libtxt: cache fallback fonts found by a Minikin font collection

If a new fallback font is discovered during paragraph layout, the fallback
font cache in txt::FontCollection will use that font in future layouts.

However, that cache is not available if the new fallback font needs to be
used for other characters within the current layout.  This PR adds a cache
to minikin::FontCollection and checks whether fonts in the cache can handle
a character before calling the fallback font provider.

See #13257

Author: jason-simmons

third_party/txt/src/minikin/FontCollection.cpp

@@ -302,8 +302,7 @@ const std::shared_ptr<FontFamily>& FontCollection::getFamilyForChar(
     // libtxt: check if the fallback font provider can match this character
     if (mFallbackFontProvider) {
       const std::shared_ptr<FontFamily>& fallback =
-          mFallbackFontProvider->matchFallbackFont(ch,
-                                                   GetFontLocale(langListId));
+          findFallbackFont(ch, vs, langListId);
       if (fallback) {
         return fallback;
       }
@@ -340,8 +339,7 @@ const std::shared_ptr<FontFamily>& FontCollection::getFamilyForChar(
     // libtxt: check if the fallback font provider can match this character
     if (mFallbackFontProvider) {
       const std::shared_ptr<FontFamily>& fallback =
-          mFallbackFontProvider->matchFallbackFont(ch,
-                                                   GetFontLocale(langListId));
+          findFallbackFont(ch, vs, langListId);
       if (fallback) {
         return fallback;
       }
@@ -365,6 +363,30 @@ const std::shared_ptr<FontFamily>& FontCollection::getFamilyForChar(
                  : mFamilies[bestFamilyIndex];
 }
 
+const std::shared_ptr<FontFamily>& FontCollection::findFallbackFont(
+    uint32_t ch,
+    uint32_t vs,
+    uint32_t langListId) const {
+  std::string locale = GetFontLocale(langListId);
+
+  const auto it = mCachedFallbackFamilies.find(locale);
+  if (it != mCachedFallbackFamilies.end()) {
+    for (const auto& fallbackFamily : it->second) {
+      if (calcCoverageScore(ch, vs, fallbackFamily)) {
+        return fallbackFamily;
+      }
+    }
+  }
+
+  const std::shared_ptr<FontFamily>& fallback =
+      mFallbackFontProvider->matchFallbackFont(ch, GetFontLocale(langListId));
+
+  if (fallback) {
+    mCachedFallbackFamilies[locale].push_back(fallback);
+  }
+  return fallback;
+}
+
 const uint32_t NBSP = 0x00A0;
 const uint32_t SOFT_HYPHEN = 0x00AD;
 const uint32_t ZWJ = 0x200C;

third_party/txt/src/minikin/FontCollection.h

@@ -17,6 +17,7 @@
 #ifndef MINIKIN_FONT_COLLECTION_H
 #define MINIKIN_FONT_COLLECTION_H
 
+#include <map>
 #include <memory>
 #include <unordered_set>
 #include <vector>
@@ -100,6 +101,9 @@ class FontCollection {
                                                       uint32_t langListId,
                                                       int variant) const;
 
+  const std::shared_ptr<FontFamily>&
+  findFallbackFont(uint32_t ch, uint32_t vs, uint32_t langListId) const;
+
   uint32_t calcFamilyScore(uint32_t ch,
                            uint32_t vs,
                            int variant,
@@ -148,6 +152,11 @@ class FontCollection {
 
   // libtxt extension: Fallback font provider.
   std::unique_ptr<FallbackFontProvider> mFallbackFontProvider;
+
+  // libtxt extension: Fallback fonts discovered after this font collection
+  // was constructed.
+  mutable std::map<std::string, std::vector<std::shared_ptr<FontFamily>>>
+      mCachedFallbackFamilies;
 };
 
 }  // namespace minikin