ifctc

A Zig implementation of Google's IFTTT (IfThisThenThat) lint. IFCTC stands for "IF Changes Then Change".

WARNING

This project isn't actively used yet. Bugs may be lurking.

Usage

Feed ifctc a unified patch file, and it will verify that all files were modified as specified. Files are read relative to the current working directory.

$ git diff --unified | ifctc

Recognized directives are:

• LINT.IfChange (or LINT.IfChange(label)), which starts a code block.

• LINT.ThenChange(paths, ...), which reports an error if its contents were modified, but not the contents in paths.

  Paths may contain a :label, in which case the code with that label must be modified.

  Paths may be relative, in which case they are relative to the file which has the directive. They may also be absolute (i.e. start with /), in which case they are relative to the directory where ifctc is invoked.

Example

As a concrete example, let's say you have two files with the same constant, which must be kept in sync:

# constants.py
MIN_VERSION = "0.2.0"

// constants.rs
const MIN_VERSION: &str = "0.2.0";

We'll add LINT directives to keep them in sync:

# LINT.IfChange
MIN_VERSION = "0.2.0"
# LINT.ThenChange(constants.rs)

// LINT.IfChange
const MIN_VERSION: &str = "0.2.0";
// LINT.ThenChange(constants.py)

And run ifctc -- everything should be okay:

$ git diff --unified | ifctc && echo ok
ok

Then, if we change one version and forget to update the other:

diff --git a/constants.rs b/constants.rs
--- a/constants.rs
+++ b/constants.rs
@@ -1,3 +1,3 @@
 // LINT.IfChange
-const MIN_VERSION: &str = "0.2.0";
+const MIN_VERSION: &str = "0.3.0";
 // LINT.ThenChange(constants.py)

Then ifctc will report an error:

$ git diff --unified | ifctc
constants.rs:3: file was not modified: constants.py

However, if both files are modified instead:

diff --git a/constants.py b/constants.py
--- a/constants.py
+++ b/constants.py
@@ -1,3 +1,3 @@
 # LINT.IfChange
-MIN_VERSION = "0.2.0"
+MIN_VERSION = "0.3.0"
 # LINT.ThenChange(constants.rs)
diff --git a/constants.rs b/constants.rs
--- a/constants.rs
+++ b/constants.rs
@@ -1,3 +1,3 @@
 // LINT.IfChange
-const MIN_VERSION: &str = "0.2.0";
+const MIN_VERSION: &str = "0.3.0";
 // LINT.ThenChange(constants.py)

Then ifctc succeeds:

$ git diff --unified | ifctc && echo ok
ok

Testing

Use zig build test to run tests.

Because the parsers in this repository have both fast paths and slow paths, they are typically tested with SplitBufferIterator, which ensures that they behave the same way on different chunk sizes.

Fuzzing is not currently used because it is not available on macOS.

Implementation notes

I made this for two reasons:

1. I needed an implementation of IFTTT, which only seems to exist here, but isn't compatible with Google's (e.g. it uses "#" for labels, instead of ":").

2. I wanted to make a small project using Zig.

And because it's Zig, I really wanted to play the game and tried to optimize the tool quite deeply, at the cost of a more complex implementation:

1. Scanning happens in multiple threads at once, without any locking.

   • We achieve this by allocating the map of possibly modified files at the start of the program, before we start scanning files. After that, most state is thread-local, and consolidated once all threads have finished scanning for changes.

2. LINT.IfChange and LINT.ThenChange directives are found using SIMD.

3. Overall, allocations and copies are kept to a minimum:

   • Lines from the patch that don't contribute to the output are skipped without copying them.

   • Nothing from scanned files is copied, except for arguments of IfChange and ThenChange directives.

   • When allocations are needed, that's usually in an arena allocator backed by a stack-fallback allocator. Unless you have long paths in your LINT.ThenChange arguments, no allocation will take place once threads have spawned.