Use decimal_shift in log and exp calculation

Author: tompng

lib/bigdecimal.rb

@@ -205,7 +205,7 @@ def self.log(x, prec)
       if x > 10 || x < 0.1
         log10 = log(BigDecimal(10), prec)
         exponent = x.exponent
-        x *= BigDecimal("1e#{-x.exponent}")
+        x = x._decimal_shift(-exponent)
         if x < 0.3
           x *= 10
           exponent -= 1
@@ -269,33 +269,30 @@ def self.exp(x, prec)
     return BigDecimal(1) if x.zero?
     return BigDecimal(1).div(exp(-x, prec), prec) if x < 0
 
-    BigDecimal.save_limit do
-      BigDecimal.limit(0)
-      # exp(x * 10**cnt) = exp(x)**(10**cnt)
-      cnt = x > 1 ? x.exponent : 0
-      prec2 = prec + BigDecimal.double_fig + cnt
-      x *= BigDecimal("1e-#{cnt}")
-      xn = BigDecimal(1)
-      y = BigDecimal(1)
-
-      # Taylor series for exp(x) around 0
-      1.step do |i|
-        n = prec2 + xn.exponent
-        break if n <= 0 || xn.zero?
-        x = x.mult(1, n)
-        xn = xn.mult(x, n).div(i, n)
-        y = y.add(xn, prec2)
-      end
-
-      # calculate exp(x * 10**cnt) from exp(x)
-      # exp(x * 10**k) = exp(x * 10**(k - 1)) ** 10
-      cnt.times do
-        y2 = y.mult(y, prec2)
-        y5 = y2.mult(y2, prec2).mult(y, prec2)
-        y = y5.mult(y5, prec2)
-      end
+    # exp(x * 10**cnt) = exp(x)**(10**cnt)
+    cnt = x > 1 ? x.exponent : 0
+    prec2 = prec + BigDecimal.double_fig + cnt
+    x = x._decimal_shift(-cnt)
+    xn = BigDecimal(1)
+    y = BigDecimal(1)
+
+    # Taylor series for exp(x) around 0
+    1.step do |i|
+      n = prec2 + xn.exponent
+      break if n <= 0 || xn.zero?
+      x = x.mult(1, n)
+      xn = xn.mult(x, n).div(i, n)
+      y = y.add(xn, prec2)
+    end
 
-      y.mult(1, prec)
+    # calculate exp(x * 10**cnt) from exp(x)
+    # exp(x * 10**k) = exp(x * 10**(k - 1)) ** 10
+    cnt.times do
+      y2 = y.mult(y, prec2)
+      y5 = y2.mult(y2, prec2).mult(y, prec2)
+      y = y5.mult(y5, prec2)
     end
+
+    y.mult(1, prec)
   end
 end