Fix Integer.sqrt to never exceed actual value

`Integer.sqrt` uses `sqrt(3)` from libm for small values.
This method must return a value less than or equal to the actual integer
square root, but libm's sqrt does not always guarantee that.

This change corrects that by decrementing the result if necessary.

Fixes [Bug #21217]

Author: mame

numeric.c

@@ -5978,7 +5978,11 @@ prefix##_isqrt(argtype n) \
         while ((t = n/x) < (argtype)x) x = (rettype)((x + t) >> 1); \
         return x; \
     } \
-    return (rettype)sqrt(argtype##_TO_DOUBLE(n)); \
+    rettype x = (rettype)sqrt(argtype##_TO_DOUBLE(n)); \
+    /* libm sqrt may returns a larger approximation than actual. */ \
+    /* Our isqrt always returns a smaller approximation. */ \
+    if (x * x > n) x--; \
+    return x; \
 }
 
 #if SIZEOF_LONG*CHAR_BIT > DBL_MANT_DIG

test/ruby/test_integer.rb

@@ -708,6 +708,10 @@ def test_square_root
     assert_equal(x, Integer.sqrt(x ** 2), "[ruby-core:95453]")
   end
 
+  def test_bug_21217
+    assert_equal(0x10000 * 2**10, Integer.sqrt(0x100000008 * 2**20))
+  end
+
   def test_fdiv
     assert_equal(1.0, 1.fdiv(1))
     assert_equal(0.5, 1.fdiv(2))