From 11def0fcc58611ae829acaafabfebe42faed930d Mon Sep 17 00:00:00 2001
From: Ariel A <24759293+aarant@users.noreply.github.com>
Date: Tue, 20 Feb 2024 20:36:48 -0500
Subject: [PATCH] Optimized IsLZ77Data to return size.

---
 include/decompress.h |  2 +-
 src/decompress.c     | 25 +++++++++----------------
 2 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/include/decompress.h b/include/decompress.h
index 11560122e0..cfa7011576 100644
--- a/include/decompress.h
+++ b/include/decompress.h
@@ -8,7 +8,7 @@ extern u8 ALIGNED(4) gDecompressionBuffer[0x4000];
 void LZDecompressWram(const u32 *src, void *dest);
 void LZDecompressVram(const u32 *src, void *dest);
 
-bool32 IsLZ77Data(const void *ptr, u32 minSize, u32 maxSize);
+u32 IsLZ77Data(const void *ptr, u32 minSize, u32 maxSize);
 
 u16 LoadCompressedSpriteSheet(const struct CompressedSpriteSheet *src);
 u16 LoadCompressedSpriteSheetByTemplate(const struct SpriteTemplate *template, s32 offset);
diff --git a/src/decompress.c b/src/decompress.c
index 76d6223871..be7bb77c7a 100644
--- a/src/decompress.c
+++ b/src/decompress.c
@@ -21,20 +21,23 @@ void LZDecompressVram(const u32 *src, void *dest)
 
 // Checks if `ptr` is likely LZ77 data
 // Checks word-alignment, min/max size, and header byte
-bool32 IsLZ77Data(const void *ptr, u32 minSize, u32 maxSize) {
+// Returns uncompressed size if true, 0 otherwise
+u32 IsLZ77Data(const void *ptr, u32 minSize, u32 maxSize) {
     const u8 *data = ptr;
     u32 size;
     // Compressed data must be word aligned
     if (((u32)ptr) & 3)
-        return FALSE;
+        return 0;
     // Check LZ77 header byte
     // See https://problemkaputt.de/gbatek.htm#biosdecompressionfunctions
     if (data[0] != 0x10)
-        return FALSE;
+        return 0;
 
     // Read 24-bit uncompressed size
     size = data[1] | (data[2] << 8) | (data[3] << 16);
-    return (size >= minSize && size <= maxSize);
+    if (size >= minSize && size <= maxSize)
+        return size;
+    return 0;
 }
 
 u16 LoadCompressedSpriteSheet(const struct CompressedSpriteSheet *src)
@@ -52,20 +55,10 @@ u16 LoadCompressedSpriteSheet(const struct CompressedSpriteSheet *src)
 u16 LoadCompressedSpriteSheetByTemplate(const struct SpriteTemplate *template, s32 offset) {
     struct SpriteTemplate myTemplate;
     struct SpriteFrameImage myImage;
-    const u8 *data = template->images->data;
     u32 size;
 
-    // (Heuristic) Check for LZ77 header
-    // See https://problemkaputt.de/gbatek.htm#biosdecompressionfunctions
-    // data[3] could be nonzero; but this would mean data >= 65536 bytes,
-    // which is 2048 tiles, far too big in practice
-    if (data[0] != 0x10 || data[3] != 0) // not compressed
-        return LoadSpriteSheetByTemplate(template, 0, offset);
-
-    // read uncompressed size from header
-    size = T1_READ_16(&data[1]);
-    // too big for compression buffer, so probably not compressed
-    if (size >= ARRAY_COUNT(gDecompressionBuffer))
+    // Check for LZ77 header and read uncompressed size, or fallback if not compressed (zero size)
+    if ((size = IsLZ77Data(template->images->data, TILE_SIZE_4BPP, sizeof(gDecompressionBuffer))) == 0)
         return LoadSpriteSheetByTemplate(template, 0, offset);
 
     LZ77UnCompWram(template->images->data, gDecompressionBuffer);