add arf_get_str; rewrite arf_fprintd

arf.h

@@ -775,6 +775,8 @@ void arf_ceil(arf_t z, const arf_t x);
 
 void arf_debug(const arf_t x);
 
+char * arf_get_str(const arf_t x, slong d);
+
 void arf_fprint(FILE * file, const arf_t x);
 
 void arf_fprintd(FILE * file, const arf_t y, slong d);

arf/fprint.c

@@ -10,7 +10,35 @@
     (at your option) any later version.  See <http://www.gnu.org/licenses/>.
 */
 
+#include <string.h>
 #include "arf.h"
+#include "arb.h"
+
+char * arf_get_str(const arf_t x, slong d)
+{
+    if (arf_is_special(x))
+    {
+        char * s = flint_malloc(5);
+
+        if (arf_is_zero(x))
+            strcpy(s, "0");
+        else if (arf_is_pos_inf(x))
+            strcpy(s, "+inf");
+        else if (arf_is_neg_inf(x))
+            strcpy(s, "-inf");
+        else
+            strcpy(s, "nan");
+
+        return s;
+    }
+    else
+    {
+        arb_t t;
+        *arb_midref(t) = *x;
+        mag_init(arb_radref(t));  /* no need to free */
+        return arb_get_str(t, FLINT_MAX(d, 1), ARB_STR_NO_RADIUS);
+    }
+}
 
 void
 arf_fprint(FILE * file, const arf_t x)
@@ -45,20 +73,8 @@ arf_fprint(FILE * file, const arf_t x)
 void
 arf_fprintd(FILE * file, const arf_t x, slong d)
 {
-    if (arf_is_finite(x) && (ARF_EXP(x) <= MPFR_EMIN_MIN + 1 ||
-                             ARF_EXP(x) >= MPFR_EMAX_MAX - 1))
-    {
-        arf_fprint(file, x);
-    }
-    else
-    {
-        mpfr_t t;
-        mpfr_init2(t, d * 3.33 + 10);
-        mpfr_set_emin(MPFR_EMIN_MIN);
-        mpfr_set_emax(MPFR_EMAX_MAX);
-        arf_get_mpfr(t, x, MPFR_RNDN);
-        mpfr_fprintf(file, "%.*Rg", FLINT_MAX(d, 1), t);
-        mpfr_clear(t);
-    }
-}
+    char * s = arf_get_str(x, d);
 
+    fprintf(file, "%s", s);
+    flint_free(s);
+}

arf/test/t-get_str.c

@@ -0,0 +1,67 @@
+/*
+    Copyright (C) 2021 Fredrik Johansson
+
+    This file is part of Arb.
+
+    Arb is free software: you can redistribute it and/or modify it under
+    the terms of the GNU Lesser General Public License (LGPL) as published
+    by the Free Software Foundation; either version 2.1 of the License, or
+    (at your option) any later version.  See <http://www.gnu.org/licenses/>.
+*/
+
+#include <string.h>
+#include "arf.h"
+#include "arb.h"
+
+int main()
+{
+    slong iter;
+    flint_rand_t state;
+
+    flint_printf("get_str....");
+    fflush(stdout);
+
+    flint_randinit(state);
+
+    for (iter = 0; iter < 1000 * arb_test_multiplier(); iter++)
+    {
+        arf_t x;
+        arb_t y;
+        char *s1, * s2;
+        slong d;
+
+        arf_init(x);
+        arb_init(y);
+
+        arf_randtest_special(x, state, 2 + n_randint(state, 200), 2 + n_randint(state, 100));
+        arb_set_arf(y, x);
+        d = 1 + n_randint(state, 200);
+
+        s1 = arf_get_str(x, d);
+        s2 = arb_get_str(y, d, ARB_STR_NO_RADIUS);
+
+        if ((arf_is_pos_inf(x) && strcmp(s1, "+inf")) ||
+            (arf_is_neg_inf(x) && strcmp(s1, "-inf")) ||
+            (arf_is_nan(x) && strcmp(s1, "nan")) ||
+            (arf_is_finite(x) && strcmp(s1, s2)))
+        {
+            flint_printf("FAIL\n");
+            arf_print(x); flint_printf("\n");
+            arb_print(y); flint_printf("\n");
+            flint_printf("%s\n", s1);
+            flint_printf("%s\n", s2);
+            flint_abort();
+        }
+
+        flint_free(s1);
+        flint_free(s2);
+
+        arf_clear(x);
+        arb_clear(y);
+    }
+
+    flint_randclear(state);
+    flint_cleanup();
+    flint_printf("PASS\n");
+    return EXIT_SUCCESS;
+}

doc/source/arf.rst

@@ -502,8 +502,12 @@ Input and output
 .. function:: void arf_printd(const arf_t x, slong d)
 
     Prints *x* as a decimal floating-point number, rounding to *d* digits.
-    This function is currently implemented using MPFR,
-    and does not support large exponents.
+    Rounding is faithful (at most 1 ulp error).
+
+.. function:: char * arf_get_str(const arf_t x, slong d)
+
+    Returns *x* as a decimal floating-point number, rounding to *d* digits.
+    Rounding is faithful (at most 1 ulp error).
 
 .. function:: void arf_fprint(FILE * file, const arf_t x)
 
@@ -512,8 +516,8 @@ Input and output
 .. function:: void arf_fprintd(FILE * file, const arf_t y, slong d)
 
     Prints *x* as a decimal floating-point number to the stream *file*,
-    rounding to *d* digits. This function is currently implemented using MPFR,
-    and does not support large exponents.
+    rounding to *d* digits.
+    Rounding is faithful (at most 1 ulp error).
 
 .. function:: char * arf_dump_str(const arf_t x)