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text-placeholder

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A flexible text template engine

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# Text Placeholder

Text Placeholder is a minimalistic text template engine designed for the manipulation of named
placeholders within textual templates.

This library operates based on two primary elements:

- **Placeholders**: Defined markers within the text templates intended to be replaced by actual
  content in the final rendition.

- **Context**: The precise data set used for the replacement of placeholders during the template
  rendering process.

For use within a [`no_std` environment](https://docs.rust-embedded.org/book/intro/no-std.html), Text
Placeholder can be configured by disabling
[the default features](https://doc.rust-lang.org/cargo/reference/features.html#the-default-feature).
This allows the library to maintain compatibility with `no_std` specifications.

## Placeholders

Placeholders are defined within certain boundaries and will be replaced once the template is parsed.

Let's define a template with placeholders named `first` and `second`:

```rust
let template = Template::new("Hello {{first}} {{second}}!");
```

Templates use the handlebars syntax as boundaries by default, but can be overridden:

```rust
let template = Template::new_with_placeholder("Hello $[first] $[second]!", "$[", "]");
```

## Context

Context is the data structure that will be used to replace your placeholders with real data.

You can think of your placeholder as a key within a `HashMap` or the name of a field within a
`struct`. In fact, these are the three types of context supported by this library:

- HashMap.
- A function
- Struct, as an **optional** feature.

### HashMap

Each placeholder should be a `key` with an associated `value` that can be converted into a `str`.

The following methods are available with a `HashMap`:

- `fill_with_hashmap`
  - replaces missing placeholders with an empty string.
  - replaces placeholders that cannot be converted to a strint with an empty string.
- `fill_with_hashmap_strict` which returns a `Error::PlaceholderError` when:
  - a placeholder is missing.
  - a placeholder value cannot be converted to a string.

#### Example

```rust
use text_placeholder::Template;
use std::collections::HashMap; // or for no_std `use hashbrown::HashMap;`

let default_template = Template::new("Hello {{first}} {{second}}!");

let mut table = HashMap::new();
table.insert("first", "text");
table.insert("second", "placeholder");

assert_eq!(default_template.fill_with_hashmap(&table), "Hello text placeholder!");

// We can also specify our own boundaries:

let custom_template = Template::new_with_placeholder("Hello $[first]] $[second]!", "$[", "]");

assert_eq!(default_template.fill_with_hashmap(&table), "Hello text placeholder!");
```

### Function

This uses a function to generate the substitution value. The value could be extracted from a HashMap
or BTreeMap, read from a config file or database, or purely computed from the key itself.

The function takes a `key` and returns an `Option<Cow<str>>` - that is it can return a borrowed
`&str`, an owned `String` or no value. Returning no value causes `fill_with_function` to fail (it's
the equivalent of `fill_with_hashmap_strict` in this way).

The function actually a `FnMut` closure, so it can also modify external state, such as keeping track
of which `key` values were used. `key` has a lifetime borrowed from the template, so it can be
stored outside of the closure.

#### Example

```rust
use text_placeholder::Template;
use std::borrow::Cow;

let template = Template::new("Hello {{first}} {{second}}!");

let mut idx = 0;
assert_eq!(
    &*template.fill_with_function(
    |key| {
      idx += 1;
      Some(Cow::Owned(format!("{key}-{idx}")))
    })
    .unwrap(),
    "Hello first-1 second-2!"
);
assert_eq!(idx, 2);
```

### Struct

Allow structs that implement the `serde::Serialize` trait to be used as context.

This is an optional feature that depends on `serde`. In order to enable it add the following to your
`Cargo.toml` file:

```toml
[dependencies]
text_placeholder = { version = "0.4", features = ["struct_context"] }
```

Each placeholder should be a `field` in your `struct` with an associated `value` that can be
converted into a `str`.

The following methods are available with a `struct`:

- `fill_with_struct`
  - replaces missing placeholders with an empty string.
  - replaces placeholders that cannot be converted to a strint with an empty string.
- `fill_with_struct_strict` which returns a `Error::PlaceholderError` when:
  - a placeholder is missing.
  - a placeholder value cannot be converted to a string.

#### Example

```rust
use text_placeholder::Template;

#[derive(Serialize)]
struct Context {
    first: String,
    second: String
}

let default_template = Template::new("Hello {{first}} {{second}}!");
let context = Context { first: "text".to_string(), second: "placeholder".to_string() };

assert_eq!(default_template.fill_with_struct(&context), "Hello text placeholder!");

// We can also specify our own boundaries:

let custom_template = Template::new_with_placeholder("Hello $[first]] $[second]!", "$[", "]");

assert_eq!(default_template.fill_with_struct(&context), "Hello text placeholder!");
```

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