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+
+<h1>The Index Library</h1>
+
+ <p><b>Table of Contents</b></p>
+ <ul>
+ <li><a href="#philosophy">Design Philosophy</a></li>
+ <li><a href="#classes">Classes</a>
+ <ul>
+ <li><a href="#entity">Entity</a></li>
+ <li><a href="#astlocation">ASTLocation</a></li>
+ <li><a href="#declreferencemap">DeclReferenceMap</a></li>
+ </ul>
+ </li>
+ <li><a href="#functions">Functions</a>
+ <ul>
+ <li><a href="#resolveloc">ResolveLocationInAST</a></li>
+ </ul>
+ </li>
+ <li><a href="#astfiles">AST Files</a></li>
+ <li><a href="#indextest">index-test tool</a>
+ <ul>
+ <li><a href="#indextestusage">Usage</a></li>
+ <li><a href="#indextestexamples">Examples</a></li>
+ </ul>
+ </li>
+</ul>
+
+<h2 id="philosophy">Design Philosophy</h2>
+
+<p> The Index library is meant to provide the basic infrastructure for
+ cross-translation-unit analysis and is primarily focused on indexing
+ related functionality. It provides an API for clients that need to
+ accurately map the AST nodes of the ASTContext to the locations in the source files.
+It also allows them to analyze information across multiple translation units.</p>
+
+<p>As a "general rule", ASTContexts are considered the primary source of
+information that a client wants about a translation unit. There will be no such class as an
+ "indexing database" that stores, for example, source locations of identifiers separately from ASTContext.
+All the information that a client needs from a translation unit will be extracted from the ASTContext.</p>
+
+<h2 id="classes">Classes</h2>
+
+<h3 id="entity">Entity</h3>
+
+<p>To be able to reason about semantically the same Decls that are contained in multiple ASTContexts, the 'Entity' class was introduced.
+An Entity is an ASTContext-independent "token" that can be created from a Decl (and a typename in the future) with
+the purpose to "resolve" it into a Decl belonging to another ASTContext. Some examples to make the concept of Entities more clear:</p>
+
+<p>
+t1.c:
+<pre class="code_example">
+void foo(void);
+void bar(void);
+</pre>
+</p>
+
+<p>
+t2.c:
+<pre class="code_example">
+void foo(void) {
+}
+</pre>
+</p>
+
+<p>
+Translation unit <code>t1.c</code> contains 2 Entities <code>foo</code> and <code>bar</code>, while <code>t2.c</code> contains 1 Entity <code>foo</code>.
+Entities are uniqued in such a way that the Entity* pointer for <code>t1.c/foo</code> is the same as the Entity* pointer for <code>t2.c/foo</code>.
+An Entity doesn't convey any information about the declaration, it is more like an opaque pointer used only to get the
+associated Decl out of an ASTContext so that the actual information for the declaration can be accessed.
+Another important aspect of Entities is that they can only be created/associated for declarations that are visible outside the
+translation unit. This means that for:
+</p>
+<p>
+t3.c:
+<pre class="code_example">
+static void foo(void);
+</pre>
+</p>
+<p>
+there can be no Entity (if you ask for the Entity* of the static function <code>foo</code> you'll get a null pointer).
+This is for 2 reasons:
+<ul>
+<li>To preserve the invariant that the same Entity* pointers refer to the same semantic Decls.
+ In the above example <code>t1.c/foo</code> and <code>t2.c/foo</code> are the same, while <code>t3.c/foo</code> is different.</li>
+<li>The purpose of Entity is to get the same semantic Decl from multiple ASTContexts. For a Decl that is not visible
+ outside of its own translation unit, you don't need an Entity since it won't appear in another ASTContext.</li>
+</ul>
+</p>
+
+<h3 id="astlocation">ASTLocation</h3>
+
+Encapsulates a "point" in the AST tree of the ASTContext.
+It represents either a Decl*, or a Stmt* along with its immediate Decl* parent.
+An example for its usage is that libIndex will provide the references of <code>foo</code> in the form of ASTLocations,
+"pointing" at the expressions that reference <code>foo</code>.
+
+<h3 id="declreferencemap">DeclReferenceMap</h3>
+
+Accepts an ASTContext and creates a mapping from NamedDecls to the ASTLocations that reference them (in the same ASTContext).
+
+<h2 id="functions">Functions</h2>
+
+<h3 id="resolveloc">ResolveLocationInAST</h3>
+
+A function that accepts an ASTContext and a SourceLocation which it resolves into an ASTLocation.
+
+<h2 id="astfiles">AST Files</h2>
+
+The precompiled headers implementation of clang (<a href="http://clang.llvm.org/docs/PCHInternals.html">PCH</a>) is ideal for storing an ASTContext in a compact form that
+will be loaded later for AST analysis. An "AST file" refers to a translation unit that was "compiled" into a precompiled header file.
+
+<h2 id="indextest">index-test tool</h2>
+
+<h3 id="indextestusage">Usage</h3>
+
+A command-line tool that exercises the libIndex API, useful for testing its features.
+As input it accepts multiple AST files (representing multiple translation units) and a few options:
+
+<p>
+<pre class="code_example">
+ -point-at [file:line:column]
+</pre>
+Resolves a [file:line:column] triplet into a ASTLocation from the first AST file. If no other option is specified, it prints the ASTLocation.
+It also prints a declaration's associated doxygen comment, if one is available.
+</p>
+
+<p>
+<pre class="code_example">
+ -print-refs
+</pre>
+Prints the ASTLocations that reference the declaration that was resolved out of the [file:line:column] triplet
+</p>
+
+<p>
+<pre class="code_example">
+ -print-defs
+</pre>
+Prints the ASTLocations that define the resolved declaration
+</p>
+
+<p>
+<pre class="code_example">
+ -print-decls
+</pre>
+Prints the ASTLocations that declare the resolved declaration
+</p>
+
+<h3 id="indextestexamples">Examples</h3>
+
+<p>
+Here's an example of using index-test:
+</p>
+
+<p>
+We have 3 files,
+</p>
+
+<p>
+foo.h:
+<pre class="code_example">
+extern int global_var;
+
+void foo_func(int param1);
+void bar_func(void);
+</pre>
+
+t1.c:
+<pre class="code_example">
+#include "foo.h"
+
+void foo_func(int param1) {
+ int local_var = global_var;
+ for (int for_var = 100; for_var < 500; ++for_var) {
+ local_var = param1 + for_var;
+ }
+ bar_func();
+}
+</pre>
+
+t2.c:
+<pre class="code_example">
+#include "foo.h"
+
+int global_var = 10;
+
+void bar_func(void) {
+ global_var += 100;
+ foo_func(global_var);
+}
+</pre>
+</p>
+
+<p>
+You first get AST files out of <code>t1.c</code> and <code>t2.c</code>:
+
+<pre class="code_example">
+$ clang-cc -emit-pch t1.c -o t1.ast
+$ clang-cc -emit-pch t2.c -o t2.ast
+</pre>
+</p>
+
+<p>
+Find the ASTLocation under this position of <code>t1.c</code>:
+<pre class="code_example">
+[...]
+void foo_func(int param1) {
+ int local_var = global_var;
+ ^
+[...]
+</pre>
+
+<pre class="code_example">
+$ index-test t1.ast -point-at t1.c:4:23
+> [Decl: Var local_var | Stmt: DeclRefExpr global_var] <t1.c:4:19, t1.c:4:19>
+</pre>
+</p>
+
+<p>
+Find the declaration:
+
+<pre class="code_example">
+$ index-test t1.ast -point-at t1.c:4:23 -print-decls
+> [Decl: Var global_var] <foo.h:1:12, foo.h:1:12>
+</pre>
+</p>
+
+<p>
+Find the references:
+
+<pre class="code_example">
+$ index-test t1.ast t2.ast -point-at t1.c:4:23 -print-refs
+> [Decl: Var local_var | Stmt: DeclRefExpr global_var] <t1.c:4:19, t1.c:4:19>
+> [Decl: Function bar_func | Stmt: DeclRefExpr global_var] <t2.c:6:3, t2.c:6:3>
+> [Decl: Function bar_func | Stmt: DeclRefExpr global_var] <t2.c:7:12, t2.c:7:12>
+</pre>
+</p>
+
+<p>
+Find definitions:
+
+<pre class="code_example">
+$ index-test t1.ast t2.ast -point-at t1.c:4:23 -print-defs
+> [Decl: Var global_var] <t2.c:3:5, t2.c:3:18>
+</pre>
+</p>
+
+</div>
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