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:
    bzr b[ax1], .z

    ; save str's location
    mov rsi, ax1

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

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

    beq ax1, rsi, .r

    inc ax0, 1
    dec ax1, 1
    jmp .l

.r:
    mov b[ax0+1], zero
    ret

.z:
    mov b[ax0], zero
    ret

;
; void strrev2(char *str)
;
; Inverses str
;
strrev2:
    bzr b[ax0], .r

    mov ax1, ax0

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

    ; increase ax0 while decreasing ax1, performing exchanges
.l:
    blteu ax1, ax0, .r

    xchg b[ax0], b[ax1]

    inc ax0, 1
    dec ax1, 1
    jmp .l

.r:
    ret