Filter to read the last 'n' lines.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@271394 13f79535-47bb-0310-9956-ffa450edef68

proposal/sandbox/filterreaders/src/main/org/apache/tools/ant/filters/TailFilter.java

@@ -0,0 +1,168 @@
+package org.apache.tools.ant.filters;
+
+import java.io.FilterReader;
+import java.io.IOException;
+import java.io.Reader;
+
+import org.apache.tools.ant.types.Parameter;
+import org.apache.tools.ant.types.Parameterizable;
+
+/**
+ * Read the last n lines.  Default is last 10 lines.
+ *
+ * Example:
+ * =======
+ *
+ * <filterreader classname="org.apache.tools.ant.filters.TailFilter">
+ *    <param name="lines" value="3"/>
+ * </filterreader>
+ *
+ * @author <a href="mailto:umagesh@apache.org">Magesh Umasankar</a>
+ */
+public final class TailFilter
+    extends FilterReader
+    implements Parameterizable
+{
+    private static final String LINES_KEY = "lines";
+
+    private Parameter[] parameters;
+
+    private boolean initialized = false;
+
+    private long linesRead = 0;
+
+    private long lines = 10;
+
+    private boolean ignoreLineFeed = false;
+
+    private char[] buffer = new char[4096];
+
+    private int returnedCharPos = -1;
+
+    private boolean completedReadAhead = false;
+
+    private int bufferPos = 0;
+
+    /**
+     * Create a new filtered reader.
+     *
+     * @param in  a Reader object providing the underlying stream.
+     */
+    public TailFilter(final Reader in) {
+        super(in);
+    }
+
+    /**
+     * Read ahead and keep in buffer last n lines only at any given
+     * point.  Grow buffer as needed.
+     */
+    public final int read() throws IOException {
+        if (!initialized) {
+            initialize();
+            initialized = true;
+        }
+
+        if (!completedReadAhead) {
+            int ch = -1;
+            while ((ch = in.read()) != -1) {
+                if (buffer.length == bufferPos) {
+                    if (returnedCharPos != -1) {
+                        final char[] tmpBuffer = new char[buffer.length];
+                        System.arraycopy(buffer, returnedCharPos + 1, tmpBuffer,
+                                         0, buffer.length - (returnedCharPos + 1));
+                        buffer = tmpBuffer;
+                        bufferPos = bufferPos - (returnedCharPos + 1);
+                        returnedCharPos = -1;
+                    } else {
+                        final char[] tmpBuffer = new char[buffer.length * 2];
+                        System.arraycopy(buffer, 0, tmpBuffer, 0, bufferPos);
+                        buffer = tmpBuffer;
+                    }
+                }
+
+                if (ignoreLineFeed) {
+                    if (ch == '\n') {
+                        ch = in.read();
+                    }
+                    ignoreLineFeed = false;
+                }
+
+                switch (ch) {
+                    case '\r':
+                        ch = '\n';
+                        ignoreLineFeed = true;
+                        //fall through
+                    case '\n':
+                        linesRead++;
+
+                        if (linesRead == lines + 1) {
+                            int i = 0;
+                            for (i = returnedCharPos + 1; buffer[i] != '\n'; i++) {
+                            }
+                            returnedCharPos = i;
+                            linesRead--;
+                        }
+                        break;
+                }
+                if (ch == -1) {
+                    break;
+                }
+
+                buffer[bufferPos] = (char) ch;
+                bufferPos++;
+            }
+            completedReadAhead = true;
+        }
+
+        ++returnedCharPos;
+        if (returnedCharPos >= bufferPos) {
+            return -1;
+        } else {
+            return buffer[returnedCharPos];
+        }
+    }
+
+    public final int read(final char cbuf[], final int off,
+                          final int len) throws IOException {
+        for (int i = 0; i < len; i++) {
+            final int ch = read();
+            if (ch == -1) {
+                if (i == 0) {
+                    return -1;
+                } else {
+                    return i;
+                }
+            }
+            cbuf[off + i] = (char) ch;
+        }
+        return len;
+    }
+
+    public final long skip(long n) throws IOException {
+        for (long i = 0; i < n; i++) {
+            if (in.read() == -1) {
+                return i;
+            }
+        }
+        return n;
+    }
+
+    /**
+     * Set Parameters
+     */
+    public final void setParameters(final Parameter[] parameters) {
+        this.parameters = parameters;
+        initialized = false;
+    }
+
+    private final void initialize() {
+        if (parameters != null) {
+            for (int i = 0; i < parameters.length; i++) {
+                if (LINES_KEY.equals(parameters[i].getName())) {
+                    lines = new Long(parameters[i].getValue()).longValue();
+                    break;
+                }
+            }
+        }
+    }
+}