; Calculating the day of the week for a given date
; ================================================
; mdfs.net/Info/Comp/PDP11/ProgTips/Date/DayOfWeek - J.G.Harston
;
; Based on code at http://6502.org/source/misc/dow.htm by Paul Guertin.
;
; This routine works for any date from 1900-01-01 to 2199-12-31.
; No range checking is done, so validate input before calling.
;
; It uses the formula
;     Weekday = (day + offset[month] + year + year/4 + fudge) mod 7
;       offset[month] adjusts the day count so 1st of a month is effectively
;         the (lastday+1)-th of the previous month.
;       fudge is -1 after 2099 as 2100 is not a leap year.
;
; On entry  r0=day, 1..31
;           r1=month, 1..12
;           r2=year-1900, 0..255
; On exit   r0=day of week 0..6 for Sun..Sat
;           Needs incrementing with INC R0 after calling to become
;           standard 1..7 range
; Size      56 bytes
;
org  &8000
.DayOfWeek
cmp  r1,#3              ; Year starts in March to make leap day the last
sbc  r2                 ; If Jan or Feb, decrement year
bpl  dowMarch           ; Not gone negative, not 1900
mov  #6,r2              ; 1900 is not a leap year, adjust base
.dowMarch
cmp  #2099-1900,r2
sbc  r0                 ; 2100 is not a leap year, so reduce day count if after 2099
movb dowMonths-1(r1),r1 ; R1=offset[month] (nb, not position-independent instruction)
add  r1,r0              ; R0=day+offset[month]
add  r2,r0              ; R0=day+offset[month]+year
asr  r2
asr  r2                 ; R2=year DIV 4
add  r2,r0              ; R0=day+offset[month]+year+(year DIV 4)
.dowMod7
sub  #7,r0
bcc  dowMod7            ; Reduce R0 to R0 MOD 7
add  #7,r0              ; Balance final sub #7, replace with add #8,r0 to return 1-7
rts  pc
.dowMonths
equb 1,4,3,6,1,4,6,2,5,0,3,5  ; Month offsets
;
; You can test this with:
;
; FOR C%=1 TO 255
; FOR B%=1 TO 12
; FOR A%=1 TO 31
;  PRINT A%;"/";B%;"/";1900+C%;" ";
;  PRINT MID$("SunMonTueWedThuFriSat",((USR DayOfWeek) AND &FF)*3+1,3)
; NEXT A%:NEXT B%:NEXT C%
;