SwiftFormat/Sources/Rules/YodaConditions.swift

//
//  YodaConditions.swift
//  SwiftFormat
//
//  Created by Nick Lockwood on 3/9/19.
//  Copyright © 2024 Nick Lockwood. All rights reserved.
//

import Foundation

public extension FormatRule {
    /// Reorders "yoda conditions" where constant is placed on lhs of a comparison
    static let yodaConditions = FormatRule(
        help: "Prefer constant values to be on the right-hand-side of expressions.",
        options: ["yoda-swap"]
    ) { formatter in
        formatter.forEachToken { i, token in
            guard case let .operator(op, .infix) = token,
                  let opIndex = ["==", "!=", "<", "<=", ">", ">="].firstIndex(of: op),
                  let prevIndex = formatter.index(of: .nonSpace, before: i),
                  formatter.isConstant(at: prevIndex), let startIndex = formatter.startOfValue(at: prevIndex),
                  !formatter.isOperator(at: formatter.index(of: .nonSpaceOrCommentOrLinebreak, before: startIndex)),
                  let nextIndex = formatter.index(of: .nonSpace, after: i), !formatter.isConstant(at: nextIndex) ||
                  formatter.isOperator(at: formatter.index(of: .nonSpaceOrCommentOrLinebreak, after: nextIndex)),
                  let endIndex = formatter.endOfExpression(at: nextIndex, upTo: [
                      .operator("&&", .infix), .operator("||", .infix),
                      .operator("?", .infix), .operator(":", .infix),
                  ])
            else {
                return
            }
            let inverseOp = ["==", "!=", ">", ">=", "<", "<="][opIndex]
            let expression = Array(formatter.tokens[nextIndex ... endIndex])
            let constant = Array(formatter.tokens[startIndex ... prevIndex])
            formatter.replaceTokens(in: nextIndex ... endIndex, with: constant)
            formatter.replaceToken(at: i, with: .operator(inverseOp, .infix))
            formatter.replaceTokens(in: startIndex ... prevIndex, with: expression)
        }
    } examples: {
        """
        ```diff
        - if 5 == foo,
        + if foo == 5,
        -    nil != bar,
        +    bar != nil,
        -    .default == baaz,
        +    baaz == .default,
          { ... }
        ```
        """
    }
}

extension Formatter {
    func valuesInRangeAreConstant(_ range: CountableRange<Int>) -> Bool {
        var index = index(of: .nonSpaceOrCommentOrLinebreak, in: range)
        while var i = index {
            switch tokens[i] {
            case .startOfScope where isConstant(at: i):
                guard let endIndex = self.index(of: .endOfScope, after: i) else {
                    return false
                }
                i = endIndex
                fallthrough
            case _ where isConstant(at: i), .delimiter(","), .delimiter(":"):
                index = self.index(of: .nonSpaceOrCommentOrLinebreak, in: i + 1 ..< range.upperBound)
            case .identifier:
                guard let nextIndex =
                    self.index(of: .nonSpaceOrComment, in: i + 1 ..< range.upperBound),
                    tokens[nextIndex] == .delimiter(":")
                else {
                    return false
                }
                // Identifier is a label
                index = nextIndex
            default:
                return false
            }
        }
        return true
    }

    func isConstant(at index: Int) -> Bool {
        var index = index
        while case .operator(_, .postfix) = tokens[index] {
            index -= 1
        }
        guard let token = token(at: index) else {
            return false
        }
        switch token {
        case .number, .identifier("true"), .identifier("false"), .identifier("nil"):
            return true
        case .endOfScope("]"), .endOfScope(")"):
            guard let startIndex = self.index(of: .startOfScope, before: index),
                  !isSubscriptOrFunctionCall(at: startIndex)
            else {
                return false
            }
            return valuesInRangeAreConstant(startIndex + 1 ..< index)
        case .startOfScope("["), .startOfScope("("):
            guard !isSubscriptOrFunctionCall(at: index),
                  let endIndex = self.index(of: .endOfScope, after: index)
            else {
                return false
            }
            return valuesInRangeAreConstant(index + 1 ..< endIndex)
        case .startOfScope, .endOfScope:
            // TODO: what if string contains interpolation?
            return token.isStringDelimiter
        case _ where options.yodaSwap == .literalsOnly:
            // Don't treat .members as constant
            return false
        case .operator(".", .prefix) where self.token(at: index + 1)?.isIdentifier == true,
             .identifier where self.token(at: index - 1) == .operator(".", .prefix) &&
                 self.token(at: index - 2) != .operator("\\", .prefix):
            return true
        default:
            return false
        }
    }

    func isOperator(at index: Int?) -> Bool {
        guard let index else {
            return false
        }
        switch tokens[index] {
        // Discount operators with higher precedence than ==
        case .operator("=", .infix),
             .operator("&&", .infix), .operator("||", .infix),
             .operator("?", .infix), .operator(":", .infix):
            return false
        case .operator(_, .infix), .keyword("as"), .keyword("is"):
            return true
        default:
            return false
        }
    }

    func startOfValue(at index: Int) -> Int? {
        var index = index
        while case .operator(_, .postfix)? = token(at: index) {
            index -= 1
        }
        if case .endOfScope? = token(at: index) {
            guard let i = self.index(of: .startOfScope, before: index) else {
                return nil
            }
            index = i
        }
        while case .operator(_, .prefix)? = token(at: index - 1) {
            index -= 1
        }
        return index
    }
}