[bigdecimal] Add BigDecimal#n_significant_digits

https://github.com/ruby/bigdecimal/commit/981dc48f95
https://github.com/ruby/bigdecimal/commit/9ecf880ec04

ext/bigdecimal/bigdecimal.c

@@ -359,8 +359,9 @@ BigDecimal_double_fig(VALUE self)
  *  internal storage properties.
  *
  *  This method is deprecated and will be removed in the future.
- *  Instead, use BigDecimal#precision for obtaining the number of decimal
- *  digits.
+ *  Instead, use BigDecimal#n_significant_digits for obtaining the number of
+ *  significant digits in scientific notation, and BigDecimal#precision for
+ *  obtaining the number of digits in decimal notation.
  *
  *     BigDecimal('5').precs #=> [9, 18]
  */
@@ -449,6 +450,32 @@ BigDecimal_precision(VALUE self)
     return SSIZET2NUM(precision);
 }
 
+static VALUE
+BigDecimal_n_significant_digits(VALUE self)
+{
+    ENTER(1);
+
+    Real *p;
+    GUARD_OBJ(p, GetVpValue(self, 1));
+
+    ssize_t n = p->Prec;
+    while (n > 0 && p->frac[n-1] == 0) --n;
+    if (n <= 0) {
+        return INT2FIX(0);
+    }
+
+    int nlz, ntz;
+
+    BDIGIT x = p->frac[0];
+    for (nlz = BASE_FIG; x > 0; x /= 10) --nlz;
+
+    x = p->frac[n-1];
+    for (ntz = 0; x > 0 && x % 10 == 0; x /= 10) ++ntz;
+
+    ssize_t n_digits = BASE_FIG * n - nlz - ntz;
+    return SSIZET2NUM(n_digits);
+}
+
 /*
  * call-seq: hash
  *
@@ -3582,6 +3609,7 @@ Init_bigdecimal(void)
     /* instance methods */
     rb_define_method(rb_cBigDecimal, "precs", BigDecimal_prec, 0);
     rb_define_method(rb_cBigDecimal, "precision", BigDecimal_precision, 0);
+    rb_define_method(rb_cBigDecimal, "n_significant_digits", BigDecimal_n_significant_digits, 0);
 
     rb_define_method(rb_cBigDecimal, "add", BigDecimal_add2, 2);
     rb_define_method(rb_cBigDecimal, "sub", BigDecimal_sub2, 2);

test/bigdecimal/test_bigdecimal.rb

@@ -1962,6 +1962,57 @@ class TestBigDecimal < Test::Unit::TestCase
     end
   end
 
+  def test_n_significant_digits_only_integer
+    assert_equal(0, BigDecimal(0).n_significant_digits)
+    assert_equal(1, BigDecimal(1).n_significant_digits)
+    assert_equal(1, BigDecimal(-1).n_significant_digits)
+    assert_equal(1, BigDecimal(10).n_significant_digits)
+    assert_equal(1, BigDecimal(-10).n_significant_digits)
+    assert_equal(3, BigDecimal(101).n_significant_digits)
+    assert_equal(3, BigDecimal(-101).n_significant_digits)
+    assert_equal(1, BigDecimal(100_000_000_000_000_000_000).n_significant_digits)
+    assert_equal(1, BigDecimal(-100_000_000_000_000_000_000).n_significant_digits)
+    assert_equal(21, BigDecimal(100_000_000_000_000_000_001).n_significant_digits)
+    assert_equal(21, BigDecimal(-100_000_000_000_000_000_001).n_significant_digits)
+    assert_equal(3, BigDecimal("111e100").n_significant_digits)
+    assert_equal(3, BigDecimal("-111e100").n_significant_digits)
+  end
+
+  def test_n_significant_digits_only_fraction
+    assert_equal(1, BigDecimal("0.1").n_significant_digits)
+    assert_equal(1, BigDecimal("-0.1").n_significant_digits)
+    assert_equal(1, BigDecimal("0.01").n_significant_digits)
+    assert_equal(1, BigDecimal("-0.01").n_significant_digits)
+    assert_equal(2, BigDecimal("0.11").n_significant_digits)
+    assert_equal(2, BigDecimal("-0.11").n_significant_digits)
+    assert_equal(1, BigDecimal("0.000_000_000_000_000_000_001").n_significant_digits)
+    assert_equal(1, BigDecimal("-0.000_000_000_000_000_000_001").n_significant_digits)
+    assert_equal(3, BigDecimal("111e-100").n_significant_digits)
+    assert_equal(3, BigDecimal("-111e-100").n_significant_digits)
+  end
+
+  def test_n_significant_digits_full
+    assert_equal(2, BigDecimal("1.1").n_significant_digits)
+    assert_equal(2, BigDecimal("-1.1").n_significant_digits)
+    assert_equal(3, BigDecimal("1.01").n_significant_digits)
+    assert_equal(3, BigDecimal("-1.01").n_significant_digits)
+    assert_equal(5, BigDecimal("11111e-2").n_significant_digits)
+    assert_equal(5, BigDecimal("-11111e-2").n_significant_digits)
+    assert_equal(21, BigDecimal("100.000_000_000_000_000_001").n_significant_digits)
+    assert_equal(21, BigDecimal("-100.000_000_000_000_000_001").n_significant_digits)
+  end
+
+  def test_n_significant_digits_special
+    BigDecimal.save_exception_mode do
+      BigDecimal.mode(BigDecimal::EXCEPTION_OVERFLOW, false)
+      BigDecimal.mode(BigDecimal::EXCEPTION_NaN, false)
+
+      assert_equal(0, BigDecimal("Infinity").n_significant_digits)
+      assert_equal(0, BigDecimal("-Infinity").n_significant_digits)
+      assert_equal(0, BigDecimal("NaN").n_significant_digits)
+    end
+  end
+
   def test_initialize_copy_dup_clone_frozen_error
     bd = BigDecimal(1)
     bd2 = BigDecimal(2)