kvisc/ka/crt/str/strrev.k

; The OS/K Team licenses this file to you under the MIT license.
; See the LICENSE file in the project root for more information.

;
; void strrev(char *buf, const char *str)
;
; buf and src must NOT overlap
;
strrev:
    cmp b[ax1], zero
    mov.z b[ax0], zero
    ret.z

    ; save str's location
    mov r10, ax1

    ; go to str's end, just before
    ; the null terminator
    mov rcx, STRLEN_MAX
    scasb.rep.nz ax1, zero
    sub ax1, ax1, 1

.2:
    ; copy, going backward though str
    ; and forward through buf
    mov rax, b[ax1]
    mov b[ax0], rax

    cmp ax1, r10
    mov.z b[ax0+1], zero
    ret.z

    add ax0, ax0, 1
    sub ax1, ax1, 1
    jmp .2


;
; void strrev2(char *str)
;
; Inverses str
;
strrev2:
    cmp b[ax0], zero
    ret.z

    mov ax1, ax0

    ; go to str's end, just before
    ; the null terminator
    mov rcx, STRLEN_MAX
    scasb.rep.nz ax1, zero
    sub ax1, ax1, 1

    ; increase ax0 while decreasing ax1, performing exchanges
.2:
    b.ae ax0, ax1, .3

    mov rax, b[ax1]
    xchg rax, b[ax0]
    mov b[ax1], rax

    add ax0, ax0, 1
    sub ax1, ax1, 1
    jmp .2

.3:
    ret