Add documentation of irep_idt and dstringt

src/util/README.md

@@ -3,7 +3,7 @@
 
 # Folder util
 
-\author Martin Brain
+\author Martin Brain, Owen Jones
 
 \section data_structures Data Structures
 
@@ -12,17 +12,17 @@ CPROVER codebase.
 
 \subsection irept Irept Data Structure
 
-There are a large number of kind of tree structured or tree-like data in
+There are a large number of kinds of tree structured or tree-like data in
 CPROVER. `irept` provides a single, unified representation for all of
 these, allowing structure sharing and reference counting of data. As
 such `irept` is the basic unit of data in CPROVER.  Each `irept`
-contains[^2] a basic unit of data (of type `dt`) which contains four
+contains[^1] a basic unit of data (of type `dt`) which contains four
 things:
 
-* `data`:   A string[^3], which is returned when the `id()` function is
+* `data`:   A string[^2], which is returned when the `id()` function is
   used.
 
-* `named_sub`:   A map from `irep_namet` (a string) to an `irept`. This
+* `named_sub`:   A map from `irep_namet` (a string) to `irept`. This
   is used for named children, i.e.  subexpressions, parameters, etc.
 
 * `comments`:   Another map from `irep_namet` to `irept` which is used
@@ -32,7 +32,7 @@ things:
   unnamed children.
 
 The `irept::pretty` function outputs the contents of an `irept` directly
-and can be used to understand an debug problems with `irept`s.
+and can be used to understand and debug problems with `irept`s.
 
 On their own `irept`s do not “mean” anything; they are effectively
 generic tree nodes. Their interpretation depends on the contents of
@@ -67,21 +67,34 @@ class’ named `typecast_exprt`. One key descendent of `exprt` is
 These are used to represent variables; the name of which can be found
 using the `get_identifier` accessor function.
 
-`codet` inherits from `exprt` and is defined in `std_code.h`. They
-represent executable code; statements in C rather than expressions. In
+`codet` inherits from `exprt` and is defined in `std_code.h`. It
+represents executable code; statements in C rather than expressions. In
 the front-end there are versions of these that hold whole code blocks,
 but in goto-programs these have been flattened so that each `irept`
 represents one sequence point (almost one line of code / one
 semi-colon). The most common descendents of `codet` are `code_assignt`
 so a common pattern is to cast the `codet` to an assignment and then
 recurse on the expression on either side.
 
-[^2]: Or references, if reference counted data sharing is enabled. It is
+[^1]: Or references, if reference counted data sharing is enabled. It is
     enabled by default; see the `SHARING` macro.
 
-[^3]: Unless `USE_STD_STRING` is set, this is actually
+[^2]: Unless `USE_STD_STRING` is set, this is actually
 a `dstring` and thus an integer which is a reference into a string table
 
+\subsection irep_idt Irep_idt and Dstringt
+
+Inside `irept`s, strings are stored as `irep_idt`s, or `irep_namet`s for
+keys to `named_sub` or `comments`. If `USE_STD_STRING` is set then both 
+`irep_idt` and `irep_namet` are `typedef`ed to `std::string`, but by default 
+it is not set and they are `typedef`ed to `dstringt`. `dstringt` has one 
+field, an unsigned integer which is an index into a static table of strings. 
+This makes it expensive to create a new string (because you have to look 
+through the whole table to see if it is already there, and add it if it 
+isn't) but very cheap to compare strings (just compare the two integers). It 
+also means that when you have lots of copies of the same string you only have
+to store the whole string once, which saves space.
+
 \dot
 digraph G {
   node [shape=box];