rockbox input handling using a "regex"

regex.txt

specification

- press UP and DOWN = scroll up or down
- short press of PLAY = play
- hold PLAY and press UP or DOWN = skip tracks

implementation

STATE1

  Press(UP)   -> Release(UP)   -> fire up, goto STATE1
  Press(DOWN) -> Release(DOWN) -> fire down, goto STATE1

  Press(PLAY) -> Release(PLAY) -> fire play, goto STATE1
              -> Timeout(hold_time) -> goto STATE2

STATE2

  Release(PLAY) -> goto STATE1
  Press(UP)     -> Release(UP)   -> fire skip prev, goto STATE2
  Press(DOWN)   -> Release(DOWN) -> fire skip next, goto STATE2
  
as a "regular expression"

  Press(UP)   Release(UP)   {fire ACTION_SCROLL_UP}
| Press(DOWN) Release(DOWN) {fire ACTION_SCROLL_DOWN}
| Press(PLAY) ( Timeout(hold_time) ( Press(UP)   Release(UP)   {fire ACTION_SKIP_NEXT}
                                   | Press(DOWN) Release(DOWN) {fire ACTION_SKIP_PREV}
                                   )* 
                                   Release(PLAY)
              | Release(PLAY) {fire ACTION_PLAY}
              )

explanation

Press(X) = matches a key press
Release(x) = matches a key release
Timeout(x) = x time elapses since the previous matched input event
{action} = do the specified action if we get to this state
           this "matches" the empty string
           
also note that there has to be implicit ignoring of non-matching input,
and implicitly the input "regex" runs in a loop.