Fix trailing zeros handling in rb_uint64_convert_to_BigDecimal

ext/bigdecimal/bigdecimal.c

@@ -2862,21 +2862,29 @@ rb_uint64_convert_to_BigDecimal(uint64_t uval, RB_UNUSED_VAR(size_t digs), int r
     }
     else {
         DECDIG buf[BIGDECIMAL_INT64_MAX_LENGTH] = {0,};
-        size_t exp = 0, ntz = 0;
-        for (; uval > 0; ++exp) {
-            DECDIG r = uval % BASE;
-            if (r == 0) ++ntz;
-            buf[BIGDECIMAL_INT64_MAX_LENGTH - exp - 1] = r;
+        DECDIG r = uval % BASE;
+        size_t len = 0, ntz = 0;
+        if (r == 0) {
+            // Count and skip trailing zeros
+            for (; r == 0 && uval > 0; ++ntz) {
+                uval /= BASE;
+                r = uval % BASE;
+            }
+        }
+        for (; uval > 0; ++len) {
+            // Store digits
+            buf[BIGDECIMAL_INT64_MAX_LENGTH - len - 1] = r;
             uval /= BASE;
+            r = uval % BASE;
         }
 
-        const size_t len = exp - ntz;
+        const size_t exp = len + ntz;
         vp = VpAllocReal(len);
         vp->MaxPrec = len;
         vp->Prec = len;
         vp->exponent = exp;
         VpSetSign(vp, 1);
-        MEMCPY(vp->frac, buf + BIGDECIMAL_INT64_MAX_LENGTH - exp, DECDIG, len);
+        MEMCPY(vp->frac, buf + BIGDECIMAL_INT64_MAX_LENGTH - len, DECDIG, len);
     }
 
     return BigDecimal_wrap_struct(obj, vp);

test/bigdecimal/helper.rb

@@ -1,8 +1,19 @@
 # frozen_string_literal: false
 require "test/unit"
 require "bigdecimal"
+require 'rbconfig/sizeof'
 
 module TestBigDecimalBase
+  if RbConfig::SIZEOF.key?("int64_t")
+    SIZEOF_DECDIG = RbConfig::SIZEOF["int32_t"]
+    BASE = 1_000_000_000
+    BASE_FIG = 9
+  else
+    SIZEOF_DECDIG = RbConfig::SIZEOF["int16_t"]
+    BASE = 1000
+    BASE_FIG = 4
+  end
+
   def setup
     @mode = BigDecimal.mode(BigDecimal::EXCEPTION_ALL)
     BigDecimal.mode(BigDecimal::EXCEPTION_ALL, true)

test/bigdecimal/test_bigdecimal.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: false
 require_relative "helper"
 require 'bigdecimal/math'
-require 'rbconfig/sizeof'
 
 class TestBigDecimal < Test::Unit::TestCase
   include TestBigDecimalBase
@@ -101,6 +100,19 @@ def test_BigDecimal_bug7522
     assert_not_same(bd, BigDecimal(bd, 1, exception: false))
   end
 
+  def test_BigDecimal_issue_192
+    # https://github.com/ruby/bigdecimal/issues/192
+    # https://github.com/rails/rails/pull/42125
+    if BASE_FIG == 9
+      int = 1_000_000_000_12345_0000
+      big = BigDecimal("0.100000000012345e19")
+    else  # BASE_FIG == 4
+      int = 1_0000_12_00
+      big = BigDecimal("0.1000012e9")
+    end
+    assert_equal(BigDecimal(int), big, "[ruby/bigdecimal#192]")
+  end
+
   def test_BigDecimal_with_invalid_string
     [
       '', '.', 'e1', 'd1', '.e', '.d', '1.e', '1.d', '.1e', '.1d',

test/bigdecimal/test_bigdecimal_util.rb

@@ -25,6 +25,8 @@ def test_Float_to_d_without_precision
     assert_equal(9.05, 9.05.to_d.to_f)
     assert_equal("9.05", 9.05.to_d.to_s('F'))
 
+    assert_equal("65.6", 65.6.to_d.to_s("F"))
+
     assert_equal(Math::PI, Math::PI.to_d.to_f)
 
     bug9214 = '[ruby-core:58858]'
@@ -60,6 +62,19 @@ def test_Float_to_d_bug13331
                  "[ruby-core:80234] [Bug #13331]")
   end
 
+  def test_Float_to_d_issue_192
+    # https://github.com/ruby/bigdecimal/issues/192
+    # https://github.com/rails/rails/pull/42125
+    if BASE_FIG == 9
+      flo = 1_000_000_000.12345
+      big = BigDecimal("0.100000000012345e10")
+    else  # BASE_FIG == 4
+      flo = 1_0000.12
+      big = BigDecimal("0.1000012e5")
+    end
+    assert_equal(flo.to_d, big, "[ruby/bigdecimal#192]")
+  end
+
   def test_Rational_to_d
     digits = 100
     delta = 1.0/10**(digits)