testscript: suggest misspelled commands

If a command is not found, we go through the list of defined commands
and check if any of them are sufficiently close to the one used.
"Sufficiently close" is defined by having a Damerau-Levenshtein distance
of 1, which feels like it hits the sweet spot between usefulness and
ease of implementation.

The negation case is still special-cased, as negation is not in the set
of defined commands.

Fixes #190

internal/textutil/almost_equal.go

@@ -0,0 +1,67 @@
+package textutil
+
+import (
+	"unicode/utf8"
+)
+
+// AlmostEqual reports whether a and b have Damerau-Levenshtein distance of at
+// most 1. That is, it reports whether a can be transformed into b by adding,
+// removing or substituting a single rune, or by swapping two adjacent runes.
+// Invalid runes are considered equal.
+//
+// It runs in O(len(a)+len(b)) time.
+func AlmostEqual(a, b string) bool {
+	for len(a) > 0 && len(b) > 0 {
+		ra, tailA := shiftRune(a)
+		rb, tailB := shiftRune(b)
+		if ra == rb {
+			a, b = tailA, tailB
+			continue
+		}
+		// check for addition/deletion/substitution
+		if equalValid(a, tailB) || equalValid(tailA, b) || equalValid(tailA, tailB) {
+			return true
+		}
+		if len(tailA) == 0 || len(tailB) == 0 {
+			return false
+		}
+		// check for swap
+		a, b = tailA, tailB
+		Ra, tailA := shiftRune(tailA)
+		Rb, tailB := shiftRune(tailB)
+		return ra == Rb && Ra == rb && equalValid(tailA, tailB)
+	}
+	if len(a) == 0 {
+		return len(b) == 0 || singleRune(b)
+	}
+	return singleRune(a)
+}
+
+// singleRune reports whether s consists of a single UTF-8 codepoint.
+func singleRune(s string) bool {
+	_, n := utf8.DecodeRuneInString(s)
+	return n == len(s)
+}
+
+// shiftRune splits off the first UTF-8 codepoint from s and returns it and the
+// rest of the string. It panics if s is empty.
+func shiftRune(s string) (rune, string) {
+	if len(s) == 0 {
+		panic(s)
+	}
+	r, n := utf8.DecodeRuneInString(s)
+	return r, s[n:]
+}
+
+// equalValid reports whether a and b are equal, if invalid code points are considered equal.
+func equalValid(a, b string) bool {
+	var ra, rb rune
+	for len(a) > 0 && len(b) > 0 {
+		ra, a = shiftRune(a)
+		rb, b = shiftRune(b)
+		if ra != rb {
+			return false
+		}
+	}
+	return len(a) == 0 && len(b) == 0
+}

internal/textutil/almost_equal_test.go

@@ -0,0 +1,100 @@
+package textutil
+
+import (
+	"math"
+	"testing"
+)
+
+func TestAlmostEqual(t *testing.T) {
+	t.Parallel()
+
+	tcs := []struct {
+		inA  string
+		inB  string
+		want bool
+	}{
+		{"", "", true},
+		{"", "a", true},
+		{"a", "a", true},
+		{"a", "b", true},
+		{"hello", "hell", true},
+		{"hello", "jello", true},
+		{"hello", "helol", true},
+		{"hello", "jelol", false},
+	}
+	for _, tc := range tcs {
+		got := AlmostEqual(tc.inA, tc.inB)
+		if got != tc.want {
+			t.Errorf("AlmostEqual(%q, %q) = %v, want %v", tc.inA, tc.inB, got, tc.want)
+		}
+		// two tests for the price of one \o/
+		if got != AlmostEqual(tc.inB, tc.inA) {
+			t.Errorf("AlmostEqual(%q, %q) == %v != AlmostEqual(%q, %q)", tc.inA, tc.inB, got, tc.inB, tc.inA)
+		}
+	}
+}
+
+func FuzzAlmostEqual(f *testing.F) {
+	f.Add("", "")
+	f.Add("", "a")
+	f.Add("a", "a")
+	f.Add("a", "b")
+	f.Add("hello", "hell")
+	f.Add("hello", "jello")
+	f.Add("hello", "helol")
+	f.Add("hello", "jelol")
+	f.Fuzz(func(t *testing.T, a, b string) {
+		if len(a) > 10 || len(b) > 10 {
+			// longer strings won't add coverage, but take longer to check
+			return
+		}
+		d := editDistance([]rune(a), []rune(b))
+		got := AlmostEqual(a, b)
+		if want := d <= 1; got != want {
+			t.Errorf("AlmostEqual(%q, %q) = %v, editDistance(%q, %q) = %d", a, b, got, a, b, d)
+		}
+		if got != AlmostEqual(b, a) {
+			t.Errorf("AlmostEqual(%q, %q) == %v != AlmostEqual(%q, %q)", a, b, got, b, a)
+		}
+	})
+}
+
+// editDistance returns the Damerau-Levenshtein distance between a and b. It is
+// inefficient, but by keeping almost verbatim to the recursive definition from
+// Wikipedia, hopefully "obviously correct" and thus suitable for the fuzzing
+// test of AlmostEqual.
+func editDistance(a, b []rune) int {
+	i, j := len(a), len(b)
+	m := math.MaxInt
+	if i == 0 && j == 0 {
+		return 0
+	}
+	if i > 0 {
+		m = min(m, editDistance(a[1:], b)+1)
+	}
+	if j > 0 {
+		m = min(m, editDistance(a, b[1:])+1)
+	}
+	if i > 0 && j > 0 {
+		d := editDistance(a[1:], b[1:])
+		if a[0] != b[0] {
+			d += 1
+		}
+		m = min(m, d)
+	}
+	if i > 1 && j > 1 && a[0] == b[1] && a[1] == b[0] {
+		d := editDistance(a[2:], b[2:])
+		if a[0] != b[0] {
+			d += 1
+		}
+		m = min(m, d)
+	}
+	return m
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}

internal/textutil/testdata/fuzz/FuzzAlmostEqual/295b316649ae86dd

@@ -0,0 +1,3 @@
+go test fuzz v1
+string("")
+string("00")

internal/textutil/testdata/fuzz/FuzzAlmostEqual/5bd9cd4e8c887808

@@ -0,0 +1,3 @@
+go test fuzz v1
+string("\x980")
+string("0\xb70")

internal/textutil/testdata/fuzz/FuzzAlmostEqual/b323cef1fc26e507

@@ -0,0 +1,3 @@
+go test fuzz v1
+string("OOOOOOOO000")
+string("0000000000000")

internal/textutil/testdata/fuzz/FuzzAlmostEqual/c6edde4256d6f5eb

@@ -0,0 +1,3 @@
+go test fuzz v1
+string("OOOOOOOO000")
+string("0000000000\x1000")

testscript/testdata/testscript_notfound.txt

@@ -0,0 +1,9 @@
+# Check that unknown commands output the right error
+
+unquote scripts/notfound.txt
+
+! testscript scripts
+stdout 'unknown command "notexist"'
+
+-- scripts/notfound.txt --
+>notexist

testscript/testdata/testscript_notfound_misspelled.txt

@@ -0,0 +1,9 @@
+# Check that misspelling a command outputs a helpful error
+
+unquote scripts/notfound.txt
+
+! testscript scripts
+stdout 'unknown command "exits" \(did you mean "exists"\?\)'
+
+-- scripts/notfound.txt --
+>exits notfound.txt

testscript/testdata/testscript_notfound_negation.txt

@@ -0,0 +1,9 @@
+# Check that missing a space on negation outputs a helpful error
+
+unquote scripts/notfound.txt
+
+! testscript scripts
+stdout 'unknown command "!exists" \(did you mean "! exists"\?\)'
+
+-- scripts/notfound.txt --
+>!exists notfound.txt

testscript/testscript.go

@@ -22,6 +22,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"sort"
 	"strings"
 	"sync/atomic"
 	"syscall"
@@ -30,6 +31,7 @@ import (
 
 	"github.com/rogpeppe/go-internal/imports"
 	"github.com/rogpeppe/go-internal/internal/os/execpath"
+	"github.com/rogpeppe/go-internal/internal/textutil"
 	"github.com/rogpeppe/go-internal/par"
 	"github.com/rogpeppe/go-internal/testenv"
 	"github.com/rogpeppe/go-internal/txtar"
@@ -669,7 +671,16 @@ func (ts *TestScript) runLine(line string) (runOK bool) {
 		cmd = ts.params.Cmds[args[0]]
 	}
 	if cmd == nil {
-		ts.Fatalf("unknown command %q", args[0])
+		// try to find spelling corrections. We arbitrarily limit the number of
+		// corrections, to not be too noisy.
+		switch c := ts.cmdSuggestions(args[0]); len(c) {
+		case 1:
+			ts.Fatalf("unknown command %q (did you mean %q?)", args[0], c[0])
+		case 2, 3, 4:
+			ts.Fatalf("unknown command %q (did you mean one of %q?)", args[0], c)
+		default:
+			ts.Fatalf("unknown command %q", args[0])
+		}
 	}
 	ts.callBuiltinCmd(args[0], func() {
 		cmd(ts, neg, args[1:])
@@ -694,6 +705,42 @@ func (ts *TestScript) callBuiltinCmd(cmd string, runCmd func()) {
 	runCmd()
 }
 
+func (ts *TestScript) cmdSuggestions(name string) []string {
+	// special case: spell-correct `!cmd` to `! cmd`
+	if strings.HasPrefix(name, "!") {
+		if _, ok := scriptCmds[name[1:]]; ok {
+			return []string{"! " + name[1:]}
+		}
+		if _, ok := ts.params.Cmds[name[1:]]; ok {
+			return []string{"! " + name[1:]}
+		}
+	}
+	var candidates []string
+	for c := range scriptCmds {
+		if textutil.AlmostEqual(name, c) {
+			candidates = append(candidates, c)
+		}
+	}
+	for c := range ts.params.Cmds {
+		if textutil.AlmostEqual(name, c) {
+			candidates = append(candidates, c)
+		}
+	}
+	if len(candidates) == 0 {
+		return nil
+	}
+	// deduplicate candidates
+	// TODO: Use slices.Compact (and maybe slices.Sort) once we can use Go 1.21
+	sort.Strings(candidates)
+	out := candidates[:1]
+	for _, c := range candidates[1:] {
+		if out[len(out)-1] == c {
+			out = append(out, c)
+		}
+	}
+	return out
+}
+
 func (ts *TestScript) applyScriptUpdates() {
 	if len(ts.scriptUpdates) == 0 {
 		return