const trimStart = (s, c) => {
  let idx = 0
  while (idx < s.length && s[idx] === c)
    idx++
  return idx > 0 ? s.substring(idx) : s
}

const optkey = (opt) => trimStart(opt, '-')

const parse = (default_options, option_arities) => (args) => {
  const positional = []
  const parsed_options = {}

  for (let i = 0; i < args.length;) {
    const arg = args[i]
    i++

    const arity = option_arities[arg]
    if (typeof(arity) === 'number') {
      if (arity+i > args.length)
        return [`${arg}: not enough arguments.`, parsed_options, positional]

      const opt = optkey(arg)
      switch (arity) {
        case 0: parsed_options[opt] = true; break
        case 1: parsed_options[opt] = args[i]; break
        default: parsed_options[opt] = args.slice(i, i+arity); break
      }
      i += arity
    } else {
      positional.push(arg)
    }
  }

  const options = Object.assign({}, default_options, parsed_options)
  return [null, options, positional]
}

const help = ({ grammar, usage }) => () => {
  if (usage) console.log(usage)
  grammar.forEach(
    ([optargs, optdesc, def]) => {
      const deftext = def ? ` (default: ${def})` : ""
      console.log(`\t${optargs.join(' ')}\t${optdesc}${deftext}`)
    }
  )
}

const CLI = ({ grammar, usage }) => {
  const details = grammar.map(([[opt, ...optargs], optdesc, def]) => [opt, optargs.length, def])
  const option_arities = Object.fromEntries(details.map(([opt, arity, _def]) => [opt, arity]))
  const default_options = Object.fromEntries(details.map(([opt, _arity, def]) => [optkey(opt), def]))
  return {
    parse: parse(default_options, option_arities),
    help: help({ grammar, usage }),
  }
}

module.exports = CLI