Definitions

ACTION

The process by which some STATE is transformed into one of its SUCCESSOR STATEs. The application of an ACTION to a particular STATE MUST yield a single SUCCESSOR STATE.

ADMISSIBLE

A HEURISTIC is ADMISSIBLE if and only if it underestimates the distance from the current state to the GOAL STATE. That is, $h(s)$ is ADMISSIBLE iff $h(s)\leq g(\mathrm{goal})-g(s)\ \forall s$.

CHILD STATE

See, SUCCESSOR STATE.

CLOSED LIST

Set of all STATEs that have been fully examined (i.e., had their SUCCESSORs generated).

CONSISTENT

See, MONOTONIC.

GOAL STATE

The terminal STATE of a PUZZLE. When the shortest sequence of MOVEs between the START STATE and some GOAL STATE is found, the PUZZLE has been solved.

HEURISTIC

A function, denoted $h$, that estimates the remaining cost from the current state to the goal state. Important special cases are ADMISSIBLE and MONOTONIC HEURISTICS.

MAY

A requirement that the product can choose to implement if desired. Can also indicate a choice among acceptable alternatives (e.g., ``The program MAY do x, y, or z.'' indicates that the choice of behavior x, y, or z is up to the designer.)

MONOTONIC

Also known as CONSISTENT. A heuristic is monotonic if and only if it underestimates the cost from every state to each of their SUCCESSOR STATES. That is, $h()$ is MONOTONIC iff $h(s)\leq c(s,a,s')+h(s')$ for any STATE $s$ having SUCCESSOR STATE $s'$ under ACTION $a$.

MOVE

An operation applied to a STATE to produce another STATE.

MUST

A requirement that the product must implement for full credit.

MUST NOT

A behavior or assumption that must not be violated. Violating a MUST NOT restriction will result in a penalty on the assignment.

NODE

See, STATE.

OPEN LIST

Set of STATEs that have been generated (as children of some other node), but not yet examined.

PARENT STATE

See, PREDECESSOR STATE.

PREDECESSOR STATE

Also called a PARENT STATE. The STATE to which a MOVE is applied to generate a SUCCESSOR STATE.

PUZZLE

A single-player game with a defined START STATE and one or more GOAL STATEs. From any STATE one or more MOVEs are possible, which transform the STATE into a SUCCESSOR STATE. The object is to find the shortest sequence of MOVES which transform the START STATE to the GOAL STATE.

RECOVERABLE ERROR

An error condition that the software can ignore, correct, or otherwise recover from. The program MUST produce a warning message and then cleanly continue with no corruption or loss of valid data.

START STATE

The initial configuration of a PUZZLE.

STATE

A possible configuration of a PUZZLE. Also known as a NODE, when conceived of as an element of the search graph.

SYNTAX ERROR

A discrepancy between the input file and the grammar given in Section 3.2.

SUCCESSOR STATE

Also called CHILD STATE. A STATE generated by the application of a MOVE to a PREDECESSOR (or PARENT) STATE.

UNRECOVERABLE ERROR

An error condition from which recovery is impossible. The program MUST produce an error message describing the condition and then cleanly halt.