# CSV [![Build Status](]( [![Coverage Status](]( [![Codacy Badge](]( [![Hex pm](]( [![Hex Docs](]( [![License](]( [![Downloads](](

[RFC 4180]( compliant, composable CSV parsing and encoding for Elixir.

## Installation

{:csv, "~> 3.0"}
to your deps in `mix.exs` like so:

defp deps do
  [{:csv, "~> 3.0"}]

## Getting all correctly formatted rows
CSV is a notoriously fickle format, with many implementations and files interpreting it differently.

For that reason, `CSV` implements a normal mode `CSV.decode` that will return a stream of `ok: ["field1", "field2"]`
and `err: "Message"` tuples. It will also **reparse lines after a previous line has opened an unterminated escape sequence**,
ensuring you get all correctly formatted rows.

The goal of this library is to allow to extract all correctly formatted rows, while displaying descriptive errors for 
incorrectly formatted rows.

In strict mode using `CSV.decode!` the library will raise an exception when it encounters the first error, aborting the

## Performance
This library uses fast binary matching and is able to parse about half a million rows of a moderately complex CSV file 
per second in a single process on a small cloud instance spec (2vCPU, 2GB Memory). CSV parsing will unlikely become a 
bottleneck in your data pipeline.

If you are reading from a large file, `CSV` will perform best when streaming with `:read_ahead` in byte mode:

```elixir!("data.csv", [read_ahead: 100_000], 1000) |> CSV.decode()

While `1000` is usually a good default number of bytes to stream, you should measure performance and fine-tune
byte size according to your environment.

## Upgrading from 2.x
The main goal for 3.x has been to streamline the library API and leverage binary matching. 

#### Upgrading should require few to no changes in most cases

- **Parallelism has been removed**, alongside its options `:num_workers` and `:worker_work_ratio`. You can safely remove them.
- `CSV` now expects line breaks to be present in the data. If you used to parse strings by applying `String.split/2` before 
  passing it to decode, you can do the same now feeding in
  the string as a single item of a list:
  ["a,b,c\nd,e,f"] |> CSV.decode()
- **`StrayQuoteError` is now `StrayEscapeCharacterError`**. If you catch this error in your code, you need to rename it.
- **The `:strip_fields` option needs to be replaced** with the `:field_transform` option:
  ```elixir!("data.csv") |> CSV.decode(field_transform: &String.trim/1)
- **`:validate_row_length` now defaults to `false`**. This option produces an error for rows with different length. Set it
  to `true` to get the same behaviour as in 2.x
- **`:escape_formulas` is now `:unescape_formulas` for `decode` and `decode!`.** It is still `:escape_formulas` for
  `encode`. Change `:escape_formulas` to `:unescape_formulas` in `decode` calls to get the same behaviour as in 2.x
- **`:escape_max_lines` now defaults to `10`** instead of `1000`. To get the same behaviour as in 2.x, use:
  ```elixir!("data.csv") |> CSV.decode(escape_max_lines: 1000)
- **`:replace` has been removed**. `CSV` will now return fields with incorrect encoding as-is. 
  You can use the new `:field_transform` option to provide a function transforming fields while they are being parsed. 
  This allows to e.g. replace incorrect encoding:
  defp replace_bad_encoding(field) do
    if String.valid?(field) do
      |> String.codepoints()
      |> codepoint -> if String.valid?(codepoint), do: codepoint, else: "?" end)
      |> Enum.join()
  end!("data.csv") |> CSV.decode(field_transform: &replace_bad_encoding/1)

**That's it!** Please open an issue if you see any other non-backward compatible behaviour so it can be documented.

### Elixir version requirements
* Elixir `1.5.0` is required for all versions above `2.5.0`.
* Elixir `1.1.0` is required for all versions above `1.1.5`.

## Design Goals
This library aims to to solve concerns related to csv parsing in data pipelines, following the UNIX philosophy:
It consumes streams or enumerables, producing streams of lists, maps or tuples depending on configuration. This simplifies
 using it in data pipelines, where CSV encoding or decoding is only one of the processing steps.

## Usage
`CSV` can decode and encode from and to a stream of bytes or lines.

### Decoding

Do this to decode data:

# Decode file line by line!("data.csv")
  |> CSV.decode()

# Decode a UTF-16 file with BOM!([:trim_bom, encoding: {:utf16, :little}])
  |> CSV.decode()

# Decode file in chunks of 1000 bytes!("data.csv", [], 1000) 
  |> CSV.decode()

# Decode a csv formatted string
  |> CSV.decode()

# Decode a list of arbitrarily chunked csv data
["list,", "of,arbitrarily", "\\nchun", "ked,csv,data\\n"] 
  |> CSV.decode()

And you'll get a stream of row tuples:
[ok: ["a", "b"], ok: ["c", "d"]]

And, potentially error tuples:
[error: "", ok: ["c", "d"]]

Use strict mode `decode!` to get a two-dimensional list, raising errors as they
occur, aborting the operation:
````elixir!("data.csv") |> CSV.decode!

#### Options

For all available options [check the docs on `decode`](
[and `decode!`](!/2)

Specify a semicolon separator:

stream |> CSV.decode(separator: ?;)

Specify a custom escape character:

stream |> CSV.decode(escape_character: ?@)

Apply a transformation to a field when parsed, e.g. trimming the field:

stream |> CSV.decode(field_transform: &String.trim/1)

Unescape formulas that have been escaped:

stream |> CSV.decode(unescape_formulas: true)

### Encoding

Do this to encode a table (two-dimensional list):

table_data |> CSV.encode

And you'll get a stream of lines ready to be written to an IO.
So, this is writing to a file:

file =!("test.csv", [:write, :utf8])
table_data |> CSV.encode |> Enum.each(&IO.write(file, &1))

#### Options

Use a semicolon separator:

your_data |> CSV.encode(separator: ?;)

Use a specific escape character:

your_data |> CSV.encode(escape_character: ?@)

You can also specify headers when encoding, which will encode map values into
the right place:

[%{"a" => "value!"}] |> CSV.encode(headers: ["z", "a"])
# ["z,a\\r\\n", ",value!\\r\\n"]

You can also specify a keyword list, the keys of the list will be used as the keys for the rows, 
but the values will be the value used for the header row name in CSV output

[%{a: "value!"}] |> CSV.encode(headers: [a: "x", b: "y"])
# ["x,y\\r\\n", "value!,\\r\\n"]

You'll surely appreciate some [more info on `encode`](

#### Polymorphic encoding

Make sure your data gets encoded the way you want - implement the `CSV.Encode`
protocol for whatever you wish to encode:

defimpl CSV.Encode, for: MyData do
  def encode(%MyData{has: fun}, env \\ []) do
    "so much #{fun}" |> CSV.Encode.encode(env)

Or similar.

#### Ensure performant encoding

The encoding protocol implements a fallback to Any for types where a simple call
o `to_string` will provide unambiguous results. Protocol dispatch for the
fallback to Any is *very* slow when protocols are not consolidated, so make sure
you [have `consolidate_protocols: true`](
in your `mix.exs` or you consolidate protocols manually for production in order
to get good performance.

There is more to know about everything :tm: - [Check the doc](

## Contributions & Bugfixes are most welcome!

Please make sure to add tests. I will not look at PRs that are
either failing or lowering coverage. Also, solve one problem at
a time.

## Copyright and License

Copyright (c) 2022 Beat Richartz

CSV source code is licensed under the [MIT License](