Adds StringSyntaxHighlighter (#363)

* Adds default tab stops

* Fixes SwiftLint warning

* Adds StringSyntaxHighlighter

* Adds documentation

* Fixes SwiftLint warnings

* Improves formatting

Author: simonbs

Sources/Runestone/Documentation.docc/Documentation.md

@@ -61,12 +61,14 @@ Syntax highlighting is based on GitHub's [Tree-sitter](https://github.com/tree-s
 
 - <doc:Syntax-Highlighting-the-Text>
 - <doc:AddingATreeSitterLanguage>
+- <doc:SyntaxHighlightingAString>
 - ``LanguageMode``
 - ``PlainTextLanguageMode``
 - ``TreeSitterLanguageMode``
 - ``TreeSitterLanguage``
 - ``TreeSitterLanguageProvider``
 - ``SyntaxNode``
+- ``StringSyntaxHighlighter``
 
 ### Indentation
 

Sources/Runestone/Documentation.docc/Extensions/StringSyntaxHighlighter.md

@@ -0,0 +1,45 @@
+# ``StringSyntaxHighlighter``
+
+## Example
+
+Create a syntax highlighter by passing a theme and language, and then call the ``StringSyntaxHighlighter/syntaxHighlight(_:)`` method to syntax highlight the provided text.
+
+```swift
+let syntaxHighlighter = StringSyntaxHighlighter(
+    theme: TomorrowTheme(),
+    language: .javaScript
+)
+let attributedString = syntaxHighlighter.syntaxHighlight(
+  """
+  function fibonacci(num) {
+    if (num <= 1) {
+      return 1
+    }
+    return fibonacci(num - 1) + fibonacci(num - 2)
+  }
+  """
+)
+```
+
+## Topics
+
+### Essentials
+
+- <doc:SyntaxHighlightingAString>
+- ``StringSyntaxHighlighter/syntaxHighlight(_:)``
+
+### Initialing the Syntax Highlighter
+
+- ``StringSyntaxHighlighter/init(theme:language:languageProvider:)``
+
+### Configuring the Appearance
+
+- ``StringSyntaxHighlighter/theme``
+- ``StringSyntaxHighlighter/kern``
+- ``StringSyntaxHighlighter/lineHeightMultiplier``
+- ``StringSyntaxHighlighter/tabLength``
+
+### Specifying the Language
+
+- ``StringSyntaxHighlighter/language``
+- ``StringSyntaxHighlighter/languageProvider``

Sources/Runestone/Documentation.docc/SyntaxHighlightingAString.md

@@ -0,0 +1,48 @@
+# Syntax Highlighting a String
+
+Learn how to syntax hightlight a string without needing to create a TextView.
+
+## Overview
+
+The <doc:StringSyntaxHighlighter> can be used to syntax highlight a string without needing to create a <doc:TextView>.
+
+Before reading this article, make sure that you have follow the guides on <doc:AddingATreeSitterLanguage> and <doc:CreatingATheme>.
+
+
+## Creating an Attributed String
+
+Create an instance of <doc:StringSyntaxHighlighter> by supplying the theme containing the colors and fonts to be used for syntax highlighting the text, as well as the language to use when parsing the text.
+
+```swift
+let syntaxHighlighter = StringSyntaxHighlighter(
+    theme: TomorrowTheme(),
+    language: .javaScript
+)
+```
+
+If the language has any embedded languages, you will need to pass an object conforming to <doc:TreeSitterLanguageProvider>, which provides the syntax highlighter with additional languages.
+
+Apply customizations to the syntax highlighter as needed.
+
+```swift
+syntaxHighlighter.kern = 0.3
+syntaxHighlighter.lineHeightMultiplier = 1.2
+syntaxHighlighter.tabLength = 2
+```
+
+With the syntax highlighter created and configured, we can syntax highlight the text.
+
+```swift
+let attributedString = syntaxHighlighter.syntaxHighlight(
+  """
+  function fibonacci(num) {
+    if (num <= 1) {
+      return 1
+    }
+    return fibonacci(num - 1) + fibonacci(num - 2)
+  }
+  """
+)
+```
+
+The attributed string can be displayed using a UILabel or UITextView.

Sources/Runestone/StringSyntaxHighlighter.swift

@@ -0,0 +1,105 @@
+import UIKit
+
+/// Syntax highlights a string.
+///
+/// An instance of `StringSyntaxHighlighter` can be used to syntax highlight a string without needing to create a `TextView`.
+public final class StringSyntaxHighlighter {
+    /// The theme to use when syntax highlighting the text.
+    public var theme: Theme
+    /// The language to use when parsing the text.
+    public var language: TreeSitterLanguage
+    /// Object that can provide embedded languages on demand. A strong reference will be stored to the language provider.
+    public var languageProvider: TreeSitterLanguageProvider?
+    /// The number of points by which to adjust kern.
+    ///
+    /// The default value is 0 meaning that kerning is disabled.
+    public var kern: CGFloat = 0
+    /// The tab length determines the width of the tab measured in space characers.
+    ///
+    /// The default value is 4 meaning that a tab is four spaces wide.
+    public var tabLength: Int = 4
+    /// The line-height is multiplied with the value.
+    public var lineHeightMultiplier: CGFloat = 1
+
+    /// Creates an object that can syntax highlight a text.
+    /// - Parameters:
+    ///   - theme: The theme to use when syntax highlighting the text.
+    ///   - language: The language to use when parsing the text
+    ///   - languageProvider: Object that can provide embedded languages on demand. A strong reference will be stored to the language provider..
+    public init(
+        theme: Theme = DefaultTheme(),
+        language: TreeSitterLanguage,
+        languageProvider: TreeSitterLanguageProvider? = nil
+    ) {
+        self.theme = theme
+        self.language = language
+        self.languageProvider = languageProvider
+    }
+
+    required init?(coder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+
+    /// Syntax highlights the text using the configured syntax highlighter.
+    /// - Parameter text: The text to be syntax highlighted.
+    /// - Returns: An attributed string containing the syntax highlighted text.
+    public func syntaxHighlight(_ text: String) -> NSAttributedString {
+        let mutableString = NSMutableString(string: text)
+        let stringView = StringView(string: mutableString)
+        let lineManager = LineManager(stringView: stringView)
+        lineManager.rebuild()
+        let languageMode = TreeSitterLanguageMode(language: language, languageProvider: languageProvider)
+        let internalLanguageMode = languageMode.makeInternalLanguageMode(
+            stringView: stringView,
+            lineManager: lineManager
+        )
+        internalLanguageMode.parse(mutableString)
+        let tabWidth = TabWidthMeasurer.tabWidth(tabLength: tabLength, font: theme.font)
+        let mutableAttributedString = NSMutableAttributedString(string: text)
+        let defaultAttributes = DefaultStringAttributes(
+            textColor: theme.textColor,
+            font: theme.font,
+            kern: kern,
+            tabWidth: tabWidth
+        )
+        defaultAttributes.apply(to: mutableAttributedString)
+        applyLineHeightMultiplier(to: mutableAttributedString)
+        let byteRange = ByteRange(from: 0, to: text.byteCount)
+        let syntaxHighlighter = internalLanguageMode.createLineSyntaxHighlighter()
+        syntaxHighlighter.theme = theme
+        let syntaxHighlighterInput = LineSyntaxHighlighterInput(
+            attributedString: mutableAttributedString,
+            byteRange: byteRange
+        )
+        syntaxHighlighter.syntaxHighlight(syntaxHighlighterInput)
+        return mutableAttributedString
+    }
+}
+
+private extension StringSyntaxHighlighter {
+    private func applyLineHeightMultiplier(to attributedString: NSMutableAttributedString) {
+        let scaledLineHeight = theme.font.totalLineHeight * lineHeightMultiplier
+        let mutableParagraphStyle = getMutableParagraphStyle(from: attributedString)
+        mutableParagraphStyle.lineSpacing = scaledLineHeight - theme.font.totalLineHeight
+        let range = NSRange(location: 0, length: attributedString.length)
+        attributedString.beginEditing()
+        attributedString.removeAttribute(.paragraphStyle, range: range)
+        attributedString.addAttribute(.paragraphStyle, value: mutableParagraphStyle, range: range)
+        attributedString.endEditing()
+    }
+
+    private func getMutableParagraphStyle(
+        from attributedString: NSMutableAttributedString
+    ) -> NSMutableParagraphStyle {
+        guard let attributeValue = attributedString.attribute(.paragraphStyle, at: 0, effectiveRange: nil) else {
+            return NSMutableParagraphStyle()
+        }
+        guard let paragraphStyle = attributeValue as? NSParagraphStyle else {
+            fatalError("Expected .paragraphStyle attribute to be instance of NSParagraphStyle")
+        }
+        guard let mutableParagraphStyle = paragraphStyle.mutableCopy() as? NSMutableParagraphStyle else {
+            fatalError("Expected mutableCopy() to return an instance of NSMutableParagraphStyle")
+        }
+        return mutableParagraphStyle
+    }
+}