Allow much larger exponent for explicitly exact numbers

Addresses #898
We do set a somewhat arbitrary limit for the exponent (65535 now)
to avoid accidentally put such numbers (or potential DoS attack)
that will cause to hog large amount of memory.

ChangeLog

@@ -1,3 +1,8 @@
+2024-12-31  Shiro Kawai  <[email protected]>
+
+	* src/number.c (read_real): Allow much larger exponent for explicitly
+	  exact numbers.  https://github.com/shirok/Gauche/issues/898
+
 2024-12-29  Shiro Kawai  <[email protected]>
 
 	* src/read.c (read_number): Propagate numeric parser error message.

src/number.c

@@ -75,6 +75,13 @@ int Scm_IsInf(double x)
    memory.  So I assume it is reasonable to limit its range. */
 #define MAX_EXPONENT  325
 
+/* If the number if explicitly exact, we can allow much larger exponent---
+   theoretically as much as the memory can hold.  In reality, though,
+   exact numbers too big are too heavy to handle.  This is somewhat
+   arbitrary limitaition:
+ */
+#define MAX_EXACT_EXPONENT  65535
+
 /* Linux gcc have those, but the declarations aren't included unless
    __USE_ISOC9X is defined.  Just in case. */
 #ifdef HAVE_TRUNC
@@ -4723,10 +4730,12 @@ static ScmObj read_real(const char **strp, int *lenp,
             if (!isdigit(c)) break;
             underscore = FALSE;
             if (!exp_overflow) {
+                /* We limit the range of exponent, so this never overflow. */
                 exponent = exponent * 10 + (c - '0');
                 /* Check obviously wrong exponent range.  More subtle check
                    will be done later. */
-                if (exponent >= MAX_EXPONENT) {
+                if ((ctx->exactness == EXACT && exponent >= MAX_EXACT_EXPONENT)
+                    || (ctx->exactness != EXACT && exponent >= MAX_EXPONENT)) {
                     exp_overflow = TRUE;
                 }
             }
@@ -4739,8 +4748,7 @@ static ScmObj read_real(const char **strp, int *lenp,
                ratnum, such large (or small) exponent is highly unusual
                and we assume we can report implementation limitation
                violation. */
-            return numerr("Such an exact number is out of implementation limitation",
-                          ctx);
+            return numerr("Exact number exponent is too big", ctx);
         }
         if (exp_minusp || SCM_EQ(fraction, SCM_MAKE_INT(0))) {
             return Scm_MakeFlonum(minusp? -0.0:0.0);

test/number.scm

@@ -474,11 +474,13 @@
 (test* "exact fractonal number" (- (real-expt 10 296))
        (string->number "#e-0.0001e300"))
 
+;; This depends on MAX_EXACT_EXPONENT in number.c
+(test* "exact fractonal number" (real-expt 10 1000)
+       (read-from-string "#e1e1000"))
+(test* "exact fractonal number" (real-expt 10 -10000)
+       (read-from-string "#e1e-10000"))
 (test* "exact fractonal number" (test-error)
-       (read-from-string "#e1e330"))
-(test* "exact fractonal number" (test-error)
-       (read-from-string "#e1e-330"))
-
+       (read-from-string "#e1e-100000"))
 
 ;;------------------------------------------------------------------
 (test-section "complex reader")