re-implement mrb_float_to_str()

The new implementation is backwards incompatible, but I couldn't find
any usage outside mruby and I also couldn't think of a different and
good name.

All ISO C99 printf conversion specifiers for floating point numbers and
an optional precision are supported.

It is largely based on code from the MIT licensed musl libc
(http://www.musl-libc.org/) and its floating point printing is exact
(unlike the current code behind Float#to_s).

1 files changed, 367 insertions, 0 deletions

diff --git a/src/fmt_fp.c b/src/fmt_fp.c
new file mode 100644
index 000000000..9345c31d1
--- /dev/null
+++ b/src/fmt_fp.c
@@ -0,0 +1,367 @@
+/*
+
+Most code in this file originates from musl (src/stdio/vfprintf.c)
+which, just like mruby itself, is licensed under the MIT license.
+
+Copyright © 2005-2014 Rich Felker, et al.
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+*/
+
+#include <limits.h>
+#include <string.h>
+#include <stdint.h>
+#include <math.h>
+#include <float.h>
+#include <ctype.h>
+
+#include "mruby.h"
+#include "mruby/string.h"
+
+struct fmt_args {
+  mrb_state *mrb;
+  mrb_value str;
+};
+
+#define MAX(a,b) ((a)>(b) ? (a) : (b))
+#define MIN(a,b) ((a)<(b) ? (a) : (b))
+
+/* Convenient bit representation for modifier flags, which all fall
+ * within 31 codepoints of the space character. */
+
+#define ALT_FORM   (1U<<('#'-' '))
+#define ZERO_PAD   (1U<<('0'-' '))
+#define LEFT_ADJ   (1U<<('-'-' '))
+#define PAD_POS    (1U<<(' '-' '))
+#define MARK_POS   (1U<<('+'-' '))
+
+static void
+out(struct fmt_args *f, const char *s, size_t l)
+{
+  mrb_str_cat(f->mrb, f->str, s, l);
+}
+
+static void
+pad(struct fmt_args *f, char c, int w, int l, int fl)
+{
+  char pad[256];
+  if (fl & (LEFT_ADJ | ZERO_PAD) || l >= w) return;
+  l = w - l;
+  memset(pad, c, l>sizeof pad ? sizeof pad : l);
+  for (; l >= sizeof pad; l -= sizeof pad)
+    out(f, pad, sizeof pad);
+  out(f, pad, l);
+}
+
+static const char xdigits[16] = {
+  '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'
+};
+
+static char*
+fmt_u(uint32_t x, char *s)
+{
+  for (; x; x /= 10) *--s = '0' + x % 10;
+  return s;
+}
+
+/* Do not override this check. The floating point printing code below
+ * depends on the float.h constants being right. If they are wrong, it
+ * may overflow the stack. */
+#if LDBL_MANT_DIG == 53
+typedef char compiler_defines_long_double_incorrectly[9-(int)sizeof(long double)];
+#endif
+
+static int
+fmt_fp(struct fmt_args *f, long double y, int w, int p, int fl, int t)
+{
+  uint32_t big[(LDBL_MANT_DIG+28)/29 + 1          // mantissa expansion
+    + (LDBL_MAX_EXP+LDBL_MANT_DIG+28+8)/9]; // exponent expansion
+  uint32_t *a, *d, *r, *z;
+  uint32_t i;
+  int e2=0, e, j, l;
+  char buf[9+LDBL_MANT_DIG/4], *s;
+  const char *prefix="-0X+0X 0X-0x+0x 0x";
+  int pl;
+  char ebuf0[3*sizeof(int)], *ebuf=&ebuf0[3*sizeof(int)], *estr;
+
+  pl=1;
+  if (signbit(y)) {
+    y=-y;
+  } else if (fl & MARK_POS) {
+    prefix+=3;
+  } else if (fl & PAD_POS) {
+    prefix+=6;
+  } else prefix++, pl=0;
+
+  if (!isfinite(y)) {
+    char *ss = (t&32)?"inf":"INF";
+    if (y!=y) ss=(t&32)?"nan":"NAN";
+    pad(f, ' ', w, 3+pl, fl&~ZERO_PAD);
+    out(f, prefix, pl);
+    out(f, ss, 3);
+    pad(f, ' ', w, 3+pl, fl^LEFT_ADJ);
+    return MAX(w, 3+pl);
+  }
+
+  y = frexpl(y, &e2) * 2;
+  if (y) e2--;
+
+  if ((t|32)=='a') {
+    long double round = 8.0;
+    int re;
+
+    if (t&32) prefix += 9;
+    pl += 2;
+
+    if (p<0 || p>=LDBL_MANT_DIG/4-1) re=0;
+    else re=LDBL_MANT_DIG/4-1-p;
+
+    if (re) {
+      while (re--) round*=16;
+      if (*prefix=='-') {
+        y=-y;
+        y-=round;
+        y+=round;
+        y=-y;
+      }
+      else {
+        y+=round;
+        y-=round;
+      }
+    }
+
+    estr=fmt_u(e2<0 ? -e2 : e2, ebuf);
+    if (estr==ebuf) *--estr='0';
+    *--estr = (e2<0 ? '-' : '+');
+    *--estr = t+('p'-'a');
+
+    s=buf;
+    do {
+      int x=y;
+      *s++=xdigits[x]|(t&32);
+      y=16*(y-x);
+      if (s-buf==1 && (y||p>0||(fl&ALT_FORM))) *s++='.';
+    } while (y);
+
+    if (p && s-buf-2 < p)
+      l = (p+2) + (ebuf-estr);
+    else
+      l = (s-buf) + (ebuf-estr);
+
+    pad(f, ' ', w, pl+l, fl);
+    out(f, prefix, pl);
+    pad(f, '0', w, pl+l, fl^ZERO_PAD);
+    out(f, buf, s-buf);
+    pad(f, '0', l-(ebuf-estr)-(s-buf), 0, 0);
+    out(f, estr, ebuf-estr);
+    pad(f, ' ', w, pl+l, fl^LEFT_ADJ);
+    return MAX(w, pl+l);
+  }
+  if (p<0) p=6;
+
+  if (y) y *= 268435456.0, e2-=28;
+
+  if (e2<0) a=r=z=big;
+  else a=r=z=big+sizeof(big)/sizeof(*big) - LDBL_MANT_DIG - 1;
+
+  do {
+    *z = y;
+    y = 1000000000*(y-*z++);
+  } while (y);
+
+  while (e2>0) {
+    uint32_t carry=0;
+    int sh=MIN(29,e2);
+    for (d=z-1; d>=a; d--) {
+      uint64_t x = ((uint64_t)*d<<sh)+carry;
+      *d = x % 1000000000;
+      carry = x / 1000000000;
+    }
+    if (carry) *--a = carry;
+    while (z>a && !z[-1]) z--;
+    e2-=sh;
+  }
+  while (e2<0) {
+    uint32_t carry=0, *b;
+    int sh=MIN(9,-e2), need=1+(p+LDBL_MANT_DIG/3+8)/9;
+    for (d=a; d<z; d++) {
+      uint32_t rm = *d & (1<<sh)-1;
+      *d = (*d>>sh) + carry;
+      carry = (1000000000>>sh) * rm;
+    }
+    if (!*a) a++;
+    if (carry) *z++ = carry;
+    /* Avoid (slow!) computation past requested precision */
+    b = (t|32)=='f' ? r : a;
+    if (z-b > need) z = b+need;
+    e2+=sh;
+  }
+
+  if (a<z) for (i=10, e=9*(r-a); *a>=i; i*=10, e++);
+  else e=0;
+
+  /* Perform rounding: j is precision after the radix (possibly neg) */
+  j = p - ((t|32)!='f')*e - ((t|32)=='g' && p);
+  if (j < 9*(z-r-1)) {
+    uint32_t x;
+    /* We avoid C's broken division of negative numbers */
+    d = r + 1 + ((j+9*LDBL_MAX_EXP)/9 - LDBL_MAX_EXP);
+    j += 9*LDBL_MAX_EXP;
+    j %= 9;
+    for (i=10, j++; j<9; i*=10, j++);
+    x = *d % i;
+    /* Are there any significant digits past j? */
+    if (x || d+1!=z) {
+      long double round = 2/LDBL_EPSILON;
+      long double small;
+      if (*d/i & 1) round += 2;
+      if (x<i/2) small=0.5;
+      else if (x==i/2 && d+1==z) small=1.0;
+      else small=1.5;
+      if (pl && *prefix=='-') round*=-1, small*=-1;
+      *d -= x;
+      /* Decide whether to round by probing round+small */
+      if (round+small != round) {
+        *d = *d + i;
+        while (*d > 999999999) {
+          *d--=0;
+          if (d<a) *--a=0;
+          (*d)++;
+        }
+        for (i=10, e=9*(r-a); *a>=i; i*=10, e++);
+      }
+    }
+    if (z>d+1) z=d+1;
+  }
+  for (; z>a && !z[-1]; z--);
+  
+  if ((t|32)=='g') {
+    if (!p) p++;
+    if (p>e && e>=-4) {
+      t--;
+      p-=e+1;
+    }
+    else {
+      t-=2;
+      p--;
+    }
+    if (!(fl&ALT_FORM)) {
+      /* Count trailing zeros in last place */
+      if (z>a && z[-1]) for (i=10, j=0; z[-1]%i==0; i*=10, j++);
+      else j=9;
+      if ((t|32)=='f')
+        p = MIN(p,MAX(0,9*(z-r-1)-j));
+      else
+        p = MIN(p,MAX(0,9*(z-r-1)+e-j));
+    }
+  }
+  l = 1 + p + (p || (fl&ALT_FORM));
+  if ((t|32)=='f') {
+    if (e>0) l+=e;
+  }
+  else {
+    estr=fmt_u(e<0 ? -e : e, ebuf);
+    while(ebuf-estr<2) *--estr='0';
+    *--estr = (e<0 ? '-' : '+');
+    *--estr = t;
+    l += ebuf-estr;
+  }
+
+  pad(f, ' ', w, pl+l, fl);
+  out(f, prefix, pl);
+  pad(f, '0', w, pl+l, fl^ZERO_PAD);
+
+  if ((t|32)=='f') {
+    if (a>r) a=r;
+    for (d=a; d<=r; d++) {
+      char *ss = fmt_u(*d, buf+9);
+      if (d!=a) while (ss>buf) *--ss='0';
+      else if (ss==buf+9) *--ss='0';
+      out(f, ss, buf+9-ss);
+    }
+    if (p || (fl&ALT_FORM)) out(f, ".", 1);
+    for (; d<z && p>0; d++, p-=9) {
+      char *ss = fmt_u(*d, buf+9);
+      while (ss>buf) *--ss='0';
+      out(f, ss, MIN(9,p));
+    }
+    pad(f, '0', p+9, 9, 0);
+  }
+  else {
+    if (z<=a) z=a+1;
+    for (d=a; d<z && p>=0; d++) {
+      char *ss = fmt_u(*d, buf+9);
+      if (ss==buf+9) *--ss='0';
+      if (d!=a) while (ss>buf) *--ss='0';
+      else {
+        out(f, ss++, 1);
+        if (p>0||(fl&ALT_FORM)) out(f, ".", 1);
+      }
+      out(f, ss, MIN(buf+9-ss, p));
+      p -= buf+9-ss;
+    }
+    pad(f, '0', p+18, 18, 0);
+    out(f, estr, ebuf-estr);
+  }
+
+  pad(f, ' ', w, pl+l, fl^LEFT_ADJ);
+
+  return MAX(w, pl+l);
+}
+
+static int
+fmt_core(struct fmt_args *f, const char *fmt, mrb_float flo)
+{
+  int p;
+
+  if (*fmt != '%') {
+    return -1;
+  }
+  ++fmt;
+
+  if (*fmt == '.') {
+    ++fmt;
+    for (p = 0; ISDIGIT(*fmt); ++fmt) {
+      p = 10 * p + (*fmt - '0');
+    }
+  }
+  else {
+    p = -1;
+  }
+
+  switch (*fmt) {
+  case 'e': case 'f': case 'g': case 'a':
+  case 'E': case 'F': case 'G': case 'A':
+    return fmt_fp(f, flo, 0, p, 0, *fmt);
+  default:
+    return -1;
+  } 
+}
+
+mrb_value
+mrb_float_to_str(mrb_state *mrb, mrb_value flo, const char *fmt)
+{
+  struct fmt_args f = { mrb, mrb_str_buf_new(mrb, 24) };
+  if (fmt_core(&f, fmt, mrb_float(flo)) < 0) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "invalid format string");
+  }
+  return f.str;
+}