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+
+<html>
+<head>
+<meta name="robots" content="noindex,nofollow">
+</head>
+<body>
+<script>
+
+const script = "sudoku.html";  //   my $script = $ENV{SCRIPT_NAME}; $script =~ s/.*\///;
+const blank = ".................................................................................";
+const givensString = (location.search.replace('?','') + blank).substr(0, 81);
+const givens = Array.from(givensString);
+const choices = Array.from({length:81}).map(_=>"123456789");
+
+const subsets = [
+
+  // rows
+  [ 0,  1,  2,  9, 10, 11, 18, 19, 20],
+  [ 3,  4,  5, 12, 13, 14, 21, 22, 23],
+  [ 6,  7,  8, 15, 16, 17, 24, 25, 26],
+  [27, 28, 29, 36, 37, 38, 45, 46, 47],
+  [30, 31, 32, 39, 40, 41, 48, 49, 50],
+  [33, 34, 35, 42, 43, 44, 51, 52, 53],
+  [54, 55, 56, 63, 64, 65, 72, 73, 74],
+  [57, 58, 59, 66, 67, 68, 75, 76, 77],
+  [60, 61, 62, 69, 70, 71, 78, 79, 80],
+
+  // columns
+  [ 0,  3,  6, 27, 30, 33, 54, 57, 60],
+  [ 1,  4,  7, 28, 31, 34, 55, 58, 61],
+  [ 2,  5,  8, 29, 32, 35, 56, 59, 62],
+  [ 9, 12, 15, 36, 39, 42, 63, 66, 69],
+  [10, 13, 16, 37, 40, 43, 64, 67, 70],
+  [11, 14, 17, 38, 41, 44, 65, 68, 71],
+  [18, 21, 24, 45, 48, 51, 72, 75, 78],
+  [19, 22, 25, 46, 49, 52, 73, 76, 79],
+  [20, 23, 26, 47, 50, 53, 74, 77, 80],
+
+  // squares
+  [ 0,  1,  2,  3,  4,  5,  6,  7,  8],
+  [ 9, 10, 11, 12, 13, 14, 15, 16, 17],
+  [18, 19, 20, 21, 22, 23, 24, 25, 26],
+  [27, 28, 29, 30, 31, 32, 33, 34, 35],
+  [36, 37, 38, 39, 40, 41, 42, 43, 44],
+  [45, 46, 47, 48, 49, 50, 51, 52, 53],
+  [54, 55, 56, 57, 58, 59, 60, 61, 62],
+  [63, 64, 65, 66, 67, 68, 69, 70, 71],
+  [72, 73, 74, 75, 76, 77, 78, 79, 80]
+];
+
+// remove choices eliminated by the at-most-one rule
+
+for (let subset of subsets) {
+  for (let i of subset) {
+    const d = givens[i];
+    if (d == ".") continue;
+    for (let j of subset) {
+      choices[j] = choices[j].replace(d, "");
+    }
+  }
+}
+
+// identify choices mandated by the at-least-one rule
+
+const unique = {};
+const digitCounts = Array.from("123456789")
+  .reduce((acc, d) => {acc[d] = 0; return acc}, {});
+for (let subset of subsets) {
+  const counts = {...digitCounts};
+  const where = {...digitCounts};
+  subset
+    .filter(i => givens[i] == ".")
+    .forEach(i => {
+      const digits = Array.from(choices[i]);
+      for (let d of digits) {
+        counts[d] += 1;
+        where[d] = i;
+      };
+    });
+  Array.from("123456789")
+    .filter(d => counts[d] == 1)
+    .forEach(d => unique[where[d]] = d)
+}
+
+// display board as table of tables of hyperlinked choices
+
+function table(first) {
+  const row = `\n<td bgcolor=#77bbff width=170 height=50 align=center>${first}`;
+  return `\n<table>` + Array.from({length:3}).map(_=>`<tr>${row}${row}${row}`).join('') + `</table>`;
+}
+
+let board = table(table("X"));
+
+function nextGivens(i, digit) {
+  let res = Array.from(givens);
+  res[i] = digit;
+  return res.join("");
+}
+
+function choicesFn(i) {
+  const uniqueChoice = unique[i];
+  return Array.from(choices[i])
+    .map(digit => {
+      if (uniqueChoice && digit != uniqueChoice) {
+        return `\n<font color=gray size=-1>${digit}</font>`;
+      } else {
+        let search = nextGivens(i, digit);
+        return `\n<a href="?${search}">${digit}</a>`;
+      }
+    }).join("");
+}
+
+for (let i=0; i <= 80; i++) {
+  const digit = givens[i];
+  const content = (digit != '.')
+    ? `<font size=8>${digit}`
+    : choicesFn(i);
+  board = board.replace('X', content);
+}
+
+</script>
+  <style>
+   a         {text-decoration: none;}
+  </style>
+  <center><br>
+    <div id=output></div>
+    <font color=gray><br><br>
+      About Sudokant, the Sudoku Servant
+      <br><table width=400><tr><td><font color=gray>
+        This server helps you solve <a href=http://www.sudoku.com/>sudoku puzzles</a>
+        by showing choices satisfying two simple rules: <i>at-most-one</i> and <i>at-least-one</i>
+        of every digit must appear in every row, column and square.
+        Choices that violate the <i>at-most-one</i> rule are omitted.
+        Alternatives to digits that must be chosen by the <i>at-least-one</i> rule are grayed.
+        <br><br>
+        Empty squares are dead ends. Back up the browser
+        to make a new choice. The above rules are insufficent to solve the hardest
+        puzzles. You can guess or look for a third rule. This program will help you
+        find these most interesting places in the hardest puzzles without becomming a
+        mechanical master of the simpler logic.
+        <br><br>Each step has a unique url that you can bookmark or send to
+        friends. Use this <a href=?>blank page</a> to enter new puzzles.
+        Contribute your insights and most diabolical puzzles to the
+        <a href=http://c2.com/cgi/wiki?SuDoku>SuDoku</a> page on my wiki.
+      </table><br>
+      <font color=gray size=-1>
+        &copy;2005 Ward Cunningham
+        &copy;2020 Ward Cunningham, Joshua Benuck, Eric Dobbs, Beth Long
+<script>
+  output.innerHTML = board
+</script>
+</body>
+</html>
