5. Fast-forward heuristic and action landmarks

In this tutorial, we will compute the fast-forward heuristic based on the best supporters of facts. Let $\Pi=\langle F,A,s_0,g\rangle$ be a STRIPS task and $s\subseteq F$ a state. When computing $h^{max}(s)$, we must compute the fixed point $\boldsymbol{s}^*$ of $\Gamma_{max}$. Recall that the best supporter $bs(p)$ of a fact $p\not\in s$ is the last action $a\in A$ responsible for the value $\boldsymbol{s}^*(p)<\infty$. If $p\in s$ or $\boldsymbol{s}^*(p)=\infty$, the best supporter is undefined which we denote as $bs(p)=\bot$.

Exercise 1: Consider the following STRIPS task $\Pi=\langle F,A,s_0,g\rangle$ where

  • $F=\{a,b,c,d,e\}$,
  • $s_0 = \{a\}$,
  • $g = \{b,e\}$,
  • $A = \{o_1,o_2,o_3,o_4\}$ defined in the table below:
action pre add cost
$o_1$ $\emptyset$ $\{b,c\}$ $4$
$o_2$ $\{a\}$ $\{c\}$ $2$
$o_3$ $\{a,c\}$ $\{d\}$ $3$
$o_4$ $\{c,d\}$ $\{e\}$ $1$

Compute the perfect heuristic $h^{ff}$ for the initial state, i.e., $h^{ff}(s_0)$.

Solution

Exercise 2: Consider the following STRIPS task $\Pi=\langle F,A,s_0,g\rangle$ where

  • $F=\{a,b,c,d,e\}$,
  • $s_0 = \{a\}$,
  • $g = \{d,e\}$,
  • $A = \{o_1,o_2,o_3,o_4,o_5\}$ defined in the table below:
action pre add cost
$o_1$ $\emptyset$ $\{b\}$ $2$
$o_2$ $\{a\}$ $\{b,c\}$ $4$
$o_3$ $\{a,b,c\}$ $\{d,e\}$ $3$
$o_4$ $\{b,c\}$ $\{e\}$ $2$
$o_5$ $\{e\}$ $\{b,c\}$ $2$

Compute $h^{ff}(s_0)$.

Solution

Exercise 3: Consider the STRIPS task $\Pi=\langle F,A,s_0,g\rangle$ where

  • $F = \{a,b,c,d,e\}$,
  • $s_0 = \{a\}$,
  • $g=\{c,d,e\}$,
  • $A=\{o_1,o_2,o_3,o_4\}$
action pre add cost
$o_1$ $\{a\}$ $\{b,c\}$ $1$
$o_2$ $\{a\}$ $\{d\}$ $2$
$o_3$ $\{b\}$ $\{c,d\}$ $3$
$o_4$ $\{a,b\}$ $\{e\}$ $4$

Find several action landmarks for $s_0$ induced by an $s$-$G$-cut and compute its elementary landmark heuristic at $s_0$.

Solution

Exercise 4: For the action landmarks from the previous exercise, compute the saturated cost partitioning of the cost function and evaluate the cost partitioned heuristic at $s_0$.

Solution

courses/pui/tutorials/tutorial05.txt · Last modified: 2024/03/12 16:21 by xhorcik