!to "test.prg", cbm
        !convtab pet
        !sl "test.labels"

        LOWER_LIMIT = 10102
        UPPER_LIMIT = 10901

        ptr1    = $9e
        tmp1    = $02
        
        strout  = $ab1e
        intout  = $bdcd
        chrout  = $ffd2

        *= $0801

basic_header
        !word .next_line
        !word 2017      ; BASIC line number.
;        !pet "ti$", $b2, 34, "000000", 34, ":" ; Set clock to 00:00:00.
        !byte $9e       ; SYS token.
        !byte "0" + (start / 1000), "0" + ((start / 100) % 10)
        !byte "0" + ((start / 10) % 10), "0" + (start % 10)
;        !pet ":", $99, "ti$" ; PRINT clock value.
        !byte 0         ; BASIC line end marker.
.next_line
        !word 0

;--------------------------------------
!zone
start:
        lda #<init_sieve_txt
        ldy #>init_sieve_txt
        jsr strout
        jsr init_sieve

        lda #<do_sieve_txt
        ldy #>do_sieve_txt
        jsr strout
        jsr do_sieve

        lda #<search_txt
        ldy #>search_txt
        jsr strout

        lda #<LOWER_LIMIT
        sta next_prime_index
        lda #>LOWER_LIMIT
        sta next_prime_index+1

        jsr next_prime
        bcc +
        brk

+       sta .prime_1
        stx .prime_1+1

.loop:  jsr next_prime
        bcs .loop_exit

        sta .prime_2
        stx .prime_2+1

        sec
        lda .prime_2
        sbc .prime_1
        tax
        lda .prime_2+1
        sbc .prime_1+1
        bne +
        cpx #2
        bne +

        lda .prime_1+1
        ldx .prime_1
        jsr intout
        lda #' '
        jsr chrout
        lda .prime_2+1
        ldx .prime_2
        jsr intout
        lda #13
        jsr chrout

+       lda .prime_2
        sta .prime_1
        lda .prime_2+1
        sta .prime_1+1

        jmp .loop
.loop_exit:

        rts

.prime_1:
        !word 0
.prime_2:
        !word 0
;--------------------------------------
!zone
init_sieve:
        lda #<sieve
        sta ptr1
        lda #>sieve
        sta ptr1+1

        lda #1
        ldy #0
        ldx #>UPPER_LIMIT
-       sta (ptr1),y
        iny
        bne -
        inc ptr1+1
        dex
        bne -

        ldy #<UPPER_LIMIT
-       sta (ptr1),y
        dey
        bne -

        lda #0
        sta sieve
        sta sieve+1

        rts
;--------------------------------------
!zone
do_sieve:
        lda #<UPPER_LIMIT
        ldx #>UPPER_LIMIT
        jsr sqrt
        sta .loop_upper
        stx .loop_upper+1

        lda #2
        sta .number
        lda #0
        sta .number+1

.outer_loop:
        lda .loop_upper
        cmp .number
        lda .loop_upper+1
        sbc .number+1
        bcc .outer_loop_exit

        clc
        lda #<sieve
        adc .number
        sta ptr1
        lda #>sieve
        adc .number+1
        sta ptr1+1

        ldy #0
        lda (ptr1),y
        beq .skip_inner_loop

        lda .number
        asl
        sta .not_prime
        lda .number+1
        rol
        sta .not_prime+1

.inner_loop:
        lda #<UPPER_LIMIT
        cmp .not_prime
        lda #>UPPER_LIMIT
        sbc .not_prime+1
        bcc .skip_inner_loop

        clc
        lda #<sieve
        adc .not_prime
        sta ptr1
        lda #>sieve
        adc .not_prime+1
        sta ptr1+1
        tya
        sta (ptr1),y

        clc
        lda .not_prime
        adc .number
        sta .not_prime
        lda .not_prime+1
        adc .number+1
        sta .not_prime+1
        bcc .inner_loop

.skip_inner_loop
        inc .number
        bne .outer_loop
        inc .number+1
        bne .outer_loop
.outer_loop_exit:

        rts

.number:        !word 0
.loop_upper:    !word 0
.not_prime:     !word 0

;--------------------------------------
!zone
sqrt:
        sta .n
        stx .n+1

        lda #0
        sta .result
        sta .result+1

        sta .one
        lda #%01000000
        sta .one+1

-       lda .one
        sec
        sbc .n
        sta tmp1
        lda .one+1
        sbc .n+1
        bcs +
        ora tmp1
        beq +

        lsr .one+1
        ror .one
        lsr .one+1
        ror .one
        jmp -
+

.loop:  lda .one
        ora .one+1
        beq .loop_exit

        clc
        lda .result
        adc .one
        sta .tmp
        lda .result+1
        adc .one+1
        sta .tmp+1

        lda .n
        cmp .tmp
        lda .n+1
        sbc .tmp+1
        bcc .skip

        sec
        lda .n
        sbc .tmp
        sta .n
        lda .n+1
        sbc .tmp+1
        sta .n+1

        lda .one
        asl
        sta .tmp
        lda .one+1
        rol
        sta .tmp+1

        clc
        lda .tmp
        adc .result
        sta .result
        lda .tmp+1
        adc .result+1
        sta .result+1
.skip:
        lsr .result+1
        ror .result

        lsr .one+1
        ror .one
        lsr .one+1
        ror .one

        jmp .loop

.loop_exit:
        lda .result
        ldx .result+1
        rts

.n:     !word 0
.one:   !word 0
.tmp:   !word 0
.result:        !word 0

;--------------------------------------
!zone
next_prime:
        lda next_prime_index
        cmp #<UPPER_LIMIT
        lda next_prime_index+1
        sbc #>UPPER_LIMIT
        bcs .no_next_prime

        clc
        lda #<sieve
        adc next_prime_index
        sta ptr1
        lda #>sieve+1
        adc next_prime_index+1
        sta ptr1+1

        inc next_prime_index
        bne +
        inc next_prime_index+1
+
        ldy #0
        lda (ptr1),y
        beq next_prime

.prime_found:
        sec
        lda next_prime_index
        sbc #1
        ldx next_prime_index+1
        bcs +
        dex
+
        clc

.no_next_prime:
        rts

;--------------------------------------
init_sieve_txt:
        !pet "initialize sieve...",13,0
do_sieve_txt:
        !pet "do sieve...",13,0
search_txt:
        !pet "search twin primes...",13,0


next_prime_index:
        !word 0
sieve:
        ;!fill UPPER_LIMIT