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Some notes on search based on Berkeley's CS 188 course and "Artificial Intelligence" Russel & Norvig 3rd Edition.

Uninformed Search - R&N 3.1-3.4

problem-solving agents

  • goal - 1st step
    • problem formulation - deciding what action and states to consider given a goal
  • uninformed - given no info about problem besides definition
    • an agent with several immediate options of unknown value can decide what to do first by examining future actions that lead to states of known value
  • 5 components
    1. initial state
    2. actions at each state
    3. transition model
    4. goal states
    5. path cost function

problems

  • toy problems
    1. vacuum world
    2. 8-puzzle (type of sliding-block puzzle)
    3. 8-queens problem
    4. Knuth conjecture
  • real-world problems
    1. route-finding
    2. TSP (and othe touring problems)
    3. VLSI layout
    4. robot navigation
    5. automatic assembly sequencing

searching for solutions

  • start at a node and make a search tree
    • frontier = open list = set of all leaf nodes available for expansion at any given point
    • search strategy determines which state to expand next
  • want to avoid redundant paths
    1. TREE-SEARCH - continuously expand the frontier
    2. GRAPH-SEARCH - tree search but also keep track of previously visited states in explored set = closed set and don’t revisit

infrastructure

  • node - data structure that contains parent, state, path-cost, action
  • metrics
    • complete - terminates in finite steps
    • optimal - finds best solution
    • time/space complexity
      • theoretical CS: $\vert V\vert +\vert E\vert $
      • b - branching factor - max number of branches of any node
      • d - depth - number of steps from the root
      • m - max length of any path in the search space
    • search cost - just time/memory
    • total cost - search cost + path cost

uninformed search = blind search

  • bfs
  • uniform-cost search - always expand node with lowest path cost g(n)
    • frontier is priority queue ordered by g
  • dfs
    • backtracking search - dfs but only one successor is generated at a time; each partially expanded node remembers which succesor to generate next
      • only O(m) memory instead of O(bm)
    • depth-limited search
      • diameter of state space - longest possible distance to goal from any start
    • iterative deepening dfs - like bfs explores entire depth before moving on
      • iterative lengthening search - instead of depth limit has path-cost limit
  • bidirectional search - search from start and goal and see if frontiers intersect
    • just because they intersect doesn’t mean it was the shortest path
    • can be difficult to search backward from goal (ex. N-queens)

A* Search and Heuristics - R&N 3.5-3.6

  • informed search - use path costs $g(n)$ and problem-specific heuristic $h(n)$
    • has evaluation function f incorporating path cost g and heuristic h
    • heuristic h = estimated cost of cheapest path from state at node n to a goal state
  • best-first - choose nodes with best f
    • greedy best-first search - let f = h: keep expanding node closest to goal
    • when f=g, reduces to uniform-cost search
  • $A^*$ search
    • $f(n) = g(n) + h(n)$ represents the estimated cost of the cheapest solution through n
    • $A^$ (with tree search) is optimal and complete if h(n) is *admissible
      • $h(n)$ never overestimates the cost to reach the goal
    • $A^$ (with graph search) is optimal and complete if h(n) is *consistent (stronger than admissible) = monotonicity
      • $h(n) \leq cost(n \to n’) + h(n’)$
      • can draw contours of f (because nondecreasing)
    • $A^$ is also *optimally efficient (guaranteed to expand fewest nodes) for any given consisten heuristic because any algorithm that that expands fewer nodes runs the risk of missing the optimal solution
      • for a heuristic, absolute error $\delta := h^-h$ and *relative error $\epsilon := \delta / h^*$
        • here $h^*$ is actual cost of root to goal
      • bad when lots of solutions with small absolute error because it must try them all
      • bad because it must store all nodes in memory
  • memory-bounded heuristic search
    • iterative-deepening $A^*$ - iterative deepening with cutoff f-cost
    • recursive best-first search - like standard best-first search but with linear space
      • each node keeps f_limit variable which is best alternative path available from any ancestor
      • as it unwinds, each node is replaced with backed-up value - best f-value of its children
        • decides whether it’s worth reexpanding subtree later
        • often flips between different good paths (h is usually less optimistic for nodes close to the goal)
    • $SMA^$ - simplified memory-bounded A - best-first until memory is full then forgot worst leaf node and add new leaf
      • store forgotten leaf node info in its parent
      • on hard problems, too much time switching between nodes
  • agents can also learn to search with metalevel learning

heuristic functions

  • effective branching factor $b^$ - if total nodes generated by A is N and solution depth is d, then b* is branching factor for uniform tree of depth d for N+1 nodes: \(N+1 = 1+b^* +(b^*)^2 + ... + (b^*)^d\)
    • want $b^*$ close to 1
  • generally want bigger heuristic because everything with $f(n) < C^*$ will be expanded
    • $h_1$ dominates $h_2$ if $h_1(n) \geq h_2(n) : \forall : n$
  • relaxed problem - removes constraints and adds edges to the graph
    • solution to original problem still solves relaxed problem
    • cost of optimal solution to a relaxed problem is an admissible heuristic for the original problem
      • also is consistent
  • when there are several good heuristics, pick $h(n) = \max[h_1(n), …, h_m(n)]$ for each node
  • pattern database - heuristic stores exact solution cost for every possible subproblem instance
    • disjoint pattern database - break into independent possible subproblems
  • can learn heuristic by solving lots of problems using useful features
    • aren’t necessarily admissible / consistent

Local Search - R&N 4.1-4.2

  • local search looks for solution not path ~ like optimization
    • maintains only current node and its neighbors

discrete space

  • hill-climbing = greedy local search
    • also stochastic hill climbing and random-restart hill climbing
  • simulated annealing - pick random move
    • if move better, then accept
    • otherwise accept with some probability p’roportional to how bad it is and accept less as time goes on
  • local beam search - pick k starts, then choose the best k states from their neighbors
    • stochastic beam search - pick best k with prob proportional to how good they are
  • genetic algorithms - population of k individuals
    • each scored by fitness function
    • pairs are selected for reproduction using crossover point
    • each location subject to random mutation
    • schema - substring in which some of the positions can be left unspecified (ex. $246**$)
      • want schema to be good representation because chunks tend to be passed on together

continuous space

  • hill-climbing / simulated annealing still work
    • could just discretize neighborhood of each state
  • use gradient
    • if possible, solve $\nabla f = 0$
    • otherwise SGD $x = x + \alpha \nabla f(x)$
      • can estimate gradient by evaluating response to small increments
  • line search - repeatedly double $\alpha$ until f starts to increase again
  • Newton-Raphson method
    • finds roots of func using 1st derive: $x_\text{root} = x - g(x) / g’(x)$
    • apply this on 1st deriv to get minimum
      • $x = x - H_f^{-1} (x) \nabla f(x)$ where H is the Hessian of 2nd derivs

Constraint satisfaction problems - R&N 6.1-6.5

  • CSP
    1. set of variables $X_1, …, X_n$
    2. set of domains $D_1, …, D_n$
    3. set of constraints $C$ specifying allowable values
  • each state is an assignment of variables
    • consistent - doesn’t violate constraints
    • complete - every variable is assigned
  • constraint graph - nodes are variables and links connect any 2 variables that participate in a constraint
    • unary constraint - restricts value of single variable
    • binary constraint
    • global constraint - arbitrary number of variables (doesn’t have to be all)
  • converting graphs to only binary constraints
    • every finite-domain constraint can be reduced to set of binary constraints w/ enough auxiliary variables
    • dual graph transformation - create a new graph with one variable for each constraint in the original graph and one binary constraint for each pair of original constraints that share variables
  • also can have preference constraints instead of absolute constraints

inference (prunes search space before backtracking)

  • node consistency - prune domains violating unary constraints
  • arc consistency - satisfy binary constraints (every node is made arc-consistent with all other nodes)
    • uses AC-3 algorithm
      • set of all arcs = binary constraints
      • pick one and apply it
        • if things changed, re-add all the neighboring arcs to the set
      • $O(cd^3)$ where $d = \vert domain\vert $, c = # arcs
    • variable can be generalized arc consistent
  • path consistency - consider constraints on triplets - PC-2 algorithm
    • extends to k-consistency (although path consistency assumes binary constraint networks)
    • strongly k-consistent - also (k-1) consistent, (k-2) consistent, … 1-consistent
      • implies $O(k^2d)$
      • establishing k-consistency time/space is exponential in k
  • global constraints can have more efficient algorithms
    • ex. assign different colors to everything
    • resource constraint = atmost constraint - sum of variable must not exceed some limit
      • bounds propagation - make sure variables can be allotted to solve resource constraint

backtracking

  • CSPs are commutative - order of choosing states doesn’t matter
  • backtracking search - depth-first search that chooses values for one variable at a time and backtracks when no legal values left
    1. variable and value ordering
      • minimum-remaining-values heuristic - assign variable with fewest choices
      • degree heuristic - pick variable involved in largest number of constraints on other unassigned variables
      • least-constraining-value heuristic - prefers value that rules out fewest choices for nieghboring variables
    2. interleaving search and inference
      • forward checking - when we assign a variable in search, check arc-consistency on its neighbors
      • maintaining arc consistency (MAC) - when we assign a variable, call AC-3, intializing with arcs to neighbors
    3. intelligent backtracking - looking backward
      • keep track of conflict set for each node (list of variable assignments that deleted things from its domain)
      • backjumping - backtracks to most recent assignment in conflict set
      • too simple - forward checking makes this redundant - conflict-directed backjumping
      • let $X_j$ be current variable and $conf(X_j)$ be conflict set. If every possible value for $X_j$ fails, backjump to the most recent variable $X_i$ in $conf(X_j)$ and set $conf(X_i) = conf(X_i) \cup conf(X_j) - X_i$ - constraint learning - findining minimum set of variables/values from conflict set that causes the problem = no-good

local search for csps

  • start with some assignment to variables
  • min-conflicts heuristic - change variable to minimize conflicts
    • can escape plateaus with tabu search - keep small list of visited states
    • could use constraint weighting

structure of problems

  • connected components of constraint graph are independent subproblems
  • tree - any 2 variables are connected by only one path
    • directed arc consistency - ordered variables $X_i$, every $X_i$ is consistent with each $X_j$ for j>i
      • tree with n nodes can be made directed arc-consisten in $O(n)$ steps - $O(nd^2)$
  • two ways to reduce constraint graphs to trees
    1. assign variables so remaining variables form a tree
      • assigned variables called cycle cutset with size c
      • $O[d^c \cdot (n-c) d^2]$
      • finding smallest cutset is hard, but can use approximation called cutset conditioning
    2. tree decomposition - view each subproblem as a mega-variable
      • tree width w - size of largest subproblem - 1
      • solvable in $O(n d^{w+1})$
  • also can look at structure in variable values
    • ex. value symmetry - can assign different colorings
      • use symmetry-breaking constraint - assign colors in alphabetical order