// module flag for command-line flag parsing
//
// - parsing flags like '--flag' or '--stuff=things' or '--things stuff'
// - handles bool, int, float and string args
// - is able to pring usage
// - handled unknown arguments as error 
// 
// Usage example:
//
//  ```v
//  module main
//  
//  import os
//  import flag
//  
//  fn main() {
//  	mut fp := flag.new_flag_parser(os.args)
//  	fp.application('flag_example_tool')
//  	fp.version('v0.0.0')
//  	fp.description('This tool is only designed to show how the flag lib is working')
//  
//  	fp.skip_executable()
//  
//  	an_int := fp.int('an_int', 666, 'some int to define 666 is default')
//  	a_bool := fp.bool('a_bool', false, 'some \'real\' flag')
//  	a_float := fp.float('a_float', 1.0, 'also floats')
//  	a_string := fp.string('a_string', 'no text', 'finally, some text')
//  
//  	additional_args := fp.finalize() or {
//  		eprintln(err)
//  		println(fp.usage())
//  		return
//  	}
//  
//  	println('
//  		  an_int: $an_int
//  		  a_bool: $a_bool
//  		 a_float: $a_float
//  		a_string: \'$a_string\'
//  	')
//  	println(additional_args.join_lines())
//  }
//  ```

module flag

// data object storing informations about a defined flag
struct Flag {
pub:
  name     string // name as it appears on command line
  usage    string // help message
  val_desc string // something like '<arg>' that appears in usage
}

// 
struct FlagParser {
pub mut: 
  args  []string                  // the arguments to be parsed
  flags []Flag                    // registered flags

  application_name        string
  application_version     string
  application_description string

  min_free_args int
  max_free_args int
}

// create a new flag set for parsing command line arguments
// TODO use INT_MAX some how
pub fn new_flag_parser(args []string) &FlagParser {
  return &FlagParser{args:args, max_free_args: 4048}
}

// change the application name to be used in 'usage' output
pub fn (fs mut FlagParser) application(n string) {
  fs.application_name = n
}

// change the application version to be used in 'usage' output
pub fn (fs mut FlagParser) version(n string) {
  fs.application_version = n
}

// change the application version to be used in 'usage' output
pub fn (fs mut FlagParser) description(n string) {
  fs.application_description = n
}

// in most cases you do not need the first argv for flag parsing
pub fn (fs mut FlagParser) skip_executable() {
  fs.args.delete(0)
}

// private helper to register a flag
fn (fs mut FlagParser) add_flag(n, u, vd string) {
  fs.flags << Flag{
    name: n,
    usage: u
    val_desc: vd
  }
}

// private: general parsing a single argument 
//  - search args for existance
//    if true
//      extract the defined value as string
//    else 
//      return an (dummy) error -> argument is not defined
//
//  - the name, usage are registered
//  - found arguments and corresponding values are removed from args list
fn (fs mut FlagParser) parse_value(n string) ?string {
  c := '--$n'
  for i, a in fs.args {
    if a == c {
      if fs.args.len > i+1 && fs.args[i+1].left(2) != '--' {
        val := fs.args[i+1]
        fs.args.delete(i+1)
        fs.args.delete(i)
        return val
      } else {
        panic('Missing argument for \'$n\'')
      }
    } else if a.len > c.len && c == a.left(c.len) && a.substr(c.len, c.len+1) == '=' {
      val := a.right(c.len+1)
      fs.args.delete(i)
      return val
    }
  }
  return error('parameter \'$n\' not found')
}

// special parsing for bool values 
// see also: parse_value
// 
// special: it is allowed to define bool flags without value
// -> '--flag' is parsed as true
// -> '--flag' is equal to '--flag=true'
fn (fs mut FlagParser) parse_bool_value(n string) ?string {
  c := '--$n'
  for i, a in fs.args {
    if a == c {
      if fs.args.len > i+1 && (fs.args[i+1] in ['true', 'false'])  {
        val := fs.args[i+1]
        fs.args.delete(i+1)
        fs.args.delete(i)
        return val
      } else {
        val := 'true'
        fs.args.delete(i)
        return val
      }
    } else if a.len > c.len && c == a.left(c.len) && a.substr(c.len, c.len+1) == '=' {
      val := a.right(c.len+1)
      fs.args.delete(i)
      return val
    }
  }
  return error('parameter \'$n\' not found')
}

// defining and parsing a bool flag 
//  if defined 
//      the value is returned (true/false)
//  else 
//      the default value is returned
//TODO error handling for invalid string to bool conversion
pub fn (fs mut FlagParser) bool(n string, v bool, u string) bool {
  fs.add_flag(n, u, '')
  parsed := fs.parse_bool_value(n) or {
    return v
  }
  return parsed == 'true'
}

// defining and parsing an int flag 
//  if defined 
//      the value is returned (int)
//  else 
//      the default value is returned
//TODO error handling for invalid string to int conversion
pub fn (fs mut FlagParser) int(n string, i int, u string) int {
  fs.add_flag(n, u, '<int>')
  parsed := fs.parse_value(n) or {
    return i
  }
  return parsed.int()
}

// defining and parsing a flaot flag 
//  if defined 
//      the value is returned (float)
//  else 
//      the default value is returned
//TODO error handling for invalid string to float conversion
pub fn (fs mut FlagParser) float(n string, f f32, u string) f32 {
  fs.add_flag(n, u, '<float>')
  parsed := fs.parse_value(n) or {
    return f
  }
  return parsed.f32()
}

// defining and parsing a string flag 
//  if defined 
//      the value is returned (string)
//  else 
//      the default value is returned
pub fn (fs mut FlagParser) string(n, v, u string) string {
  fs.add_flag(n, u, '<arg>')
  parsed := fs.parse_value(n) or {
    return v
  }
  return parsed
}

// add an additional check for free arguments 
// this will cause an error in finalize() if free args are out of range
// (min, ..., max)
pub fn (fs mut FlagParser) limit_free_args(min, max int) {
  if min > max {
    panic('flag.limit_free_args expect min < max, got $min >= $max')
  }
  fs.min_free_args = min
  fs.max_free_args = max
}

const (
  // used for formating usage message
  SPACE = '                            '
)

// collect all given information and 
pub fn (fs FlagParser) usage() string {
  mut use := '\n'
  use += 'usage ${fs.application_name} [options] [ARGS]\n'
  use += '\n'

  if fs.flags.len > 0 {
    use += 'options:\n'
    for f in fs.flags {
      flag_desc := '  --$f.name $f.val_desc'
      space := if flag_desc.len > SPACE.len-2 {
        '\n$SPACE'
      } else {
        SPACE.right(flag_desc.len)
      }
      use += '$flag_desc$space$f.usage\n'
    }
  }

  use += '\n'
  use += '$fs.application_name $fs.application_version\n'
  if fs.application_description != '' {
    use += '\n'
    use += 'description:\n'
    use += '$fs.application_description'
  }
  return use
}

// finalize argument parsing -> call after all arguments are defined
//
// all remaining arguments are returned in the same order they are defined on
// commandline
//
// if additional flag are found (things starting with '--') an error is returned 
// error handling is up to the application developer
pub fn (fs FlagParser) finalize() ?[]string {
  for a in fs.args {
    if a.left(2) == '--' {
      return error('Unknown argument \'${a.right(2)}\'')
    }
  }
  if fs.args.len < fs.min_free_args {
    return error('Expect at least ${fs.min_free_args} arguments')
  }
  if fs.args.len >= fs.max_free_args {
    if fs.max_free_args > 0 {
      return error('Expect at most ${fs.max_free_args} arguments')
    } else {
      return error('Expect no arguments')
    }
  }
  return fs.args
}