;*******************************************************************************
; GetWordFromList
; Reads a 16-bit value from a list of values.
;-------------------------------------------------------------------------------
; input a ..... index
; input hl .... pointer to list
;-------------------------------------------------------------------------------
; output hl ... word read from it
;-------------------------------------------------------------------------------
; modifies .... a,de,hl
;*******************************************************************************

GetWordFromList:
    ld      e, a                        ; Extend index to 16-bit
    ld      d, 0
    
    add     hl, de                      ; Add it to the base address twice to
    add     hl, de                        ; compute the pointer to the word
    
    ld      e, (hl)                     ; Read word
    inc     hl
    ld      d, (hl)
    
    ex      de, hl                      ; Return word in HL
    ret                                 ; End of subroutine

;*******************************************************************************
; GetTrackState
; Helper function to convert the track state offset (as in FMCHAN_TRACK and
; PSGCHAN_TRACK) into a full pointer to the track state.
;-------------------------------------------------------------------------------
; input e ..... offset for track state
;-------------------------------------------------------------------------------
; output ix ... pointer to track state
;-------------------------------------------------------------------------------
; modifies .... de, ix
;*******************************************************************************

GetTrackStateAddr:
    ld      d, 0                        ; Extend offset to 16-bit
    ld      ix, TrackData               ; Base address for tracks
    add     ix, de                      ; Compute pointer
    ret                                 ; End of subroutine

;*******************************************************************************
; GetTrackInstrument
; Gets a pointer to the current instrument for a track.
;-------------------------------------------------------------------------------
; input ix .... pointer to track state
;-------------------------------------------------------------------------------
; output de ... pointer to instrument data
;-------------------------------------------------------------------------------
; modifies .... a,de,hl
;*******************************************************************************

GetTrackInstrument:
    ld      a, (ix+TRACK_INSTRUMENT)    ; Get current instrument ID
    
    or      a                           ; Instrument ID 0 is reserved for the
    jr      z, GetNullInstrument          ; "null" instrument (used to stop a
                                          ; channel)
    
    dec     a                           ; Instrument list starts at ID = 1
    ld      hl, (InstrumentList)        ; Pointer to instrument list
    call    GetWordFromList             ; Retrieve pointer to instrument
    ex      de, hl                      ; Return pointer in DE
    
    ret                                 ; End of subroutine

GetNullInstrument:
    ld      de, NullInstrument          ; ID 0 = null instrument
    ret                                 ; End of subroutine

;*******************************************************************************
; GetTrackPitch
; Gets the actual pitch of a track (with transpose applied), and splits it into
; octave and semitone. Used by the code that deals with the hardware.
;-------------------------------------------------------------------------------
; input ix .... pointer to track state
;-------------------------------------------------------------------------------
; output a .... semitone
; output c .... octave
;-------------------------------------------------------------------------------
; modifies .... a,bc
;*******************************************************************************

GetTrackPitch:
    ld      a, (ix+TRACK_PITCH)         ; Get current pitch and add the
    add     a, (ix+TRACK_TRANSPOSE)       ; transpose to it
    
    ld      bc, $0CFF                   ; Divide the pitch by 12 in order to
SplitPitch:                               ; get the semitones (remaineder) and
    inc     c                             ; octave (quotient)
    sub     b
    jr      nc, SplitPitch
    add     b
    
    ret                                 ; End of subroutine