sudoku checker

is_valid_solution.js

/* Please write the following function:

 is_valid_solution(string grid) => boolean

The input, grid, is a string representing a grid starting in the top­left corner and moving row­by­row down to the bottom­right.

For instance, the (valid) grid pictured above would be:

    grid = "835416927296857431417293658569134782123678549748529163652781394981345276374962815"

The output is a boolean representing whether it is a valid solution.
*/
module.exports = is_valid_solution

function is_valid_solution(grid) {
    // early exit conditions:
    // is not a string, is not 81 characters long, contains characters other than 1-9
    if (typeof grid !== 'string' || grid.length !== 81 || grid.replace(/[1-9]/g, '').length !== 0 ) return false

    let rows = getRowsFromGridString(grid) // [ [ n1, n2, ..., n9 ], [row2], ..., [row9] ]
    let columns = transposeRowsToColumns(rows) // [ [ n1, n2, ..., n9 ], [col2], ..., [col9] ]

    // check rows, columns and 3x3 boxes; if any are false
    return checkForDupes(rows) && checkForDupes(columns) && checkAllBoxesForDupes(rows)
}

// helper functions
function getRowsFromGridString(string) {
    let arr = []
    for (let i = 0; i < string.length; i += 9) {
        // slice off 9 characters and split
        let row = string.slice(i, i + 9).split('')
        arr.push(row)
    }
    return arr;
}

function transposeRowsToColumns(nestedArray) {
    return nestedArray[0].map((row, i) => nestedArray.map(col => col[i]))
}

function checkForDupes(array) {
    let ret = true
    array.forEach((row) => {
        row.forEach((n, i) => {
            if (row.includes(n, i + 1)) ret = false
        })
    })
    return ret
}

// check each 3x3 box for duplicates
// returns false if duplicates exist
function checkAllBoxesForDupes(grid) {
    // rows
    for (let rowStart = 0; rowStart < grid.length; rowStart += 3) {
        // columns
        for (let colStart = 0; colStart < grid.length; colStart += 3) {
            if (!checkOneBox(grid, rowStart, colStart)) return false
        }
    }
    return true
}

// inspect a square for dupes, default 3x3
// returns false if duplicates exist
function checkOneBox(grid, rowStart, colStart, squareLen = 3) {
    let check = ''
    // rows
    for (let i = rowStart; i < rowStart + squareLen; i += 1) {
        // columns
        for (let j = colStart; j < colStart + squareLen; j += 1) {
            let curr = grid[i][j]

            if (check.includes(curr)) return false
            else check += curr
        }
    }
    return true
}

// quick testing
const assert = require('assert')
// no dupes; valid puzzle
assert.equal(is_valid_solution("835416927296857431417293658569134782123678549748529163652781394981345276374962815"), true)


// invalid inputs
assert.equal(is_valid_solution('hello'), false)
assert.equal(is_valid_solution(null), false)
assert.equal(is_valid_solution(835416927296857431417293658569134782123678549748529163652781394981345276374962815), false)
assert.equal(is_valid_solution("035416927296857431417293658569134782123678549748529163652781394981345276374962815"), false)

// dupe in row: 0,0 && 1,0 === 8
assert.equal(is_valid_solution("885416927296857431417293658569134782123678549748529163652781394981345276374962815"), false)
// dupe in row: 8,9 && 9,9 === 1
assert.equal(is_valid_solution("835416927296857431417293658569134782123678549748529163652781394981345276374962811"), false)
// dupe in column: 0,0 && 0,1 === 8
assert.equal(is_valid_solution("835416927896857431417293658569134782123678549748529163652781394981345276374962815"), false)
// dupe in column: 9,8 && 9,9 === 5
assert.equal(is_valid_solution("835416927296857431417293658569134782123678549748529163652781394981345275374962815"), false)
// dupe in 3x3 box: 0,0 && 3,3 === 8
assert.equal(is_valid_solution("835416927296857431418293658569134782123678549748529163652781394981345276374962815"), false)
// dupe in 3x3 box: 6,6 && 9,9 === 5
assert.equal(is_valid_solution("835416927296857431417293658569134782123678549748529163652781594981345276374962815"), false)