package config

import (
	"flag"
	"fmt"
)

// Extract the program configuration from the command-line arguments.
func FromArgs() (*Config, error) {
	// Relevant flags.
	verbose := flag.Bool("v", false, "Verbosity. If set, the program will print logs.")
	explanation := flag.Bool("x", false, "Explanation. Whether or not to show all reduction steps.")
	statistics := flag.Bool("s", false, "Statistics. If set, the process will print various statistics about the run.")
	profile := flag.String("p", "", "CPU profiling. If an output file is defined, the program will profile its execution and dump its results into it.")
	file := flag.String("f", "", "File. If set, read source from the specified file.")
	output := flag.String("o", "", "Output. If set, write result to the specified file. Use '-' for stdout (default).")
	interpreter := flag.String("i", "lambda", "Interpreter. The reduction engine to use: 'lambda' or 'debruijn'.")
	flag.Parse()

	// Validate interpreter flag.
	var interpType Interpreter
	switch *interpreter {
	case "lambda":
		interpType = LambdaInterpreter
	case "debruijn":
		interpType = DeBruijnInterpreter
	default:
		return nil, fmt.Errorf("invalid interpreter: %s (must be 'lambda' or 'debruijn')", *interpreter)
	}

	// Parse source type.
	var source Source
	if *file != "" {
		// File flag takes precedence.
		if flag.NArg() > 0 {
			return nil, fmt.Errorf("cannot specify both -f flag and positional argument")
		}
		source = FileSource{Path: *file}
	} else if flag.NArg() == 0 {
		return nil, fmt.Errorf("no input given")
	} else if flag.NArg() > 1 {
		return nil, fmt.Errorf("more than 1 command-line argument")
	} else {
		// Positional argument.
		if flag.Arg(0) == "-" {
			source = StdinSource{}
		} else {
			source = StringSource{Data: flag.Arg(0)}
		}
	}

	// Parse destination type.
	var destination Destination
	if *output == "" || *output == "-" {
		destination = StdoutDestination{}
	} else {
		destination = FileDestination{Path: *output}
	}

	return &Config{
		Source:      source,
		Destination: destination,
		Verbose:     *verbose,
		Explanation: *explanation,
		Profile:     *profile,
		Statistics:  *statistics,
		Interpreter: interpType,
	}, nil
}