Objective

Write next_state, the transition function of a download state machine: it returns the state reached from current when e fires, and current unchanged when that pair has no entry in the table.

Steps

• Both enums already sit in your file. state lists IDLE, RUNNING, PAUSED, DONE, FAILED, in that order. event lists START, PAUSE, RESUME, FINISH, ERROR. Grademe compiles its own copy of the two definitions, so renaming or reordering an enumerator breaks the build.
• Six pairs move the machine, and only these six: IDLE + START -> RUNNING, RUNNING + PAUSE -> PAUSED, RUNNING + FINISH -> DONE, RUNNING + ERROR -> FAILED, PAUSED + RESUME -> RUNNING, PAUSED + ERROR -> FAILED.
• Every other pair is an invalid transition. Return current unchanged, with no error code and no output of your own.
• So IDLE + ERROR stays IDLE, since a download that never started has nothing to fail at. PAUSED + FINISH stays PAUSED, since a paused transfer reports no completion. RUNNING + START stays RUNNING.
• IDLE is the initial state and START is the only event it answers.
• DONE and FAILED are absorbing states. Both keep returning themselves, whatever the event: nothing restarts a finished download and nothing revives a failed one.
• An e outside the five enumerators reaches you sooner or later, because an enum in C is an integer type and nothing forbids a caller from writing (event)42. Treat that value like any pair absent from the table and return current.
• Return an enumerator of state. Grademe prints the name of the state you hand back, feeding your function one event at a time and showing the whole path, so a chain of calls reads IDLE -> RUNNING -> PAUSED.

Objective

Write next_state, the transition function of a download state machine: it returns the state reached from current when e fires, and current unchanged when that pair has no entry in the table.

Steps

• Both enums already sit in your file. state lists IDLE, RUNNING, PAUSED, DONE, FAILED, in that order. event lists START, PAUSE, RESUME, FINISH, ERROR. Grademe compiles its own copy of the two definitions, so renaming or reordering an enumerator breaks the build.
• Six pairs move the machine, and only these six: IDLE + START -> RUNNING, RUNNING + PAUSE -> PAUSED, RUNNING + FINISH -> DONE, RUNNING + ERROR -> FAILED, PAUSED + RESUME -> RUNNING, PAUSED + ERROR -> FAILED.
• Every other pair is an invalid transition. Return current unchanged, with no error code and no output of your own.
• So IDLE + ERROR stays IDLE, since a download that never started has nothing to fail at. PAUSED + FINISH stays PAUSED, since a paused transfer reports no completion. RUNNING + START stays RUNNING.
• IDLE is the initial state and START is the only event it answers.
• DONE and FAILED are absorbing states. Both keep returning themselves, whatever the event: nothing restarts a finished download and nothing revives a failed one.
• An e outside the five enumerators reaches you sooner or later, because an enum in C is an integer type and nothing forbids a caller from writing (event)42. Treat that value like any pair absent from the table and return current.
• Return an enumerator of state. Grademe prints the name of the state you hand back, feeding your function one event at a time and showing the whole path, so a chain of calls reads IDLE -> RUNNING -> PAUSED.