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        A[val] = i+1;
        V[i] = val;
      }
      if (i >= plen && perm_to_num(plen, V, &num))
        XSRETURN_UV(num);
    }
    DISPATCHPP();
    objectify_result(aTHX_ svp, ST(0));
    XSRETURN(1);

void randperm(IN UV n, IN UV k = 0)
  PREINIT:
    UV i, *S;
    dMY_CXT;
  PPCODE:
    if (items == 1) k = n;
    if (k > n) k = n;
    if (k == 0) XSRETURN_EMPTY;
    New(0, S, k, UV);
    randperm(MY_CXT.randcxt, n, k, S);
    EXTEND(SP, (EXTEND_TYPE)k);
    for (i = 0; i < k; i++) {
      if (n < 2*CINTS)  PUSH_NPARITY(S[i]);
      else              PUSHs(sv_2mortal(newSVuv(S[i])));
    }
    Safefree(S);

void shuffle(...)
  PROTOTYPE: @
  PREINIT:
    SSize_t i, j;
    void* randcxt;
    dMY_CXT;
  PPCODE:
    if (items == 0)
      XSRETURN_EMPTY;
    for (i = 0, randcxt = MY_CXT.randcxt; i < items-1; i++) {
      j = urandomm64(randcxt, items-i);
      { SV* t = ST(i); ST(i) = ST(i+j); ST(i+j) = t; }
    }
    XSRETURN(items);

void vecsample(IN SV* svk, ...)
  PROTOTYPE: $@
  PREINIT:
    void   *randcxt;
    UV      k;
    Size_t  nitems, i;
    dMY_CXT;
  PPCODE:
    if (items == 1)
      XSRETURN_EMPTY;
    randcxt = MY_CXT.randcxt;
    /*
     * Fisher-Yates shuffle with first 'k' selections returned.
     *
     * There is only one algorithm here, no shortcuts other than
     * detecting an empty list.
     *
     * With a list input, the input is on the stack ST(1),ST(2),...
     * We move the last item to ST(0) then shuffle 'k' iterations.
     *
     * With an array reference input, we cannot modify the input at all.
     * We create an index array and shuffle using that.  Remembering to
     * act like the last item is at the front so we match the list results.
     * We optimize by pushing each selection onto the return stack as
     * we find it rather than pushing them all at the end with another loop.
     */
    if (items > 2 || !SvROK(ST(1)) || SvTYPE(SvRV(ST(1))) != SVt_PVAV) {
      /* Standard form, where we are given an array of items */
      nitems = items-1;
      if (_validate_and_set(&k, aTHX_ svk, IFLAG_POS) == 0 || k > nitems)
        k = nitems;
      ST(0) = ST(items-1); /* Move last value to the first stack entry. */
      for (i = 0; i < k; i++) {
        uint32_t j = urandomm32(randcxt, nitems-i);
        { SV* t = ST(i); ST(i) = ST(i+j); ST(i+j) = t; }
      }
    } else { /* We are given a single array reference.  Select from it. */
      DECL_ARREF(avp);
      USE_ARREF(avp, ST(1), SUBNAME, AR_READ);
      nitems = len_avp;

      if (_validate_and_set(&k, aTHX_ svk, IFLAG_POS) == 0 || k > nitems)
        k = nitems;
      if (k == 0)
        XSRETURN_EMPTY;
      if (nitems < 65536) {
        uint16_t *I;
        New(0, I, nitems, uint16_t);
        I[0] = nitems-1;  for (i = 1; i < nitems; i++)  I[i] = i-1;
        EXTEND(SP, (EXTEND_TYPE)k);
        for (i = 0; i < k; i++) {
          uint32_t j = urandomm32(randcxt, nitems-i);
          uint16_t t = I[i+j];  I[i+j] = I[i];
          PUSHs(FETCH_ARREF(avp,t));
        }
        Safefree(I);
      } else {
        size_t *I;
        New(0, I, nitems, size_t);
        I[0] = nitems-1;  for (i = 1; i < nitems; i++)  I[i] = i-1;
        EXTEND(SP, (EXTEND_TYPE)k);
        for (i = 0; i < k; i++) {
          size_t j = urandomm64(randcxt, nitems-i);
          size_t t = I[i+j];  I[i+j] = I[i];
          PUSHs(FETCH_ARREF(avp,t));
        }
        Safefree(I);
      }
    }
    XSRETURN(k);

void is_happy(SV* svn, UV base = 10, UV k = 2)
  PREINIT:
    UV n, sum;
    int h, status;
  PPCODE:
    if (base < 2 || base > 36) croak("is_happy: invalid base %"UVuf, base);
    if (k > 10) croak("is_happy: invalid exponent %"UVuf, k);
    status = _validate_and_set(&n, aTHX_ svn, IFLAG_POS);