From 06615702d85d5640984ab0e6a7b3155df4d261a3 Mon Sep 17 00:00:00 2001
From: j-james
Date: Mon, 14 Nov 2022 23:56:36 -0800
Subject: Add Sudoku solver

---
 sudoku/sudoku.nim | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 100 insertions(+)
 create mode 100644 sudoku/sudoku.nim

diff --git a/sudoku/sudoku.nim b/sudoku/sudoku.nim
new file mode 100644
index 0000000..aa0672b
--- /dev/null
+++ b/sudoku/sudoku.nim
@@ -0,0 +1,100 @@
+# Simple, inefficient sudoku solver.
+# But it's cute. Written on a plane.
+import std/[sequtils, sugar]
+type Board = seq[seq[int]]
+
+const input: Board = @[
+  @[0,5,8, 6,0,0, 0,4,0],
+  @[1,0,0, 7,0,0, 0,9,0],
+  @[0,6,0, 0,0,4, 8,0,0],
+  @[0,8,0, 9,0,0, 0,0,0],
+  @[4,0,0, 0,1,0, 0,0,2],
+  @[0,0,0, 0,0,3, 0,6,0],
+  @[0,0,2, 3,0,0, 0,7,0],
+  @[0,9,0, 0,0,1, 0,0,4],
+  @[0,1,0, 0,0,7, 5,3,0]
+]
+
+func valid_input(board: Board): bool =
+  for j in 0 ..< 9:
+    for i in 0 ..< 9:
+      if input[i][j] != 0 and input[i][j] != board[i][j]:
+        return false
+  return true
+
+assert input.valid_input()
+
+func valid_rows(board: Board): bool =
+  for i in 0 ..< 9:
+    var used: seq[int]
+    for j in 0 ..< 9:
+      var num = board[i][j]
+      if num in used:
+        return false
+      if num != 0:
+        used.add(num)
+  return true
+
+assert input.valid_rows()
+
+func valid_columns(board: Board): bool =
+  for j in 0 ..< 9:
+    var used: seq[int]
+    for i in 0 ..< 9:
+      var num = board[i][j]
+      if num in used:
+        return false
+      if num != 0:
+        used.add(num)
+  return true
+
+assert input.valid_rows()
+
+func valid_boxes(board: Board): bool =
+  for l in 0 ..< 3:
+    for k in 0 ..< 3:
+      var used: seq[int]
+      for j in 0 ..< 3:
+        for i in 0 ..< 3:
+          var num = board[3*k + i][3*l + j]
+          if num in used:
+            return false
+          if num != 0:
+            used.add(num)
+  return true
+
+assert input.valid_boxes()
+
+func valid(board: Board): bool =
+  return board.valid_input() and board.valid_boxes() and
+         board.valid_rows() and board.valid_columns()
+
+assert input.valid()
+
+proc print(board: Board) =
+  for j in 0 ..< 9:
+    for i in 0 ..< 9:
+      stdout.write(board[j][i])
+      stdout.write(" ")
+      if i == 2 or i == 5:
+        stdout.write("| ")
+    stdout.write("\n")
+    if j == 2 or j == 5:
+      stdout.write("---------------------")
+      stdout.write("\n")
+  stdout.write("\n\n")
+
+proc generate_boards(board: Board) =
+  var board = board
+  for j in 0 ..< 9:
+    for i in 0 ..< 9:
+      if board[i][j] == 0:
+        for k in 1 .. 9:
+          board[i][j] = k
+          if board.valid():
+            generate_boards(board)
+  if board.valid():
+    board.print()
+    quit()
+
+generate_boards(input)
-- 
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