package repl

import (
	"bufio"
	"fmt"
	"io"
	"math"
	"strings"

	"matheval/evaluator"
	"matheval/lexer"
	"matheval/parser"
)

const prompt = ">> "

// Run starts the read-eval-print loop, reading from r and writing to w.
func Run(r io.Reader, w io.Writer) {
	scanner := bufio.NewScanner(r)
	fmt.Fprint(w, prompt)

	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		if line == "" {
			fmt.Fprint(w, prompt)
			continue
		}

		result, err := evalLine(line)
		if err != nil {
			fmt.Fprintf(w, "error: %s\n", err)
		} else {
			fmt.Fprintln(w, formatResult(result))
		}

		fmt.Fprint(w, prompt)
	}
}

// evalLine tokenizes, parses, and evaluates a single expression string.
func evalLine(line string) (float64, error) {
	tokens, err := lexer.Tokenize(line)
	if err != nil {
		return 0, err
	}

	tree, err := parser.Parse(tokens)
	if err != nil {
		return 0, err
	}

	return evaluator.Eval(tree)
}

// formatResult formats a float64 for display.
// Whole numbers are printed without decimal points.
func formatResult(val float64) string {
	if val == math.Trunc(val) && !math.IsInf(val, 0) && !math.IsNaN(val) {
		return fmt.Sprintf("%g", val)
	}
	return fmt.Sprintf("%g", val)
}