substantial/updeep

/* eslint no-shadow:0, no-param-reassign:0 prefer-rest-params:0 */
export const _ = '@@updeep/placeholder'

function countArguments(args, max) {
  let n = args.length
  if (n > max) n = max

  while (args[n - 1] === _) {
    n -= 1
  }

  return n
}

export function curry1(fn) {
  return function curried(a, ...args) {
    const [b, c] = args
    const n = countArguments(arguments)

    if (n >= 1) return fn(a, b, c)
    return curried
  }
}

export function curry2(fn) {
  return function curried(a, b, ...args) {
    const [c, d] = args
    const n = countArguments(arguments, 2)

    if (b === _ || c === _ || d === _) {
      throw new Error(
        'Can only use placeholder on first argument of this function.'
      )
    }

    if (n >= 2) {
      if (a === _) return curry1((a, c, d) => fn(a, b, c, d))
      return fn(a, b, c, d)
    }

    if (n === 1) return curry1((b, c, d) => fn(a, b, c, d))
    return curried
  }
}

export function curry3(fn) {
  return function curried(a, b, c, ...args) {
    const [d, e] = args
    const n = countArguments(arguments, 3)

    if (c === _ || d === _ || e === _) {
      throw new Error(
        'Can only use placeholder on first or second argument of this function.'
      )
    }

    if (n >= 3) {
      if (a === _) {
        if (b === _) return curry2((a, b, d, e) => fn(a, b, c, d, e))
        return curry1((a, d, e) => fn(a, b, c, d, e))
      }
      if (b === _) return curry1((b, d, e) => fn(a, b, c, d, e))
      return fn(a, b, c, d, e)
    }

    if (n === 2) {
      if (a === _) return curry2((a, c, d, e) => fn(a, b, c, d, e))
      return curry1((c, d, e) => fn(a, b, c, d, e))
    }

    if (n === 1) return curry2((b, c, d, e) => fn(a, b, c, d, e))

    return curried
  }
}

export function curry4(fn) {
  return function curried(a, b, c, d, ...args) {
    const [e, f] = args
    const n = countArguments(arguments, 4)

    if (d === _ || e === _ || f === _) {
      throw new Error(
        'Can only use placeholder on first, second or third argument of this function.'
      )
    }

    if (n >= 4) {
      if (a === _) {
        if (b === _) {
          if (c === _) return curry3((a, b, c, e, f) => fn(a, b, c, d, e, f))
          return curry2((a, b, e, f) => fn(a, b, c, d, e, f))
        }
        if (c === _) return curry2((a, c, e, f) => fn(a, b, c, d, e, f))
        return curry1((a, e, f) => fn(a, b, c, d, e, f))
      }
      if (b === _) {
        if (c === _) return curry2((b, c, e, f) => fn(a, b, c, d, e, f))
        return curry1((b, e, f) => fn(a, b, c, d, e, f))
      }
      if (c === _) return curry1((c, e, f) => fn(a, b, c, d, e, f))
      return fn(a, b, c, d, e, f)
    }

    if (n === 3) {
      if (a === _) {
        if (b === _) return curry3((a, b, d, e, f) => fn(a, b, c, d, e, f))
        return curry2((a, d, e, f) => fn(a, b, c, d, e, f))
      }
      if (b === _) return curry2((b, d, e, f) => fn(a, b, c, d, e, f))
      return curry1((d, e, f) => fn(a, b, c, d, e, f))
    }

    if (n === 2) {
      if (a === _) return curry3((a, c, d, e, f) => fn(a, b, c, d, e, f))
      return curry2((c, d, e, f) => fn(a, b, c, d, e, f))
    }

    if (n === 1) return curry3((b, c, d, e, f) => fn(a, b, c, d, e, f))
    return curried
  }
}

export default function curry(fn, length = fn.length) {
  return [fn, curry1, curry2, curry3, curry4][length](fn)
}