patterns: Added BZip3 File Header Pattern (WerWolv#329)

* Added: BZip3 Compression

* Fixed: Bound the SmallBlock data by parent's size

* Improved: Now uses the name 'Chunk' for block wrappers, such that the name 'block' matches with what the BZ3 API does

* Improved: Import rather than Include std::mem

* Added: Missing 'description' field in pragma

README.md

@@ -42,6 +42,7 @@ Everything will immediately show up in ImHex's Content Store and gets bundled wi
 | BSON | `application/bson` | [`patterns/bson.hexpat`](patterns/bson.hexpat) | BSON (Binary JSON) format |
 | bplist | | [`patterns/bplist.hexpat`](patterns/bplist.hexpat) | Apple's binary property list format (bplist) |
 | BSP | | [`patterns/bsp_goldsrc.hexpat`](patterns/bsp_goldsrc.hexpat) | GoldSrc engine maps format (used in Half-Life 1) |
+| BZIP3 | | [`patterns/bzip3.hexpat`](patterns/bzip3.hexpat) | GoldSrc engine maps format (used in Half-Life 1) |
 | CCHVA | | [`patterns/cchva.hexpat`](patterns/cchva.hexpat) | Command and Conquer Voxel Animation |
 | CCVXL | | [`patterns/ccvxl.hexpat`](patterns/ccvxl.hexpat) | Command and Conquer Voxel Model |
 | CCPAL | | [`patterns/ccpal.hexpat`](patterns/ccpal.hexpat) | Command and Conquer Voxel Palette |

patterns/bzip3.hexpat

@@ -0,0 +1,64 @@
+#pragma author Sewer56
+#pragma description Parses BZip3 compression (file format) by Kamila Szewczyk
+#pragma MIME application/x-bzip3
+#pragma endian little
+#pragma magic [42 5A 33 76 31] @ 0x00
+import std.mem;
+
+// Helper function for bit counting
+fn popcount(u8 b) {
+    u32 count = 0;
+    while (b != 0) {
+        count = count + (b & 1);
+        b = b >> 1;
+    }
+    return count;
+};
+
+// Frame header structure
+struct FrameHeader {
+    char magic[5];         // "BZ3v1"
+    u32 blockSize;         // Maximum block size
+};
+
+// Small block header (for blocks < 64 bytes)
+struct SmallBlock {
+    u32 crc32;           // CRC32 checksum
+    u32 literal;         // Always 0xFFFFFFFF for small blocks
+    u8 data[parent.compressedSize - 8]; // Uncompressed data
+};
+
+// Regular block (blocks > 64 bytes)
+struct Block {
+    u32 crc32;            // CRC32 checksum of uncompressed data
+    u32 bwtIndex;         // Burrows-Wheeler transform index
+    u8  model;            // Compression model flags
+
+    if ((model & 0x02) != 0)     
+        u32 lzpSize;      // Size after LZP compression
+    if ((model & 0x04) != 0)     
+        u32 rleSize;      // Size after RLE compression
+
+    u8 data[parent.compressedSize - (popcount(model) * 4 + 9)];
+};
+
+// Main block structure
+struct Chunk {
+    u32 compressedSize;   // Size of compressed block
+    u32 origSize;         // Original uncompressed size
+
+    if (compressedSize < 64) {
+        SmallBlock block;
+    } else {
+        Block block;
+    }
+};
+
+// Main parsing structure
+struct BZip3File {
+    FrameHeader header;
+    // Read blocks until end of file
+    Chunk chunks[while(!std::mem::eof())];
+};
+
+BZip3File file @ 0x0;