patterns: Add LZ4 Frame Format pattern (#523)

* patterns: Add LZ4 Frame Format pattern

Adds a pattern for the LZ4 Frame Format supporting standard, skippable,
and legacy frames. Includes validation for version, block maximum size,
and end mark.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs: Add LZ4 to README pattern list

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: paxcut <53811119+paxcut@users.noreply.github.com>

Author: 3 people

README.md

@@ -146,6 +146,7 @@ Everything will immediately show up in ImHex's Content Store and gets bundled wi
 | Lua 5.3 | | [`patterns/lua53.hexpat`](patterns/lua53.hexpat) | Lua 5.3 bytecode |
 | Lua 5.4 | | [`patterns/lua54.hexpat`](patterns/lua54.hexpat) | Lua 5.4 bytecode |
 | LCE Savefile | | [`patterns/lcesave.hexpat`](patterns/lcesave.hexpat) | Minecraft Legacy Console Edition save file |
+| LZ4 | `application/x-lz4` | [`patterns/lz4.hexpat`](patterns/lz4.hexpat) | LZ4 Frame Format |
 | LZNT1 | | [`patterns/lznt1.hexpat`](patterns/lznt1.hexpat) | LZNT1 compressed data format |
 | Mach-O | `application/x-mach-binary` | [`patterns/macho.hexpat`](patterns/macho.hexpat) | Mach-O executable |
 | MIDI | `audio/midi` | [`patterns/midi.hexpat`](patterns/midi.hexpat) | MIDI header, event fields provided |

patterns/lz4.hexpat

@@ -0,0 +1,123 @@
+#pragma author Gemini
+#pragma description LZ4 Frame Format
+#pragma MIME application/x-lz4
+#pragma endian little
+
+import std.io;
+import std.sys;
+import std.mem;
+import std.core;
+
+enum MagicNumber : u32 {
+    Standard  = 0x184D2204,
+    Legacy    = 0x184C2102,
+    
+    Skippable_0 = 0x184D2A50,
+    Skippable_1 = 0x184D2A51,
+    Skippable_2 = 0x184D2A52,
+    Skippable_3 = 0x184D2A53,
+    Skippable_4 = 0x184D2A54,
+    Skippable_5 = 0x184D2A55,
+    Skippable_6 = 0x184D2A56,
+    Skippable_7 = 0x184D2A57,
+    Skippable_8 = 0x184D2A58,
+    Skippable_9 = 0x184D2A59,
+    Skippable_A = 0x184D2A5A,
+    Skippable_B = 0x184D2A5B,
+    Skippable_C = 0x184D2A5C,
+    Skippable_D = 0x184D2A5D,
+    Skippable_E = 0x184D2A5E,
+    Skippable_F = 0x184D2A5F
+};
+
+bitfield FLG {
+    dictionary_id_present    : 1;
+    reserved_1               : 1;
+    content_checksum_present : 1;
+    content_size_present     : 1;
+    block_checksum_present   : 1;
+    block_independence       : 1;
+    version                  : 2;
+};
+
+bitfield BD {
+    reserved_0      : 4;
+    block_max_size  : 3;
+    reserved_7      : 1;
+};
+
+struct FrameDescriptor {
+    FLG flg;
+    BD bd;
+
+    std::assert(flg.version == 1, "Invalid LZ4 frame version (must be 01)");
+    std::assert(bd.block_max_size >= 4, "Invalid block maximum size");
+
+    if (flg.content_size_present) {
+        u64 content_size;
+    }
+    if (flg.dictionary_id_present) {
+        u32 dictionary_id;
+    }
+    
+    u8 header_checksum;
+};
+
+bitfield BlockSize {
+    size         : 31;
+    uncompressed : 1;
+};
+
+struct DataBlock<auto block_checksum_present> {
+    BlockSize size;
+
+    u8 data[size.size];
+    if (block_checksum_present) {
+        u32 block_checksum;
+    }
+};
+
+struct StandardFrame {
+    FrameDescriptor descriptor;
+    
+    DataBlock<descriptor.flg.block_checksum_present> blocks[while($ + 4 <= std::mem::size() && std::mem::read_unsigned($, 4) != 0)];
+
+    if ($ + 4 <= std::mem::size()) {
+        u32 end_mark;
+        std::assert(end_mark == 0x00000000, "Invalid EndMark");
+    }
+
+    if (descriptor.flg.content_checksum_present && $ + 4 <= std::mem::size()) {
+        u32 content_checksum;
+    }
+};
+
+struct SkippableFrame {
+    u32 frame_size;
+    u8 user_data[frame_size];
+};
+
+struct LegacyBlock {
+    u32 compressed_size;
+    u8 data[compressed_size];
+};
+
+struct LegacyFrame {
+    LegacyBlock blocks[while($ + 4 <= std::mem::size() && std::mem::read_unsigned($, 4) != 0x184D2204 && std::mem::read_unsigned($, 4) != 0x184C2102 && (std::mem::read_unsigned($, 4) & 0xFFFFFFF0) != 0x184D2A50)];
+};
+
+struct Frame {
+    MagicNumber magic;
+    
+    if (magic == MagicNumber::Standard) {
+        StandardFrame frame;
+    } else if (magic == MagicNumber::Legacy) {
+        LegacyFrame frame;
+    } else if ((u32(magic) & 0xFFFFFFF0) == 0x184D2A50) {
+        SkippableFrame frame;
+    } else {
+        std::error("Unknown Magic Number");
+    }
+};
+
+Frame frames[while($ < std::mem::size())] @ 0x00;