SwiftLint/Source/SwiftLintBuiltInRules/Rules/Style/VerticalWhitespaceRule.swift

import Foundation
import SwiftSyntax

@SwiftSyntaxRule(explicitRewriter: true, correctable: true)
struct VerticalWhitespaceRule: Rule {
    var configuration = VerticalWhitespaceConfiguration()

    static let description = RuleDescription(
        identifier: "vertical_whitespace",
        name: "Vertical Whitespace",
        description: VerticalWhitespaceConfiguration.defaultDescriptionReason,
        kind: .style,
        nonTriggeringExamples: [
            Example("let abc = 0\n"),
            Example("let abc = 0\n\n"),
            Example("/* bcs \n\n\n\n*/"),
            Example("// bca \n\n"),
            Example("class CCCC {\n  \n}"),
            Example("""
            // comment

            import Foundation
            """),
            Example("""

            // comment

            import Foundation
            """),
        ],
        triggeringExamples: [
            Example("let aaaa = 0\n\n\n"),
            Example("struct AAAA {}\n\n\n\n"),
            Example("class BBBB {}\n\n\n"),
            Example("class CCCC {\n  \n  \n}"),
            Example("""


            import Foundation
            """),
        ],
        corrections: [
            Example("let b = 0\n\n\nclass AAA {}\n"): Example("let b = 0\n\nclass AAA {}\n"),
            Example("let c = 0\n\n\nlet num = 1\n"): Example("let c = 0\n\nlet num = 1\n"),
            Example("// bca \n\n\n"): Example("// bca \n\n"),
            Example("class CCCC {\n  \n  \n  \n}"): Example("class CCCC {\n  \n}"),
        ] // End of line autocorrections are handled by Trailing Newline Rule.
    )
}

private extension VerticalWhitespaceRule {
    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
        /// The number of additional newlines to expect before the first token.
        private var firstTokenAdditionalNewlines = 1

        override func visit(_ token: TokenSyntax) -> SyntaxVisitorContinueKind {
            // Reset immediately. Only the first token has an additional leading newline.
            defer { firstTokenAdditionalNewlines = 0 }

            // The strategy here is to keep track of the position of the _first_ violating newline
            // in each consecutive run, and report the violation when the run _ends_.

            if token.leadingTrivia.isEmpty {
                return .visitChildren
            }

            var consecutiveNewlines = 0
            var currentPosition = token.position
            var violationPosition: AbsolutePosition?

            func process(_ count: Int, _ offset: Int) {
                for _ in 0..<(count + firstTokenAdditionalNewlines) {
                    if consecutiveNewlines > configuration.maxEmptyLines, violationPosition == nil {
                        violationPosition = currentPosition
                    }
                    consecutiveNewlines += 1
                    currentPosition = currentPosition.advanced(by: offset)
                }
            }

            for piece in token.leadingTrivia {
                switch piece {
                case .newlines(let count), .carriageReturns(let count), .formfeeds(let count), .verticalTabs(let count):
                    process(count, 1)
                case .carriageReturnLineFeeds(let count):
                    process(count, 2) // CRLF is 2 bytes
                case .spaces, .tabs:
                    currentPosition += piece.sourceLength
                default:
                    // A comment breaks the chain of newlines.
                    firstTokenAdditionalNewlines = 0
                    if let violationPosition {
                        report(violationPosition, consecutiveNewlines)
                    }
                    violationPosition = nil
                    consecutiveNewlines = 0
                    currentPosition += piece.sourceLength
                }
            }
            if let violationPosition {
                report(violationPosition, consecutiveNewlines)
            }

            return .visitChildren
        }

        private func report(_ position: AbsolutePosition, _ newlines: Int) {
            violations.append(ReasonedRuleViolation(
                position: position,
                reason: configuration.configuredDescriptionReason + "; currently \(newlines - 1)"
            ))
        }
    }

    final class Rewriter: ViolationsSyntaxRewriter<ConfigurationType> {
        override func visit(_ token: TokenSyntax) -> TokenSyntax {
            var result = [TriviaPiece]()
            var pendingWhitespace = [TriviaPiece]()
            var consecutiveNewlines = 0

            func process(_ count: Int, _ create: (Int) -> TriviaPiece) {
                let linesToPreserve = min(count, max(0, configuration.maxEmptyLines + 1 - consecutiveNewlines))
                consecutiveNewlines += count

                if count > linesToPreserve {
                    numberOfCorrections += count - linesToPreserve
                }

                if linesToPreserve > 0 {
                    // We can still add this piece, even if we adjusted its count lower.
                    // Pull in any pending whitespace along with it.
                    result.append(contentsOf: pendingWhitespace)
                    result.append(create(linesToPreserve))
                    pendingWhitespace.removeAll()
                } else {
                    // We're now in violation. Dump pending whitespace so it's excluded from the result.
                    pendingWhitespace.removeAll()
                }
            }

            for piece in token.leadingTrivia {
                switch piece {
                case .newlines(let count):
                    process(count, TriviaPiece.newlines)
                case .carriageReturns(let count):
                    process(count, TriviaPiece.carriageReturns)
                case .carriageReturnLineFeeds(let count):
                    process(count, TriviaPiece.carriageReturnLineFeeds)
                case .formfeeds(let count):
                    process(count, TriviaPiece.formfeeds)
                case .verticalTabs(let count):
                    process(count, TriviaPiece.verticalTabs)
                case .spaces, .tabs:
                    pendingWhitespace.append(piece)
                default:
                    // Reset and pull in pending whitespace
                    consecutiveNewlines = 0
                    result.append(contentsOf: pendingWhitespace)
                    result.append(piece)
                    pendingWhitespace.removeAll()
                }
            }
            // Pull in any remaining pending whitespace
            if !pendingWhitespace.isEmpty {
                result.append(contentsOf: pendingWhitespace)
            }

            return super.visit(token.with(\.leadingTrivia, Trivia(pieces: result)))
        }
    }
}