src/appl/pretty.c

/*
 *  R : A Computer Language for Statistical Data Analysis
 *  Copyright (C) 1995-2018  The R Core Team
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, a copy is available at
 *  https://www.R-project.org/Licenses/
 */

/* Pretty Intervals
 * ----------------
 * Constructs m "pretty" values which cover the given interval	*lo <= *up
 *	m ~= *ndiv + 1	(i.e., ndiv := approximate number of INTERVALS)
 *
 * It is not quite clear what should happen for	 *lo = *up;
 * S itself behaves quite funilly, then.
 *
 * In my opinion, a proper 'pretty' should always ensure
 * *lo < *up, and hence *ndiv >=1 in the result.
 * However, in S and here, we allow  *lo == *up, and *ndiv = 0.
 * Note however, that we are NOT COMPATIBLE to S. [Martin M.]
 *
 * NEW (0.63.2): ns, nu are double (==> no danger of integer overflow)
 *
 * We determine
 * if the interval (up - lo) is ``small'' [<==>	 i_small == TRUE, below].
 * For the ``i_small'' situation, there is a parameter  shrink_sml,
 * the factor by which the "scale" is shrunk.		~~~~~~~~~~
 * It is advisable to set it to some (smaller) integer power of 2,
 * since this enables exact floating point division.
 */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#ifdef ENABLE_NLS
#include <libintl.h>
#define _(String) gettext (String)
#else
#define _(String) (String)
#endif

#include <math.h>
#include <float.h>
#include <R_ext/Applic.h>
#include <Rmath.h>
#include <R_ext/Error.h>
#ifdef DEBUGpr
# include <R_ext/Print.h>
#endif

#ifdef HAVE_VISIBILITY_ATTRIBUTE
# define attribute_hidden __attribute__ ((visibility ("hidden")))
#else
# define attribute_hidden
#endif

attribute_hidden
double R_pretty(double *lo, double *up, int *ndiv, int min_n,
		double shrink_sml,
		const double high_u_fact[],
		// (h, h5) = c(high.u.bias, u5.bias) in base::pretty.default()
		int eps_correction, int return_bounds)
{
/* From version 0.65 on, we had rounding_eps := 1e-5, before, r..eps = 0
 * then, 1e-7 was consistent with seq.default() and seq.int() till 2010-02-03,
 * where it was changed to 1e-10 for seq*(), and in 2017-08-14 for pretty(): */
#define rounding_eps 1e-10

#define h  high_u_fact[0]
#define h5 high_u_fact[1]

    double dx, cell, unit, base, U;
    double ns, nu;
    int k;
    Rboolean i_small;

    dx = *up - *lo;
    /* cell := "scale"	here */
    if(dx == 0 && *up == 0) { /*  up == lo == 0	 */
	cell = 1;
	i_small = TRUE;
    } else {
	cell = fmax2(fabs(*lo),fabs(*up));
	/* U = upper bound on cell/unit */
	U = 1 + ((h5 >= 1.5*h+.5) ? 1/(1+h) : 1.5/(1+h5));
	U *= imax2(1,*ndiv) * DBL_EPSILON; // avoid overflow for large ndiv
	/* added times 3, as several calculations here */
	i_small = dx < cell * U * 3;
    }

    /*OLD: cell = FLT_EPSILON+ dx / *ndiv; FLT_EPSILON = 1.192e-07 */
    if(i_small) {
	if(cell > 10)
	    cell = 9 + cell/10;
	cell *= shrink_sml;
	if(min_n > 1) cell /= min_n;
    } else {
	cell = dx;
	if(*ndiv > 1) cell /= *ndiv;
    }

    if(cell < 20*DBL_MIN) {
      warning(_("Internal(pretty()): very small range.. corrected"));
      cell = 20*DBL_MIN;
    } else if(cell * 10 > DBL_MAX) {
      warning(_("Internal(pretty()): very large range.. corrected"));
      cell = .1*DBL_MAX;
    }
    /* NB: the power can be negative and this relies on exact
       calculation, which glibc's exp10 does not achieve */
    base = pow(10.0, floor(log10(cell))); /* base <= cell < 10*base */

    /* unit : from { 1,2,5,10 } * base
     *	 such that |u - cell| is small,
     * favoring larger (if h > 1, else smaller)  u  values;
     * favor '5' more than '2'  if h5 > h  (default h5 = .5 + 1.5 h) */
    unit = base;
    if((U = 2*base)-cell <  h*(cell-unit)) { unit = U;
    if((U = 5*base)-cell < h5*(cell-unit)) { unit = U;
    if((U =10*base)-cell <  h*(cell-unit)) unit = U; }}
    /* Result: c := cell,  u := unit,  b := base
     *	c in [	1,	      (2+ h) /(1+h) ] b ==> u=  b
     *	c in ( (2+ h)/(1+h),  (5+2h5)/(1+h5)] b ==> u= 2b
     *	c in ( (5+2h)/(1+h), (10+5h) /(1+h) ] b ==> u= 5b
     *	c in ((10+5h)/(1+h),	         10 ) b ==> u=10b
     *
     *	===>	2/5 *(2+h)/(1+h)  <=  c/u  <=  (2+h)/(1+h)	*/

    ns = floor(*lo/unit+rounding_eps);
    nu = ceil (*up/unit-rounding_eps);
#ifdef DEBUGpr
    REprintf("pretty(lo=%g,up=%g,ndiv=%d,min_n=%d,shrink=%g,high_u=(%g,%g),"
	     "eps=%d)\n\t dx=%g; is.small:%d. ==> cell=%g; unit=%g\n",
	     *lo, *up, *ndiv, min_n, shrink_sml, h, h5,
	      eps_correction,	dx, (int)i_small, cell, unit);
#endif
    if(eps_correction && (eps_correction > 1 || !i_small)) {
	if(*lo != 0.) *lo *= (1- DBL_EPSILON); else *lo = -DBL_MIN;
	if(*up != 0.) *up *= (1+ DBL_EPSILON); else *up = +DBL_MIN;
    }

#ifdef DEBUGpr
    if(ns*unit > *lo)
	REprintf("\t ns= %.0f -- while(ns*unit > *lo) ns--;\n", ns);
#endif
    while(ns*unit > *lo + rounding_eps*unit) ns--;

#ifdef DEBUGpr
    if(nu*unit < *up)
	REprintf("\t nu= %.0f -- while(nu*unit < *up) nu++;\n", nu);
#endif
    while(nu*unit < *up - rounding_eps*unit) nu++;

    k = (int)(0.5 + nu - ns);
    if(k < min_n) {
	/* ensure that	nu - ns	 == min_n */
#ifdef DEBUGpr
	REprintf("\tnu-ns=%.0f-%.0f=%d SMALL -> ensure nu-ns= min_n=%d\n",
		 nu,ns, k, min_n);
#endif
	k = min_n - k;
	if(ns >= 0.) {
	    nu += k/2;
	    ns -= k/2 + k%2;/* ==> nu-ns = old(nu-ns) + min_n -k = min_n */
	} else {
	    ns -= k/2;
	    nu += k/2 + k%2;
	}
	*ndiv = min_n;
    }
    else {
	*ndiv = k;
    }
    if(return_bounds) {// used in pretty.default(), ensure result covers original range
	if(ns * unit < *lo) *lo = ns * unit;
	if(nu * unit > *up) *up = nu * unit;
    } else { // used in graphics GEpretty()
	*lo = ns;
	*up = nu;
    }
#ifdef DEBUGpr
    REprintf("\t ns=%.0f ==> lo=%g\n", ns, *lo);
    REprintf("\t nu=%.0f ==> up=%g  ==> ndiv = %d\n", nu, *up, *ndiv);
#endif
    return unit;
#undef h
#undef h5
}