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, 6 insertions, 143 deletions

diff --git a/src/numeric.c b/src/numeric.c
index e01e661a5..8b6ec4c88 100644
--- a/src/numeric.c
+++ b/src/numeric.c
@@ -18,13 +18,9 @@
 #define floor(f) floorf(f)
 #define ceil(f) ceilf(f)
 #define fmod(x,y) fmodf(x,y)
-#define FLO_MAX_DIGITS 7
-#define FLO_MAX_SIGN_LENGTH 3
-#define FLO_EPSILON FLT_EPSILON
+#define MRB_FLO_TO_STR_FMT "%.7g"
 #else
-#define FLO_MAX_DIGITS 14
-#define FLO_MAX_SIGN_LENGTH 10
-#define FLO_EPSILON DBL_EPSILON
+#define MRB_FLO_TO_STR_FMT "%.14g"
 #endif
 
 MRB_API mrb_float
@@ -107,142 +103,6 @@ num_div(mrb_state *mrb, mrb_value x)
  *  representation.
  */
 
-static mrb_value
-mrb_flo_to_str(mrb_state *mrb, mrb_float flo)
-{
-  double n = (double)flo;
-  int max_digits = FLO_MAX_DIGITS;
-
-  if (isnan(n)) {
-    return mrb_str_new_lit(mrb, "NaN");
-  }
-  else if (isinf(n)) {
-    if (n < 0) {
-      return mrb_str_new_lit(mrb, "-inf");
-    }
-    else {
-      return mrb_str_new_lit(mrb, "inf");
-    }
-  }
-  else {
-    int digit;
-    int m = 0;
-    int exp;
-    mrb_bool e = FALSE;
-    char s[48];
-    char *c = &s[0];
-    int length = 0;
-
-    if (signbit(n)) {
-      n = -n;
-      *(c++) = '-';
-    }
-
-    if (n != 0.0) {
-      if (n > 1.0) {
-        exp = (int)floor(log10(n));
-      }
-      else {
-        exp = (int)-ceil(-log10(n));
-      }
-    }
-    else {
-      exp = 0;
-    }
-
-    /* preserve significands */
-    if (exp < 0) {
-      int i, beg = -1, end = 0;
-      double f = n;
-      double fd = 0;
-      for (i = 0; i < FLO_MAX_DIGITS; ++i) {
-        f = (f - fd) * 10.0;
-        fd = floor(f + FLO_EPSILON);
-        if (fd != 0) {
-          if (beg < 0) beg = i;
-          end = i + 1;
-        }
-      }
-      if (beg >= 0) length = end - beg;
-      if (length > FLO_MAX_SIGN_LENGTH) length = FLO_MAX_SIGN_LENGTH;
-    }
-
-    if (abs(exp) + length >= FLO_MAX_DIGITS) {
-      /* exponent representation */
-      e = TRUE;
-      n = n / pow(10.0, exp);
-      if (isinf(n)) {
-        if (s < c) {            /* s[0] == '-' */
-          return mrb_str_new_lit(mrb, "-0.0");
-        }
-        else {
-          return mrb_str_new_lit(mrb, "0.0");
-        }
-      }
-    }
-    else {
-      /* un-exponent (normal) representation */
-      if (exp > 0) {
-        m = exp;
-      }
-    }
-
-    /* puts digits */
-    while (max_digits >= 0) {
-      double weight = (m < 0) ? 0.0 : pow(10.0, m);
-      double fdigit = (m < 0) ? n * 10.0 : n / weight;
-
-      if (fdigit < 0) fdigit = n = 0;
-      if (m < -1 && fdigit < FLO_EPSILON) {
-        if (e || exp > 0 || m <= -abs(exp)) {
-          break;
-        }
-      }
-      digit = (int)floor(fdigit + FLO_EPSILON);
-      if (m == 0 && digit > 9) {
-        n /= 10.0;
-        exp++;
-        continue;
-      }
-      *(c++) = '0' + digit;
-      n = (m < 0) ? n * 10.0 - digit : n - (digit * weight);
-      max_digits--;
-      if (m-- == 0) {
-        *(c++) = '.';
-      }
-    }
-    if (c[-1] == '0') {
-      while (&s[0] < c && c[-1] == '0') {
-        c--;
-      }
-      c++;
-    }
-
-    if (e) {
-      *(c++) = 'e';
-      if (exp > 0) {
-        *(c++) = '+';
-      }
-      else {
-        *(c++) = '-';
-        exp = -exp;
-      }
-
-      if (exp >= 100) {
-        *(c++) = '0' + exp / 100;
-        exp -= exp / 100 * 100;
-      }
-
-      *(c++) = '0' + exp / 10;
-      *(c++) = '0' + exp % 10;
-    }
-
-    *c = '\0';
-
-    return mrb_str_new(mrb, &s[0], c - &s[0]);
-  }
-}
-
 /* 15.2.9.3.16(x) */
 /*
  *  call-seq:
@@ -257,7 +117,10 @@ mrb_flo_to_str(mrb_state *mrb, mrb_float flo)
 static mrb_value
 flo_to_s(mrb_state *mrb, mrb_value flt)
 {
-  return mrb_flo_to_str(mrb, mrb_float(flt));
+  if (isnan(mrb_float(flt))) {
+    return mrb_str_new_lit(mrb, "NaN");
+  }
+  return mrb_float_to_str(mrb, flt, MRB_FLO_TO_STR_FMT);
 }
 
 /* 15.2.9.3.2  */