LibGfx/WebP: Move some to-be-shared code to WebPSharedLossless.h

No behavior change. No measurable performance different either.

(I tried `hyperfine 'Build/lagom/bin/image --no-output foo.webp'`
for a few input images before and after this change, and I didn't
see a difference. I also tried if moving both
Gfx::CanonicalCode::read_symbol() and
Compress::CanonicalCode::read_symbol() inline, and that didn't
help either.)

Meta/gn/secondary/Userland/Libraries/LibGfx/BUILD.gn

@@ -97,6 +97,7 @@ shared_library("LibGfx") {
     "ImageFormats/WebPLoader.cpp",
     "ImageFormats/WebPLoaderLossless.cpp",
     "ImageFormats/WebPLoaderLossy.cpp",
+    "ImageFormats/WebPSharedLossless.cpp",
     "ImageFormats/WebPWriter.cpp",
     "ImageFormats/WebPWriterLossless.cpp",
     "ImmutableBitmap.cpp",

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -71,6 +71,7 @@ set(SOURCES
     ImageFormats/WebPLoader.cpp
     ImageFormats/WebPLoaderLossless.cpp
     ImageFormats/WebPLoaderLossy.cpp
+    ImageFormats/WebPSharedLossless.cpp
     ImageFormats/WebPWriter.cpp
     ImageFormats/WebPWriterLossless.cpp
     ImmutableBitmap.cpp

Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossless.cpp

@@ -12,6 +12,7 @@
 #include <AK/Vector.h>
 #include <LibCompress/Deflate.h>
 #include <LibGfx/ImageFormats/WebPLoaderLossless.h>
+#include <LibGfx/ImageFormats/WebPSharedLossless.h>
 
 // Lossless format: https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification
 
@@ -49,77 +50,6 @@ ErrorOr<VP8LHeader> decode_webp_chunk_VP8L_header(ReadonlyBytes vp8l_data)
     return VP8LHeader { width, height, is_alpha_used, vp8l_data.slice(5) };
 }
 
-namespace {
-
-// WebP-lossless's CanonicalCodes are almost identical to deflate's.
-// One difference is that codes with a single element in webp-lossless consume 0 bits to produce that single element,
-// while they consume 1 bit in Compress::CanonicalCode. This class wraps Compress::CanonicalCode to handle the case
-// where the codes contain just a single element, and dispatches to Compress::CanonicalCode else.
-class CanonicalCode {
-public:
-    CanonicalCode()
-        : m_code(0)
-    {
-    }
-
-    static ErrorOr<CanonicalCode> from_bytes(ReadonlyBytes);
-    ErrorOr<u32> read_symbol(LittleEndianInputBitStream&) const;
-
-private:
-    explicit CanonicalCode(u32 single_symbol)
-        : m_code(single_symbol)
-    {
-    }
-
-    explicit CanonicalCode(Compress::CanonicalCode code)
-        : m_code(move(code))
-    {
-    }
-
-    Variant<u32, Compress::CanonicalCode> m_code;
-};
-
-ErrorOr<CanonicalCode> CanonicalCode::from_bytes(ReadonlyBytes bytes)
-{
-    auto non_zero_symbol_count = 0;
-    auto last_non_zero_symbol = -1;
-    for (size_t i = 0; i < bytes.size(); i++) {
-        if (bytes[i] != 0) {
-            non_zero_symbol_count++;
-            last_non_zero_symbol = i;
-        }
-    }
-
-    if (non_zero_symbol_count == 1)
-        return CanonicalCode(last_non_zero_symbol);
-
-    return CanonicalCode(TRY(Compress::CanonicalCode::from_bytes(bytes)));
-}
-
-ErrorOr<u32> CanonicalCode::read_symbol(LittleEndianInputBitStream& bit_stream) const
-{
-    return TRY(m_code.visit(
-        [](u32 single_code) -> ErrorOr<u32> { return single_code; },
-        [&bit_stream](Compress::CanonicalCode const& code) { return code.read_symbol(bit_stream); }));
-}
-
-// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#61_overview
-// "From here on, we refer to this set as a prefix code group."
-class PrefixCodeGroup {
-public:
-    PrefixCodeGroup() = default;
-    PrefixCodeGroup(PrefixCodeGroup&&) = default;
-    PrefixCodeGroup(PrefixCodeGroup const&) = delete;
-
-    CanonicalCode& operator[](int i) { return m_codes[i]; }
-    CanonicalCode const& operator[](int i) const { return m_codes[i]; }
-
-private:
-    Array<CanonicalCode, 5> m_codes;
-};
-
-}
-
 // https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#621_decoding_and_building_the_prefix_codes
 static ErrorOr<CanonicalCode> decode_webp_chunk_VP8L_prefix_code(LittleEndianInputBitStream& bit_stream, size_t alphabet_size)
 {

Userland/Libraries/LibGfx/ImageFormats/WebPSharedLossless.cpp

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2024, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/ImageFormats/WebPSharedLossless.h>
+
+namespace Gfx {
+
+ErrorOr<CanonicalCode> CanonicalCode::from_bytes(ReadonlyBytes bytes)
+{
+    auto non_zero_symbol_count = 0;
+    auto last_non_zero_symbol = -1;
+    for (size_t i = 0; i < bytes.size(); i++) {
+        if (bytes[i] != 0) {
+            non_zero_symbol_count++;
+            last_non_zero_symbol = i;
+        }
+    }
+
+    if (non_zero_symbol_count == 1)
+        return CanonicalCode(last_non_zero_symbol);
+
+    return CanonicalCode(TRY(Compress::CanonicalCode::from_bytes(bytes)));
+}
+
+ErrorOr<u32> CanonicalCode::read_symbol(LittleEndianInputBitStream& bit_stream) const
+{
+    return TRY(m_code.visit(
+        [](u32 single_code) -> ErrorOr<u32> { return single_code; },
+        [&bit_stream](Compress::CanonicalCode const& code) { return code.read_symbol(bit_stream); }));
+}
+
+}

Userland/Libraries/LibGfx/ImageFormats/WebPSharedLossless.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2024, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibCompress/Deflate.h>
+
+namespace Gfx {
+
+// WebP-lossless's CanonicalCodes are almost identical to deflate's.
+// One difference is that codes with a single element in webp-lossless consume 0 bits to produce that single element,
+// while they consume 1 bit in Compress::CanonicalCode. This class wraps Compress::CanonicalCode to handle the case
+// where the codes contain just a single element, and dispatches to Compress::CanonicalCode else.
+class CanonicalCode {
+public:
+    CanonicalCode()
+        : m_code(0)
+    {
+    }
+
+    static ErrorOr<CanonicalCode> from_bytes(ReadonlyBytes);
+    ErrorOr<u32> read_symbol(LittleEndianInputBitStream&) const;
+
+private:
+    explicit CanonicalCode(u32 single_symbol)
+        : m_code(single_symbol)
+    {
+    }
+
+    explicit CanonicalCode(Compress::CanonicalCode code)
+        : m_code(move(code))
+    {
+    }
+
+    Variant<u32, Compress::CanonicalCode> m_code;
+};
+
+// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#61_overview
+// "From here on, we refer to this set as a prefix code group."
+class PrefixCodeGroup {
+public:
+    PrefixCodeGroup() = default;
+    PrefixCodeGroup(PrefixCodeGroup&&) = default;
+    PrefixCodeGroup(PrefixCodeGroup const&) = delete;
+
+    CanonicalCode& operator[](int i) { return m_codes[i]; }
+    CanonicalCode const& operator[](int i) const { return m_codes[i]; }
+
+private:
+    Array<CanonicalCode, 5> m_codes;
+};
+
+}