Add more decimal calculations

src/main/java/me/mrletsplay/shareclientcore/document/Util.java

@@ -44,7 +44,27 @@ public class Util {
 		return null;
 	}
 
-	public static void getIncrement(Identifier[] i1, Identifier[] i2, int offset) {
+	public static int[] getIncremented(Identifier[] i1, Identifier[] i2, int offset) {
+		// TODO: can potentially be optimized by just calculating the index of the first non-zero digit (e.g. don't return array and discard it -> just return the index)
+		int[] delta = subtract(i2, i1, offset);
+		int firstNonZero = 0;
+		for(int i = 0; i < delta.length; i++) {
+			if(delta[i] != 0) {
+				firstNonZero = i;
+				break;
+			}
+		}
+
+		// Because of math, (firstNonZero + 1) >= i1.length (I think)
+		// then make the array 1 longer so we have space for the increment (which is one order of magnitude smaller)
+		int[] incremented = new int[firstNonZero + 2];
+		for(int i = 0; i < incremented.length; i++) {
+			incremented[i] = i <= i1.length ? i1[i].digit() : 0;
+		}
+
+		add1AtIndex(incremented, firstNonZero + 1); // last digit might be zero, which is ambigious, inc again
+		if(incremented[incremented.length - 1] == 0) add1AtIndex(incremented, firstNonZero + 1);
+		return incremented;
 	}
 
 	/**
@@ -67,7 +87,26 @@ public class Util {
 			}
 		}
 
+		if(carry == 1) throw new RuntimeException("Can't subtract numbers (are you sure a > b?)");
+
 		return diff;
 	}
 
+	public static void add1AtIndex(int[] a, int index) {
+		// Technically, some of the overflow preventions are probably overkill since the biggest carry we can get is 1 (we're only adding a 1 somewhere after all)
+
+		int carry = 0;
+		for(int i = a.length - 1; i >= 0; i--) {
+			int dA = a[i];
+			int dB = (index == i ? 1 : 0) + carry;
+
+			carry = dB - (BASE - dA) + 1; // Calculate carry and be overflow-safe
+			if(carry < 0) carry = 0;
+
+			a[i] = carry != 0 ? -BASE + dA + dB : dA + dB; // Calculate sum of digits and be overflow-safe
+		}
+
+		if(carry == 1) throw new RuntimeException("Can't add numbers: Too large");
+	}
+
 }

src/test/java/me/mrletsplay/shareclientcore/DecimalTest.java

@@ -1,6 +1,7 @@
 package me.mrletsplay.shareclientcore;
 
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
 
@@ -13,21 +14,56 @@ public class DecimalTest {
 	public void testSimpleSubtraction() {
 		Identifier[] a = new Identifier[] { new Identifier(1, 0), new Identifier(2, 0), new Identifier(3, 0) };
 		Identifier[] b = new Identifier[] { new Identifier(1, 0), new Identifier(1, 0), new Identifier(3, 0) };
-		assertArrayEquals(Util.subtract(a, b, 0), new int[] { 0, 1, 0 });
+		assertArrayEquals(new int[] { 0, 1, 0 }, Util.subtract(a, b, 0));
 	}
 
 	@Test
 	public void testCarrySubtraction() {
 		Identifier[] a = new Identifier[] { new Identifier(2, 0), new Identifier(0, 0), new Identifier(4, 0) };
 		Identifier[] b = new Identifier[] { new Identifier(1, 0), new Identifier(0, 0), new Identifier(5, 0) };
-		assertArrayEquals(Util.subtract(a, b, 0), new int[] { 0, 9, 9 });
+		assertArrayEquals(new int[] { 0, Util.BASE - 1, Util.BASE - 1 }, Util.subtract(a, b, 0));
 	}
 
 	@Test
 	public void testOffsetSubtraction() {
 		Identifier[] a = new Identifier[] { new Identifier(1, 0), new Identifier(0, 0), new Identifier(5, 0) };
 		Identifier[] b = new Identifier[] { new Identifier(1, 0), new Identifier(0, 0), new Identifier(4, 0) };
-		assertArrayEquals(Util.subtract(a, b, 1), new int[] { 0, 1 });
+		assertArrayEquals(new int[] { 0, 1 }, Util.subtract(a, b, 1));
+	}
+
+	@Test
+	public void testReversedInputFails() {
+		Identifier[] a = new Identifier[] { new Identifier(1, 0), new Identifier(1, 0), new Identifier(3, 0) };
+		Identifier[] b = new Identifier[] { new Identifier(1, 0), new Identifier(2, 0), new Identifier(3, 0) };
+		assertThrows(RuntimeException.class, () -> Util.subtract(a, b, 0));
+	}
+
+	@Test
+	public void testSimpleAdd1() {
+		int[] a = new int[] { 0, 0 };
+		Util.add1AtIndex(a, 1);
+		assertArrayEquals(new int[] { 0, 1 }, a);
+	}
+
+	@Test
+	public void testCarryAdd1() {
+		int[] a = new int[] { 0, Util.BASE - 1 };
+		Util.add1AtIndex(a, 1);
+		assertArrayEquals(new int[] { 1, 0 }, a);
+	}
+
+	@Test
+	public void testCarryAdd1_2() {
+		int[] a = new int[] { 0, Util.BASE - 1, Util.BASE - 1 };
+		Util.add1AtIndex(a, 2);
+		assertArrayEquals(new int[] { 1, 0, 0 }, a);
+	}
+
+	@Test
+	public void testCarryAdd1_3() {
+		int[] a = new int[] { 0, Util.BASE - 1, Util.BASE - 1 };
+		Util.add1AtIndex(a, 1);
+		assertArrayEquals(new int[] { 1, 0, Util.BASE - 1 }, a);
 	}
 
 }