## Developing the Library’s Functionality with Test-Driven Development

Now that we’ve extracted the logic into *src/lib.rs* and left the argument
collecting and error handling in *src/main.rs*, it’s much easier to write tests
for the core functionality of our code. We can call functions directly with
various arguments and check return values without having to call our binary
from the command line. Feel free to write some tests for the functionality in
the `Config::new` and `run` functions on your own.

In this section, we’ll add the searching logic to the `minigrep` program by
using the Test-driven development (TDD) process. This software development
technique follows these steps:

1. Write a test that fails and run it to make sure it fails for the reason you
   expect.
2. Write or modify just enough code to make the new test pass.
3. Refactor the code you just added or changed and make sure the tests
   continue to pass.
4. Repeat from step 1!

This process is just one of many ways to write software, but TDD can help drive
code design as well. Writing the test before you write the code that makes the
test pass helps to maintain high test coverage throughout the process.

We’ll test drive the implementation of the functionality that will actually do
the searching for the query string in the file contents and produce a list of
lines that match the query. We’ll add this functionality in a function called
`search`.

### Writing a Failing Test

Because we don’t need them anymore, let’s remove the `println!` statements from
*src/lib.rs* and *src/main.rs* that we used to check the program’s behavior.
Then, in *src/lib.rs*, we’ll add a `tests` module with a test function, as we
did in [Chapter 11][ch11-anatomy]<!-- ignore -->. The test function specifies
the behavior we want the `search` function to have: it will take a query and
the text to search for the query in, and it will return only the lines from the
text that contain the query. Listing 12-15 shows this test, which won’t compile
yet.

<span class="filename">Filename: src/lib.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch12-an-io-project/listing-12-15/src/lib.rs:here}}
```

<span class="caption">Listing 12-15: Creating a failing test for the `search`
function we wish we had</span>

This test searches for the string `"duct"`. The text we’re searching is three
lines, only one of which contains `"duct"` (Note that the backslash after the
opening double quote tells Rust not to put a newline character at the beginning
of the contents of this string literal). We assert that the value returned from
the `search` function contains only the line we expect.

We aren’t able to run this test and watch it fail because the test doesn’t even
compile: the `search` function doesn’t exist yet! So now we’ll add just enough
code to get the test to compile and run by adding a definition of the `search`
function that always returns an empty vector, as shown in Listing 12-16. Then
the test should compile and fail because an empty vector doesn’t match a vector
containing the line `"safe, fast, productive."`

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch12-an-io-project/listing-12-16/src/lib.rs:here}}
```

<span class="caption">Listing 12-16: Defining just enough of the `search`
function so our test will compile</span>

Notice that we need an explicit lifetime `'a` defined in the signature of
`search` and used with the `contents` argument and the return value. Recall in
[Chapter 10][ch10-lifetimes]<!-- ignore --> that the lifetime parameters
specify which argument lifetime is connected to the lifetime of the return
value. In this case, we indicate that the returned vector should contain string
slices that reference slices of the argument `contents` (rather than the
argument `query`).

In other words, we tell Rust that the data returned by the `search` function
will live as long as the data passed into the `search` function in the
`contents` argument. This is important! The data referenced *by* a slice needs
to be valid for the reference to be valid; if the compiler assumes we’re making
string slices of `query` rather than `contents`, it will do its safety checking
incorrectly.

If we forget the lifetime annotations and try to compile this function, we’ll
get this error:

```console
{{#include ../listings/ch12-an-io-project/output-only-02-missing-lifetimes/output.txt}}
```

Rust can’t possibly know which of the two arguments we need, so we need to tell
it. Because `contents` is the argument that contains all of our text and we
want to return the parts of that text that match, we know `contents` is the
argument that should be connected to the return value using the lifetime syntax.

Other programming languages don’t require you to connect arguments to return
values in the signature. Although this might seem strange, it will get easier
over time. You might want to compare this example with the [“Validating
References with Lifetimes”][validating-references-with-lifetimes]<!-- ignore
--> section in Chapter 10.

Now let’s run the test:

```console
{{#include ../listings/ch12-an-io-project/listing-12-16/output.txt}}
```

Great, the test fails, exactly as we expected. Let’s get the test to pass!

### Writing Code to Pass the Test

Currently, our test is failing because we always return an empty vector. To fix
that and implement `search`, our program needs to follow these steps:

* Iterate through each line of the contents.
* Check whether the line contains our query string.
* If it does, add it to the list of values we’re returning.
* If it doesn’t, do nothing.
* Return the list of results that match.

Let’s work through each step, starting with iterating through lines.

#### Iterating Through Lines with the `lines` Method

Rust has a helpful method to handle line-by-line iteration of strings,
conveniently named `lines`, that works as shown in Listing 12-17. Note this
won’t compile yet.

<span class="filename">Filename: src/lib.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch12-an-io-project/listing-12-17/src/lib.rs:here}}
```

<span class="caption">Listing 12-17: Iterating through each line in `contents`
</span>

The `lines` method returns an iterator. We’ll talk about iterators in depth in
[Chapter 13][ch13-iterators]<!-- ignore -->, but recall that you saw this way of using an
iterator in [Listing 3-5][ch3-iter]<!-- ignore -->, where we used a `for` loop
with an iterator to run some code on each item in a collection.

#### Searching Each Line for the Query

Next, we’ll check whether the current line contains our query string.
Fortunately, strings have a helpful method named `contains` that does this for
us! Add a call to the `contains` method in the `search` function, as shown in
Listing 12-18. Note this still won’t compile yet.

<span class="filename">Filename: src/lib.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch12-an-io-project/listing-12-18/src/lib.rs:here}}
```

<span class="caption">Listing 12-18: Adding functionality to see whether the
line contains the string in `query`</span>

#### Storing Matching Lines

We also need a way to store the lines that contain our query string. For that,
we can make a mutable vector before the `for` loop and call the `push` method
to store a `line` in the vector. After the `for` loop, we return the vector, as
shown in Listing 12-19.

<span class="filename">Filename: src/lib.rs</span>

```rust,ignore
{{#rustdoc_include ../listings/ch12-an-io-project/listing-12-19/src/lib.rs:here}}
```

<span class="caption">Listing 12-19: Storing the lines that match so we can
return them</span>

Now the `search` function should return only the lines that contain `query`,
and our test should pass. Let’s run the test:

```console
{{#include ../listings/ch12-an-io-project/listing-12-19/output.txt}}
```

Our test passed, so we know it works!

At this point, we could consider opportunities for refactoring the
implementation of the search function while keeping the tests passing to
maintain the same functionality. The code in the search function isn’t too bad,
but it doesn’t take advantage of some useful features of iterators. We’ll
return to this example in [Chapter 13][ch13-iterators]<!-- ignore -->, where we’ll
explore iterators in detail, and look at how to improve it.

#### Using the `search` Function in the `run` Function

Now that the `search` function is working and tested, we need to call `search`
from our `run` function. We need to pass the `config.query` value and the
`contents` that `run` reads from the file to the `search` function. Then `run`
will print each line returned from `search`:

<span class="filename">Filename: src/lib.rs</span>

```rust,ignore
{{#rustdoc_include ../listings/ch12-an-io-project/no-listing-02-using-search-in-run/src/lib.rs:here}}
```

We’re still using a `for` loop to return each line from `search` and print it.

Now the entire program should work! Let’s try it out, first with a word that
should return exactly one line from the Emily Dickinson poem, “frog”:

```console
{{#include ../listings/ch12-an-io-project/no-listing-02-using-search-in-run/output.txt}}
```

Cool! Now let’s try a word that will match multiple lines, like “body”:

```console
{{#include ../listings/ch12-an-io-project/output-only-03-multiple-matches/output.txt}}
```

And finally, let’s make sure that we don’t get any lines when we search for a
word that isn’t anywhere in the poem, such as “monomorphization”:

```console
{{#include ../listings/ch12-an-io-project/output-only-04-no-matches/output.txt}}
```

Excellent! We’ve built our own mini version of a classic tool and learned a lot
about how to structure applications. We’ve also learned a bit about file input
and output, lifetimes, testing, and command line parsing.

To round out this project, we’ll briefly demonstrate how to work with
environment variables and how to print to standard error, both of which are
useful when you’re writing command line programs.

[validating-references-with-lifetimes]:
ch10-03-lifetime-syntax.html#validating-references-with-lifetimes
[ch11-anatomy]: ch11-01-writing-tests.html#the-anatomy-of-a-test-function
[ch10-lifetimes]: ch10-03-lifetime-syntax.html
[ch3-iter]: ch03-05-control-flow.html#looping-through-a-collection-with-for
[ch13-iterators]: ch13-02-iterators.html
