Fix uint64 conversion

Stop using logarithm to compute the number of components. Instead, use the theoretical maximum number of components for buffer, and count up the actual number of components during conversion.
ruby · Feb 4, 2021 · 9067b35 · 9067b35
1 parent 7479923
commit 9067b35
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Showing 2 changed files with 27 additions and 12 deletions.
diff --git 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
@@ -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