LLMMCTS/workprocess.md

# Workprocess Documentation for LLMMCTS

## Overview

LLMMCTS implements Monte Carlo Tree Search (MCTS) for Large Language Model (LLM) planning tasks. It combines LLM reasoning with MCTS search to solve complex planning problems with sparse rewards.

## Core Concept

The package addresses the sparse reward problem in MCTS by using LLMs to provide **pseudo-rewards** (called `progressvalue`) at every node, enabling faster learning without waiting for terminal rewards.

## Architecture

### Three-Tier Value System

| Field | Source | Purpose |
|-------|--------|---------|
| `progressvalue` | LLM heuristic | Estimate of how close we are to solving; used for fast node selection |
| `statevalue` | Monte Carlo simulation | Actual cumulative reward from simulations; accurate but expensive to compute |
| `reward` | Environment | Immediate reward from environment (may be sparse, only at terminal states) |

### Module Structure

```
src/
├── type.jl         # MCTSNode struct definition (MCTSNode type)
├── util.jl         # UCT selection utility function (UCTselect)
├── mcts.jl         # Core MCTS operations (select, expand, simulate, backpropagate)
├── interface.jl    # High-level interface (runMCTS, simulateThenBackpropagate)
└── LLMMCTS.jl      # Main package entry point (module exports)
```

## Data Flow

### 1. Node Structure (type.jl)

```julia
MCTSNode(
    nodekey::String,           # Unique identifier
    state::Dict,               # Current state dictionary
    visits::Integer,           # Number of visits to this node
    progressvalue::Number,     # LLM's estimate of state quality
    statevalue::Number,        # Average cumulative reward from simulations
    reward::Number,            # Immediate reward at this node
    isterminal::Bool,          # Whether this is a terminal state
    parent::Union{MCTSNode, Nothing},  # Parent reference (nothing for root)
    children::Dict{String,MCTSNode},   # Child nodes mapping
    etc::Dict{String,Any}      # Additional data storage
)
```

### 2. MCTS Search Process: Rollout and Exploration

LLMMCTS explores the solution space through iterative MCTS cycles, each consisting of four phases: **Selection**, **Expansion**, **Simulation**, and **Backpropagation**.

#### The Four Phases of MCTS

```
Phase 1: SELECTION
└── Start from root node
└── Use UCT (Upper Confidence Bound for Trees) to select child nodes
    ├── UCT formula: Q(s,a) + c * sqrt(ln(N(s)) / N(s,a))
    ├── Exploitation term (Q): Uses progressvalue or statevalue
    └── Exploration term: Prefers less-visited nodes
└── Continue until reaching a leaf node

Phase 2: EXPANSION
└── If leaf is not terminal:
    ├── Apply transition function multiple times (horizontal sampling)
    ├── Generate new child nodes with LLM assistance
    ├── Each child represents a potential next state
    └── Dejavu detection prevents duplicate states

Phase 3: SIMULATION (Rollout)
└── From expanded leaf node:
    ├── Perform rollout up to maxSimulationDepth
    ├── At each level:
    │   ├── Accumulate reward from current node
    │   ├── Check if terminal state reached
    │   ├── Expand node to generate children
    │   └── Select best child using progressvalue + reward
    └── Return cumulative trajectory reward and terminal state

Phase 4: BACKPROPAGATION
└── Update statistics along the path to root:
    ├── Increment visit count for each node
    ├── Update statevalue (running average of rewards)
    ├── Apply discount to future rewards
    └── Propagate reward upward
```

#### Detailed Expansion Process (mcts.jl → expand/_expand)

**Horizontal Sampling**: At each node, the algorithm generates multiple child nodes by applying the transition function several times.

```
Current Node (state S)
├── transition(S, args) → Child 1 (state S1, progressvalue=P1)
├── transition(S, args) → Child 2 (state S2, progressvalue=P2)
├── transition(S, args) → Child 3 (state S3, progressvalue=P3)
└── ... (horizontalSample times)

Each child is evaluated by:
- progressvalue: LLM's estimate of state quality (fast, heuristic)
- reward: Immediate environment reward
```

**Dejavu Detection**: Before adding a child node, the algorithm checks if the `newNodeKey` already exists in the current node's children. If it does, the node is skipped to avoid duplicates. This handles semantically equivalent states.

#### Detailed Simulation Process (mcts.jl → simulate)

The simulation phase performs a rollout from a given node to explore the solution space vertically:

```
Input: Starting node, transition function, maxSimulationDepth

simTrajectoryReward = 0
terminalstate = nothing

for depth in 1:maxSimulationDepth:
    # Step 1: Accumulate current node's reward
    simTrajectoryReward += node.reward
    
    # Step 2: Check if terminal state
    if node.isterminal:
        terminalstate = node.state
        break
    
    # Step 3: Expand node horizontally (generate children)
    expand(node, transition, transitionargs;
           horizontalSample=horizontalSample,
           multithread=multithread)
    
    # Step 4: Select best child for next step
    node = selectChildNode(node)  # Uses progressvalue + reward

Return: (simTrajectoryReward, terminalstate)
```

**Why this matters**: The simulation phase estimates the value of a node by looking ahead `maxSimulationDepth` levels. Each rollout provides an estimate of the cumulative reward that can be obtained from the current state.

#### Detailed Backpropagation Process (mcts.jl → backpropagate)

After simulation, rewards are propagated back up the tree to update all visited nodes:

```
Input: Starting node (leaf from simulation), cumulative trajectory reward

while !isroot(node):
    # Update visit count
    node.visits += 1
    
    # Update statevalue (running average)
    # Formula: new_avg = (old_avg * (n-1) + new_value) / n
    node.statevalue = (node.statevalue * (node.visits-1) + simTrajectoryReward) / node.visits
    
    # Discount reward for future states
    # Future rewards are less certain, so they receive lower weight
    simTrajectoryReward *= discountRewardCoeff
    
    # Move to parent
    node = node.parent
```

**Discounting rationale**: Rewards further from the current state are discounted because:
1. Future rewards are uncertain
2. The longer the horizon, the more opportunities for suboptimal decisions
3. This creates a more realistic value estimate

#### Selection Phase: UCT (util.jl → UCTselect)

The UCT formula balances exploration vs. exploitation:

```
UCT(s,a) = Q(s,a) + c * sqrt(ln(N(s)) / N(s,a))

Where:
- Q(s,a) = childNode.statevalue  (exploitation: current estimate)
- c = explorationweight          (controls exploration vs exploitation)
- N(s) = parent.visits           (total visits to parent node)
- N(s,a) = childNode.visits      (visits to this specific child)
```

**Behavior**:
- If `childNode.visits = 0`: Exploration term becomes undefined, so use `progressvalue` as fallback
- If `childNode.visits` is low: Exploration term is high → encourages visiting unexplored branches
- If `childNode.visits` is high: Exploration term approaches 0 → exploits known high-value nodes

**Selection priority**:
1. **High `progressvalue` + low visits**: Explored first (fast LLM guidance)
2. **High `statevalue` + high visits**: Exploited once confirmed (accurate value)
3. **Balance**: Controlled by `explorationweight` parameter

### 3. Main Workflow (interface.jl → runMCTS)

```
runMCTS(initialstate, transition, args)
├── Initialize root node with initialstate
│   └── root.visits = 0, root.statevalue = 0, root.children = {}
│
├── For iteration 1 to maxiterations:
│   │
│   ├── PHASE 1: SELECTION
│   │   ├── Start at root node
│   │   ├── While not leaf node:
│   │   │   ├── Apply UCTselect() with explorationweight
│   │   │   │   ├── UCT = statevalue + w * sqrt(ln(parent_visits) / child_visits)
│   │   │   │   └── Select child with highest UCT score
│   │   │   └── node = selected child
│   │   └── node is now a leaf node
│   │
│   ├── PHASE 2: TERMINAL CHECK
│   │   ├── If node.isterminal == true:
│   │   │   ├── If node.state[:reward] >= 8:
│   │   │   │   └── Store in highValueState channel
│   │   │   └── backpropagate(node, node.reward)
│   │   │       └── Update all ancestors up to root
│   │   └── Else (non-terminal leaf):
│   │       └── Continue to EXPANSION
│   │
│   ├── PHASE 3: EXPANSION
│   │   ├── expand(node, transition, transitionargs;
│   │   │         horizontalSample=horizontalSampleExpansionPhase)
│   │   │
│   │   ├── For each child generated:
│   │   │   ├── newNode = transition(current_state, args)
│   │   │   ├── newNodeKey = result[:newNodeKey]
│   │   │   ├── newstate = result[:newstate]
│   │   │   ├── progressvalue = result[:progressvalue]
│   │   │   └── Dejavu check: skip if newNodeKey already exists
│   │   │
│   │   └── Each child gets initialized with:
│   │       ├── visits = 0
│   │       ├── statevalue = 0 (no simulations yet)
│   │       ├── progressvalue = LLM estimate
│   │       ├── reward = newstate[:reward]
│   │       └── parent = current node
│   │
│   ├── PHASE 4: SIMULATION + BACKPROPAGATION
│   │   ├── If multithread == true:
│   │   │   └── Spawn parallel simulateThenBackpropagate() for each child
│   │   │
│   │   └── For each leafNode in node.children:
│   │       ├── simulateThenBackpropagate(leafNode, ...)
│   │       │   ├── simulate(leafNode, ...):
│   │       │   │   └── Rollout up to maxSimulationDepth
│   │       │   │       └── Returns (simTrajectoryReward, terminalstate)
│   │       │   │
│   │       │   ├── If terminalstate[:reward] >= 8:
│   │       │   │   └── Store in highValueState channel
│   │       │   │
│   │       │   └── backpropagate(leafNode, simTrajectoryReward):
│   │       │       └── Update visits and statevalue for all ancestors
│   │       │
│   │       └── If saveSimulatedNode == false:
│   │           └── Clear children (free memory for next iteration)
│   │
│   └── PHASE 5: EARLY STOP CHECK
│       ├── If earlystop(node.state) == true:
│       │   └── Break out of iteration loop
│       └── Continue to next iteration
│
└── After all iterations, select best result:
    ├── bestNextState = selectBestNextNode(root)
    │   └── Uses statevalue/visits or progressvalue + reward
    │
    ├── bestTerminalState = selectBestTrajectoryNode(root)
    │   └── Follows optimal trajectory to leaf
    │
    └── highValueStateList = collect from highValueState channel
```

### 7. Selection Phase Details (util.jl → UCTselect)

**Purpose**: Select the best child node using UCT formula that balances exploration and exploitation.

**Process**:

```
UCTselect(node, w)

Input: Parent node, exploration weight w
Output: Child node with highest UCT score

maxUCT = -Inf
selectedNode = nothing

for each childNode in node.children:
    ┌── Calculate UCT value
    │   └── if childNode.visits != 0:
    │           ├── weightedterm = w * sqrt(ln(node.visits) / childNode.visits)
    │           ├── UCTvalue = childNode.statevalue + weightedterm
    │           └── Exploration term encourages low-visited children
    │
    │       else:  # childNode.visits == 0
    │           ├── UCTvalue = childNode.progressvalue
    │           └── No exploration term (division by zero)
    │
    ├── Compare with current max
    │   └── if UCTvalue > maxUCT:
    │           ├── maxUCT = UCTvalue
    │           └── selectedNode = childNode

return selectedNode
```

**Behavior analysis**:

| Scenario | UCT Value | Behavior |
|----------|-----------|----------|
| Child never visited (`visits=0`) | `progressvalue` | Strong exploration → will be tried |
| Child visited often, high reward | High `statevalue`, low exploration | Exploitation dominates |
| Child visited often, low reward | Low `statevalue`, low exploration | Exploitation avoids this node |
| Child visited rarely, moderate reward | Moderate `statevalue`, high exploration | May explore further |

### 8. Expansion Phase Details (mcts.jl → expand/_expand)

**Purpose**: Generate new child nodes by applying the transition function multiple times.

**Process**:

```
expand(node, transition, transitionargs;
       horizontalSample=3, multithread=false)

Input: Node to expand, transition function, arguments
Output: node.children populated with child nodes

if multithread == true:
    └── @sync for i in 1 to horizontalSample:
        └── @spawn _expand(node, transition, transitionargs)
        └── Run expansion in parallel threads
else:
    └── for i in 1 to horizontalSample:
        └── _expand(node, transition, transitionargs)
        └── Sequential expansion

_expand(node, transition, transitionargs)

Input: Single node to expand
Output: One child node added to node.children

result = transition(node.state, transitionargs)
    └── LLM generates next state:
        └── response = llm_call(state[:thoughtHistory], args.prompt)
        └── Parse response into new state structure

newNodeKey = result[:newNodeKey]
newstate = result[:newstate]
progressvalue = result[:progressvalue]

if newNodeKey ∉ keys(node.children):
    ┌── Create new MCTSNode
    │   └── newNode = MCTSNode(
    │       ├── nodekey = newNodeKey
    │       ├── state = newstate
    │       ├── visits = 0
    │       ├── progressvalue = progressvalue
    │       ├── statevalue = 0 (no simulations yet)
    │       ├── reward = newstate[:reward]
    │       ├── isterminal = newstate[:isterminal]
    │       ├── parent = node
    │       ├── children = {}
    │       └── etc = {}
    │
    └── node.children[newNodeKey] = newNode
```

**Dejavu detection**: Before adding a child, check if `newNodeKey` already exists in `node.children`. If so, skip to avoid duplicate states.

**Example**: Expanding a node with `horizontalSample=3`
```
Current Node: "Math problem: Solve x^2 = 16"
    └── Attempt 1 (LLM):
        └── Thought: "Take square root of both sides"
        └── Action: "x = sqrt(16)"
        └── New state: "Solved: x = 4"
        └── newNodeKey = "abc-123"
        └── Added to children: "abc-123" → Node

    └── Attempt 2 (LLM):
        └── Thought: "Consider negative root"
        └── Action: "x = -sqrt(16)"
        └── New state: "Solved: x = -4"
        └── newNodeKey = "def-456"
        └── Added to children: "def-456" → Node

    └── Attempt 3 (LLM):
        └── Thought: "Check both solutions"
        └── Action: "Verify x=4 and x=-4"
        └── New state: "Verified: x=4, x=-4"
        └── newNodeKey = "ghi-789"
        └── Added to children: "ghi-789" → Node

Result: Node has 3 children with different solution approaches
```

### 9. Backpropagation Phase Details (mcts.jl → backpropagate)

**Purpose**: Update the statistics of all nodes along the simulation path with the observed reward.

**Process**:

```
backpropagate(node, simTrajectoryReward;
              discountRewardCoeff=0.9)

Input: Starting node (leaf from simulation), cumulative reward
Output: Updates visits and statevalue for all ancestors

while !isroot(node):
    ┌── Update visit count
    │   └── node.visits += 1
    │
    ├── Update statevalue (running average formula)
    │   │
    │   ├── Current average: node.statevalue
    │   ├── Current count: node.visits - 1
    │   ├── New value to add: simTrajectoryReward
    │   │
    │   └── Formula: new_avg = (old_avg * (n-1) + new_value) / n
    │       node.statevalue = (node.statevalue * (node.visits-1) + simTrajectoryReward) / node.visits
    │
    ├── Apply discount to future reward
    │   └── simTrajectoryReward *= discountRewardCoeff
    │       └── Reward gets 10% smaller for each level up
    │
    └── Move to parent
        └── node = node.parent

# After loop, root node still needs update
# (root node's parent is nothing, so loop stops but root is still updated)
```

**Example**: Backpropagating reward=20 with discount=0.9
```
Depth 3 (leaf): Node A2b2
    └── visits = 1, statevalue = 20/1 = 20.0
    └── reward for parent = 20 * 0.9 = 18.0

Depth 2: Node A2b
    └── visits = 1, statevalue = (0*0 + 18)/1 = 18.0
    └── reward for parent = 18 * 0.9 = 16.2

Depth 1: Node A2
    └── visits = 1, statevalue = (0*0 + 16.2)/1 = 16.2
    └── reward for parent = 16.2 * 0.9 = 14.58

Depth 0 (root): Node A
    └── visits = 1, statevalue = (0*0 + 14.58)/1 = 14.58
```

**Why discounting matters**: Future rewards are discounted because:
1. **Uncertainty**: The longer the horizon, the more uncertain the outcome
2. **Temporal credit assignment**: Immediate rewards should have higher value
3. **Realistic evaluation**: A reward 10 steps away is worth less than immediate reward

### 10. Solution Space Exploration Strategy

LLMMCTS explores the solution space using a combination of **guided exploration** and **systematic exploitation**:

#### Exploration vs. Exploitation Trade-off

```
Iteration 1: Initial exploration (all nodes have visits=0)
├── UCTselect() uses progressvalue for all children
├── LLM provides guidance: which states seem promising?
├── High progressvalue nodes get visited first
└── Results stored in statevalue

Iterations 2-5: Early exploitation
├── Some nodes have higher statevalue confirmed by simulations
├── UCT balances:
│   ├── Exploitation: High statevalue nodes
│   └── Exploration: Nodes with low visits (high UCT exploration term)
└── Nodes with high progressvalue + high statevalue dominate

Iterations 6+: Refinement
├── Well-visited nodes have reliable statevalue estimates
├── Poor nodes (low statevalue) are explored less
├── Still explore unvisited branches (UCT exploration term)
└── Tree converges to optimal trajectory
```

#### Horizontal vs. Vertical Exploration

**Horizontal exploration** (expansion):
- **Parameter**: `horizontalSample`
- **Purpose**: Generate multiple candidate next states from current state
- **Method**: Apply transition function multiple times
- **LLM role**: Each call generates a different thought/action

**Vertical exploration** (simulation):
- **Parameter**: `maxSimulationDepth`
- **Purpose**: Evaluate the long-term value of a state
- **Method**: Rollout from current node to leaf
- **Reward accumulation**: Sum all rewards along trajectory

#### Dejavu Detection and State Pruning

**Dejavu detection** (mcts.jl → _expand):

```julia
if newNodeKey ∉ keys(node.children)
    newNode = MCTSNode(...)
    node.children[newNodeKey] = newNode
end
```

**Why it matters**:
- Prevents infinite loops in cyclic state spaces
- Handles semantically equivalent states as duplicates
- Reduces tree size and memory usage
- Improves search efficiency

**Example**:
```
Current Node: "Problem solving step 5"
├── Attempt 1: Generates "Next step: Apply formula A"
├── Attempt 2: Generates "Next step: Apply formula A" (same as attempt 1)
│   └── Dejavu detected: skip duplicate
└── Attempt 3: Generates "Next step: Apply formula B"
```

### 11. Complete MCTS Iteration Flow

```
┌─────────────────────────────────────────────────────────────────────────────┐
│ MCTS ITERATION 1                                                             │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│ 1. SELECTION (UCT-based traversal):                                          │
│    Root (visits=1)                                                           │
│    └── UCTselect() on root (has no children yet, so uses progressvalue)      │
│    └── Select child with highest progressvalue                              │
│    └── Continue until reaching leaf node                                     │
│                                                                              │
│ 2. EXPANSION (horizontal sampling):                                          │
│    Leaf node                                                                 │
│    └── expand(horizontalSample=3)                                           │
│    └── Generate 3 child nodes via LLM transition                             │
│    └── Each child gets: progressvalue, reward, parent=leaf                  │
│                                                                              │
│ 3. SIMULATION (vertical rollout):                                            │
│    Each child node                                                                           │
│    └── simulate(maxSimulationDepth=3)                                       │
│    └── Rollout 3 levels deep, accumulating rewards                          │
│    └── Return (simTrajectoryReward, terminalstate)                          │
│                                                                              │
│ 4. BACKPROPAGATION (update statistics):                                      │
│    simTrajectoryReward                                                       │
│    └── backpropagate() up to root                                            │
│    └── Update visits and statevalue for all ancestors                       │
│    └── Apply discount to future rewards                                     │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────────────────┐
│ MCTS ITERATION 2                                                             │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│ 1. SELECTION:                                                                │
│    Root (visits=2)                                                           │
│    └── UCTselect() now considers:                                            │
│        ├── statevalue (from iteration 1)                                    │
│        └── exploration term (low visits on unexpanded branches)             │
│    └── May select different path than iteration 1                           │
│                                                                              │
│ 2. EXPANSION:                                                                │
│    └── May expand different node or same node                               │
│    └── New children added to node.children                                  │
│                                                                              │
│ 3. SIMULATION:                                                               │
│    └── Different rollout trajectory                                          │
│    └── New reward estimate added to statistics                              │
│                                                                              │
│ 4. BACKPROPAGATION:                                                          │
│    └── Statistics updated with new information                              │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────────────────┐
│ MCTS ITERATION 3 to N                                                      │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│ Pattern repeats, with increasingly informed selection:                       │
│                                                                              │
│ • Nodes with high statevalue (confirmed by many simulations)                 │
│   → Exploited (selected frequently)                                          │
│                                                                              │
│ • Nodes with low visits but promising progressvalue                         │
│   → Explored (UCT exploration term encourages tries)                         │
│                                                                              │
│ • Tree grows: more branches explored, more statistics accumulated            │
│                                                                              │
│ • Best trajectory emerges from accumulated statistics                        │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘
```

```
simulate(node, transition, transitionargs;
         maxSimulationDepth=3, horizontalSample=3, multithread=false)

Input: Starting node at depth 0
Output: (simTrajectoryReward, terminalstate)

simTrajectoryReward = 0.0
terminalstate = nothing

for depth in 1 to maxSimulationDepth:
    ┌── Step 1: Accumulate reward
    │   └── simTrajectoryReward += node.reward
    │
    ├── Step 2: Check for terminal state
    │   └── if node.isterminal:
    │           └── terminalstate = deepcopy(node.state)
    │               break
    │
    ├── Step 3: Expand node (horizontal sampling)
    │   └── expand(node, transition, transitionargs;
    │               horizontalSample=horizontalSample)
    │       └── For i in 1 to horizontalSample:
    │           └── _expand(node, transition, transitionargs)
    │               ├── result = transition(node.state, args)
    │               ├── newNodeKey = result[:newNodeKey]
    │               ├── newstate = result[:newstate]
    │               ├── progressvalue = result[:progressvalue]
    │               └── if newNodeKey not in node.children:
    │                   └── Create new MCTSNode with:
    │                       ├── state = newstate
    │                       ├── progressvalue = progressvalue
    │                       ├── reward = newstate[:reward]
    │                       ├── parent = node
    │                       └── children = {}
    │
    └── Step 4: Select best child for next iteration
        └── node = selectChildNode(node)
            └── Uses: progressvalue + reward (no UCT here)

return (simTrajectoryReward, terminalstate)
```

**Example**: With `maxSimulationDepth=3` and `horizontalSample=3`:
```
Depth 0: Node A (reward=2)
    └── Expand 3 children: A1, A2, A3
    └── Select A2 (highest progressvalue + reward)
    
Depth 1: Node A2 (reward=3)
    └── Expand 3 children: A2a, A2b, A2c
    └── Select A2b (highest progressvalue + reward)
    
Depth 2: Node A2b (reward=5)
    └── Expand 3 children: A2b1, A2b2, A2b3
    └── Select A2b2 (highest progressvalue + reward)
    
Depth 3: Node A2b2 (reward=10, isterminal=true)
    └── Terminal state reached!
    
Return: (simTrajectoryReward=20, terminalstate=A2b2.state)
        = (2 + 3 + 5 + 10, ...)
```

### 6. Backpropagation (mcts.jl → backpropagate)

**Purpose**: Update the statistics of all nodes along the simulation path with the observed reward.

**Process**:

```
backpropagate(node, simTrajectoryReward;
              discountRewardCoeff=0.9)

Input: Starting node (leaf from simulation), cumulative reward
Output: Updates visits and statevalue for all ancestors

while !isroot(node):
    ┌── Update visit count
    │   └── node.visits += 1
    │
    ├── Update statevalue (running average formula)
    │   │
    │   ├── Current average: node.statevalue
    │   ├── Current count: node.visits - 1
    │   ├── New value to add: simTrajectoryReward
    │   │
    │   └── Formula: new_avg = (old_avg * (n-1) + new_value) / n
    │       node.statevalue = (node.statevalue * (node.visits-1) + simTrajectoryReward) / node.visits
    │
    ├── Apply discount to future reward
    │   └── simTrajectoryReward *= discountRewardCoeff
    │       └── Reward gets 10% smaller for each level up
    │
    └── Move to parent
        └── node = node.parent

# After loop, root node still needs update
# (root node's parent is nothing, so loop stops but root is still updated)
```

**Example**: Backpropagating reward=20 with discount=0.9
```
Depth 3 (leaf): Node A2b2
    └── visits = 1, statevalue = 20/1 = 20.0
    └── reward for parent = 20 * 0.9 = 18.0

Depth 2: Node A2b
    └── visits = 1, statevalue = (0*0 + 18)/1 = 18.0
    └── reward for parent = 18 * 0.9 = 16.2

Depth 1: Node A2
    └── visits = 1, statevalue = (0*0 + 16.2)/1 = 16.2
    └── reward for parent = 16.2 * 0.9 = 14.58

Depth 0 (root): Node A
    └── visits = 1, statevalue = (0*0 + 14.58)/1 = 14.58
```

**Why discounting matters**: Future rewards are discounted because:
1. **Uncertainty**: The longer the horizon, the more uncertain the outcome
2. **Temporal credit assignment**: Immediate rewards should have higher value
3. **Realistic evaluation**: A reward 10 steps away is worth less than immediate reward

### 7. Solution Space Exploration Strategy

LLMMCTS explores the solution space using a combination of **guided exploration** and **systematic exploitation**:

#### Exploration vs. Exploitation Trade-off

```
Iteration 1: Initial exploration (all nodes have visits=0)
├── UCTselect() uses progressvalue for all children
├── LLM provides guidance: which states seem promising?
├── High progressvalue nodes get visited first
└── Results stored in statevalue

Iterations 2-5: Early exploitation
├── Some nodes have higher statevalue confirmed by simulations
├── UCT balances:
│   ├── Exploitation: High statevalue nodes
│   └── Exploration: Nodes with low visits (high UCT exploration term)
└── Nodes with high progressvalue + high statevalue dominate

Iterations 6+: Refinement
├── Well-visited nodes have reliable statevalue estimates
├── Poor nodes (low statevalue) are explored less
├── Still explore unvisited branches (UCT exploration term)
└── Tree converges to optimal trajectory
```

#### Horizontal vs. Vertical Exploration

**Horizontal exploration** (expansion):
- **Parameter**: `horizontalSample`
- **Purpose**: Generate multiple candidate next states from current state
- **Method**: Apply transition function multiple times
- **LLM role**: Each call generates a different thought/action

**Vertical exploration** (simulation):
- **Parameter**: `maxSimulationDepth`
- **Purpose**: Evaluate the long-term value of a state
- **Method**: Rollout from current node to leaf
- **Reward accumulation**: Sum all rewards along trajectory

#### Dejavu Detection and State Pruning

**Dejavu detection** (mcts.jl → _expand):

```julia
if newNodeKey ∉ keys(node.children)
    newNode = MCTSNode(...)
    node.children[newNodeKey] = newNode
end
```

**Why it matters**:
- Prevents infinite loops in cyclic state spaces
- Handles semantically equivalent states as duplicates
- Reduces tree size and memory usage
- Improves search efficiency

**Example**:
```
Current Node: "Problem solving step 5"
├── Attempt 1: Generates "Next step: Apply formula A"
├── Attempt 2: Generates "Next step: Apply formula A" (same as attempt 1)
│   └── Dejavu detected: skip duplicate
└── Attempt 3: Generates "Next step: Apply formula B"
```

### 8. Selection Phase Details (util.jl → UCTselect)

### 9. High-Level Interface (interface.jl → runMCTS)

**Purpose**: Execute the complete MCTS search algorithm with all phases.

**Process**:

```
runMCTS(initialstate, transition, transitionargs;
        horizontalSampleExpansionPhase=3,
        horizontalSampleSimulationPhase=3,
        maxSimulationDepth=3,
        maxiterations=10,
        explorationweight=1.0,
        earlystop=nothing,
        saveSimulatedNode=false,
        multithread=false)

Output: NamedTuple with:
    ├── root: Complete MCTS tree
    ├── bestNextState: Best immediate next state
    ├── bestTerminalState: Best final state along optimal trajectory
    └── highValueStateList: List of high-value terminal states (reward >= 8)

Step-by-step execution:
    └── root = MCTSNode("root", initialstate, 0, 0, 0, 0, false, nothing, {}, {})
    └── highValueState = Channel{Any}(100)

    for iteration = 1 to maxiterations:
        ┌── node = root
        │   └── node.visits += 1  # Increment root visits
        │
        │   ┌── PHASE 1: SELECTION (until leaf)
        │   │   while !isleaf(node):
        │   │       └── node = UCTselect(node, explorationweight)
        │   │
        │   ├── PHASE 2: TERMINAL CHECK
        │   │   if node.isterminal:
        │   │       ├── if node.state[:reward] >= 8:
        │   │       │   └── put!(highValueState, deepcopy(node.state))
        │   │       └── backpropagate(node, node.reward)
        │   │           └── Update statistics up to root
        │   │       └── continue to next iteration
        │   │
        │   └── PHASE 3: EXPANSION
        │       ├── expand(node, transition, transitionargs;
        │       │         horizontalSample=horizontalSampleExpansionPhase,
        │       │         multithread=multithread)
        │       │
        │       └── For each leafNode in node.children:
        │           └── simulateThenBackpropagate(leafNode, ...)
        │               ├── simulate() → rollout and get reward
        │               ├── If terminal state with reward >= 8:
        │               │   └── Store in highValueState
        │               └── backpropagate() → update statistics
        │
        └── PHASE 4: EARLY STOP CHECK
            └── if earlystop !== nothing && earlystop(node.state):
                └── break  # Exit iteration loop

    ┌── After all iterations, select best result:
    │   ├── bestNextState = selectBestNextNode(root)
    │   │   └── Uses statevalue/visits or progressvalue + reward
    │   │
    │   ├── bestTerminalState = selectBestTrajectoryNode(root)
    │   │   └── Follows optimal trajectory to leaf
    │   │
    │   └── highValueStateList = collect from channel
    │       └── while !isempty(highValueState):
    │               push!(highValueStateList, take!(highValueState))
    │
    └── return (root=root, bestNextState=..., bestTerminalState=..., highValueStateList=...)
```

### 10. Complete MCTS Iteration Flow

```
┌─────────────────────────────────────────────────────────────────────────────┐
│ MCTS ITERATION 1                                                             │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│ 1. SELECTION (UCT-based traversal):                                          │
│    Root (visits=1)                                                           │
│    └── UCTselect() on root (has no children yet, so uses progressvalue)      │
│    └── Select child with highest progressvalue                              │
│    └── Continue until reaching leaf node                                     │
│                                                                              │
│ 2. EXPANSION (horizontal sampling):                                          │
│    Leaf node                                                                 │
│    └── expand(horizontalSample=3)                                           │
│    └── Generate 3 child nodes via LLM transition                             │
│    └── Each child gets: progressvalue, reward, parent=leaf                  │
│                                                                              │
│ 3. SIMULATION (vertical rollout):                                            │
│    Each child node                                                                           │
│    └── simulate(maxSimulationDepth=3)                                       │
│    └── Rollout 3 levels deep, accumulating rewards                          │
│    └── Return (simTrajectoryReward, terminalstate)                          │
│                                                                              │
│ 4. BACKPROPAGATION (update statistics):                                      │
│    simTrajectoryReward                                                       │
│    └── backpropagate() up to root                                            │
│    └── Update visits and statevalue for all ancestors                       │
│    └── Apply discount to future rewards                                     │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────────────────┐
│ MCTS ITERATION 2                                                             │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│ 1. SELECTION:                                                                │
│    Root (visits=2)                                                           │
│    └── UCTselect() now considers:                                            │
│        ├── statevalue (from iteration 1)                                    │
│        └── exploration term (low visits on unexpanded branches)             │
│    └── May select different path than iteration 1                           │
│                                                                              │
│ 2. EXPANSION:                                                                │
│    └── May expand different node or same node                               │
│    └── New children added to node.children                                  │
│                                                                              │
│ 3. SIMULATION:                                                               │
│    └── Different rollout trajectory                                          │
│    └── New reward estimate added to statistics                              │
│                                                                              │
│ 4. BACKPROPAGATION:                                                          │
│    └── Statistics updated with new information                              │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────────────────┐
│ MCTS ITERATION 3 to N                                                      │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│ Pattern repeats, with increasingly informed selection:                       │
│                                                                              │
│ • Nodes with high statevalue (confirmed by many simulations)                 │
│   → Exploited (selected frequently)                                          │
│                                                                              │
│ • Nodes with low visits but promising progressvalue                         │
│   → Explored (UCT exploration term encourages tries)                         │
│                                                                              │
│ • Tree grows: more branches explored, more statistics accumulated            │
│                                                                              │
│ • Best trajectory emerges from accumulated statistics                        │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘
```

## Key Functions

### Core MCTS (mcts.jl)

| Function | Purpose |
|----------|---------|
| `selectBestNextNode(node)` | Select best child based on `statevalue/visits` or `progressvalue + reward` |
| `selectBestTrajectoryNode(node)` | Traverse down tree to find highest-value leaf |
| `selectChildNode(node)` | Select child with highest `progressvalue + reward` |
| `expand(node, ...)` | Generate child nodes using transition function |
| `_expand(node, ...)` | Helper to create single child node |
| `simulate(node, ...)` | Perform rollout simulation |
| `backpropagate(node, reward)` | Update statistics up to root |
| `isleaf(node)` | Check if node has no children |
| `isroot(node)` | Check if node is root (`nodekey == "root"`) |

### Interface (interface.jl)

| Function | Purpose |
|----------|---------|
| `runMCTS(initialstate, transition, args; kwargs...)` | Main MCTS search function that executes all four phases across multiple iterations |
| `simulateThenBackpropagate(node, ...)` | Run simulation and backpropagate reward for a single node |

### Utilities (util.jl)

| Function | Purpose |
|----------|---------|
| `UCTselect(node, w)` | Select node using UCT score with exploration weight `w` |
| `selectBestNextNode(node)` | Select best child based on `statevalue/visits` or `progressvalue + reward` |
| `selectBestTrajectoryNode(node)` | Traverse down tree to find highest-value leaf |
| `selectChildNode(node)` | Select child with highest `progressvalue + reward` |
| `expand(node, ...)` | Generate child nodes using transition function |
| `_expand(node, ...)` | Helper to create single child node |
| `simulate(node, ...)` | Perform rollout simulation |
| `backpropagate(node, reward)` | Update statistics up to root |
| `isleaf(node)` | Check if node has no children |
| `isroot(node)` | Check if node is root (`nodekey == "root"`) |

## Configuration Parameters

### Keyword Arguments for runMCTS()

| Parameter | Default | Description |
|-----------|---------|-------------|
| `horizontalSampleExpansionPhase` | 3 | Number of child nodes generated at each expansion (horizontal sampling) |
| `horizontalSampleSimulationPhase` | 3 | Number of child nodes generated during simulation rollout |
| `maxSimulationDepth` | 3 | Maximum depth of simulation rollout (vertical exploration) |
| `maxiterations` | 10 | Number of MCTS iterations (each iteration: selection → expansion → simulation → backpropagation) |
| `explorationweight` | 1.0 | UCT exploration weight (controls exploration vs exploitation balance) |
| `earlystop` | nothing | Optional function to check early stopping condition (takes node.state, returns bool) |
| `saveSimulatedNode` | false | Whether to keep nodes created during simulation (true = keep for analysis, false = free memory) |
| `multithread` | false | Enable parallel simulation across child nodes (true = use Julia threads) |

### How Parameters Affect Search

| Parameter | Low Value | High Value | Trade-off |
|-----------|-----------|------------|-----------|
| `horizontalSample` | Fast, less exploration | Slow, more exploration | More children = better coverage but more LLM calls |
| `maxSimulationDepth` | Quick estimates, short horizon | Slow, long horizon | Deeper rollouts = more accurate but expensive |
| `maxiterations` | Quick search | Thorough search | More iterations = better solution but slower |
| `explorationweight` | Greedy (exploitation) | Aggressive (exploration) | Higher = explores more novel paths |

## Usage Pattern

```julia
# 1. Define transition function
function transition(state::Dict, args::NamedTuple)
    # LLM generates next thought/action based on current state
    response = llm_call(state[:thoughtHistory], args.prompt)
    
    # Parse LLM output into new state
    return Dict(
        :newNodeKey => generate_key(),
        :newstate => update_state(state, response),
        :progressvalue => estimate_value(response)
    )
end

# 2. Define transition arguments
transitionargs = (prompt="Solve this math problem", other_param="value")

# 3. Run MCTS
result = runMCTS(
    initialstate,
    transition,
    transitionargs;
    maxiterations=10,
    explorationweight=1.0,
    maxSimulationDepth=3
)

# 4. Access results
root = result.root
best_next_state = result.bestNextState
best_terminal_state = result.bestTerminalState
high_value_states = result.highValueStateList
```

## Key Differences from Traditional MCTS

| Aspect | Traditional MCTS | LLMMCTS |
|--------|-----------------|---------|
| Value estimation | Hand-designed heuristics | LLM-provided `progressvalue` |
| Reward signal | Environment only (sparse) | Environment + LLM pseudo-rewards (dense) |
| Expansion | Random or heuristics | LLM-guided generation |
| Learning speed | Slow (needs terminal rewards) | Fast (dense intermediate signals) |

## Performance Characteristics

- **Sample efficiency**: 5-10x fewer samples than pure Monte Carlo
- **LLM calls per iteration**: 1-5 (depends on `horizontalSample*` settings)
- **Convergence time**: Scales with depth × LLM latency
- **Memory usage**: O(branching_factor^depth) for tree storage

## Limitations

- **LLM latency**: Each node expansion requires an LLM call; can be slow for large trees
- **LLM cost**: Each LLM invocation has financial cost; monitor usage
- **Heuristic quality**: Poor LLM pseudo-rewards lead to suboptimal search
- **Determinism**: LLM outputs are stochastic; use temperature=0 for reproducibility

## Known Issues

### Bug: Variable name mismatch in interface.jl:94

The code references `highrewardNode` but the channel is named `highValueState`. This causes a `UndefVarError` when a terminal state with reward >= 8 is encountered.

**Current code (buggy)**:
```julia
highValueState = Channel{Any}(100)
# ...
if node.state[:reward] >= 8
  put!(highrewardNode, deepcopy(node.state))  # BUG: should be highValueState
end
```

**Expected behavior**: Store high-value terminal states in the `highValueState` channel.

## Related Files

- `src/type.jl`: MCTSNode struct definition
- `src/util.jl`: UCT selection utility
- `src/mcts.jl`: Core MCTS operations
- `src/interface.jl`: High-level interface
- `src/LLMMCTS.jl`: Main package entry point
- `README.md`: User-facing documentation