# API File Generation

There are certain pieces of `NeuralNetworksTypes.h`, `Types.h`,
`OperandTypes.h`, `OperationTypes.h`, and of our various `*.hal`/`*.aidl` files
that ought to be kept in sync -- most notably the operand type and operation
type definitions and descriptions. To avoid having to do this manually, a tool
`generate_api.py` is employed to combine a single *specification file* with one
*template file* per API file to produce that API file. The
script `generate_api.sh` invokes `generate_api.py` once per API file, passing
appropriate arguments.

## `generate_api.sh`

The environment variable `ANDROID_BUILD_TOP` must be set.

Invoked with no arguments or with the `--mode=update` argument, this script
regenerates each API file in place, by invoking `generate_api.py` once per
generated file.

Invoked with the `--mode=hook` argument, this script checks whether NDK and
Canonical files need to be regenerated.

When the `--dryrun` argument is present, this script shows how it would invoke
`generate_api.py` but does not actually regenerate files or check whether they
need to be regenerated.

## `generate_api.py`

This tool generates a single output file from an input specification file and an
input template file.  It takes the following mandatory arguments:

* `--output OUTPUT` path to generated output file (such as `Types.h`)
* `--specification SPECIFICATION` path to input specification file
* `--template TEMPLATE` path to input template file
* `--kind KIND` token identifying kind of file to generate

The "kind" is an arbitrary token that the specification file can reference with
the `%kind` directive to help generate different text in different situations.
It has no meaning to the tool itself.  Today, the following kinds are used:
`ndk` (when generating `NeuralNetworksTypes.h`), `canonical` (when generating
`Types.h`, `OperandTypes.h`, and `OperationTypes.h`), `hal_1.0` (when generating
`1.0/types.hal`), `hal_1.1`, `hal_1.2`, `hal_1.3` and `aidl` (when
generating `OperandType.aidl` and `OperationType.aidl`).

## Template File Syntax

Every line of the template file is copied verbatim to the output file *unless*
that line begins with `%`.

A line that begins with `%%` is a comment, and is ignored.

A line that begins with `%` and is not a comment is a *directive*.

### Directives

#### `%insert *name*`

Copy the *section* with the specified *name* from the specification file to the
output file.  The section is defined by a `%section` directive in the
specification file.

#### `%insert-indented *count* *name*`

Similar to `%insert *name*`, but each non-empty copied line is prefixed with
*count* space characters.  *count* must be a non-negative integer.

## Specification File Syntax

The specification file consists of comments, *directives*, and other text.

A line that begins with `%%` is a comment, and is ignored.

A line that begins with `%` and is not a comment is a *directive*.

The meaning of a line that is neither a comment nor a directive depends on the
context -- the *region* in which that line appears.

### Regions

The specification file is divided into *regions*, which are sequences of lines
delimited by certain directives.

Certain regions can enclose certain other regions, but this is very limited:

* A conditional region can enclose a section region.
* A section region can enclose a conditional region.

Equivalently:

* A conditional region can be enclosed by a section region.
* A section region can be enclosed by a conditional region.

#### null region

A *null region* is a sequence of lines that is not part of any other region.
For example, a specification file that contains no directives other than
`%define` and `%define-kinds` consists of a single null region.

Within a null region, all lines other than directives are treated as comments
and are ignored.

#### conditional region

A *conditional region* is a sequence of lines immediately preceded by the `%kind
*list*` directive and immediately followed by the `%/kind` directive.  The
`%kind` directive establishes a condition state **on** or **off** (see the
description of the directive for details).  When the condition is **on**, the
lines in the region are processed normally (i.e., directives have their usual
effect, and non-directive lines are added to the enclosing section region, if
any).  When the condition is **off**, lines in the region other than the `%else`
directive are ignored *except* that even ignored directives undergo some level
of syntactic and semantic checking.

#### section region

A *section region* is a sequence of lines immediately preceded by the `%section
*name*` directive and immediately followed by the `%/section` directive.  Every
line in the sequence that doesn't begin with `%` undergoes macro substitution,
and the resulting lines are associated with the section name.  They can be
inserted into the generated output file as directed by the template file's
`%insert` and `%insert-indented` directives.  They can be added to another
section region with the with the specification file's `%insert` and
`%insert-indented` directives.

This is the mechanism by which a specification file contributes text to the
generated output file.

### Directives

#### `%define *name* *body*`

Defines a macro identified by the token *name*.  The *body* is separated from
the *name* by exactly one whitespace character, and extends to the end of the
line -- it may contain whitespace itself.  For example,

  %define test  this body begins and ends with a space character 

Macro substitution occurs within a section region: a substring `%{*name*}` is
replaced with the corresponding *body*.  Macro substitution is *not* recursive:
A substring `%{*name2*}` in *body* will not undergo macro substitution, except
as discussed for *macro arguments* below.

Permitted in regions: null, conditional, section

##### macro arguments

The more general form of a macro invocation is `%{*name* *arglist*}`, where
*arglist* is a list of whitespace-separated arguments.  Within the *body*, a
substring of the form `%{argnum}` will be replaced by the corresponding argument
from *arglist*.  For example, if the definition is

```
%define test second is %{2}, first is %{1}
```

then the macro invocation

```
%{test alpha beta}
```

is expanded to

```
second is beta, first is alpha
```

The only check on the number of arguments supplied at macro invocation time is
that there must be at least as many arguments as the highest `%{argnum}`
reference in the macro body.  In the example above, `%{test alpha}` would be an
error, but `%{test alpha beta gamma}` would not.

#### `%insert *name*`

Adds all lines from the named section region to the current section region.

Permitted in regions: section

#### `%insert-indented *count* *name*`

Similar to `%insert *name*`, but each non-empty added line is prefixed
with *count* space characters.  *count* must be a non-negative integer.

Permitted in regions: section

#### `%kind *list*`, `%else`, `%/kind`

`%kind *list*` creates a *conditional region* terminated by `%/kind`.

The *list* consists of a space-delimited list of tokens, any of which may end in
`*` to indicate a *wildcard pattern* or `+` to indicate a *lowest version
pattern*.  Any other pattern is a *simple pattern*.  The condition is **on** in
three cases:
* One of the simple pattern tokens equals the "kind"
* One of the wildcard pattern tokens less the `*` is a prefix of the "kind"
* One of the lowest version pattern tokens less the `+` matches the "kind" or
  the "kind" matches any token to the right from the lowest version pattern in
  the list passed to %define-kinds

In all other cases, the condition is **off**.

Within the region, the condition is inverted every time the `%else` directive
appears.

Permitted in regions: null, section

#### `%define-kinds *list*`

This directive has two purposes:

* Validity-checking.  If the "kind" is not on the space-delimited *list* of tokens,
  `generate_api.py` terminates with an error.
* Ordering the possible kinds for the *lowest version pattern* (see the section
  above for the explanation of the pattern).

Only one such directive is allowed per specification file.

Permitted in regions: null, section

#### `%section *name*`, `%/section`

`%section *name*` creates a *section region* terminated by `%/section`.

Permitted in regions: null, conditional