add mdtest command to generate and run tests from a markdown file

mdtest/example1.go.tpl

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"github.com/alexflint/go-arg/v2"
+	{{if contains .Code "fmt."}}"fmt"{{end}}
+	{{if contains .Code "strings."}}"strings"{{end}}
+)
+
+func main() {
+	{{.Code}}
+}

mdtest/example2.go.tpl

@@ -0,0 +1,9 @@
+package main
+
+import (
+	"github.com/alexflint/go-arg/v2"
+	{{if contains .Code "fmt."}}"fmt"{{end}}
+    {{if contains .Code "strings."}}"strings"{{end}}
+)
+
+{{.Code}}

mdtest/mdtest.go

@@ -0,0 +1,179 @@
+// mdtest executes code blocks in markdown and checks that they run as expected
+package main
+
+import (
+	"bytes"
+	"context"
+	_ "embed"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"text/template"
+	"time"
+
+	"github.com/alexflint/go-arg/v2"
+)
+
+// var pattern = "```go(.*)```\\s*```\\s*\\$(.*)\\n(.*)```"
+var pattern = "(?s)```go([^`]*?)```\\s*```([^`]*?)```" //go(.*)```\\s*```\\s*\\$(.*)\\n(.*)```"
+
+var re = regexp.MustCompile(pattern)
+
+var funcs = map[string]any{
+	"contains": strings.Contains,
+}
+
+//go:embed example1.go.tpl
+var templateSource1 string
+
+//go:embed example2.go.tpl
+var templateSource2 string
+
+var t1 = template.Must(template.New("example1.go").Funcs(funcs).Parse(templateSource1))
+var t2 = template.Must(template.New("example2.go").Funcs(funcs).Parse(templateSource2))
+
+type payload struct {
+	Code string
+}
+
+func runCode(ctx context.Context, code []byte, cmd string) ([]byte, error) {
+	dir, err := os.MkdirTemp("", "")
+	if err != nil {
+		return nil, fmt.Errorf("error creating temp dir to build and run code: %w", err)
+	}
+
+	fmt.Println(dir)
+	fmt.Println(strings.Repeat("-", 80))
+
+	srcpath := filepath.Join(dir, "src.go")
+	binpath := filepath.Join(dir, "example")
+
+	// If the code contains a main function then use t2, otherwise use t1
+	t := t1
+	if strings.Contains(string(code), "func main") {
+		t = t2
+	}
+
+	var b bytes.Buffer
+	err = t.Execute(&b, payload{Code: string(code)})
+	if err != nil {
+		return nil, fmt.Errorf("error executing template for source file: %w", err)
+	}
+
+	fmt.Println(b.String())
+	fmt.Println(strings.Repeat("-", 80))
+
+	err = os.WriteFile(srcpath, b.Bytes(), os.ModePerm)
+	if err != nil {
+		return nil, fmt.Errorf("error writing temporary source file: %w", err)
+	}
+
+	compiler, err := exec.LookPath("go")
+	if err != nil {
+		return nil, fmt.Errorf("could not find path to go compiler: %w", err)
+	}
+
+	buildCmd := exec.CommandContext(ctx, compiler, "build", "-o", binpath, srcpath)
+	out, err := buildCmd.CombinedOutput()
+	if err != nil {
+		return nil, fmt.Errorf("error building source: %w. Compiler said:\n%s", err, string(out))
+	}
+
+	// replace "./example" with full path to compiled program
+	var env, args []string
+	var found bool
+	for _, part := range strings.Split(cmd, " ") {
+		if found {
+			args = append(args, part)
+		} else if part == "./example" {
+			found = true
+		} else {
+			env = append(env, part)
+		}
+	}
+
+	runCmd := exec.CommandContext(ctx, binpath, args...)
+	runCmd.Env = env
+	output, err := runCmd.CombinedOutput()
+	if err != nil {
+		return nil, fmt.Errorf("error runing example: %w. Program said:\n%s", err, string(output))
+	}
+
+	// Clean up the temp dir
+	if err := os.RemoveAll(dir); err != nil {
+		return nil, fmt.Errorf("error deleting temp dir: %w", err)
+	}
+
+	return output, nil
+}
+
+func Main() error {
+	ctx := context.Background()
+
+	var args struct {
+		Input string `arg:"positional,required"`
+	}
+	arg.MustParse(&args)
+
+	buf, err := os.ReadFile(args.Input)
+	if err != nil {
+		return err
+	}
+
+	fmt.Println(strings.Repeat("=", 80))
+
+	matches := re.FindAllSubmatchIndex(buf, -1)
+	for k, match := range matches {
+		codebegin, codeend := match[2], match[3]
+		code := buf[codebegin:codeend]
+
+		shellbegin, shellend := match[4], match[5]
+		shell := buf[shellbegin:shellend]
+
+		lines := strings.Split(string(shell), "\n")
+		for i := 0; i < len(lines); i++ {
+			if strings.HasPrefix(lines[i], "$") && strings.Contains(lines[i], "./example") {
+				cmd := strings.TrimSpace(strings.TrimPrefix(lines[i], "$"))
+
+				var output []string
+				i++
+				for i < len(lines) && !strings.HasPrefix(lines[i], "$") {
+					output = append(output, lines[i])
+					i++
+				}
+
+				expected := strings.TrimSpace(strings.Join(output, "\n"))
+
+				fmt.Println(string(code))
+				fmt.Println(strings.Repeat("-", 80))
+				fmt.Println(string(cmd))
+				fmt.Println(strings.Repeat("-", 80))
+				fmt.Println(string(expected))
+				fmt.Println(strings.Repeat("-", 80))
+
+				ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
+				defer cancel()
+
+				actual, err := runCode(ctx, code, cmd)
+				if err != nil {
+					return fmt.Errorf("error running example %d: %w\nCode was:\n%s", k, err, string(code))
+				}
+
+				fmt.Println(string(actual))
+				fmt.Println(strings.Repeat("=", 80))
+			}
+		}
+	}
+	fmt.Printf("found %d matches\n", len(matches))
+	return nil
+}
+
+func main() {
+	if err := Main(); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+}