Scala/Scala.js library for manipulating Fancy Ansi colored strings
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readme.md

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LandingExample

"com.lihaoyi" %% "fansi" % "0.2.5"
"com.lihaoyi" %%% "fansi" % "0.2.5" // Scala.js or Scala-Native

Fansi is a Scala library to make it easy to deal with fancy colored Ansi strings within your command-line programs.

While "normal" use of Ansi escapes with java.lang.String, you find yourself concatenating colors:

val colored = Console.RED + "Hello World Ansi!" + Console.RESET

To build your colored string. This works the first time, but is error prone on larger strings: e.g. did you remember to put a Console.RESET where it's necessary? Do you need to end with one to avoid leaking the color to the entire console after printing it?

Furthermore, some operations are fundamentally difficult or error-prone with this approach. For example,

val colored: String = Console.RED + "Hello World Ansi!" + Console.RESET

// How to efficiently get the length of this string on-screen? We could try
// using regexes to remove and Ansi codes, but that's slow and inefficient. 
// And it's easy to accidentally call `colored.length` and get a invalid length
val length = ??? 

// How to make the word `World` blue, while preserving the coloring of the 
// `Ansi!` text after? What if the string came from somewhere else and you 
// don't know what color that text was originally?
val blueWorld = ??? 

// What if I want to underline "World" instead of changing it's color, while
// still preserving the original color?
val underlinedWorld = ???

// What if I want to apply underlines to "World" and the two characters on 
// either side, after I had already turned "World" blue?
val underlinedBlue = ???

While simple to describe, these tasks are all error-prone and difficult to do using normal java.lang.Strings containing Ansi color codes. This is especially so if, unlike the toy example above, colored is coming from some other part of your program and you're not sure what or how-many Ansi color codes it already contains.

With Fansi, doing all these tasks is simple, error-proof and efficient:

val colored: fansi.Str = fansi.Color.Red("Hello World Ansi!")
// Or fansi.Str("Hello World Ansi!").overlay(fansi.Color.Red) 

val length = colored.length // Fast and returns the non-colored length of string

val blueWorld = colored.overlay(fansi.Color.Blue, 6, 11)

val underlinedWorld = colored.overlay(fansi.Underlined.On, 6, 11)

val underlinedBlue = blueWorld.overlay(fansi.Underlined.On, 4, 13)

And it just works:

LandingExample

Why Fansi?

Unlike normal java.lang.Strings with Ansi escapes embedded inside, fansi.Str allows you to perform a range of operations in an efficient manner:

  • Extracting the non-Ansi plainText version of the string

  • Get the non-Ansi length

  • Concatenate colored Ansi strings without worrying about leaking colors between them

  • Applying colors to certain portions of an existing fansi.Str, and ensuring that the newly-applied colors get properly terminated while existing colors are unchanged

  • Splitting colored Ansi strings at a plainText index

  • Rendering to colored java.lang.Strings with Ansi escapes embedded, which can be passed around or concatenated without worrying about leaking colors.

These are tasks which are possible to do with normal java.lang.String, but are tedious, error-prone and typically inefficient. Often, you can get by with adding copious amounts of Console.RESETs when working with colored java.lang.Strings, but even that easily results in errors when you RESET too much and stomp over colors that already exist:

StringError

fansi.Str allows you to perform these tasks safely and easily:

FansiRocks

Fansi is also very efficient: fansi.Str uses just 3x as much memory as java.lang.String to hold all the additional formatting information. Its operations are probably about the same factor slower, as they are all implemented using fast arraycopys and while-loops similar to java.lang.String. That means that - unlike fiddling with Ansi-codes using regexes - you generally do not need to worry about performance when dealing with fansi.Strs. Just treat them as you would java.lang.Strings: splitting them, substringing them, and applying or removing colors or other styles at-will.

Fansi was originally a part of the Ammonite REPL, but is now a standalone zero-dependency library anyone can use if they want to easily and efficiently deal with colored Ansi strings.

Using Fansi

The main operations you need to know are:

  • fansi.Str(raw: CharSequence): fansi.String, to construct colored Ansi strings from a java.lang.String, with or without existing Ansi color codes inside it.

  • fansi.Str, the primary data-type that you will use to pass-around colored Ansi strings and manipulate them: concatenating, splitting, applying or removing colors, etc.

fansi.Str

  • fansi.Attrs are the individual modifications you can make to an fansi.Str's formatting. Examples are:
    • fansi.Bold.{On, Off}
    • fansi.Reversed.{On, Off}
    • fansi.Underlined.{On, Off}
    • fansi.Color.*
    • fansi.Back.*
    • fansi.Attr.Reset

fansi.Attr

  • fansi.Attrs represents a group of zero or more fansi.Attrs. These that can be passed around together, combined via ++ or applied to fansi.Strs all at once. Any individual fansi.Attr can be used when fansi.Attrs is required, as can fansi.Attrs.empty.

fansi.Attrs

  • Using any of the fansi.Attr or fansi.Attrs mentioned above, e.g. fansi.Color.Red, using fansi.Color.Red("hello world ansi!") to create a fansi.Str with that text and color, or fansi.Str("hello world ansi!").overlay(fansi.Color.Blue, 6, 11)

  • .render to convert a fansi.Str back into a java.lang.String with all necessary Ansi color codes within it

Fansi also supports 8-bit 256-colors through fansi.Color.Full and fansi.Back.Full, as well as 24-bit 16-million-colors through fansi.Color.True and fansi.Back.True:

docs/TrueColor.png

Note that Fansi only performs the rendering of the colors to an ANSI-encoded string. Final rendering will depend on whichever terminal you print the string to, whether it is able to display these sets of colors or not.

Digging Deeper

If you want to dig into deeper, there are a few more APIs you can use:

  • fansi.Str.join(args: fansi.Str*) to conveniently join together multiple fansi.Strs all at once, more efficient than ++ for large numbers of inputs
  • getColors/getColor and getChars/getChar methods on fansi.Str to extract the raw data for your own use
  • fansi.Str.fromArrays to piece it back together

This allows you to perform fast, mutable array operations on the color/character arrays if you know what you're doing and want to perform operations that are inconvenient or slow to do through fansi.Str's immutable API. For example, if you want to do a bunch of work with colored strings and then at-the-end render everything to HTML, you can manually walk over the color/character arrays yourself and decide where to print HTML tags to give the text colors.

fansi.Str currently has a relatively skeletal API: it is slightly smaller than what java.lang.String has, and definitely much less than what is available on scala.RichString's extension methods. Feel free to implement your own custom operations using fromArrays if you can't find what you want on fansi.Str, or send a patch if you think it's arguably general enough to be included in Fansi itself.

  • fansi.Attrs.emitAnsiCodes Lets you manually emit the different java.lang.Strings that correspond to changes in color in an Ansi string.

For example, if you want to emit the Ansi codes that correspond to the transition from "No Color" to "Red", you can use

fansi.Attrs.emitAnsiCodes(0, fansi.Color.Red.applyMask) // "\u001b[31m"

Or the Ansi codes from "Red" to "No Color"

fansi.Attrs.emitAnsiCodes(fansi.Color.Red.applyMask, 0) // "\u001b[39m"

Or for any other combination of attributes

val attrs = fansi.Color.Red ++ fansi.Back.Blue ++ fansi.Underlined.On
fansi.Attrs.emitAnsiCodes(0, attrs.applyMask) // "\u001b[31m\u001b[44m\u001b[4m"

You can also pass in an errorMode when parsing a string via ansi.Str(...) to tell Fansi how to behave if it finds Ansi escapes it can't handle. You have the options:

  • fansi.ErrorMode.Throw is the default, to throw an exception and fail the parse if it sees an Ansi escape it does not recognize.
  • fansi.ErrorMode.Sanitize to remove the escape character but leave the remnants of the escape-sequence in the result that people can see
  • fansi.ErrorMode.Strip to remove those escape sequences entirely so that no trace of them remains in the final result

Scaladoc

Changelog

0.2.5

  • Support for Scala-Native

0.2.4

  • Added fansi.Str.join, fansi.Str#getChar, fansi.Str#getColor
  • Created a Patreon Page to raise money to try and fund the development of Fansi, PPrint, Ammonite and my other open-source libraries. If you've use these libraries in the past and enjoyed doing so, please chip in to support development!

0.2.3

  • Publish for Scala 2.12

0.2.2

  • Reduce memory usage by 134mb by not initializing huge lookup tables to parse truecolor colors.

0.2.1

  • Fix #7: Parsing of true colors broken

0.2.0

  • Added the fansi.Color.True and fansi.Back.True colors, allowing you to specify Attrs that represent the 24-bit 16-million-color "True Color" range.

0.1.3

  • Fixed a bug in substring that incorrectly threw an out of bounds exception for end == length
  • Exposed the fansi.Attrs.emitAnsiCodes function
  • Renamed Attrs.empty to Attrs.Empty for consistency with all the others

0.1.2

  • Removed infinite-loop if parsing strings with Ansi escapes that are not recognized by Fansi
  • Added fansi.ErrorMode parameter, to control behavior when un-recognized Ansi escapes are found.

0.1.1

  • Doubled the speed of the .render operation
  • Doubled the speed of the .overlay operation
  • Added the .overlayAll method on fansi.Str, to allow you to quickly apply multiple overlays onto the same string

0.1.0

  • First release