;********************************************************************************
; @module ringbuffer
; @type utility
; @details
;   Size of the ringbuffer is RBUF_MEM_END - RBUF_MEM_START - 2, since two bytes
;   are used by the read and write pointer
;   The RBUF_NAME variable must be defined, the functions will then be exported
;   as rb_<RBUF_NAME>_<function> where <function> = init, read or write
; @requires
;   RBUF_MEM_START: First address of ringbuffer memory space
;   RBUF_MEM_END: Last address of ringbuffer memory space
;   RBUF_NAME: Name of the ringbuffer
;********************************************************************************
.code

.ifndef RBUF_MEM_START
    .fatal "RBUF_MEM_START not defined"
.endif
.ifndef RBUF_MEM_END
    .fatal "RBUF_MEM_END not defined"
.endif

; can not detect if RBUF_NAME is defined, if it is not you will get 
; "String constant expected" Error on the line below
.define _RBUF_NAME .concat("rb_", RBUF_NAME)
RB_WRITE    = RBUF_MEM_START        ; write pointer, relative to RB_WRITE
RB_READ     = RBUF_MEM_START + 1    ; read ponter, relative to RB_START
RB_START    = RBUF_MEM_START + 2
RB_LENGTH   = RBUF_MEM_END - RBUF_MEM_START - 2

.if RB_LENGTH < 1
    .fatal "buffer size too small, must be at least 1"
.endif
.if RB_LENGTH > $ff
    .fatal "buffer size too large, must be <= $ff"
.endif


;********************************************************************************
; @function Initialize the buffer
;********************************************************************************
.ident(.concat(_RBUF_NAME, "_init")):
.scope
    stz RB_WRITE
    stz RB_READ
    rts
.endscope

;********************************************************************************
; @function Read a value from the buffer
; @details
;   If there is no value to be read, the Pz will be set
; @returns A: value
; @modifies: A, X
;********************************************************************************
.ident(.concat(_RBUF_NAME, "_read")):
.scope
    ldx RB_READ
    cpx RB_WRITE
    beq @rb_read_rts  ; if buffer empty
    lda RB_START,x
    inx ; increment RB_READ pointer, not using macro bec. of unknown Pz
    cpx #RB_LENGTH
    beq @read_wrap
    stx RB_READ 
@rb_read_rts:
    rts
@read_wrap:  ; ptr == RB_LENGTH -> ptr = 0
    stz RB_READ
    ; make sure Pz is not set
    ldx #$01
    rts
.endscope

;********************************************************************************
; @function Write a value to the buffer
; @param A: value to store
; @modifies: X
;********************************************************************************
.ident(.concat(_RBUF_NAME, "_write")):
.scope
    ; lda kp_VALUES, x  ; load the char in a
    ldx RB_WRITE
    sta RB_START,x
    inx  ; increment write pointer
    cpx #RB_LENGTH
    beq @write_wrap
    stx RB_WRITE
@check_buf_full:  ; increment read if buffer is full
    cpx RB_READ
    beq @read_inc
    rts
@write_wrap:  ; ptr == RB_LENGTH -> ptr = 0
    stz RB_WRITE
    ldx #0
    bra @check_buf_full
@read_inc:
    ldx RB_READ
    inx
    cpx #RB_LENGTH
    beq @read_wrap
    stx RB_READ
    rts
@read_wrap:  ; ptr == RB_LENGTH -> ptr = 0
    stz RB_READ
    rts
.endscope

.undefine _RBUF_NAME
.undefine RBUF_NAME