Add suggestion on cmd not found

acmd.go

@@ -159,9 +159,29 @@ func (r *Runner) run() error {
 			return c.Do(r.ctx, params)
 		}
 	}
+
+	if suggestion := r.suggestCommand(cmd); suggestion != "" {
+		fmt.Fprintf(r.cfg.Output, "%q is not a subcommand, did you mean %q?\n", cmd, suggestion)
+	}
 	return fmt.Errorf("no such command %q", cmd)
 }
 
+// suggestCommand for not found earlier command.
+func (r *Runner) suggestCommand(cmd string) string {
+	const maxMatchDist = 2
+	minDist := maxMatchDist + 1
+	match := ""
+
+	for _, c := range r.cmds {
+		dist := strDistance(cmd, c.Name)
+		if dist < minDist {
+			minDist = dist
+			match = c.Name
+		}
+	}
+	return match
+}
+
 var defaultUsage = func(w io.Writer) func(cfg Config, cmds []Command) {
 	return func(cfg Config, cmds []Command) {
 		if cfg.AppDescription != "" {

acmd_test.go

@@ -1,6 +1,7 @@
 package acmd
 
 import (
+	"bytes"
 	"context"
 	"testing"
 )
@@ -50,3 +51,51 @@ func TestRunner_init(t *testing.T) {
 		}
 	}
 }
+
+func TestRunner_suggestCommand(t *testing.T) {
+	testCases := []struct {
+		cmds []Command
+		args []string
+		want string
+	}{
+		{
+			cmds: []Command{
+				{Name: "for", Do: nopFunc},
+				{Name: "foo", Do: nopFunc},
+				{Name: "bar", Do: nopFunc},
+			},
+			args: []string{"fooo"},
+			want: `"fooo" is not a subcommand, did you mean "foo"?` + "\n",
+		},
+		{
+			cmds: []Command{},
+			args: []string{"hell"},
+			want: `"hell" is not a subcommand, did you mean "help"?` + "\n",
+		},
+		{
+			cmds: []Command{},
+			args: []string{"verZION"},
+			want: "",
+		},
+		{
+			cmds: []Command{},
+			args: []string{"verZion"},
+			want: `"verZion" is not a subcommand, did you mean "version"?` + "\n",
+		},
+	}
+
+	for _, tc := range testCases {
+		buf := &bytes.Buffer{}
+		r := RunnerOf(tc.cmds, Config{
+			Args:   tc.args,
+			Output: buf,
+		})
+		if err := r.Run(); err == nil {
+			t.Fatal()
+		}
+
+		if got := buf.String(); got != tc.want {
+			t.Logf("want %q got %q", tc.want, got)
+		}
+	}
+}

levenshtein.go

@@ -0,0 +1,48 @@
+package acmd
+
+// strDistance between 2 strings using Levenshtein distance algorithm.
+func strDistance(a, b string) int {
+	switch {
+	case a == "":
+		return len(b)
+	case b == "":
+		return len(a)
+	case a == b:
+		return 0
+	}
+
+	if len(a) > len(b) {
+		a, b = b, a
+	}
+	lenA, lenB := len(a), len(b)
+
+	x := make([]int, lenA+1)
+	for i := 0; i < len(x); i++ {
+		x[i] = i
+	}
+
+	for i := 1; i <= lenB; i++ {
+		prev := i
+		for j := 1; j <= lenA; j++ {
+			current := x[j-1] // match
+			if b[i-1] != a[j-1] {
+				current = min3(x[j-1]+1, prev+1, x[j]+1)
+			}
+			x[j-1], prev = prev, current
+		}
+		x[lenA] = prev
+	}
+	return x[lenA]
+}
+
+func min3(a, b, c int) int {
+	if a < b {
+		if a < c {
+			return a
+		}
+	}
+	if b < c {
+		return b
+	}
+	return c
+}

levenshtein_test.go

@@ -0,0 +1,30 @@
+package acmd
+
+import "testing"
+
+func Test_strDistance(t *testing.T) {
+	testCases := []struct {
+		a, b string
+		want int
+	}{
+		{"", "", 0},
+		{"a", "a", 0},
+		{"", "hello", 5},
+		{"hello", "", 5},
+		{"hello", "hello", 0},
+		{"ab", "aa", 1},
+		{"ab", "ba", 2},
+		{"ab", "aaa", 2},
+		{"bbb", "a", 3},
+		{"kitten", "sitting", 3},
+		{"distance", "difference", 5},
+		{"resume and cafe", "resumes and cafes", 2},
+	}
+
+	for _, tc := range testCases {
+		dist := strDistance(tc.a, tc.b)
+		if dist != tc.want {
+			t.Errorf("for (%q , %q) want %d, got %d", tc.a, tc.b, tc.want, dist)
+		}
+	}
+}