import { emptySet, add, length, forEach, has, remove } from "../structures/set.js";
import { getDepth, Slot } from "./value_slot.js";
import { compareSlot } from "./compare.js";
import { makeTypeParser } from "../parser/type_parser.js";
import { newDynamic } from "../primitives/dynamic.js";

const emptySetOfSlots = emptySet(compareSlot);

export const findLCA = slotA => slotB => {
  if (compareSlot(slotA)(slotB) === 0) {
    return slotA;
  }
  if (getDepth(slotA) > getDepth(slotB)) {
    // we can assume that slotA.variant === "write"
    return findLCA(slotA.value.slot)(slotB);
  }
  else {
    // we can assume that slotB.variant === "write"
    return findLCA(slotA)(slotB.value.slot);
  }
};

// returns
//  {slotA} if slotA is younger
//  {slotB} if slotB is younger
//  {slotA, slotB} if they are concurrent (conflicting)
export const merge = slotA => slotB => {
  const lca = findLCA(slotA)(slotB);
  if (compareSlot(lca)(slotA) === 0) {
    return add(emptySetOfSlots)(slotB);
  }
  if (compareSlot(lca)(slotB) === 0) {
    return add(emptySetOfSlots)(slotA);
  }
  const setWithA = add(emptySetOfSlots)(slotA);
  const setWithAandB = add(setWithA)(slotB);
  return setWithAandB;
};

export const mergeN = slots => {
  let result = slots;
  forEach(slots)(slotA => {
    forEach(slots)(slotB => {
      // compare all non-identical pairs
      if (compareSlot(slotA)(slotB) !== 0) {
        const m = merge(slotA)(slotB);
        if (length(m) === 1) {
          // if in the pair, one is an ancestor of the other,
          // only keep the other
          if (has(m)(slotA)) {
            result = remove(result)(slotB);
          }
        }
      }
    });
  });
  return result;
};

const mkType = makeTypeParser({
  extraPrimitives: [
    ["Slot" , Slot],
  ],
});

export const ModuleMerge = [
  ["merge" , newDynamic(merge)(mkType("Slot -> Slot -> {Slot}"))],
  ["mergeN", newDynamic(merge)(mkType("{Slot} -> {Slot}"      ))],
];