June 2018 Update

Task/Sudoku/Kotlin/sudoku.kotlin

@@ -0,0 +1,84 @@
+// version 1.2.10
+
+class Sudoku(rows: List<String>) {
+    private val grid = IntArray(81)
+    private var solved = false
+
+    init {
+        require(rows.size == 9 && rows.all { it.length == 9 }) {
+            "Grid must be 9 x 9"
+        }
+        for (i in 0..8) {
+            for (j in 0..8 ) grid[9 * i + j] = rows[i][j] - '0'
+        }
+    }
+
+    fun solve() {
+        println("Starting grid:\n\n$this")
+        placeNumber(0)
+        println(if (solved) "Solution:\n\n$this" else "Unsolvable!")
+    }
+
+    private fun placeNumber(pos: Int) {
+        if (solved) return
+        if (pos == 81) {
+            solved = true
+            return
+        }
+        if (grid[pos] > 0) {
+            placeNumber(pos + 1)
+            return
+        }
+        for (n in 1..9) {
+            if (checkValidity(n, pos % 9, pos / 9)) {
+                grid[pos] = n
+                placeNumber(pos + 1)
+                if (solved) return
+                grid[pos] = 0
+            }
+        }
+    }
+
+    private fun checkValidity(v: Int, x: Int, y: Int): Boolean {
+        for (i in 0..8) {
+            if (grid[y * 9 + i] == v || grid[i * 9 + x] == v) return false
+        }
+        val startX = (x / 3) * 3
+        val startY = (y / 3) * 3
+        for (i in startY until startY + 3) {
+            for (j in startX until startX + 3) {
+                if (grid[i * 9 + j] == v) return false
+            }
+        }
+        return true
+    }
+
+    override fun toString(): String {
+        val sb = StringBuilder()
+        for (i in 0..8) {
+            for (j in 0..8) {
+                sb.append(grid[i * 9 + j])
+                sb.append(" ")
+                if (j == 2 || j == 5) sb.append("| ")
+            }
+            sb.append("\n")
+            if (i == 2 || i == 5) sb.append("------+-------+------\n")
+        }
+        return sb.toString()
+    }
+}
+
+fun main(args: Array<String>) {
+    val rows = listOf(
+        "850002400",
+        "720000009",
+        "004000000",
+        "000107002",
+        "305000900",
+        "040000000",
+        "000080070",
+        "017000000",
+        "000036040"
+    )
+    Sudoku(rows).solve()
+}