improve performance with parsing bigints with lots of digits (#826)

Author: pjfanning

src/main/java/com/fasterxml/jackson/core/io/BigDecimalParser.java

@@ -37,7 +37,7 @@ public static BigDecimal parse(final char[] chars, final int off, final int len)
             return parseBigDecimal(chars, off, len, len / 10);
 
         // 20-Aug-2022, tatu: Although "new BigDecimal(...)" only throws NumberFormatException
-        //    operatons by "parseBigDecimal()" can throw "ArithmeticException", so handle both:
+        //    operations by "parseBigDecimal()" can throw "ArithmeticException", so handle both:
         } catch (ArithmeticException | NumberFormatException e) {
             String desc = e.getMessage();
             // 05-Feb-2021, tatu: Alas, JDK mostly has null message so:

src/main/java/com/fasterxml/jackson/core/io/NumberInput.java

@@ -8,6 +8,10 @@
 
 public final class NumberInput
 {
+    // numbers with more than these characters are better parsed with BigDecimalParser
+    // parsing numbers with many digits in Java is slower than O(n)
+    private final static int LARGE_INT_SIZE = 1250;
+
     /**
      * Formerly used constant for a value that was problematic on certain
      * pre-1.8 JDKs.
@@ -395,6 +399,9 @@ public static BigDecimal parseBigDecimal(char[] ch) throws NumberFormatException
      * @since v2.14
      */
     public static BigInteger parseBigInteger(String s) throws NumberFormatException {
+        if (s.length() > LARGE_INT_SIZE) {
+            return BigDecimalParser.parse(s).toBigInteger();
+        }
         return new BigInteger(s);
     }
 }

src/test/java/com/fasterxml/jackson/core/io/TestNumberInput.java

@@ -1,5 +1,7 @@
 package com.fasterxml.jackson.core.io;
 
+import java.math.BigInteger;
+
 public class TestNumberInput
     extends com.fasterxml.jackson.core.BaseTest
 {
@@ -24,5 +26,20 @@ public void testParseFloat()
         assertEquals("1.4E-45", Float.toString(NumberInput.parseFloat(exampleFloat2)));
         assertEquals("1.4E-45", Float.toString(NumberInput.parseFloat(exampleFloat2, true)));
     }
+
+    public void testParseLongBigInteger()
+    {
+        StringBuilder stringBuilder = new StringBuilder();
+        for (int i = 0; i < 1000; i++) {
+            stringBuilder.append(7);
+        }
+        String test1000 = stringBuilder.toString();
+        assertEquals(new BigInteger(test1000), NumberInput.parseBigInteger(test1000));
+        for (int i = 0; i < 1000; i++) {
+            stringBuilder.append(7);
+        }
+        String test2000 = stringBuilder.toString();
+        assertEquals(new BigInteger(test2000), NumberInput.parseBigInteger(test2000));
+    }
 }