package junit.framework;

public class ComparisonCompactor {

	private static final String ELLIPSIS= "...";

	private static final String DELTA_END= "]";

	private static final String DELTA_START= "[";

	private int fContextLength;

	private String fExpected;

	private String fActual;

	private int fPrefix;

	private int fSuffix;

	public ComparisonCompactor(int contextLength, String expected, String actual) {

		fContextLength= contextLength;

		fExpected= expected;

		fActual= actual;

	}

	public String compact(String message) {

		if (fExpected == null || fActual == null || areStringsEqual())

			return Assert.format(message, fExpected, fActual);

		findCommonPrefix();

		findCommonSuffix();

		String expected= compactString(fExpected);

		String actual= compactString(fActual);

		return Assert.format(message, expected, actual);

	}

	private String compactString(String source) {

		String result= DELTA_START + source.substring(fPrefix, source.length() - fSuffix + 1) + DELTA_END;

		if (fPrefix > 0)

			result= computeCommonPrefix() + result;

		if (fSuffix > 0)

			result= result + computeCommonSuffix();

		return result;

	}

	private void findCommonPrefix() {

		fPrefix= 0;

		int end= Math.min(fExpected.length(), fActual.length());

		for (; fPrefix < end; fPrefix++) {

			if (fExpected.charAt(fPrefix) != fActual.charAt(fPrefix))

				break;

		}

	}

	private void findCommonSuffix() {

		int expectedSuffix= fExpected.length() - 1;

		int actualSuffix= fActual.length() - 1;

		for (; actualSuffix >= fPrefix && expectedSuffix >= fPrefix; actualSuffix--, expectedSuffix--) {

			if (fExpected.charAt(expectedSuffix) != fActual.charAt(actualSuffix))

				break;

		}

		fSuffix=  fExpected.length() - expectedSuffix;

	}

	private String computeCommonPrefix() {

		return (fPrefix > fContextLength ? ELLIPSIS : "") + fExpected.substring(Math.max(0, fPrefix - fContextLength), fPrefix);

	}

	private String computeCommonSuffix() {

		int end= Math.min(fExpected.length() - fSuffix + 1 + fContextLength, fExpected.length());

		return fExpected.substring(fExpected.length() - fSuffix + 1, end) + (fExpected.length() - fSuffix + 1 < fExpected.length() - fContextLength ? ELLIPSIS : "");

	}

	private boolean areStringsEqual() {

		return fExpected.equals(fActual);

	}

}