grpc-swift/Sources/GRPCCodeGen/Internal/Renderer/TextBasedRenderer.swift

/*
 * Copyright 2023, gRPC Authors All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftOpenAPIGenerator open source project
//
// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//
import Foundation

/// An object for building up a generated file line-by-line.
///
/// After creation, make calls such as `writeLine` to build up the file,
/// and call `rendered` at the end to get the full file contents.
final class StringCodeWriter {

  /// The stored lines of code.
  private var lines: [String]

  /// The current nesting level.
  private var level: Int

  /// The indentation for each level as the number of spaces.
  internal let indentation: Int

  /// Whether the next call to `writeLine` will continue writing to the last
  /// stored line. Otherwise a new line is appended.
  private var nextWriteAppendsToLastLine: Bool = false

  /// Creates a new empty writer.
  init(indentation: Int) {
    self.level = 0
    self.lines = []
    self.indentation = indentation
  }

  /// Concatenates the stored lines of code into a single string.
  /// - Returns: The contents of the full file in a single string.
  func rendered() -> String { lines.joined(separator: "\n") }

  /// Writes a line of code.
  ///
  /// By default, a new line is appended to the file.
  ///
  /// To continue the last line, make a call to `nextLineAppendsToLastLine`
  /// before calling `writeLine`.
  /// - Parameter line: The contents of the line to write.
  func writeLine(_ line: String) {
    let newLine: String
    if nextWriteAppendsToLastLine && !lines.isEmpty {
      let existingLine = lines.removeLast()
      newLine = existingLine + line
    } else {
      let indentation = Array(repeating: " ", count: self.indentation * level).joined()
      newLine = indentation + line
    }
    lines.append(newLine)
    nextWriteAppendsToLastLine = false
  }

  /// Increases the indentation level by 1.
  func push() { level += 1 }

  /// Decreases the indentation level by 1.
  /// - Precondition: Current level must be greater than 0.
  func pop() {
    precondition(level > 0, "Cannot pop below 0")
    level -= 1
  }

  /// Executes the provided closure with one level deeper indentation.
  /// - Parameter work: The closure to execute.
  /// - Returns: The result of the closure execution.
  func withNestedLevel<R>(_ work: () -> R) -> R {
    push()
    defer { pop() }
    return work()
  }

  /// Sets a flag on the writer so that the next call to `writeLine` continues
  /// the last stored line instead of starting a new line.
  ///
  /// Safe to call repeatedly, it gets reset by `writeLine`.
  func nextLineAppendsToLastLine() { nextWriteAppendsToLastLine = true }
}

@available(*, unavailable)
extension TextBasedRenderer: Sendable {}

/// A renderer that uses string interpolation and concatenation
/// to convert the provided structure code into raw string form.
struct TextBasedRenderer: RendererProtocol {

  func render(
    structured: StructuredSwiftRepresentation
  ) throws
    -> SourceFile
  {
    let namedFile = structured.file
    renderFile(namedFile.contents)
    let string = writer.rendered()
    return SourceFile(name: namedFile.name, contents: string)
  }

  /// The underlying writer.
  private let writer: StringCodeWriter

  /// Creates a new empty renderer.
  static var `default`: TextBasedRenderer { .init(indentation: 4) }

  init(indentation: Int) {
    self.writer = StringCodeWriter(indentation: indentation)
  }

  // MARK: - Internals

  /// Returns the current contents of the writer as a string.
  func renderedContents() -> String { writer.rendered() }

  /// Renders the specified Swift file.
  func renderFile(_ description: FileDescription) {
    if let topComment = description.topComment {
      renderComment(topComment)
      writer.writeLine("")
    }
    if let imports = description.imports {
      renderImports(imports)
      writer.writeLine("")
    }
    for (codeBlock, isLast) in description.codeBlocks.enumeratedWithLastMarker() {
      renderCodeBlock(codeBlock)
      if !isLast {
        writer.writeLine("")
      }
    }
  }

  /// Renders the specified comment.
  func renderComment(_ comment: Comment) {
    let prefix: String
    let commentString: String
    switch comment {
    case .inline(let string):
      prefix = "//"
      commentString = string
    case .doc(let string):
      prefix = "///"
      commentString = string
    case .mark(let string, sectionBreak: true):
      prefix = "// MARK: -"
      commentString = string
    case .mark(let string, sectionBreak: false):
      prefix = "// MARK:"
      commentString = string
    case .preFormatted(let string):
      prefix = ""
      commentString = string
    }

    let lines = commentString.transformingLines { line, isLast in
      // The last line of a comment that is blank should be dropped.
      // Pre formatted documentation might contain such lines.
      if line.isEmpty && prefix.isEmpty && isLast {
        return nil
      } else {
        let formattedPrefix = !prefix.isEmpty && !line.isEmpty ? "\(prefix) " : prefix
        return "\(formattedPrefix)\(line)"
      }
    }
    lines.forEach(writer.writeLine)
  }

  /// Renders the specified import statements.
  func renderImports(_ imports: [ImportDescription]?) { (imports ?? []).forEach(renderImport) }

  /// Renders a single import statement.
  func renderImport(_ description: ImportDescription) {
    func render(preconcurrency: Bool) {
      let spiPrefix = description.spi.map { "@_spi(\($0)) " } ?? ""
      let preconcurrencyPrefix = preconcurrency ? "@preconcurrency " : ""
      let accessLevel = description.accessLevel.map { "\($0) " } ?? ""

      if let item = description.item {
        writer.writeLine(
          "\(preconcurrencyPrefix)\(spiPrefix)\(accessLevel)import \(item.kind) \(description.moduleName).\(item.name)"
        )
      } else if let moduleTypes = description.moduleTypes {
        for type in moduleTypes {
          writer.writeLine("\(preconcurrencyPrefix)\(spiPrefix)\(accessLevel)import \(type)")
        }
      } else {
        writer.writeLine(
          "\(preconcurrencyPrefix)\(spiPrefix)\(accessLevel)import \(description.moduleName)"
        )
      }
    }

    switch description.preconcurrency {
    case .always: render(preconcurrency: true)
    case .never: render(preconcurrency: false)
    case .onOS(let operatingSystems):
      writer.writeLine("#if \(operatingSystems.map { "os(\($0))" }.joined(separator: " || "))")
      render(preconcurrency: true)
      writer.writeLine("#else")
      render(preconcurrency: false)
      writer.writeLine("#endif")
    }
  }

  /// Renders the specified access modifier.
  func renderedAccessModifier(_ accessModifier: AccessModifier) -> String {
    switch accessModifier {
    case .public: return "public"
    case .package: return "package"
    case .internal: return "internal"
    case .fileprivate: return "fileprivate"
    case .private: return "private"
    }
  }

  /// Renders the specified identifier.
  func renderIdentifier(_ identifier: IdentifierDescription) {
    switch identifier {
    case .pattern(let string): writer.writeLine(string)
    case .type(let existingTypeDescription):
      renderExistingTypeDescription(existingTypeDescription)
    }
  }

  /// Renders the specified member access expression.
  func renderMemberAccess(_ memberAccess: MemberAccessDescription) {
    if let left = memberAccess.left {
      renderExpression(left)
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(".\(memberAccess.right)")
  }

  /// Renders the specified function call argument.
  func renderFunctionCallArgument(_ arg: FunctionArgumentDescription) {
    if let left = arg.label {
      writer.writeLine("\(left): ")
      writer.nextLineAppendsToLastLine()
    }
    renderExpression(arg.expression)
  }

  /// Renders the specified function call.
  func renderFunctionCall(_ functionCall: FunctionCallDescription) {
    renderExpression(functionCall.calledExpression)
    writer.nextLineAppendsToLastLine()
    writer.writeLine("(")
    let arguments = functionCall.arguments
    if arguments.count > 1 {
      writer.withNestedLevel {
        for (argument, isLast) in arguments.enumeratedWithLastMarker() {
          renderFunctionCallArgument(argument)
          if !isLast {
            writer.nextLineAppendsToLastLine()
            writer.writeLine(",")
          }
        }
      }
    } else {
      writer.nextLineAppendsToLastLine()
      if let argument = arguments.first { renderFunctionCallArgument(argument) }
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(")")
    if let trailingClosure = functionCall.trailingClosure {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" ")
      renderClosureInvocation(trailingClosure)
    }
  }

  /// Renders the specified assignment expression.
  func renderAssignment(_ assignment: AssignmentDescription) {
    renderExpression(assignment.left)
    writer.nextLineAppendsToLastLine()
    writer.writeLine(" = ")
    writer.nextLineAppendsToLastLine()
    renderExpression(assignment.right)
  }

  /// Renders the specified switch case kind.
  func renderSwitchCaseKind(_ kind: SwitchCaseKind) {
    switch kind {
    case let .`case`(expression, associatedValueNames):
      let associatedValues: String
      let maybeLet: String
      if !associatedValueNames.isEmpty {
        associatedValues = "(" + associatedValueNames.joined(separator: ", ") + ")"
        maybeLet = "let "
      } else {
        associatedValues = ""
        maybeLet = ""
      }
      writer.writeLine("case \(maybeLet)")
      writer.nextLineAppendsToLastLine()
      renderExpression(expression)
      writer.nextLineAppendsToLastLine()
      writer.writeLine(associatedValues)
    case .multiCase(let expressions):
      writer.writeLine("case ")
      writer.nextLineAppendsToLastLine()
      for (expression, isLast) in expressions.enumeratedWithLastMarker() {
        renderExpression(expression)
        writer.nextLineAppendsToLastLine()
        if !isLast { writer.writeLine(", ") }
        writer.nextLineAppendsToLastLine()
      }
    case .`default`: writer.writeLine("default")
    }
  }

  /// Renders the specified switch case.
  func renderSwitchCase(_ switchCase: SwitchCaseDescription) {
    renderSwitchCaseKind(switchCase.kind)
    writer.nextLineAppendsToLastLine()
    writer.writeLine(":")
    writer.withNestedLevel { renderCodeBlocks(switchCase.body) }
  }

  /// Renders the specified switch expression.
  func renderSwitch(_ switchDesc: SwitchDescription) {
    writer.writeLine("switch ")
    writer.nextLineAppendsToLastLine()
    renderExpression(switchDesc.switchedExpression)
    writer.nextLineAppendsToLastLine()
    writer.writeLine(" {")
    for caseDesc in switchDesc.cases { renderSwitchCase(caseDesc) }
    writer.writeLine("}")
  }

  /// Renders the specified if statement.
  func renderIf(_ ifDesc: IfStatementDescription) {
    let ifBranch = ifDesc.ifBranch
    writer.writeLine("if ")
    writer.nextLineAppendsToLastLine()
    renderExpression(ifBranch.condition)
    writer.nextLineAppendsToLastLine()
    writer.writeLine(" {")
    writer.withNestedLevel { renderCodeBlocks(ifBranch.body) }
    writer.writeLine("}")
    for branch in ifDesc.elseIfBranches {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" else if ")
      writer.nextLineAppendsToLastLine()
      renderExpression(branch.condition)
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" {")
      writer.withNestedLevel { renderCodeBlocks(branch.body) }
      writer.writeLine("}")
    }
    if let elseBody = ifDesc.elseBody {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" else {")
      writer.withNestedLevel { renderCodeBlocks(elseBody) }
      writer.writeLine("}")
    }
  }

  /// Renders the specified switch expression.
  func renderDoStatement(_ description: DoStatementDescription) {
    writer.writeLine("do {")
    writer.withNestedLevel { renderCodeBlocks(description.doStatement) }
    if let catchBody = description.catchBody {
      writer.writeLine("} catch {")
      if !catchBody.isEmpty {
        writer.withNestedLevel { renderCodeBlocks(catchBody) }
      } else {
        writer.nextLineAppendsToLastLine()
      }
    }
    writer.writeLine("}")
  }

  /// Renders the specified value binding expression.
  func renderValueBinding(_ valueBinding: ValueBindingDescription) {
    writer.writeLine("\(renderedBindingKind(valueBinding.kind)) ")
    writer.nextLineAppendsToLastLine()
    renderFunctionCall(valueBinding.value)
  }

  /// Renders the specified keyword.
  func renderedKeywordKind(_ kind: KeywordKind) -> String {
    switch kind {
    case .return: return "return"
    case .try(hasPostfixQuestionMark: let hasPostfixQuestionMark):
      return "try\(hasPostfixQuestionMark ? "?" : "")"
    case .await: return "await"
    case .throw: return "throw"
    case .yield: return "yield"
    }
  }

  /// Renders the specified unary keyword expression.
  func renderUnaryKeywordExpression(_ expression: UnaryKeywordDescription) {
    writer.writeLine(renderedKeywordKind(expression.kind))
    guard let expr = expression.expression else { return }
    writer.nextLineAppendsToLastLine()
    writer.writeLine(" ")
    writer.nextLineAppendsToLastLine()
    renderExpression(expr)
  }

  /// Renders the specified closure invocation.
  func renderClosureInvocation(_ invocation: ClosureInvocationDescription) {
    writer.writeLine("{")
    if !invocation.argumentNames.isEmpty {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" \(invocation.argumentNames.joined(separator: ", ")) in")
    }
    if let body = invocation.body { writer.withNestedLevel { renderCodeBlocks(body) } }
    writer.writeLine("}")
  }

  /// Renders the specified binary operator.
  func renderedBinaryOperator(_ op: BinaryOperator) -> String { op.rawValue }

  /// Renders the specified binary operation.
  func renderBinaryOperation(_ operation: BinaryOperationDescription) {
    renderExpression(operation.left)
    writer.nextLineAppendsToLastLine()
    writer.writeLine(" \(renderedBinaryOperator(operation.operation)) ")
    writer.nextLineAppendsToLastLine()
    renderExpression(operation.right)
  }

  /// Renders the specified inout expression.
  func renderInOutDescription(_ description: InOutDescription) {
    writer.writeLine("&")
    writer.nextLineAppendsToLastLine()
    renderExpression(description.referencedExpr)
  }

  /// Renders the specified optional chaining expression.
  func renderOptionalChainingDescription(_ description: OptionalChainingDescription) {
    renderExpression(description.referencedExpr)
    writer.nextLineAppendsToLastLine()
    writer.writeLine("?")
  }

  /// Renders the specified tuple expression.
  func renderTupleDescription(_ description: TupleDescription) {
    writer.writeLine("(")
    writer.nextLineAppendsToLastLine()
    let members = description.members
    for (member, isLast) in members.enumeratedWithLastMarker() {
      renderExpression(member)
      if !isLast {
        writer.nextLineAppendsToLastLine()
        writer.writeLine(", ")
      }
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(")")
  }

  /// Renders the specified expression.
  func renderExpression(_ expression: Expression) {
    switch expression {
    case .literal(let literalDescription): renderLiteral(literalDescription)
    case .identifier(let identifierDescription):
      renderIdentifier(identifierDescription)
    case .memberAccess(let memberAccessDescription): renderMemberAccess(memberAccessDescription)
    case .functionCall(let functionCallDescription): renderFunctionCall(functionCallDescription)
    case .assignment(let assignment): renderAssignment(assignment)
    case .switch(let switchDesc): renderSwitch(switchDesc)
    case .ifStatement(let ifDesc): renderIf(ifDesc)
    case .doStatement(let doStmt): renderDoStatement(doStmt)
    case .valueBinding(let valueBinding): renderValueBinding(valueBinding)
    case .unaryKeyword(let unaryKeyword): renderUnaryKeywordExpression(unaryKeyword)
    case .closureInvocation(let closureInvocation): renderClosureInvocation(closureInvocation)
    case .binaryOperation(let binaryOperation): renderBinaryOperation(binaryOperation)
    case .inOut(let inOut): renderInOutDescription(inOut)
    case .optionalChaining(let optionalChaining):
      renderOptionalChainingDescription(optionalChaining)
    case .tuple(let tuple): renderTupleDescription(tuple)
    }
  }

  /// Renders the specified literal expression.
  func renderLiteral(_ literal: LiteralDescription) {
    func write(_ string: String) { writer.writeLine(string) }
    switch literal {
    case let .string(string):
      // Use a raw literal if the string contains a quote/backslash.
      if string.contains("\"") || string.contains("\\") {
        write("#\"\(string)\"#")
      } else {
        write("\"\(string)\"")
      }
    case let .int(int): write("\(int)")
    case let .bool(bool): write(bool ? "true" : "false")
    case .nil: write("nil")
    case .array(let items):
      writer.writeLine("[")
      if !items.isEmpty {
        writer.withNestedLevel {
          for (item, isLast) in items.enumeratedWithLastMarker() {
            renderExpression(item)
            if !isLast {
              writer.nextLineAppendsToLastLine()
              writer.writeLine(",")
            }
          }
        }
      } else {
        writer.nextLineAppendsToLastLine()
      }
      writer.writeLine("]")

    case .dictionary(let items):
      writer.writeLine("[")
      if items.isEmpty {
        writer.nextLineAppendsToLastLine()
        writer.writeLine(":")
        writer.nextLineAppendsToLastLine()
      } else {
        writer.withNestedLevel {
          for (item, isLast) in items.enumeratedWithLastMarker() {
            renderExpression(item.key)
            writer.nextLineAppendsToLastLine()
            writer.writeLine(": ")
            writer.nextLineAppendsToLastLine()
            renderExpression(item.value)
            if !isLast {
              writer.nextLineAppendsToLastLine()
              writer.writeLine(",")
            }
          }
        }
      }
      writer.writeLine("]")
    }
  }

  /// Renders the specified where clause requirement.
  func renderedWhereClauseRequirement(_ requirement: WhereClauseRequirement) -> String {
    switch requirement {
    case .conformance(let left, let right): return "\(left): \(right)"
    }
  }

  /// Renders the specified where clause.
  func renderedWhereClause(_ clause: WhereClause) -> String {
    let renderedRequirements = clause.requirements.map(renderedWhereClauseRequirement)
    return "where \(renderedRequirements.joined(separator: ", "))"
  }

  /// Renders the specified extension declaration.
  func renderExtension(_ extensionDescription: ExtensionDescription) {
    if let accessModifier = extensionDescription.accessModifier {
      writer.writeLine(renderedAccessModifier(accessModifier) + " ")
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("extension \(extensionDescription.onType)")
    writer.nextLineAppendsToLastLine()
    if !extensionDescription.conformances.isEmpty {
      writer.writeLine(": \(extensionDescription.conformances.joined(separator: ", "))")
      writer.nextLineAppendsToLastLine()
    }
    if let whereClause = extensionDescription.whereClause {
      writer.writeLine(" " + renderedWhereClause(whereClause))
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(" {")
    for (declaration, isLast) in extensionDescription.declarations.enumeratedWithLastMarker() {
      writer.withNestedLevel {
        renderDeclaration(declaration)
        if !isLast {
          writer.writeLine("")
        }
      }
    }
    writer.writeLine("}")
  }

  /// Renders the specified type reference to an existing type.
  func renderExistingTypeDescription(_ type: ExistingTypeDescription) {
    switch type {
    case .any(let existingTypeDescription):
      writer.writeLine("any ")
      writer.nextLineAppendsToLastLine()
      renderExistingTypeDescription(existingTypeDescription)
    case .generic(let wrapper, let wrapped):
      renderExistingTypeDescription(wrapper)
      writer.nextLineAppendsToLastLine()
      writer.writeLine("<")
      writer.nextLineAppendsToLastLine()
      renderExistingTypeDescription(wrapped)
      writer.nextLineAppendsToLastLine()
      writer.writeLine(">")
    case .optional(let existingTypeDescription):
      renderExistingTypeDescription(existingTypeDescription)
      writer.nextLineAppendsToLastLine()
      writer.writeLine("?")
    case .member(let components):
      writer.writeLine(components.joined(separator: "."))
    case .array(let existingTypeDescription):
      writer.writeLine("[")
      writer.nextLineAppendsToLastLine()
      renderExistingTypeDescription(existingTypeDescription)
      writer.nextLineAppendsToLastLine()
      writer.writeLine("]")
    case .dictionaryValue(let existingTypeDescription):
      writer.writeLine("[String: ")
      writer.nextLineAppendsToLastLine()
      renderExistingTypeDescription(existingTypeDescription)
      writer.nextLineAppendsToLastLine()
      writer.writeLine("]")
    case .some(let existingTypeDescription):
      writer.writeLine("some ")
      writer.nextLineAppendsToLastLine()
      renderExistingTypeDescription(existingTypeDescription)
    case .closure(let closureSignatureDescription):
      renderClosureSignature(closureSignatureDescription)
    }
  }

  /// Renders the specified typealias declaration.
  func renderTypealias(_ alias: TypealiasDescription) {
    var words: [String] = []
    if let accessModifier = alias.accessModifier {
      words.append(renderedAccessModifier(accessModifier))
    }
    words.append(contentsOf: [
      "typealias", alias.name, "=",
    ])
    writer.writeLine(words.joinedWords() + " ")
    writer.nextLineAppendsToLastLine()
    renderExistingTypeDescription(alias.existingType)
  }

  /// Renders the specified binding kind.
  func renderedBindingKind(_ kind: BindingKind) -> String {
    switch kind {
    case .var: return "var"
    case .let: return "let"
    }
  }

  /// Renders the specified variable declaration.
  func renderVariable(_ variable: VariableDescription) {
    do {
      if let accessModifier = variable.accessModifier {
        writer.writeLine(renderedAccessModifier(accessModifier) + " ")
        writer.nextLineAppendsToLastLine()
      }
      if variable.isStatic {
        writer.writeLine("static ")
        writer.nextLineAppendsToLastLine()
      }
      writer.writeLine(renderedBindingKind(variable.kind) + " ")
      writer.nextLineAppendsToLastLine()
      renderExpression(variable.left)
      if let type = variable.type {
        writer.nextLineAppendsToLastLine()
        writer.writeLine(": ")
        writer.nextLineAppendsToLastLine()
        renderExistingTypeDescription(type)
      }
    }

    if let right = variable.right {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" = ")
      writer.nextLineAppendsToLastLine()
      renderExpression(right)
    }

    if let body = variable.getter {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" {")
      writer.withNestedLevel {
        let hasExplicitGetter =
          !variable.getterEffects.isEmpty || variable.setter != nil || variable.modify != nil
        if hasExplicitGetter {
          let keywords = variable.getterEffects.map(renderedFunctionKeyword).joined(separator: " ")
          let line = "get \(keywords) {"
          writer.writeLine(line)
          writer.push()
        }
        renderCodeBlocks(body)
        if hasExplicitGetter {
          writer.pop()
          writer.writeLine("}")
        }
        if let modify = variable.modify {
          writer.writeLine("_modify {")
          writer.withNestedLevel { renderCodeBlocks(modify) }
          writer.writeLine("}")
        }
        if let setter = variable.setter {
          writer.writeLine("set {")
          writer.withNestedLevel { renderCodeBlocks(setter) }
          writer.writeLine("}")
        }
      }
      writer.writeLine("}")
    }
  }

  /// Renders the specified struct declaration.
  func renderStruct(_ structDesc: StructDescription) {
    if let accessModifier = structDesc.accessModifier {
      writer.writeLine(renderedAccessModifier(accessModifier) + " ")
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("struct \(structDesc.name)")
    writer.nextLineAppendsToLastLine()
    if !structDesc.conformances.isEmpty {
      writer.writeLine(": \(structDesc.conformances.joined(separator: ", "))")
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(" {")
    if !structDesc.members.isEmpty {
      writer.withNestedLevel {
        for (member, isLast) in structDesc.members.enumeratedWithLastMarker() {
          renderDeclaration(member)
          if !isLast {
            writer.writeLine("")
          }
        }
      }
    } else {
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("}")
  }

  /// Renders the specified protocol declaration.
  func renderProtocol(_ protocolDesc: ProtocolDescription) {
    if let accessModifier = protocolDesc.accessModifier {
      writer.writeLine("\(renderedAccessModifier(accessModifier)) ")
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("protocol \(protocolDesc.name)")
    writer.nextLineAppendsToLastLine()
    if !protocolDesc.conformances.isEmpty {
      let conformances = protocolDesc.conformances.joined(separator: ", ")
      writer.writeLine(": \(conformances)")
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(" {")
    if !protocolDesc.members.isEmpty {
      writer.withNestedLevel {
        for (member, isLast) in protocolDesc.members.enumeratedWithLastMarker() {
          renderDeclaration(member)
          if !isLast {
            writer.writeLine("")
          }
        }
      }
    } else {
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("}")
  }

  /// Renders the specified enum declaration.
  func renderEnum(_ enumDesc: EnumDescription) {
    if enumDesc.isFrozen {
      writer.writeLine("@frozen ")
      writer.nextLineAppendsToLastLine()
    }
    if let accessModifier = enumDesc.accessModifier {
      writer.writeLine("\(renderedAccessModifier(accessModifier)) ")
      writer.nextLineAppendsToLastLine()
    }
    if enumDesc.isIndirect {
      writer.writeLine("indirect ")
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("enum \(enumDesc.name)")
    writer.nextLineAppendsToLastLine()
    if !enumDesc.conformances.isEmpty {
      writer.writeLine(": \(enumDesc.conformances.joined(separator: ", "))")
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(" {")
    if !enumDesc.members.isEmpty {
      writer.withNestedLevel { for member in enumDesc.members { renderDeclaration(member) } }
    } else {
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("}")
  }

  /// Renders the specified enum case associated value.
  func renderEnumCaseAssociatedValue(_ value: EnumCaseAssociatedValueDescription) {
    var words: [String] = []
    if let label = value.label { words.append(label + ":") }
    writer.writeLine(words.joinedWords())
    writer.nextLineAppendsToLastLine()
    renderExistingTypeDescription(value.type)
  }

  /// Renders the specified enum case declaration.
  func renderEnumCase(_ enumCase: EnumCaseDescription) {
    writer.writeLine("case \(enumCase.name)")
    switch enumCase.kind {
    case .nameOnly: break
    case .nameWithRawValue(let rawValue):
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" = ")
      writer.nextLineAppendsToLastLine()
      renderLiteral(rawValue)
    case .nameWithAssociatedValues(let values):
      if values.isEmpty { break }
      for (value, isLast) in values.enumeratedWithLastMarker() {
        renderEnumCaseAssociatedValue(value)
        if !isLast {
          writer.nextLineAppendsToLastLine()
          writer.writeLine(", ")
        }
      }
    }
  }

  /// Renders the specified declaration.
  func renderDeclaration(_ declaration: Declaration) {
    switch declaration {
    case let .commentable(comment, nestedDeclaration):
      renderCommentableDeclaration(comment: comment, declaration: nestedDeclaration)
    case let .deprecated(deprecation, nestedDeclaration):
      renderDeprecatedDeclaration(deprecation: deprecation, declaration: nestedDeclaration)
    case let .guarded(availability, nestedDeclaration):
      renderGuardedDeclaration(availability: availability, declaration: nestedDeclaration)
    case .variable(let variableDescription): renderVariable(variableDescription)
    case .extension(let extensionDescription): renderExtension(extensionDescription)
    case .struct(let structDescription): renderStruct(structDescription)
    case .protocol(let protocolDescription): renderProtocol(protocolDescription)
    case .enum(let enumDescription): renderEnum(enumDescription)
    case .typealias(let typealiasDescription): renderTypealias(typealiasDescription)
    case .function(let functionDescription): renderFunction(functionDescription)
    case .enumCase(let enumCase): renderEnumCase(enumCase)
    }
  }

  /// Renders the specified function kind.
  func renderedFunctionKind(_ functionKind: FunctionKind) -> String {
    switch functionKind {
    case .initializer(let isFailable): return "init\(isFailable ? "?" : "")"
    case .function(let name, let isStatic):
      return (isStatic ? "static " : "") + "func \(name)"

    }
  }

  /// Renders the specified function keyword.
  func renderedFunctionKeyword(_ keyword: FunctionKeyword) -> String {
    switch keyword {
    case .throws: return "throws"
    case .async: return "async"
    case .rethrows: return "rethrows"
    }
  }

  /// Renders the specified function signature.
  func renderClosureSignature(_ signature: ClosureSignatureDescription) {
    if signature.sendable {
      writer.writeLine("@Sendable ")
      writer.nextLineAppendsToLastLine()
    }
    if signature.escaping {
      writer.writeLine("@escaping ")
      writer.nextLineAppendsToLastLine()
    }

    writer.writeLine("(")
    let parameters = signature.parameters
    let separateLines = parameters.count > 1
    if separateLines {
      writer.withNestedLevel {
        for (parameter, isLast) in signature.parameters.enumeratedWithLastMarker() {
          renderClosureParameter(parameter)
          if !isLast {
            writer.nextLineAppendsToLastLine()
            writer.writeLine(",")
          }
        }
      }
    } else {
      writer.nextLineAppendsToLastLine()
      if let parameter = parameters.first {
        renderClosureParameter(parameter)
        writer.nextLineAppendsToLastLine()
      }
    }
    writer.writeLine(")")

    let keywords = signature.keywords
    for keyword in keywords {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" " + renderedFunctionKeyword(keyword))
    }

    if let returnType = signature.returnType {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" -> ")
      writer.nextLineAppendsToLastLine()
      renderExpression(returnType)
    }
  }

  /// Renders the specified function signature.
  func renderFunctionSignature(_ signature: FunctionSignatureDescription) {
    do {
      if let accessModifier = signature.accessModifier {
        writer.writeLine(renderedAccessModifier(accessModifier) + " ")
        writer.nextLineAppendsToLastLine()
      }
      let generics = signature.generics
      writer.writeLine(
        renderedFunctionKind(signature.kind)
      )
      if !generics.isEmpty {
        writer.nextLineAppendsToLastLine()
        writer.writeLine("<")
        for (genericType, isLast) in generics.enumeratedWithLastMarker() {
          writer.nextLineAppendsToLastLine()
          renderExistingTypeDescription(genericType)
          if !isLast {
            writer.nextLineAppendsToLastLine()
            writer.writeLine(", ")
          }
        }
        writer.nextLineAppendsToLastLine()
        writer.writeLine(">")
      }
      writer.nextLineAppendsToLastLine()
      writer.writeLine("(")
      let parameters = signature.parameters
      let separateLines = parameters.count > 1
      if separateLines {
        writer.withNestedLevel {
          for (parameter, isLast) in signature.parameters.enumeratedWithLastMarker() {
            renderParameter(parameter)
            if !isLast {
              writer.nextLineAppendsToLastLine()
              writer.writeLine(",")
            }
          }
        }
      } else {
        writer.nextLineAppendsToLastLine()
        if let parameter = parameters.first { renderParameter(parameter) }
        writer.nextLineAppendsToLastLine()
      }
      writer.writeLine(")")
    }

    do {
      let keywords = signature.keywords
      if !keywords.isEmpty {
        for keyword in keywords {
          writer.nextLineAppendsToLastLine()
          writer.writeLine(" " + renderedFunctionKeyword(keyword))
        }
      }
    }

    if let returnType = signature.returnType {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" -> ")
      writer.nextLineAppendsToLastLine()
      renderExpression(returnType)
    }

    if let whereClause = signature.whereClause {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" " + renderedWhereClause(whereClause))
    }
  }

  /// Renders the specified function declaration.
  func renderFunction(_ functionDescription: FunctionDescription) {
    renderFunctionSignature(functionDescription.signature)
    guard let body = functionDescription.body else { return }
    writer.nextLineAppendsToLastLine()
    writer.writeLine(" {")
    if !body.isEmpty {
      writer.withNestedLevel { renderCodeBlocks(body) }
    } else {
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine("}")
  }

  /// Renders the specified parameter declaration.
  func renderParameter(_ parameterDescription: ParameterDescription) {
    if let label = parameterDescription.label {
      writer.writeLine(label)
    } else {
      writer.writeLine("_")
    }
    writer.nextLineAppendsToLastLine()
    if let name = parameterDescription.name, name != parameterDescription.label {
      // If the label and name are the same value, don't repeat it.
      writer.writeLine(" ")
      writer.nextLineAppendsToLastLine()
      writer.writeLine(name)
      writer.nextLineAppendsToLastLine()
    }
    writer.writeLine(": ")
    writer.nextLineAppendsToLastLine()

    if parameterDescription.inout {
      writer.writeLine("inout ")
      writer.nextLineAppendsToLastLine()
    }

    if let type = parameterDescription.type {
      renderExistingTypeDescription(type)
    }

    if let defaultValue = parameterDescription.defaultValue {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" = ")
      writer.nextLineAppendsToLastLine()
      renderExpression(defaultValue)
    }
  }

  /// Renders the specified parameter declaration for a closure.
  func renderClosureParameter(_ parameterDescription: ParameterDescription) {
    let name = parameterDescription.name
    let label: String
    if let declaredLabel = parameterDescription.label {
      label = declaredLabel
    } else {
      label = "_"
    }

    if let name = name {
      writer.writeLine(label)
      if name != parameterDescription.label {
        // If the label and name are the same value, don't repeat it.
        writer.writeLine(" ")
        writer.nextLineAppendsToLastLine()
        writer.writeLine(name)
        writer.nextLineAppendsToLastLine()
      }
    }

    if parameterDescription.inout {
      writer.writeLine("inout ")
      writer.nextLineAppendsToLastLine()
    }

    if let type = parameterDescription.type {
      renderExistingTypeDescription(type)
    }

    if let defaultValue = parameterDescription.defaultValue {
      writer.nextLineAppendsToLastLine()
      writer.writeLine(" = ")
      writer.nextLineAppendsToLastLine()
      renderExpression(defaultValue)
    }
  }

  /// Renders the specified declaration with a comment.
  func renderCommentableDeclaration(comment: Comment?, declaration: Declaration) {
    if let comment { renderComment(comment) }
    renderDeclaration(declaration)
  }

  /// Renders the specified declaration with a deprecation annotation.
  func renderDeprecatedDeclaration(deprecation: DeprecationDescription, declaration: Declaration) {
    renderDeprecation(deprecation)
    renderDeclaration(declaration)
  }

  func renderDeprecation(_ deprecation: DeprecationDescription) {
    let things: [String] = [
      "*", "deprecated", deprecation.message.map { "message: \"\($0)\"" },
      deprecation.renamed.map { "renamed: \"\($0)\"" },
    ]
    .compactMap({ $0 })
    let line = "@available(\(things.joined(separator: ", ")))"
    writer.writeLine(line)
  }

  /// Renders the specified declaration with an availability guard annotation.
  func renderGuardedDeclaration(availability: AvailabilityDescription, declaration: Declaration) {
    renderAvailability(availability)
    renderDeclaration(declaration)
  }

  func renderAvailability(_ availability: AvailabilityDescription) {
    var line = "@available("
    for osVersion in availability.osVersions {
      line.append("\(osVersion.os.name) \(osVersion.version), ")
    }
    line.append("*)")
    writer.writeLine(line)
  }

  /// Renders the specified code block item.
  func renderCodeBlockItem(_ description: CodeBlockItem) {
    switch description {
    case .declaration(let declaration): renderDeclaration(declaration)
    case .expression(let expression): renderExpression(expression)
    }
  }

  /// Renders the specified code block.
  func renderCodeBlock(_ description: CodeBlock) {
    if let comment = description.comment { renderComment(comment) }
    let item = description.item
    renderCodeBlockItem(item)
  }

  /// Renders the specified code blocks.
  func renderCodeBlocks(_ blocks: [CodeBlock]) { blocks.forEach(renderCodeBlock) }
}

extension Array {

  /// Returns a collection of tuples, where the first element is
  /// the collection element and the second is a Boolean value indicating
  /// whether it is the last element in the collection.
  /// - Returns: A collection of tuples.
  fileprivate func enumeratedWithLastMarker() -> [(Element, isLast: Bool)] {
    let count = count
    return enumerated().map { index, element in (element, index == count - 1) }
  }
}

extension Array where Element == String {
  /// Returns a string where the elements of the array are joined
  /// by a space character.
  /// - Returns: A string with the elements of the array joined by space characters.
  fileprivate func joinedWords() -> String { joined(separator: " ") }
}

extension String {

  /// Returns an array of strings, where each string represents one line
  /// in the current string.
  /// - Returns: An array of strings, each representing one line in the original string.
  fileprivate func asLines() -> [String] {
    split(omittingEmptySubsequences: false, whereSeparator: \.isNewline).map(String.init)
  }

  /// Returns a new string where the provided closure transforms each line.
  /// The closure takes a string representing one line as a parameter.
  /// - Parameter work: The closure that transforms each line.
  /// - Returns: A new string where each line has been transformed using the given closure.
  fileprivate func transformingLines(_ work: (String, Bool) -> String?) -> [String] {
    asLines().enumeratedWithLastMarker().compactMap(work)
  }
}

extension TextBasedRenderer {

  /// Returns the provided expression rendered as a string.
  /// - Parameter expression: The expression.
  /// - Returns: The string representation of the expression.
  static func renderedExpressionAsString(_ expression: Expression) -> String {
    let renderer = TextBasedRenderer.default
    renderer.renderExpression(expression)
    return renderer.renderedContents()
  }
}