Interpolation between two integers taking N moves without using floats and avoiding division and modulus.

Intlerp.java

package org.magnos.path;

import static org.junit.Assert.*;

import org.junit.Test;

public class TestIntlerp 
{
	
	public class Intlerp 
	{
		public int value, jump, error, errorIncrement, errorMax, sign;
		
		public void reset(int start, int end, int moves) {
			int signedGap = end - start;
			int gap = Math.abs( signedGap );
			value = start;
			sign = Integer.signum( signedGap );
			jump = signedGap / moves;
			errorMax = moves;
			errorIncrement = Math.min( gap - (moves * Math.abs( jump )), moves );
			error = moves >> 1;
		}
		
		public int next() {
			value += jump;
			error += errorIncrement;
			if (error >= errorMax) {
				value += sign;
				error -= errorMax;
			}
			return value;
		}
	}

	@Test
	public void test()
	{
		final int BOUNDS = 128;
		final int N_MAX = 256;
		
		Intlerp x = new Intlerp();
		
		for (int a = -BOUNDS; a < BOUNDS; a++)
		{
			for (int b = -BOUNDS; b < BOUNDS; b++)
			{
				for (int N = 2; N < N_MAX; N++)
				{
					runTest( x, a, b, N );
				}
			}
		}
		
	}
	
	public void runTest( Intlerp x, int a, int b, int N )
	{
		x.reset(a, b, N);
		
		for (int i = 0; i < N; i++) {
			x.next();
		}
		
		assertEquals( String.format( "%d to %d in %d", a, b, N), b, x.value );
	}
	
}