16M state. N=20 → 매 effectively infinite.
- **Branching factor (b)** × **depth (d)** → b^d.
- **Pruning** (alpha-beta, constraint propagation, branch-and-bound) → 매 effective space ↓.

### 매 응용
1. **Pathfinding**: 매 grid/graph 의 매 cell/node space.
2. **Game AI**: 매 chess/go 의 매 game tree.
3. **Planning**: 매 STRIPS, PDDL 의 매 action sequence space.
4. **NAS**: 매 neural architecture 의 매 hyperparameter space.
5. **LLM reasoning**: 매 chain-of-thought / tree-of-thought 의 매 reasoning tree.

## 💻 패턴

### Pattern 1: Generic search space (BFS)
```python
from collections import deque

def bfs(initial, successors, is_goal):
    frontier = deque([(initial, [])])
    visited = {initial}
    while frontier:
        state, path = frontier.popleft()
        if is_goal(state):
            return path + [state]
        for nxt in successors(state):
            if nxt not in visited:
                visited.add(nxt)
                frontier.append((nxt, path + [state]))
    return None
```

### Pattern 2: A* with admissible heuristic (search space reduction)
```python
import heapq

def astar(initial, successors, is_goal, heuristic, cost):
    pq = [(heuristic(initial), 0, initial, [])]
    seen = {}
    while pq:
        _, g, s, path = heapq.heappop(pq)
        if is_goal(s):
            return path + [s]
        if s in seen and seen[s] <= g:
            continue
        seen[s] = g
        for nxt in successors(s):
            new_g = g + cost(s, nxt)
            f = new_g + heuristic(nxt)
            heapq.heappush(pq, (f, new_g, nxt, path + [s]))
```

### Pattern 3: Constraint propagation (CSP)
```python
# 매 search space 의 매 prune via 매 arc-consistency.
def ac3(domains, constraints):
    queue = [(x, y) for x in domains for y in constraints.get(x, [])]
    while queue:
        x, y = queue.pop(0)
        if revise(domains, x, y, constraints):
            if not domains[x]:
                return False  # 매 inconsistent
            for z in constraints.get(x, []) - {y}:
                queue.append((z, x))
    return True
```

### Pattern 4: Tree-of-Thoughts (LLM reasoning space)
```python
# 매 LLM 의 매 reasoning step 을 매 search node 로.
async def tot_search(problem, max_depth=5, beam=3):
    frontier = [{"state": problem, "trace": []}]
    for d in range(max_depth):
        cands = []
        for node in frontier:
            thoughts = await llm.expand(node["state"], k=beam)
            for t in thoughts:
                cands.append({"state": t, "trace": node["trace"] + [t]})
        # 매 evaluator (LLM-as-judge) 가 매 top-beam pick.
        scored = await llm.evaluate(cands)
        frontier = sorted(scored, key=lambda x: -x["score"])[:beam]
        if any(is_goal(n["state"]) for n in frontier):
            break
    return frontier[0]["trace"]
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 매 small finite space | BFS / DFS — 매 complete |
| 매 large but heuristic-able | A* / IDA* |
| 매 huge stochastic | MCTS (UCT) |
| 매 continuous space | gradient-based / Bayesian opt |
| 매 LLM reasoning | Tree-of-Thoughts / Graph-of-Thoughts |
| 매 constraint-rich | CSP solver (Z3, OR-Tools) |

**기본값**: 매 first 매 reformulate problem 으로 매 space 의 size ↓ — 매 algorithm choice 보다 효과 큼.

## 🔗 Graph
- 부모: [[Combinatorial Optimization]]
- 변형: [[State Space]] · [[Hypothesis Space]]
- 응용: [[MCTS]]

## 🤖 LLM 활용
**언제**: 매 problem 의 매 search space modeling 의 매 design 도움.
**언제 X**: 매 매우 narrow domain (chess engine 등) — specialized solver 가 우위.

## ❌ 안티패턴
- **No pruning**: 매 brute-force on b^d=10^15 — 매 wall-clock 의 절망.
- **Wrong representation**: 매 redundant states (symmetry 의 explode) — canonicalize 필요.
- **Heuristic over-engineering**: 매 inadmissible heuristic 의 매 optimality 깨짐.

## 🧪 검증 / 중복
- Verified (Russell & Norvig *AIMA* 4th ed; Yao et al. ToT 2023).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Search Space components/scaling/BFS/A*/CSP/ToT 정리 |