commit ea695a3055fac6a90612823d7624f80df96a5fd3
Author: Ray Miller <ray@metail.co.uk>
Date:   Tue Aug 13 16:56:53 2019 +0100

    Initial check-in

diff --git a/grid.go b/grid.go
new file mode 100644
index 0000000..14d23ef
--- /dev/null
+++ b/grid.go
@@ -0,0 +1,120 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+type Grid [81]int
+
+func (g Grid) ElementAt(row, col int) int {
+	return g[col+row*9]
+}
+
+func (g Grid) WithElementAt(row, col int, val int) Grid {
+	g[col+row*9] = val
+	return g
+}
+
+func (g Grid) String() string {
+	var sb strings.Builder
+	for r := 0; r < 9; r++ {
+		fmt.Fprintf(&sb, "%d %d %d | %d %d %d | %d %d %d\n",
+			g.ElementAt(r, 0), g.ElementAt(r, 1), g.ElementAt(r, 2),
+			g.ElementAt(r, 3), g.ElementAt(r, 4), g.ElementAt(r, 5),
+			g.ElementAt(r, 6), g.ElementAt(r, 7), g.ElementAt(r, 8))
+		if r == 2 || r == 5 {
+			sb.WriteString("------+-------+------\n")
+		}
+	}
+	return sb.String()
+}
+
+func (g Grid) Neighbours(row, col int) IntSet {
+	neighbours := IntSet{}
+
+	// Neighbouring row
+	for c := 0; c < 9; c++ {
+		if v := g.ElementAt(row, c); v != 0 {
+			neighbours.Insert(v)
+		}
+	}
+
+	// Neighbouring col
+	for r := 0; r < 9; r++ {
+		if v := g.ElementAt(r, col); v != 0 {
+			neighbours.Insert(v)
+		}
+	}
+
+	// Top-left corner of the 3x3 subgrid
+	tlRow := (row / 3) * 3
+	tlCol := (col / 3) * 3
+
+	// Neighbouring subgrid
+	for r := tlRow; r < tlRow+3; r++ {
+		for c := tlCol; c < tlCol+3; c++ {
+			if v := g.ElementAt(r, c); v != 0 {
+				neighbours.Insert(v)
+			}
+		}
+	}
+
+	return neighbours
+}
+
+func (g Grid) FirstEmptyCell() int {
+	for i, v := range g {
+		if v == 0 {
+			return i
+		}
+	}
+	return -1
+}
+
+func (g Grid) Candidates(row, col int) []int {
+	neighbours := g.Neighbours(row, col)
+	var candidates []int
+	for _, v := range []int{1, 2, 3, 4, 5, 6, 7, 8, 9} {
+		if !neighbours.Contains(v) {
+			candidates = append(candidates, v)
+		}
+	}
+	return candidates
+}
+
+func (g Grid) Solve() (Grid, error) {
+	i := g.FirstEmptyCell()
+	if i == -1 {
+		// No unfilled cells: we have found a solution!
+		return g, nil
+	}
+	row := i / 9
+	col := i % 9
+	candidates := g.Candidates(row, col)
+	// Try each of the candidates in turn
+	for _, v := range candidates {
+		result, err := g.WithElementAt(row, col, v).Solve()
+		if err == nil {
+			return result, nil
+		}
+	}
+	// There were no valid candidates
+	return g, fmt.Errorf("No solutions found")
+}
+
+func ParseGrid(data []byte) (Grid, error) {
+	G := Grid{}
+	i := 0
+	for _, x := range data {
+		x := int(x) - int('0')
+		if x >= 0 && x <= 9 {
+			G[i] = x
+			i++
+		}
+	}
+	if i < len(G) {
+		return G, fmt.Errorf("Invalid input: found %d elements, expected %d", i, len(G))
+	}
+	return G, nil
+}
diff --git a/grid_test.go b/grid_test.go
new file mode 100644
index 0000000..c5dc5ba
--- /dev/null
+++ b/grid_test.go
@@ -0,0 +1,190 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+)
+
+func TestElementAt(t *testing.T) {
+	G := Grid{} // an empty grid
+	for row := 0; row < 9; row++ {
+		for col := 0; col < 9; col++ {
+			v := G.ElementAt(row, col)
+			if v != 0 {
+				t.Errorf("ElementAt(%d, %d) = %d, expected 0", row, col, v)
+			}
+		}
+	}
+}
+
+func TestElementAtPanic(t *testing.T) {
+	defer func() {
+		if r := recover(); r == nil {
+			t.Errorf("ElementAt did not panic when out of bounds")
+		}
+	}()
+	G := Grid{}
+	G.ElementAt(10, 10)
+}
+
+func TestWithElementAt(t *testing.T) {
+	G := Grid{}
+	for i := 0; i < 10; i++ {
+		row := rand.Intn(9)
+		col := rand.Intn(9)
+		val := rand.Intn(10)
+		G = G.WithElementAt(row, col, val)
+		if G.ElementAt(row, col) != val {
+			t.Errorf("WithElementAt(%d, %d, %d) did not return expected value", row, col, val)
+		}
+	}
+}
+
+func TestParseGrid(t *testing.T) {
+	data := []byte(`010020706
+700913040
+380004001
+000007010
+500109003
+090500000
+200300094
+040762005
+105090070`)
+	G, err := ParseGrid(data)
+	if err != nil {
+		t.Errorf("Unexpected error parsing grid: %v", err)
+	}
+	// Check that row 2 is as expected
+	row := 2
+	expected := []int{3, 8, 0, 0, 0, 4, 0, 0, 1}
+	for col, want := range expected {
+		got := G.ElementAt(row, col)
+		if got != want {
+			t.Errorf("ElementAt(%d,%d) = %d, expected %d", row, col, got, want)
+		}
+	}
+	fmt.Println(G)
+}
+
+func TestParseGridError(t *testing.T) {
+	data := []byte(`01002070
+700913040
+380004001
+000007010
+500109003
+090500000
+200300094
+040762005
+105090070`)
+	_, err := ParseGrid(data)
+	if err == nil {
+		t.Errorf("ParseGrid() did not return an expected error")
+	}
+}
+
+var tests = []struct {
+	data           []byte
+	grid           Grid
+	firstEmptyCell int
+	neighbours     []int
+	candidates     []int
+}{
+	{
+		[]byte("010020706700913040380004001000007010500109003090500000200300094040762005105090070"),
+		Grid{},
+		0,
+		[]int{1, 2, 3, 5, 6, 7, 8},
+		[]int{4, 9},
+	},
+	{
+		[]byte("000000800785090006040700002068300051400000007570009320900002060800060719006000000"),
+		Grid{},
+		0,
+		[]int{4, 5, 7, 8, 9},
+		[]int{1, 2, 3, 6},
+	},
+	{
+		[]byte("320000809400600027005000040000401000002000900000502000080000100640003008107000032"),
+		Grid{},
+		2,
+		[]int{2, 3, 4, 5, 7, 8, 9},
+		[]int{1, 6},
+	},
+	{
+		[]byte("000008000040100390000900561006094000003000200000320800372001000095003020000200000"),
+		Grid{},
+		0,
+		[]int{3, 4, 8},
+		[]int{1, 2, 5, 6, 7, 9},
+	},
+}
+
+func init() {
+	for i := 0; i < len(tests); i++ {
+		grid, err := ParseGrid(tests[i].data)
+		if err != nil {
+			panic(err)
+		}
+		tests[i].grid = grid
+	}
+}
+
+func TestFirstEmptyCell(t *testing.T) {
+	for i, x := range tests {
+		got := tests[i].grid.FirstEmptyCell()
+		if got != x.firstEmptyCell {
+			t.Errorf("FirstEmptyCell(grid[%d]) = %d, expected %d", i, got, x.firstEmptyCell)
+		}
+	}
+}
+
+func TestNeighbours(t *testing.T) {
+	for i, x := range tests {
+		r := x.firstEmptyCell / 9
+		c := x.firstEmptyCell % 9
+		got := x.grid.Neighbours(r, c)
+		if len(got) != len(x.neighbours) {
+			t.Errorf("len(Neighbours(grid[%d]) = %d, expected %d", i, len(got), len(x.neighbours))
+		}
+		for _, v := range x.neighbours {
+			if !got.Contains(v) {
+				t.Errorf("Neighbours(grid[%d]) did not contain %d", i, v)
+			}
+		}
+	}
+}
+
+func TestCandidates(t *testing.T) {
+	for i, x := range tests {
+		r := x.firstEmptyCell / 9
+		c := x.firstEmptyCell % 9
+		got := x.grid.Candidates(r, c)
+		if len(got) != len(x.candidates) {
+			t.Errorf("len(Candidates(grid[%d]) = %d, expected %d", i, len(got), len(x.candidates))
+		}
+		for _, v := range x.candidates {
+			found := false
+			for _, w := range got {
+				if v == w {
+					found = true
+					break
+				}
+			}
+			if !found {
+				t.Errorf("Candidates(grid[%d]) did not contain %d", i, v)
+			}
+		}
+	}
+}
+
+func TestSolve(t *testing.T) {
+	for i, x := range tests {
+		res, err := x.grid.Solve()
+		if err != nil {
+			t.Errorf("Could not solve grid %d", i)
+		} else {
+			fmt.Println(res)
+		}
+	}
+}
diff --git a/intset.go b/intset.go
new file mode 100644
index 0000000..232b6c8
--- /dev/null
+++ b/intset.go
@@ -0,0 +1,21 @@
+package main
+
+type IntSet []int
+
+// Contains returns true if x is in the set, otherwise false
+func (s *IntSet) Contains(x int) bool {
+	for _, v := range *s {
+		if v == x {
+			return true
+		}
+	}
+	return false
+}
+
+// Insert adds x to the set if it is not already present
+func (s *IntSet) Insert(x int) {
+	if s.Contains(x) {
+		return
+	}
+	*s = append(*s, x)
+}
diff --git a/intset_test.go b/intset_test.go
new file mode 100644
index 0000000..dae575c
--- /dev/null
+++ b/intset_test.go
@@ -0,0 +1,41 @@
+package main
+
+import "testing"
+
+func TestContains(t *testing.T) {
+	s := IntSet{}
+	if s.Contains(0) {
+		t.Errorf("The empty set should not contain 0")
+	}
+	xs := []int{1, 2, 3}
+	s = IntSet(xs)
+	for _, x := range xs {
+		if !s.Contains(x) {
+			t.Errorf("Expected set to contain %d", x)
+		}
+	}
+	if s.Contains(0) {
+		t.Errorf("The set {1,2,3} should not contain 0")
+	}
+}
+
+func TestInsert(t *testing.T) {
+	s := IntSet{}
+	xs := []int{1, 2, 3}
+	for _, x := range xs {
+		if s.Contains(x) {
+			t.Errorf("%d has not yet been added to the set", x)
+		}
+		s.Insert(x)
+		if !s.Contains(x) {
+			t.Errorf("Expected the set to contain %d", x)
+		}
+	}
+	if len(s) != len(xs) {
+		t.Errorf("The set should contain %d elements", len(xs))
+	}
+	s.Insert(1)
+	if len(s) != len(xs) {
+		t.Errorf("Inserting an element twice should not affect size of the set")
+	}
+}
diff --git a/sudoku.go b/sudoku.go
new file mode 100644
index 0000000..d45499c
--- /dev/null
+++ b/sudoku.go
@@ -0,0 +1,31 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintln(os.Stderr, "Usage: sudoku FILENAME")
+		os.Exit(2)
+	}
+	filename := os.Args[1]
+	data, err := ioutil.ReadFile(filename)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error reading puzzle file: %v\n", err)
+		os.Exit(3)
+	}
+	grid, err := ParseGrid(data)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error parsing grid: %v\n", err)
+		os.Exit(3)
+	}
+	solution, err := grid.Solve()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(3)
+	}
+	fmt.Println(solution)
+}