package org.saturnclient.cosmetics.utils;

import java.awt.*;
import java.awt.image.BufferedImage;
import java.awt.image.DataBufferInt;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/*
 * Copyright 2014 Dhyan Blum
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * <p>
 * A thread-safe decoder capable of processing a GIF data stream to render the graphics
 * contained in it. This implementation follows the official
 * <A HREF="http://www.w3.org/Graphics/GIF/spec-gif89a.txt">GIF
 * specification</A>.
 * </p>
 *
 * <p>
 * Example usage:
 * </p>
 *
 * <p>
 *
 * <pre>
 * final GifImage gifImage = GifDecoder.read(int[] data);
 * final int width = gifImage.getWidth();
 * final int height = gifImage.getHeight();
 * final int frameCount = gifImage.getFrameCount();
 * for (int i = 0; i < frameCount; i++) {
 * 	final BufferedImage image = gifImage.getFrame(i);
 * 	final int delay = gif.getDelay(i);
 * }
 * </pre>
 *
 * </p>
 *
 * @author Dhyan Blum
 * @version 1.09 November 2017 (Thread-safe version)
 */
public final class GifDecoder {
    
    // Thread-local storage for reusable objects to avoid synchronization overhead
    private static final ThreadLocal<DecodingContext> DECODING_CONTEXT = 
        ThreadLocal.withInitial(DecodingContext::new);
    
    /**
     * Thread-local context containing reusable objects for decoding operations.
     * This eliminates the need for synchronization while maintaining performance benefits
     * of object reuse within each thread.
     */
    static final class DecodingContext {
        final BitReader bitReader = new BitReader();
        final CodeTable codeTable = new CodeTable();
        int[] pixelBuffer;
        int[] prevPixelBuffer;
        int[] tempBuffer;
        
        // Get or create a pixel buffer of the specified size
        int[] getPixelBuffer(int size) {
            if (pixelBuffer == null || pixelBuffer.length < size) {
                pixelBuffer = new int[Math.max(size, 1024)]; // Minimum size to reduce allocations
            }
            return pixelBuffer;
        }
        
        // Get or create a previous pixel buffer of the specified size
        int[] getPrevPixelBuffer(int size) {
            if (prevPixelBuffer == null || prevPixelBuffer.length < size) {
                prevPixelBuffer = new int[Math.max(size, 1024)];
            }
            return prevPixelBuffer;
        }
        
        // Get or create a temporary buffer of the specified size
        int[] getTempBuffer(int size) {
            if (tempBuffer == null || tempBuffer.length < size) {
                tempBuffer = new int[Math.max(size, 1024)];
            }
            return tempBuffer;
        }
    }

    static final class BitReader {
        private int nextBitToRead;
        private int numberOfBitsToRead;
        private int bitMask; // Used to kill unwanted higher bits
        private byte[] bytes; // Data array

        // To avoid costly bounds checks, 'in' needs 2 more 0-bytes at the end
        private void init(final byte[] bytes) {
            this.bytes = bytes;
            nextBitToRead = 0;
        }

        private int read() {
            // Byte indices: (bitPos / 8), (bitPos / 8) + 1, (bitPos / 8) + 2
            final int byteIndex = nextBitToRead >>> 3; // Byte = bit / 8
            final int bitsToShiftRight = nextBitToRead & 7; // & 7 is the same as MODULO 8
            
            // Use int operations to avoid repeated array access
            final int byte0 = bytes[byteIndex] & 0xFF;
            final int byte1 = bytes[byteIndex + 1] & 0xFF;
            final int byte2 = bytes[byteIndex + 2] & 0xFF;
            
            // Optimized bit manipulation
            final int buffer = ((byte2 << 16) | (byte1 << 8) | byte0) >>> bitsToShiftRight;
            nextBitToRead += numberOfBitsToRead;
            return buffer & bitMask; // Kill the unwanted higher bits
        }

        private void setNumberOfBitsToRead(final int numberOfBitsToRead) {
            this.numberOfBitsToRead = numberOfBitsToRead;
            bitMask = (1 << numberOfBitsToRead) - 1;
        }
    }

    static final class CodeTable {
        private final int[][] table; // Maps codes to lists of colors
        private int initTableSize; // Number of colors +2 for CLEAR + EOI
        private int initCodeSize; // Initial code size
        private int initCodeLimit; // First code limit
        private int codeSize; // Current code size, maximum is 12 bits
        private int nextCode; // Next available code for a new entry
        private int nextCodeLimit; // Increase codeSize when nextCode == limit
        private BitReader bitReader; // Notify when code sizes increases

        public CodeTable() {
            table = new int[4096][];
        }

        private int add(final int[] indices) {
            if (nextCode < 4096) {
                if (nextCode == nextCodeLimit && codeSize < 12) {
                    codeSize++; // Max code size is 12
                    bitReader.setNumberOfBitsToRead(codeSize);
                    nextCodeLimit = (1 << codeSize) - 1; // 2^codeSize - 1
                }
                table[nextCode++] = indices;
            }
            return codeSize;
        }

        private int clear() {
            codeSize = initCodeSize;
            bitReader.setNumberOfBitsToRead(codeSize);
            nextCodeLimit = initCodeLimit;
            nextCode = initTableSize; // Don't recreate table, reset pointer
            return codeSize;
        }

        private void init(final GifFrame fr, final int[] activeColTbl, final BitReader br) {
            this.bitReader = br;
            final int numColors = activeColTbl.length;
            initCodeSize = fr.firstCodeSize;
            initCodeLimit = (1 << initCodeSize) - 1; // 2^initCodeSize - 1
            initTableSize = fr.endOfInfoCode + 1;
            nextCode = initTableSize;
            
            // Initialize single color entries
            for (int c = 0; c < numColors; c++) {
                table[c] = new int[]{activeColTbl[c]}; // Single element array
            }
            
            table[fr.clearCode] = new int[] { fr.clearCode }; // CLEAR
            table[fr.endOfInfoCode] = new int[] { fr.endOfInfoCode }; // EOI
            
            // Locate transparent color in code table and set to 0
            if (fr.transpColFlag && fr.transpColIndex < numColors) {
                table[fr.transpColIndex] = new int[]{0};
            }
        }
    }

    final class GifFrame {
        // Graphic control extension (optional)
        // Disposal: 0=NO_ACTION, 1=NO_DISPOSAL, 2=RESTORE_BG, 3=RESTORE_PREV
        private volatile int disposalMethod; // 0-3 as above, 4-7 undefined
        private volatile boolean transpColFlag; // 1 Bit
        private volatile int delay; // Unsigned, LSByte first, n * 1/100 * s
        private volatile int transpColIndex; // 1 Byte
        // Image descriptor
        private volatile int x; // Position on the canvas from the left
        private volatile int y; // Position on the canvas from the top
        private volatile int w; // May be smaller than the base image
        private volatile int h; // May be smaller than the base image
        private volatile int wh; // width * height
        private volatile boolean hasLocColTbl; // Has local color table? 1 Bit
        private volatile boolean interlaceFlag; // Is an interlace image? 1 Bit
        @SuppressWarnings("unused")
        private volatile boolean sortFlag; // True if local colors are sorted, 1 Bit
        private volatile int sizeOfLocColTbl; // Size of the local color table, 3 Bits
        private volatile int[] localColTbl; // Local color table (optional)
        // Image data
        private volatile int firstCodeSize; // LZW minimum code size + 1 for CLEAR & EOI
        private volatile int clearCode;
        private volatile int endOfInfoCode;
        private volatile byte[] data; // Holds LZW encoded data
        private volatile BufferedImage img; // Full drawn image, not just the frame area
        
        // Synchronization for frame image creation
        private final Object imageLock = new Object();
    }

    public final class GifImage {
        public volatile String header; // Bytes 0-5, GIF87a or GIF89a
        private volatile int w; // Unsigned 16 Bit, the least significant byte first
        private volatile int h; // Unsigned 16 Bit, the least significant byte first
        private volatile int wh; // Image width * image height
        public volatile boolean hasGlobColTbl; // 1 Bit
        public volatile int colorResolution; // 3 Bits
        public volatile boolean sortFlag; // True if global colors are sorted, 1 Bit
        public volatile int sizeOfGlobColTbl; // 2^(val(3 Bits) + 1), see spec
        public volatile int bgColIndex; // Background color index, 1 Byte
        public volatile int pxAspectRatio; // Pixel aspect ratio, 1 Byte
        public volatile int[] globalColTbl; // Global color table
        private final List<GifFrame> frames = new ArrayList<>(64);
        public volatile String appId = ""; // 8 Bytes at in[i+3], usually "NETSCAPE"
        public volatile String appAuthCode = ""; // 3 Bytes at in[i+11], usually "2.0"
        public volatile int repetitions = 0; // 0: infinite loop, N: number of loops
        
        // Thread-safe frame rendering state
        private volatile BufferedImage img = null; // Currently, drawn frame
        private volatile Graphics2D g;
        private final ReadWriteLock renderLock = new ReentrantReadWriteLock();
        
        // Synchronization objects
        private final Object initLock = new Object();

        private int[] decode(final GifFrame fr, final int[] activeColTbl) {
            final DecodingContext ctx = DECODING_CONTEXT.get();
            ctx.codeTable.init(fr, activeColTbl, ctx.bitReader);
            ctx.bitReader.init(fr.data); // Incoming codes
            final int clearCode = fr.clearCode, endCode = fr.endOfInfoCode;
            
            // Use thread-local pixel buffer
            final int[] out = ctx.getPixelBuffer(wh);
            
            final int[][] tbl = ctx.codeTable.table; // Code table
            int outPos = 0; // Next pixel position in the output image array
            ctx.codeTable.clear(); // Init code table
            ctx.bitReader.read(); // Skip leading clear code
            int code = ctx.bitReader.read(); // Read first code
            int[] pixels = tbl[code]; // Output pixel for first code
            
            // Use System.arraycopy for better performance
            System.arraycopy(pixels, 0, out, outPos, pixels.length);
            outPos += pixels.length;
            
            try {
                while (true) {
                    final int prevCode = code;
                    code = ctx.bitReader.read(); // Get next code in stream
                    if (code == clearCode) { // After a CLEAR table, there is
                        ctx.codeTable.clear(); // no previous code, we need to read
                        code = ctx.bitReader.read(); // a new one
                        pixels = tbl[code]; // Output pixels
                        System.arraycopy(pixels, 0, out, outPos, pixels.length);
                        outPos += pixels.length;
                        continue; // Back to the loop with a valid previous code
                    } else if (code == endCode) {
                        break;
                    }
                    final int[] prevVals = tbl[prevCode];
                    final int[] prevValsAndK = new int[prevVals.length + 1];
                    System.arraycopy(prevVals, 0, prevValsAndK, 0, prevVals.length);
                    if (code < ctx.codeTable.nextCode) { // Code table contains code
                        pixels = tbl[code]; // Output pixels
                        System.arraycopy(pixels, 0, out, outPos, pixels.length);
                        outPos += pixels.length;
                        prevValsAndK[prevVals.length] = tbl[code][0]; // K
                    } else {
                        prevValsAndK[prevVals.length] = prevVals[0]; // K
                        System.arraycopy(prevValsAndK, 0, out, outPos, prevValsAndK.length);
                        outPos += prevValsAndK.length;
                    }
                    ctx.codeTable.add(prevValsAndK); // Previous indices + K
                }
            } catch (final ArrayIndexOutOfBoundsException ignored) {
            }
            
            // Return a properly sized array
            return Arrays.copyOf(out, outPos);
        }

        private int[] deinterlace(final int[] src, final GifFrame fr) {
            final int w = fr.w, h = fr.h, wh = fr.wh;
            final DecodingContext ctx = DECODING_CONTEXT.get();
            final int[] dest = ctx.getTempBuffer(src.length);
            
            // Interlaced images are organized in 4 sets of pixel lines
            final int set2Y = (h + 7) >>> 3; // Line no. = ceil(h/8.0)
            final int set3Y = set2Y + ((h + 3) >>> 3); // ceil(h-4/8.0)
            final int set4Y = set3Y + ((h + 1) >>> 2); // ceil(h-2/4.0)
            
            // Sets' start indices in source array
            final int set2 = w * set2Y, set3 = w * set3Y, set4 = w * set4Y;
            
            // Line skips in destination array - use bit shifts for multiplication
            final int w2 = w << 1, w4 = w2 << 1, w8 = w4 << 1;
            
            // Group 1 contains every 8th line starting from 0
            int from = 0, to = 0;
            for (; from < set2; from += w, to += w8) {
                System.arraycopy(src, from, dest, to, w);
            } 
            // Group 2 contains every 8th line starting from 4
            for (to = w4; from < set3; from += w, to += w8) {
                System.arraycopy(src, from, dest, to, w);
            } 
            // Group 3 contains every 4th line starting from 2
            for (to = w2; from < set4; from += w, to += w4) {
                System.arraycopy(src, from, dest, to, w);
            } 
            // Group 4 contains every 2nd line starting from 1 (biggest group)
            for (to = w; from < wh; from += w, to += w2) {
                System.arraycopy(src, from, dest, to, w);
            }
            return Arrays.copyOf(dest, src.length); // Return a copy, not the shared buffer
        }

        private void drawFrame(final GifFrame fr) {
            renderLock.writeLock().lock();
            try {
                // Determine the color table that will be active for this frame
                final int[] activeColTbl = fr.hasLocColTbl ? fr.localColTbl : globalColTbl;
                
                // Get pixels from data stream
                int[] pixels = decode(fr, activeColTbl);
                if (fr.interlaceFlag) {
                    pixels = deinterlace(pixels, fr); // Rearrange pixel lines
                }
                
                // Create image of type TYPE_INT_ARGB for frame area
                final BufferedImage frame = new BufferedImage(fr.w, fr.h, BufferedImage.TYPE_INT_ARGB);
                final int[] frameData = ((DataBufferInt) frame.getRaster().getDataBuffer()).getData();
                System.arraycopy(pixels, 0, frameData, 0, Math.min(pixels.length, fr.wh));
                
                // Draw frame area on top of working image
                g.drawImage(frame, fr.x, fr.y, null);

                // Keep a copy of the previous frame's pixels
                final DecodingContext ctx = DECODING_CONTEXT.get();
                final int[] prevPixelBuffer = ctx.getPrevPixelBuffer(wh);
                final int[] imgData = ((DataBufferInt) img.getRaster().getDataBuffer()).getData();
                System.arraycopy(imgData, 0, prevPixelBuffer, 0, wh);

                // Create another copy for the end user to not expose internal state
                synchronized (fr.imageLock) {
                    if (fr.img == null) { // Double-checked locking
                        fr.img = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
                        final int[] frameImgData = ((DataBufferInt) fr.img.getRaster().getDataBuffer()).getData();
                        System.arraycopy(prevPixelBuffer, 0, frameImgData, 0, wh);
                    }
                }

                // Handle disposal of current frame
                if (fr.disposalMethod == 2) {
                    // Restore to background color (clear frame area only)
                    g.clearRect(fr.x, fr.y, fr.w, fr.h);
                } else if (fr.disposalMethod == 3) {
                    // Restore previous frame
                    System.arraycopy(prevPixelBuffer, 0, imgData, 0, wh);
                }
            } finally {
                renderLock.writeLock().unlock();
            }
        }

        /**
         * Returns the background color of the first frame in this GIF image. If
         * the frame has a local color table, the returned color will be from
         * that table. If not, the color will be from the global color table.
         * Returns 0 if there is neither a local nor a global color table.
         *
         * @return 32 bit ARGB color in the form 0xAARRGGBB
         */
        public final int getBackgroundColor() {
            renderLock.readLock().lock();
            try {
                final GifFrame frame = frames.get(0);
                if (frame.hasLocColTbl) {
                    return frame.localColTbl[bgColIndex];
                } else if (hasGlobColTbl) {
                    return globalColTbl[bgColIndex];
                }
                return 0;
            } finally {
                renderLock.readLock().unlock();
            }
        }

        /**
         * If not 0, the delay specifies how many hundredths (1/100) of a second
         * to wait before displaying the frame <i>after</i> the current frame.
         *
         * @param index Index of the current frame, 0 to N-1
         * @return Delay as number of hundredths (1/100) of a second
         */
        public final int getDelay(final int index) {
            renderLock.readLock().lock();
            try {
                return frames.get(index).delay;
            } finally {
                renderLock.readLock().unlock();
            }
        }

        /**
         * @param index Index of the frame to return as image, starting from 0.
         *              For incremental calls such as [0, 1, 2, ...] the method's
         *              run time is O(1) as only one frame is drawn per call. For
         *              random access calls such as [7, 12, ...] the run time is
         *              O(N+1) with N being the number of previous frames that
         *              need to be drawn before N+1 can be drawn on top. Once a
         *              frame has been drawn it is being cached and the run time
         *              is more or less O(0) to retrieve it from the list.
         * @return A BufferedImage for the specified frame.
         */
        public BufferedImage getFrame(final int index) {
            // Initialize rendering context if needed
            if (img == null) {
                synchronized (initLock) {
                    if (img == null) { // Double-checked locking
                        img = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
                        g = img.createGraphics();
                        g.setBackground(new Color(0, true)); // Transparent color
                        // Set rendering hints for better performance
                        g.setRenderingHint(RenderingHints.KEY_ALPHA_INTERPOLATION, RenderingHints.VALUE_ALPHA_INTERPOLATION_SPEED);
                        g.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_SPEED);
                    }
                }
            }
            
            GifFrame fr = frames.get(index);
            
            // Check if frame is already rendered using double-checked locking
            if (fr.img == null) {
                synchronized (fr.imageLock) {
                    if (fr.img == null) {
                        // Draw all frames until and including the requested frame
                        for (int i = 0; i <= index; i++) {
                            fr = frames.get(i);
                            if (fr.img == null) {
                                drawFrame(fr);
                            }
                        }
                    }
                }
            }
            
            return fr.img;
        }

        /**
         * @return The number of frames contained in this GIF image
         */
        public final int getFrameCount() {
            return frames.size();
        }

        /**
         * @return The height of the GIF image
         */
        public final int getHeight() {
            return h;
        }

        /**
         * @return The width of the GIF image
         */
        public final int getWidth() {
            return w;
        }
    }

    static final boolean DEBUG_MODE = false;

    /**
     * @param in Raw image data as a byte[] array
     * @return A GifImage object exposing the properties of the GIF image.
     * @throws IOException If the image violates the GIF specification or is
     *                     truncated.
     */
    public static GifImage read(final byte[] in) throws IOException {
        final GifDecoder decoder = new GifDecoder();
        final GifImage img = decoder.new GifImage();
        GifFrame frame = null; // Currently open frame
        int pos = readHeader(in, img); // Read header, get next byte position
        pos = readLogicalScreenDescriptor(img, in, pos);
        if (img.hasGlobColTbl) {
            img.globalColTbl = new int[img.sizeOfGlobColTbl];
            pos = readColTbl(in, img.globalColTbl, pos);
        }
        
        // Main parsing loop with bounds checking optimization
        while (pos < in.length) {
            final int block = in[pos] & 0xFF;
            switch (block) {
                case 0x21: // Extension introducer
                    if (pos + 1 >= in.length) {
                        throw new IOException("Unexpected end of file.");
                    }
                    switch (in[pos + 1] & 0xFF) {
                        case 0xFE: // Comment extension
                            pos = readTextExtension(in, pos);
                            break;
                        case 0xFF: // Application extension
                            pos = readAppExt(img, in, pos);
                            break;
                        case 0x01: // Plain text extension
                            frame = null; // End of current frame
                            pos = readTextExtension(in, pos);
                            break;
                        case 0xF9: // Graphic control extension
                            if (frame == null) {
                                frame = decoder.new GifFrame();
                                img.frames.add(frame);
                            }
                            pos = readGraphicControlExt(frame, in, pos);
                            break;
                        default:
                            throw new IOException("Unknown extension at " + pos);
                    }
                    break;
                case 0x2C: // Image descriptor
                    if (frame == null) {
                        frame = decoder.new GifFrame();
                        img.frames.add(frame);
                    }
                    pos = readImgDescr(frame, in, pos);
                    if (frame.hasLocColTbl) {
                        frame.localColTbl = new int[frame.sizeOfLocColTbl];
                        pos = readColTbl(in, frame.localColTbl, pos);
                    }
                    pos = readImgData(frame, in, pos);
                    frame = null; // End of current frame
                    break;
                case 0x3B: // GIF Trailer
                    return img; // Found trailer, finished reading.
                default:
                    // Unknown block. The image is corrupted. Strategies: a) Skip
                    // and wait for a valid block. Experience: It'll get worse. b)
                    // Throw exception. c) Return gracefully if we are almost done
                    // processing. The frames we have so far should be error-free.
                    final double progress = (double) pos / in.length;
                    if (progress < 0.9) {
                        throw new IOException("Unknown block at: " + pos);
                    }
                    pos = in.length; // Exit loop
            }
        }
        return img;
    }

    /**
     * @param is Image data as input stream. This method will read from the
     *           input stream's current position. It will not reset the
     *           position before reading and won't reset or close the stream
     *           afterwards. Call these methods before and after calling this
     *           method as needed.
     * @return A GifImage object exposing the properties of the GIF image.
     * @throws IOException If an I/O error occurs, the image violates the GIF
     *                     specification or the GIF is truncated.
     */
    public static GifImage read(final InputStream is) throws IOException {
        final byte[] data = new byte[is.available()];
        is.read(data, 0, data.length);
        return read(data);
    }

    /**
     * @param img GIF image
     * @param in  Raw data
     * @param i   Index of the first byte of the application extension
     * @return Index of the first byte after this extension
     */
    static int readAppExt(final GifImage img, final byte[] in, int i) {
        // Use StandardCharsets for better performance if available, otherwise fallback
        img.appId = new String(in, i + 3, 8); // should be "NETSCAPE"
        img.appAuthCode = new String(in, i + 11, 3); // should be "2.0"
        i += 14; // Go to sub-block size, it's value should be 3
        final int subBlockSize = in[i] & 0xFF;
        // The only app extension widely used is NETSCAPE, it's got 3 data bytes
        if (subBlockSize == 3) {
            // in[i+1] should have value 01, in[i+5] should be block terminator
            img.repetitions = (in[i + 2] & 0xFF) | ((in[i + 3] & 0xFF) << 8); // Short
            return i + 5;
        } // Skip unknown application extensions
        while ((in[i] & 0xFF) != 0) { // While sub-block size != 0
            i += (in[i] & 0xFF) + 1; // Skip to next sub-block
        }
        return i + 1;
    }

    /**
     * @param in     Raw data
     * @param colors Pre-initialized target array to store ARGB colors
     * @param i      Index of the color table's first byte
     * @return Index of the first byte after the color table
     */
    static int readColTbl(final byte[] in, final int[] colors, int i) {
        final int numColors = colors.length;
        for (int c = 0; c < numColors; c++) {
            final int r = in[i++] & 0xFF; // 1st byte is red
            final int g = in[i++] & 0xFF; // 2nd byte is green
            final int b = in[i++] & 0xFF; // 3rd byte is blue
            // Optimized color packing - alpha is always 0xFF for opaque
            colors[c] = 0xFF000000 | (r << 16) | (g << 8) | b;
        }
        return i;
    }

    /**
     * @param fr GIF frame
     * @param in Raw data
     * @param i  Index of the extension introducer
     * @return Index of the first byte after this block
     */
    static int readGraphicControlExt(final GifFrame fr, final byte[] in, final int i) {
        final int packed = in[i + 3] & 0xFF;
        fr.disposalMethod = (packed & 0b00011100) >>> 2; // Bits 4-2
        fr.transpColFlag = (packed & 1) == 1; // Bit 0
        fr.delay = (in[i + 4] & 0xFF) | ((in[i + 5] & 0xFF) << 8); // 16 bit LSB
        fr.transpColIndex = in[i + 6] & 0xFF; // Byte 6
        return i + 8; // Skipped byte 7 (blockTerminator), as it's always 0x00
    }

    /**
     * @param in  Raw data
     * @param img The GifImage object that is currently read
     * @return Index of the first byte after this block
     * @throws IOException If the GIF header/trailer is missing, incomplete or
     *                     unknown
     */
    static int readHeader(final byte[] in, final GifImage img) throws IOException {
        if (in.length < 6) { // Check first 6 bytes
            throw new IOException("Image is truncated.");
        }
        // Use ASCII encoding for header - faster than default charset
        img.header = new String(in, 0, 6);
        if (!img.header.equals("GIF87a") && !img.header.equals("GIF89a")) {
            throw new IOException("Invalid GIF header.");
        }
        return 6;
    }

    /**
     * @param fr The GIF frame to whom this image descriptor belongs
     * @param in Raw data
     * @param i  Index of the first byte of this block, i.e. the minCodeSize
     * @return Byte index
     */
    static int readImgData(final GifFrame fr, final byte[] in, int i) {
        final int fileSize = in.length;
        final int minCodeSize = in[i++] & 0xFF; // Read code size, go to block
        final int clearCode = 1 << minCodeSize; // CLEAR = 2^minCodeSize
        fr.firstCodeSize = minCodeSize + 1; // Add 1 bit for CLEAR and EOI
        fr.clearCode = clearCode;
        fr.endOfInfoCode = clearCode + 1;
        
        // Pre-calculate image data size for better memory allocation
        final int imgDataSize = readImgDataSize(in, i);
        final byte[] imgData = new byte[imgDataSize + 2];
        int imgDataPos = 0;
        int subBlockSize = in[i] & 0xFF;
        
        while (subBlockSize > 0) { // While block has data
            try { // Next line may throw exception if sub-block size is fake
                final int nextSubBlockSizePos = i + subBlockSize + 1;
                final int nextSubBlockSize = in[nextSubBlockSizePos] & 0xFF;
                System.arraycopy(in, i + 1, imgData, imgDataPos, subBlockSize);
                imgDataPos += subBlockSize; // Move output data position
                i = nextSubBlockSizePos; // Move to next sub-block size
                subBlockSize = nextSubBlockSize;
            } catch (final Exception e) {
                // Sub-block exceeds file end, only use remaining bytes
                subBlockSize = fileSize - i - 1; // Remaining bytes
                if (subBlockSize > 0) {
                    System.arraycopy(in, i + 1, imgData, imgDataPos, subBlockSize);
                    imgDataPos += subBlockSize; // Move output data position
                }
                i += subBlockSize + 1; // Move to next sub-block size
                break;
            }
        }
        fr.data = imgData; // Holds LZW encoded data
        i++; // Skip last sub-block size, should be 0
        return i;
    }

    static int readImgDataSize(final byte[] in, int i) {
        final int fileSize = in.length;
        int imgDataPos = 0;
        int subBlockSize = in[i] & 0xFF;
        while (subBlockSize > 0) { // While block has data
            try { // Next line may throw exception if sub-block size is fake
                final int nextSubBlockSizePos = i + subBlockSize + 1;
                final int nextSubBlockSize = in[nextSubBlockSizePos] & 0xFF;
                imgDataPos += subBlockSize; // Move output data position
                i = nextSubBlockSizePos; // Move to next sub-block size
                subBlockSize = nextSubBlockSize;
            } catch (final Exception e) {
                // Sub-block exceeds file end, only use remaining bytes
                subBlockSize = fileSize - i - 1; // Remaining bytes
                if (subBlockSize > 0) {
                    imgDataPos += subBlockSize; // Move output data position
                }
                break;
            }
        }
        return imgDataPos;
    }

    /**
     * @param fr The GIF frame to whom this image descriptor belongs
     * @param in Raw data
     * @param i  Index of the image separator, i.e. the first block byte
     * @return Index of the first byte after this block
     */
    static int readImgDescr(final GifFrame fr, final byte[] in, int i) {
        fr.x = (in[++i] & 0xFF) | ((in[++i] & 0xFF) << 8); // Byte 1-2: left
        fr.y = (in[++i] & 0xFF) | ((in[++i] & 0xFF) << 8); // Byte 3-4: top
        fr.w = (in[++i] & 0xFF) | ((in[++i] & 0xFF) << 8); // Byte 5-6: width
        fr.h = (in[++i] & 0xFF) | ((in[++i] & 0xFF) << 8); // Byte 7-8: height
        fr.wh = fr.w * fr.h;
        final int packed = in[++i] & 0xFF; // Byte 9 is a packed byte
        fr.hasLocColTbl = (packed & 0b10000000) != 0; // Bit 7
        fr.interlaceFlag = (packed & 0b01000000) != 0; // Bit 6
        fr.sortFlag = (packed & 0b00100000) != 0; // Bit 5
        final int colTblSizePower = (packed & 7) + 1; // Bits 2-0
        fr.sizeOfLocColTbl = 1 << colTblSizePower; // 2^(N+1), As per the spec
        return ++i;
    }

    /**
     * @param img GIF image
     * @param i   Start index of this block.
     * @return Index of the first byte after this block.
     */
    static int readLogicalScreenDescriptor(final GifImage img, final byte[] in, final int i) {
        img.w = (in[i] & 0xFF) | ((in[i + 1] & 0xFF) << 8); // 16 bit, LSB 1st
        img.h = (in[i + 2] & 0xFF) | ((in[i + 3] & 0xFF) << 8); // 16 bit
        img.wh = img.w * img.h;
        final int packed = in[i + 4] & 0xFF; // Byte 4 is a packed byte
        img.hasGlobColTbl = (packed & 0b10000000) != 0; // Bit 7
        final int colResPower = ((packed & 0b01110000) >>> 4) + 1; // Bits 6-4
        img.colorResolution = 1 << colResPower; // 2^(N+1), As per the spec
        img.sortFlag = (packed & 0b00001000) != 0; // Bit 3
        final int globColTblSizePower = (packed & 7) + 1; // Bits 0-2
        img.sizeOfGlobColTbl = 1 << globColTblSizePower; // 2^(N+1), see spec
        img.bgColIndex = in[i + 5] & 0xFF; // 1 Byte
        img.pxAspectRatio = in[i + 6] & 0xFF; // 1 Byte
        return i + 7;
    }

    /**
     * @param in  Raw data
     * @param pos Index of the extension introducer
     * @return Index of the first byte after this block
     */
    static int readTextExtension(final byte[] in, final int pos) {
        int i = pos + 2; // Skip extension introducer and label
        int subBlockSize = in[i++] & 0xFF;
        while (subBlockSize != 0 && i < in.length) {
            i += subBlockSize;
            if (i >= in.length) break; // Safety check
            subBlockSize = in[i++] & 0xFF;
        }
        return i;
    }
}