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cc65/libsrc/common/strcspn.s
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ArmAsm

;
; Ullrich von Bassewitz, 11.06.1998
;
; size_t strcspn (const char* s1, const char* s2);
;
.export _strcspn
.import popax
.importzp ptr1, ptr2, tmp1, tmp2, tmp3
_strcspn:
sta ptr2 ; Save s2
stx ptr2+1
jsr popax ; Get s1
sta ptr1
stx ptr1+1
ldx #0 ; low counter byte
stx tmp1 ; high counter byte
ldy #$00
L1: lda (ptr1),y ; get next char from s1
beq L6 ; jump if done
sta tmp2 ; save char
iny
bne L2
inc ptr1+1
L2: sty tmp3 ; save index into s1
ldy #0 ; get index into s2
L3: lda (ptr2),y ;
beq L4 ; jump if done
cmp tmp2
beq L6
iny
bne L3
; The character was not found in s2. Increment the counter and start over
L4: ldy tmp3 ; reload index
inx
bne L1
inc tmp1
bne L1
; The character was found, or we reached the end of s1. Return count of
; characters
L6: txa ; get low counter byte
ldx tmp1 ; get high counter byte
rts