/* Totally minimal "typed functions" */

/* helper: dest is really a TF and imps is either nothing, an imp, or
 * an array of implementations. author is the entity responsible for these
 * implementations.
 */
const addImps = (dest, imps, author) => {
   if (imps) {
      if (!Array.isArray(imps[0])) imps = [imps]
   } else {
      imps = []
   }
   if (author) {
      if (dest.authors.has(author)) {
         const [count, index] = dest.authors.get(author)
         if (count) dest.imps.splice(index, count)
      }
      dest.authors.set(author, [imps.length, dest.imps.length])
   }
   for (const imp of imps) dest.imps.push(imp)
}

/* Create a TF, optionally adding some initial imps and further optionally
 * registering them to an author. There are two advantages to author
 * registration: (1) if the same author adds implementations, the prior imps
 * will be deleted, and (2) the author can be invalidated, and the TF will
 * lazily call the author back with the given data the next time it is 
 * called, so that it can re-add the imps.
 */
export default function poortf (name, imps, author, data) {
   /* This is the (function) object we will return */
   function fn () {
      for (const imp of fn.imps) {
         if (imp[0](arguments)) return imp[1].apply(null, arguments)
      }
      throw new TypeError(
         `TF ${fn.name}: No match for ${arguments[0]}, ${arguments[1]}, ...`)
   }

   /* Now dress it up for use */
   Object.defineProperty(fn, 'name', {value: name})
   fn.imps = []
   fn.authors = new Map() // tracks who made each implementation
   fn.authorData = new Map() // tracks author callback data
   addImps(fn, imps, author)
   if (author) fn.authorData.set(author, data)
   fn.addImps = (newI, author, data) => {
      addImps(fn, newI, author)
      if (author) fn.authorData.set(author, data)
   }
   fn.invalidate = author => {
      addImps(fn, null, author) // deletes the author's imps
      fn.unshiftImp([() => true, (...args) => fn.reloadAndRecall(args)])
   }
   fn.unshiftImp = newImp => {
      fn.imps.unshift(newImp)
      for (const record in fn.authors.values()) {
         record[1] += 1
      }
   }
   fn.shiftImp = () => {
      const oldImp = fn.imps.shift()
      for (const record in fn.authors.values()) {
         record[1] -= 1
      }
   }
   fn.reloadAndRecall = (argArray) => {
      fn.shiftImp() // throw ourself away, ha ha
      for (const author of fn.authorData.keys()) {
         if (fn.authors.get(author)[0] === 0) {
            // imps count of 0 means the author was invalidated
            author(fn.authorData.get(author))
         }
      }
      return fn.apply(null, argArray)
   }
   return fn
}

export function isTF(x) {
   return typeof x === 'function' && 'imps' in x && Array.isArray(x.imps)
}