Introduce flatbuffer (#272)

Co-authored-by: star9029 <hengxings783@gmail.com>

include/Index/MergedIndex.h

@@ -59,12 +59,12 @@ struct MergedIndex {
     /// A map between source file path and its header contexts.
     llvm::StringMap<HeaderContexts> contexts;
 
-    /// All merged symbol relations.
-    llvm::DenseMap<SymbolHash, llvm::DenseMap<Relation, roaring::Roaring>> relations;
-
     /// All merged symbol occurrences.
     llvm::DenseMap<Occurrence, roaring::Roaring> occurrences;
 
+    /// All merged symbol relations.
+    llvm::DenseMap<SymbolHash, llvm::DenseMap<Relation, roaring::Roaring>> relations;
+
     void remove(llvm::StringRef path);
 
     void merge(llvm::StringRef path, std::uint32_t include, FileIndex& index);

include/Index/schema.fbs

@@ -0,0 +1,65 @@
+namespace clice.index.binary;
+
+struct Range {
+    begin: uint;
+    end: uint;
+}
+
+struct Occurrence {
+    range: Range;
+    target: ulong;
+}
+
+struct Relation {
+    kind: int;
+    padding: int;
+    range: Range;
+    target_symbol: ulong;
+}
+
+table CacheEntry {
+    sha256: string;
+    canonical_id: uint;
+}
+
+struct Context {
+    include_: uint;
+    canonical_id: uint;
+}
+
+table HeaderContexts {
+    version: uint;
+    includes: [Context];
+}
+
+table HeaderContextsEntry {
+    path: string;
+    contexts: HeaderContexts;
+}
+
+table OccurrenceEntry {
+    occurrence: Occurrence;
+    context: [ubyte];
+}
+
+table RelationEntry {
+    relation: Relation;
+    context: [ubyte];
+}
+
+table SymbolRelationsEntry {
+    symbol: ulong;
+    relations: [RelationEntry];
+}
+
+table MergedIndex {
+    max_canonical_id: uint;
+
+    canonical_cache: [CacheEntry];
+
+    contexts: [HeaderContextsEntry];
+
+    occurrences: [OccurrenceEntry];
+
+    relations: [SymbolRelationsEntry];
+}