Improve compatibility to CRuby for `Float#to_s`

Bfore:

  Float::INFINITY.to_s  #=> "inf"
  50.0.to_s             #=> "50"
  1e20.to_s             #=> "1e+20"

After / CRuby:

  Float::INFINITY.to_s  #=> "Infinity"
  50.0.to_s             #=> "50.0"
  1e20.to_s             #=> "1.0e+20"

2 files changed, 77 insertions, 4 deletions

diff --git a/src/numeric.c b/src/numeric.c
index 089cc744d..fc8460300 100644
--- a/src/numeric.c
+++ b/src/numeric.c
@@ -10,6 +10,7 @@
 #endif
 #include <limits.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include <mruby.h>
 #include <mruby/array.h>
@@ -23,9 +24,9 @@
 #define floor(f) floorf(f)
 #define ceil(f) ceilf(f)
 #define fmod(x,y) fmodf(x,y)
-#define MRB_FLO_TO_STR_FMT "%.8g"
+#define FLO_TO_STR_PREC 8
 #else
-#define MRB_FLO_TO_STR_FMT "%.16g"
+#define FLO_TO_STR_PREC 16
 #endif
 #endif
 
@@ -177,10 +178,49 @@ num_div(mrb_state *mrb, mrb_value x)
 static mrb_value
 flo_to_s(mrb_state *mrb, mrb_value flt)
 {
-  if (isnan(mrb_float(flt))) {
+  mrb_float f = mrb_float(flt);
+
+  if (isinf(f)) {
+    return f < 0 ? mrb_str_new_lit(mrb, "-Infinity")
+                 : mrb_str_new_lit(mrb, "Infinity");
+  }
+  else if (isnan(f)) {
     return mrb_str_new_lit(mrb, "NaN");
   }
-  return mrb_float_to_str(mrb, flt, MRB_FLO_TO_STR_FMT);
+  else {
+    char fmt[] = "%." MRB_STRINGIZE(FLO_TO_STR_PREC) "g";
+    mrb_value str = mrb_float_to_str(mrb, flt, fmt);
+    mrb_int len;
+    char *begp;
+
+    insert_dot_zero:
+    begp = RSTRING_PTR(str);
+    len = RSTRING_LEN(str);
+    for (char *p = begp, *endp = p + len; p < endp; ++p) {
+      if (*p == '.') {
+        return str;
+      }
+      else if (*p == 'e') {
+        ptrdiff_t e_pos = p - begp;
+        mrb_str_cat(mrb, str, ".0", 2);
+        p = RSTRING_PTR(str) + e_pos;
+        memmove(p + 2, p, len - e_pos);
+        memcpy(p, ".0", 2);
+        return str;
+      }
+    }
+
+    if (FLO_TO_STR_PREC + (begp[0] == '-') <= len) {
+      --fmt[sizeof(fmt) - 3];  /* %.16g(%.8g) -> %.15g(%.7g) */
+      str = mrb_float_to_str(mrb, flt, fmt);
+      goto insert_dot_zero;
+    }
+    else {
+      mrb_str_cat(mrb, str, ".0", 2);
+    }
+
+    return str;
+  }
 }
 
 /* 15.2.9.3.2  */
diff --git a/test/t/float.rb b/test/t/float.rb
index 68fd31b9a..eac5c5792 100644
--- a/test/t/float.rb
+++ b/test/t/float.rb
@@ -206,4 +206,37 @@ assert('Float#>>') do
   assert_equal(-1, -23.0 >> 128)
 end
 
+assert('Float#to_s') do
+  uses_float = 4e38.infinite?  # enable MRB_USE_FLOAT?
+
+  assert_equal("Infinity", Float::INFINITY.to_s)
+  assert_equal("-Infinity", (-Float::INFINITY).to_s)
+  assert_equal("NaN", Float::NAN.to_s)
+  assert_equal("0.0", 0.0.to_s)
+  assert_equal("-0.0", -0.0.to_s)
+  assert_equal("-3.21", -3.21.to_s)
+  assert_equal("50.0", 50.0.to_s)
+  assert_equal("0.00021", 0.00021.to_s)
+  assert_equal("-0.00021", -0.00021.to_s)
+  assert_equal("2.1e-05", 0.000021.to_s)
+  assert_equal("-2.1e-05", -0.000021.to_s)
+  assert_equal("1.0e+20", 1e20.to_s)
+  assert_equal("-1.0e+20", -1e20.to_s)
+  assert_equal("1.0e+16", 10000000000000000.0.to_s)
+  assert_equal("-1.0e+16", -10000000000000000.0.to_s)
+  assert_equal("100000.0", 100000.0.to_s)
+  assert_equal("-100000.0", -100000.0.to_s)
+  if uses_float
+    assert_equal("1.0e+08", 100000000.0.to_s)
+    assert_equal("-1.0e+08", -100000000.0.to_s)
+    assert_equal("1.0e+07", 10000000.0.to_s)
+    assert_equal("-1.0e+07", -10000000.0.to_s)
+  else
+    assert_equal("1.0e+15", 1000000000000000.0.to_s)
+    assert_equal("-1.0e+15", -1000000000000000.0.to_s)
+    assert_equal("100000000000000.0", 100000000000000.0.to_s)
+    assert_equal("-100000000000000.0", -100000000000000.0.to_s)
+  end
+end
+
 end # const_defined?(:Float)