Sudoku solver in Swift

_sudoku.swift

/*
Copyright (c) 2015 Kristopher Johnson

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

/// A Mark is an value in the range 1...9
///
/// An assertion failure will be triggered if an attempt is made
/// to create a mark with a value outside the valid range.  If
/// assertions are disabled, then any invalid values will be treated
/// as the value 1.
public struct Mark {
    public let value: Int

    public init(_ value: Int) {
        assert(1 <= value && value <= 9, "only values 1...9 are valid for a mark")

        switch value {
        case 1...9: self.value = value
        default:    self.value = 1
        }
    }
}

/// A Sudoku square is empty or has a mark with value 1...9.
public enum Square: IntegerLiteralConvertible {
    case Empty
    case Marked(Mark)

    /// Initialize a square value from an integer literal
    ///
    /// Any value in the range 1...9 will be treated as a mark.
    /// Any other value will result in an empty square.
    public init(integerLiteral value: IntegerLiteralType) {
        switch value {
        case 1...9: self = .Marked(Mark(value))
        default:    self = .Empty
        }
    }

    var isEmpty: Bool {
        switch self {
        case .Empty:   return true
        case .Marked(_): return false
        }
    }

    func isMarkedWithValue(value: Int) -> Bool {
        switch self {
        case .Empty:            return false
        case .Marked(let mark): return mark.value == value
        }
    }
}

// A Sudoku puzzle is a 9x9 matrix of squares
public typealias Sudoku = [[Square]]

/// Find a solution for a sudoku puzzle
public func solveSudoku(s: Sudoku) -> Sudoku? {
    if let (row, col) = findEmptySquare(s) {
        for mark in 1...9 {
            if canTryMark(mark, atRow: row, column: col, inSudoku: s) {
                let candidate = copySudoku(s, withMark: mark, atRow: row, column: col)
                if let solution = solveSudoku(candidate) {
                    return solution
                }
            }
        }

        // No solution
        return nil
    }
    else {
        // No empty squares, so it's solved
        return s
    }
}

/// Find all solutions for a sudoku puzzle, invoking a user-provided function for each solution.
///
/// If the user-provided function returns false, then iteration of solutions will stop.
public func findAllSolutions(s: Sudoku, processAndContinue: (Sudoku) -> Bool) {
    // This will be set true if processAndContinue() returns false
    var stop = false

    // Local functions can't refer to themselves, so to let findSolutionsUntilStop()
    // make a recursive call to itself, we need to have another local variable that
    // holds a reference to it.
    var recursiveCall: (Sudoku) -> () = { _ in return }

    // This is the same as solveSudoku(), except that it invokes processAndContinue()
    // when it finds a solution, and if that returns true, it keeps searching for
    // solutions.
    func findSolutionsUntilStop(s: Sudoku) {
        if let (row, col) = findEmptySquare(s) {
            for mark in 1...9 {
                if stop {
                    break
                }

                if canTryMark(mark, atRow: row, column: col, inSudoku: s) {
                    let candidate = copySudoku(s, withMark: mark, atRow: row, column: col)
                    recursiveCall(candidate)
                }
            }
        }
        else {
            // No empty squares, so this is a solution
            if !processAndContinue(s) {
                stop = true
            }
        }
    }

    recursiveCall = findSolutionsUntilStop

    findSolutionsUntilStop(s)
}

/// Print a Sudoku as a 9x9 matrix
///
/// Empty squares are printed as dots.
public func printSudoku(s: Sudoku) {
    for row in s {
        for square in row {
            switch (square) {
            case .Empty:            print(".")
            case .Marked(let mark): print(mark.value)
            }
        }
        println()
    }
}

/// Create a copy of a Sudoku with an additional mark
private func copySudoku(s: Sudoku, withMark mark: Int, atRow row: Int, column col: Int) -> Sudoku {
    var result = Sudoku(s)

    var newRow  = Array(s[row])
    newRow[col] = .Marked(Mark(mark))
    result[row] = newRow

    return result
}

/// Find an empty square in a Sudoku board
///
/// :returns: (row, column), or nil if there are no empty squares
private func findEmptySquare(s: Sudoku) -> (Int, Int)? {
    for row in 0..<9 { for col in 0..<9 {
        if s[row][col].isEmpty { return (row, col) }
    }}
    return nil
}

/// Determine whether putting the specified mark at the specified square would violate uniqueness constrains
private func canTryMark(mark: Int, atRow row: Int, column col: Int, inSudoku s: Sudoku) -> Bool {
    return !doesSudoku(s, containMark: mark, inRow: row)
        && !doesSudoku(s, containMark: mark, inColumn: col)
        && !doesSudoku(s, containMark: mark, in3x3BoxWithRow: row, column: col)
}

/// Determine whether a specified mark already exists in a specified row
private func doesSudoku(s: Sudoku, containMark mark: Int, inRow row: Int) -> Bool {
    for col in 0..<9 {
        if s[row][col].isMarkedWithValue(mark) { return true }
    }
    return false
}

/// Determine whether a specified mark already exists in a specified column
private func doesSudoku(s: Sudoku, containMark mark: Int, inColumn col: Int) -> Bool {
    for row in 0..<9 {
        if s[row][col].isMarkedWithValue(mark) { return true }
    }
    return false
}

/// Determine whether a specified mark already exists in a specified 3x3 subgrid
private func doesSudoku(s: Sudoku, containMark mark: Int, in3x3BoxWithRow row: Int, column col: Int) -> Bool {
    let boxMinRow = (row / 3) * 3
    let boxMaxRow = boxMinRow + 2
    let boxMinCol = (col / 3) * 3
    let boxMaxCol = boxMinCol + 2

    for row in boxMinRow...boxMaxRow {
        for col in boxMinCol...boxMaxCol {
            if s[row][col].isMarkedWithValue(mark) { return true }
        }
    }
    return false
}

main.swift

// Example puzzle from http://en.wikipedia.org/wiki/Sudoku
let example: Sudoku = [
    [5, 3, 0,  0, 7, 0,  0, 0, 0],
    [6, 0, 0,  1, 9, 5,  0, 0, 0],
    [0, 9, 8,  0, 0, 0,  0, 6, 0],

    [8, 0, 0,  0, 6, 0,  0, 0, 3],
    [4, 0, 0,  8, 0, 3,  0, 0, 1],
    [7, 0, 0,  0, 2, 0,  0, 0, 6],

    [0, 6, 0,  0, 0, 0,  2, 8, 0],
    [0, 0, 0,  4, 1, 9,  0, 0, 5],
    [0, 0, 0,  0, 8, 0,  0, 7, 0],
]

println("\nPuzzle:")
printSudoku(example)
if let solutionForExample = solveSudoku(example) {
    println("\nSolution:")
    printSudoku(solutionForExample)
}
else {
    println("No solution")
}


// Find all solutions to this puzzle (there are 20)

let diagonals: Sudoku = [
    [9, 0, 0,  0, 0, 0,  6, 0, 0],
    [0, 8, 0,  0, 0, 0,  0, 5, 0],
    [0, 0, 7,  0, 0, 0,  0, 0, 4],

    [3, 0, 0,  6, 0, 0,  9, 0, 0],
    [0, 2, 0,  0, 5, 0,  0, 8, 0],
    [0, 0, 1,  0, 0, 4,  0, 0, 7],

    [6, 0, 0,  9, 0, 0,  3, 0, 0],
    [0, 5, 0,  0, 8, 0,  0, 2, 0],
    [0, 0, 4,  0, 0, 7,  0, 0, 1],
]

println("\nPuzzle:")
printSudoku(diagonals)
var solutionCount: Int = 0
findAllSolutions(diagonals) { solution in
    ++solutionCount

    println("\nSolution \(solutionCount):")
    printSudoku(solution)

    // Return true to continue
    return true
}
if solutionCount == 0 {
    println("No solutions")
}

Equivalent code in TypeScript: https://gist.github.com/kristopherjohnson/81fd9d4bfe951690b74b

and in Haskell: https://gist.github.com/kristopherjohnson/46200d04db07736407bd

I was using this code recently and had to make some minor updates to get it running under Swift 3. My changes can be found here: https://gist.github.com/MustafaHaddara/1bc5197f4ef72a387639306942f21b42

Thanks @kristopherjohnson for implementing this