## 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.

In this section, we’ll add the searching logic to the `minigrep` program
using the test-driven development (TDD) process with the following 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!

Though it’s just one of many ways to write software, TDD can help drive code
design. 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*, 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, 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 yet able to run this test and watch it fail because the test doesn’t
even compile: the `search` function doesn’t exist yet! In accordance with TDD
principles, 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 to define an explicit lifetime `'a` in the signature of
`search` and use that lifetime 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 explicitly. 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, but this practice 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>

At the moment, we’re building up functionality. To get it to compile, we need
to return a value from the body as we indicated we would in the function
signature.

#### Storing Matching Lines

To finish this function, we need a way to store the matching lines that we want
to return. 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
