2 files changed, 27 insertions, 12 deletions
diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index b9ba0ea62d..704f04510e 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -2727,23 +2727,22 @@ rb_uint64_convert_to_BigDecimal(uint64_t uval, RB_UNUSED_VAR(size_t digs), int r
         vp->frac[0] = (DECDIG)uval;
     }
     else {
-        const size_t len = (size_t)ceil(log10((double)uval) / BASE_FIG);
-
-        vp = VpAllocReal(len);
-        vp->MaxPrec = len;
-        vp->Prec = len;
-        vp->exponent = len;
-        VpSetSign(vp, 1);
-
-        size_t i, ntz = 0;
-        for (i = 0; i < len; ++i) {
+        DECDIG buf[BIGDECIMAL_INT64_MAX_LENGTH] = {0,};
+        size_t exp = 0, ntz = 0;
+        for (; uval > 0; ++exp) {
             DECDIG r = uval % BASE;
-            vp->frac[len - i - 1] = r;
             if (r == 0) ++ntz;
+            buf[BIGDECIMAL_INT64_MAX_LENGTH - exp - 1] = r;
             uval /= BASE;
         }
 
-        vp->Prec -= ntz;
+        const size_t len = exp - 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);
     }
 
     return BigDecimal_wrap_struct(obj, vp);
diff --git a/ext/bigdecimal/bigdecimal.h b/ext/bigdecimal/bigdecimal.h
index acc00b8127..bd1c46743e 100644
--- a/ext/bigdecimal/bigdecimal.h
+++ b/ext/bigdecimal/bigdecimal.h
@@ -54,9 +54,25 @@
 #if SIZEOF_DECDIG == 4
 # define BIGDECIMAL_BASE ((DECDIG)1000000000U)
 # define BIGDECIMAL_COMPONENT_FIGURES 9
+/*
+ * The number of components required for a 64-bit integer.
+ *
+ *   INT64_MAX:   9_223372036_854775807
+ *   UINT64_MAX: 18_446744073_709551615
+ */
+# define BIGDECIMAL_INT64_MAX_LENGTH 3
+
 #elif SIZEOF_DECDIG == 2
 # define BIGDECIMAL_BASE ((DECDIG)10000U)
 # define BIGDECIMAL_COMPONENT_FIGURES 4
+/*
+ * The number of components required for a 64-bit integer.
+ *
+ *   INT64_MAX:   922_3372_0368_5477_5807
+ *   UINT64_MAX: 1844_6744_0737_0955_1615
+ */
+# define BIGDECIMAL_INT64_MAX_LENGTH 5
+
 #else
 # error Unknown size of DECDIG
 #endif