Improve Number.prototype.toString for radix other than 10

- fix the conversions for integers and exact fractions
- approximate approach for other cases.
- bypass floating point conversions for JS_TAG_INT values
- avoid divisions for base 10 integer conversions

quickjs.c

@@ -11328,6 +11328,8 @@ static JSValue js_bigdecimal_to_string(JSContext *ctx, JSValueConst val)
 #endif /* CONFIG_BIGNUM */
 
 /* 2 <= base <= 36 */
+static char const digits[36] = "0123456789abcdefghijklmnopqrstuvwxyz";
+
 static char *i64toa(char *buf_end, int64_t n, unsigned int base)
 {
     char *q = buf_end;
@@ -11339,15 +11341,20 @@ static char *i64toa(char *buf_end, int64_t n, unsigned int base)
         n = -n;
     }
     *--q = '\0';
+    if (base == 10) {
+        /* division by known base uses multiplication */
+        do {
+            digit = (uint64_t)n % 10;
+            n = (uint64_t)n / 10;
+            *--q = '0' + digit;
+        } while (n != 0);
+    } else {
         do {
             digit = (uint64_t)n % base;
             n = (uint64_t)n / base;
-        if (digit < 10)
-            digit += '0';
-        else
-            digit += 'a' - 10;
-        *--q = digit;
+            *--q = digits[digit];
         } while (n != 0);
+    }
     if (is_neg)
         *--q = '-';
     return q;
@@ -11595,6 +11602,80 @@ static JSValue js_dtoa(JSContext *ctx,
     return JS_NewString(ctx, buf);
 }
 
+static JSValue js_dtoa_radix(JSContext *ctx, double d, int radix)
+{
+    char buf[2200], *ptr, *ptr2;
+    /* d is finite */
+    int sign = d < 0;
+    int digit;
+    double frac, d0;
+    int64_t n0 = 0;
+    d = fabs(d);
+    d0 = trunc(d);
+    frac = d - d0;
+    ptr = buf + 1100;
+    *ptr = '\0';
+    if (d0 <= MAX_SAFE_INTEGER) {
+        int64_t n = n0 = (int64_t)d0;
+        while (n >= radix) {
+            digit = n % radix;
+            n = n / radix;
+            *--ptr = digits[digit];
+        }
+        *--ptr = digits[(int)n];
+    } else {
+        /* no decimals */
+        while (d0 >= radix) {
+            digit = fmod(d0, radix);
+            d0 = trunc(d0 / radix);
+            if (d0 >= MAX_SAFE_INTEGER)
+                digit = 0;
+            *--ptr = digits[digit];
+        }
+        *--ptr = digits[(int)d0];
+        goto done;
+    }
+    if (frac != 0) {
+        double log2_radix = log2(radix);
+        double prec = 1023 + 51;  // handle subnormals
+        ptr2 = buf + 1100;
+        *ptr2++ = '.';
+        while (frac != 0 && n0 <= MAX_SAFE_INTEGER/2 && prec > 0) {
+            frac *= radix;
+            digit = trunc(frac);
+            frac -= digit;
+            *ptr2++ = digits[digit];
+            n0 = n0 * radix + digit;
+            prec -= log2_radix;
+        }
+        *ptr2 = '\0';
+        if (frac * radix >= radix / 2) {
+            char nine = digits[radix - 1];
+            // round to closest
+            while (ptr2[-1] == nine)
+                *--ptr2 = '\0';
+            if (ptr2[-1] == '.') {
+                *--ptr2 = '\0';
+                while (ptr2[-1] == nine)
+                    *--ptr2 = '0';
+            }
+            if (ptr2 - 1 == ptr)
+                *--ptr = '1';
+            else
+                ptr2[-1] += 1;
+        } else {
+            while (ptr2[-1] == '0')
+                *--ptr2 = '\0';
+            if (ptr2[-1] == '.')
+                *--ptr2 = '\0';
+        }
+    }
+done:
+    ptr[-1] = '-';
+    ptr -= sign;
+    return JS_NewString(ctx, ptr);
+}
+
 JSValue JS_ToStringInternal(JSContext *ctx, JSValueConst val, BOOL is_ToPropertyKey)
 {
     uint32_t tag;
@@ -40975,8 +41056,16 @@ static JSValue js_number_toString(JSContext *ctx, JSValueConst this_val,
         if (base < 0)
             goto fail;
     }
+    if (JS_VALUE_GET_TAG(val) == JS_TAG_INT) {
+        char buf1[70], *ptr;
+        ptr = i64toa(buf1 + sizeof(buf1), JS_VALUE_GET_INT(val), base);
+        return JS_NewString(ctx, ptr);
+    }
     if (JS_ToFloat64Free(ctx, &d, val))
         return JS_EXCEPTION;
+    if (base != 10 && isfinite(d)) {
+        return js_dtoa_radix(ctx, d, base);
+    }
     return js_dtoa(ctx, d, base, 0, JS_DTOA_VAR_FORMAT);
  fail:
     JS_FreeValue(ctx, val);