<!--
    This readme is created with https://crates.io/crates/cargo-rdme

    To update: edit `cargo_diff/src/lib.rs` then run:

    ```
    $ cargo rdme -w cache_diff
    ```

    Note: All intra-doc links need a certain type of formattting for rdme to expand
    them to the correct docs.rs links. More info found here:

        https://github.com/orium/cargo-rdme/blob/964a939c8c86a2e6aa3f6a8f89cf75b64ab92f6a/README.md#intralinks
-->

# cache_diff

<!-- cargo-rdme start -->

Generate clean, human readable diffs between two cache structs

### What

The `CacheDiff` trait provides a way to compare two structs and generate a list of differences between them.
When it returns an empty list, the two structs are identical.

You can manually implement the trait, or you can use the `#[derive(CacheDiff)]` macro to automatically generate the implementation.

Attributes are:

  - `cache_diff(rename = "<new name>")` Specify custom name for the field
  - `cache_diff(ignore)` Ignores the given field
  - `cache_diff(display = <function>)` Specify a function to call to display the field

### Why

Cloud Native Buildpacks (CNBs) written in Rust using [libcnb.rs](https://github.com/heroku/libcnb.rs) use
a serializable struct to represent the state of a cache. When that data changes, we need to invalidate the
cache, but also report back to the user what changed.

Due to the CNB layer implementation, this struct is often called "metadata".

### Install

```shell
$ cargo add cache_diff
```

For ANSI colored output, add the [`bullet_stream`](https://github.com/heroku-buildpacks/bullet_stream) feature:

```shell
$ cargo add cache_diff --features bullet_stream
```

### Derive usage

By default a `#[derive(CacheDiff)]` will generate a `diff` function that compares each field in the struct.
You can disable this dependency by specifying `features = []`.

```rust
use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,
}
let diff = Metadata { version: "3.4.0".to_string() }
    .diff(&Metadata { version: "3.3.0".to_string() });

assert_eq!(diff.join(" "), "version (`3.3.0` to `3.4.0`)");
```

Struct fields must implement [`PartialEq`](std::cmp::PartialEq) and [`Display`](std::fmt::Display). Also note that [`PartialEq`](std::cmp::PartialEq) on the top level
cache struct is not  used or required. If you want to customize equality logic, you can implement
the `CacheDiff` trait manually:

```rust
use cache_diff::CacheDiff;

#[derive(Debug)]
struct Metadata {
    version: String,
}

// Implement the trait manually
impl CacheDiff for Metadata {
   fn diff(&self, old: &Self) -> Vec<String> {
        let mut diff = vec![];
        // This evaluation logic differs from the derive macro
        if !self.custom_compare_eq(old) {
            diff.push(format!("Cache is different ({old:?} to {self:?})"));
        }

        diff
   }
}

impl Metadata {
  fn custom_compare_eq(&self, old: &Self) -> bool {
      todo!()
  }
}
```

### Ordering

The order of output will match the struct field definition from top to bottom:

```rust
use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,
    distro: String,
}
let now = Metadata { version: "3.4.0".to_string(), distro: "Ubuntu".to_string() };
let diff = now.diff(&Metadata { version: "3.3.0".to_string(), distro: "Alpine".to_string() });

assert_eq!(diff.join(", "), "version (`3.3.0` to `3.4.0`), distro (`Alpine` to `Ubuntu`)");
```

### Rename attributes

If your field name is not descriptive enough, you can rename it:

```rust
use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    #[cache_diff(rename="Ruby version")]
    version: String,
}
let now = Metadata { version: "3.4.0".to_string() };
let diff = now.diff(&Metadata { version: "3.3.0".to_string() });

assert_eq!(diff.join(" "), "Ruby version (`3.3.0` to `3.4.0`)");
```

### Ignore attributes

If the struct contains fields that should not be included in the diff comparison, you can ignore them:

```rust
use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,

    #[cache_diff(ignore)]
    changed_by: String
}
let now = Metadata { version: "3.4.0".to_string(), changed_by: "Alice".to_string() };
let diff = now.diff(&Metadata { version: now.version.clone(), changed_by: "Bob".to_string() });

assert!(diff.is_empty());
```

### Handle structs missing display

Not all structs implement the [`Display`](std::fmt::Display) trait, for example [`std::path::PathBuf`](std::path::PathBuf) requires that you call `display()` on it.

The `#[derive(CacheDiff)]` macro will automatically handle the following conversions for you:

- `std::path::PathBuf` (via [`std::path::Path::display`](std::path::Path::display))

However, if you have a custom struct that does not implement [`Display`](std::fmt::Display), you can specify a function to call instead:

```rust
use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    #[cache_diff(display = my_function)]
    version: NoDisplay,
}

#[derive(PartialEq)]
struct NoDisplay(String);
fn my_function(s: &NoDisplay) -> String {
    format!("custom {}", s.0)
}

let now = Metadata { version: NoDisplay("3.4.0".to_string())};
let diff = now.diff(&Metadata { version: NoDisplay("3.3.0".to_string())});

assert_eq!(diff.join(" "), "version (`custom 3.3.0` to `custom 3.4.0`)");
```

<!-- cargo-rdme end -->

## Releasing

Releases can be performed via `cargo release`:

```
$ cargo install cargo-release
```

Release readiness for all crates can be checked by running:

```
$ cargo release --workspace --exclude usage --dry-run
```

When satisfied, contributors with permissions can release by running:

```
$ cargo release --workspace --exclude usage --execute
```