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+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright 2019 Olivier Matz <zer0@droids-corp.org>
+
+Architecture
+============
+
+Most of *libecoli* is written in C. It provides a C API to ease the
+creation of interactive command line interfaces. It is split in several
+parts:
+
+- the core: it provides the main API to parse and complete strings.
+- ecoli nodes: this is the modular part of *libecoli*. Each node
+  implements a specific parsing (integers, strings, regular expressions,
+  sequence, ...)
+- yaml parser: load an ecoli tree described in yaml.
+- editline: helpers to instantiate an *editline* and connect it to
+  *libecoli*.
+- utilities: logging, string, strings vector, hash table, ...
+
+The grammar tree
+----------------
+
+The *ecoli nodes* are organized in a tree. An *ecoli grammar tree*
+describes how the input is parsed and completed. Let's take a simple
+example:
+
+.. figure:: simple-tree.svg
+
+   A simple *ecoli grammar tree*.
+
+We can also represent it as text like this::
+
+  sh_lex(
+    seq(
+      str(foo),
+      option(
+        str(bar)
+      )
+    )
+  )
+
+This tree matches the following strings:
+
+- ``foo``
+- ``foo bar``
+
+But, for instance, it does **not** match the following strings:
+
+- ``bar``
+- ``foobar``
+- (empty string)
+
+Parsing an input
+----------------
+
+When the *libecoli* parses an input, it browses the tree (depth first
+search) and returns an *ecoli parse tree*. Let's decompose what is done
+when ``ec_node_parse_strvec(root_node, input)`` is called, step by step:
+
+1. The initial input is a string vector ``["foo bar"]``.
+2. The *sh_lex* node splits the input as a shell would have done it, in
+   ``["foo", "bar"]``, and passes it to its child.
+3. The *seq* node (sequence) passes the input to its first child.
+4. The *str* node matches if the first element of the string vector is
+   ``foo``. This is the case, so it returns the number of eaten
+   strings (1) to its parent.
+5. The *seq* node passes the remaining part ``["bar"]`` to its next
+   child.
+6. The option node passes the string vector to its child, which
+   matches. It returns 1 to its parent.
+7. The *seq* node has no more child, it returns the number of eaten
+   strings in the vector (2).
+8. Finally, *sh_lex* compares the returned value to the number of
+   strings in its initial vector, and return 1 (match) to its caller.
+
+If a node does not match, it returns ``EC_PARSE_NOMATCH`` to its parent,
+and it is propagated until it reaches a node that handle the case. For
+instance, the *or* node is able to handle this error. If a child does
+not match, the next one is tried until it finds one that matches.
+
+The parse tree
+--------------
+
+Let's take another simple example.
+
+.. figure:: simple-tree2.svg
+
+   Another simple *ecoli grammar tree*.
+
+In that case, there is no lexer (the *sh_lex* node), so the input must
+be a string vector that is already split. For instance, it matches:
+
+- ``["foo"]``
+- ``["bar", "1"]``
+- ``["bar", "100"]``
+
+But it does **not** match:
+
+- ``["bar 1"]``, because there is no *sh_lex* node on the top of the tree
+- ``["bar"]``
+- ``[]`` (empty vector)
+
+At the time the input is parsed, a *parse tree* is built. When it
+matches, it describes which part of the *ecoli grammar tree* that
+actually matched, and what input matched for each node.
+
+Passing ``["bar", "1"]`` to the previous tree would result in the
+following *parse tree*:
+
+.. figure:: parse-tree.svg
+
+   The *ecoli parse tree*, result of parsing.
+
+Each node of the *parse tree* references the node of the *grammar tree*
+that matched. It also stores the string vector that matched.
+
+.. figure:: parse-tree2.svg
+
+   The same *ecoli parse tree*, showing the references to the grammar
+   tree.
+
+We can see that not all of the grammar nodes are referenced, since the
+str("foo") node was not parsed. Similarly, one grammar node can be
+referenced several times in the parse tree, as we can see it in the
+following example:
+
+.. figure:: simple-tree3.svg
+
+   A simple *ecoli grammar tree*, that matches ``[]``, ``[foo]``,
+   ``[foo, foo]``, ...
+
+Here is the resulting *parse tree* when the input vector is ``[foo, foo,
+foo]``:
+
+.. figure:: parse-tree3.svg
+
+   The resulting *parse tree*.
+
+Node identifier
+---------------
+
+XXX
+
+Todo
+----
+
+- completions
+- C example
+- ec_config
+- parse yaml
+- params are consumed
+- nodes
+- attributes