1 files changed, 185 insertions, 0 deletions
diff --git a/internal/repl/repl.go b/internal/repl/repl.go
new file mode 100644
index 0000000..0f690e9
--- /dev/null
+++ b/internal/repl/repl.go
@@ -0,0 +1,185 @@
+package repl
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"os/signal"
+	"path/filepath"
+	"strings"
+	"syscall"
+
+	"codeberg.org/snonux/perc/internal/calculator"
+	"github.com/mattn/go-isatty"
+
+	"github.com/c-bata/go-prompt"
+)
+
+const historyFile = ".perc_history"
+
+// executor runs a calculation command and returns the result
+func executor(input string) {
+	input = strings.TrimSpace(input)
+	if input == "" {
+		return
+	}
+
+	// Check if it's a built-in command
+	if cmd, ok := isBuiltinCommand(input); ok {
+		output, err := ExecuteCommand(cmd)
+		if err != nil {
+			fmt.Printf("Error: %v\n", err)
+		}
+		if output != "" {
+			fmt.Println(output)
+		}
+		// Don't add built-in commands to history
+		return
+	}
+
+	// Run the calculation
+	result, err := calculator.Parse(input)
+	if err != nil {
+		fmt.Printf("Error: %v\n", err)
+		return
+	}
+	fmt.Println(result)
+}
+
+// isBuiltinCommand checks if input starts with a built-in command
+func isBuiltinCommand(input string) (string, bool) {
+	args := strings.Fields(input)
+	if len(args) == 0 {
+		return "", false
+	}
+
+	cmd := strings.ToLower(args[0])
+	for _, builtin := range builtinCommands {
+		if cmd == builtin {
+			return input, true
+		}
+	}
+	return "", false
+}
+
+// getHistoryPath returns the path to the history file
+func getHistoryPath() string {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return ""
+	}
+	return filepath.Join(home, historyFile)
+}
+
+// loadHistory loads history from file
+func loadHistory() []string {
+	historyPath := getHistoryPath()
+	if historyPath == "" {
+		return nil
+	}
+
+	file, err := os.Open(historyPath)
+	if err != nil {
+		return nil
+	}
+	defer file.Close()
+
+	var history []string
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		history = append(history, scanner.Text())
+	}
+	return history
+}
+
+// saveHistory saves history to file
+func saveHistory(history []string) error {
+	historyPath := getHistoryPath()
+	if historyPath == "" {
+		return nil
+	}
+
+	// Keep only last 1000 entries to prevent unlimited growth
+	if len(history) > 1000 {
+		history = history[len(history)-1000:]
+	}
+
+	file, err := os.Create(historyPath)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	writer := bufio.NewWriter(file)
+	for _, entry := range history {
+		if _, err := writer.WriteString(entry + "\n"); err != nil {
+			return err
+		}
+	}
+	return writer.Flush()
+}
+
+// RunREPL starts the interactive REPL
+func RunREPL() error {
+	// Check if stdin is a TTY
+	if !isatty.IsTerminal(os.Stdin.Fd()) {
+		fmt.Fprintln(os.Stderr, "REPL mode requires a TTY. Use 'perc <calculation>' for non-interactive mode.")
+		return fmt.Errorf("stdin is not a TTY")
+	}
+
+	history := loadHistory()
+
+	p := prompt.New(
+		executor,
+		completer,
+		prompt.OptionTitle("perc - Percentage Calculator"),
+		prompt.OptionPrefix("perc> "),
+		prompt.OptionLivePrefix(func() (string, bool) {
+			return "perc> ", true
+		}),
+		prompt.OptionHistory(history),
+	)
+
+	// Handle SIGINT gracefully
+	sigChan := make(chan os.Signal, 1)
+	signal.Notify(sigChan, syscall.SIGINT)
+
+	go func() {
+		<-sigChan
+		fmt.Println("\nUse 'quit' or 'exit' to exit, or Ctrl+D")
+	}()
+
+	// Run the prompt
+	p.Run()
+
+	// Note: History is not saved automatically in this version
+	// The prompt library stores it in memory but doesn't expose a getter
+
+	return nil
+}
+
+// completer provides auto-completion for built-in commands
+func completer(d prompt.Document) []prompt.Suggest {
+	text := d.GetWordBeforeCursor()
+	if text == "" {
+		return nil
+	}
+
+	var suggestions []prompt.Suggest
+	for _, cmd := range builtinCommands {
+		if strings.HasPrefix(strings.ToLower(cmd), strings.ToLower(text)) {
+			suggestions = append(suggestions, prompt.Suggest{Text: cmd, Description: getCommandDescription(cmd)})
+		}
+	}
+	return suggestions
+}
+
+func getCommandDescription(cmd string) string {
+	descriptions := map[string]string{
+		"help":   "Show help information",
+		"clear":  "Clear the screen",
+		"quit":   "Exit the REPL",
+		"exit":   "Exit the REPL",
+	}
+	return descriptions[cmd]
+}