🖇 Infinitely generating linked list with memoisation (or an nth-generator).
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const always = x => _ => x;
const infiniteList = {
/**
* InfiniteList Constructor. Iterates infinitely until index value is found.
*
* @param {any} start
* @param {any} next
* @returns InfiniteLinkedListItem object
*/
create (start, next) {
// Closure magic!
let cache = [];
let j = 0;
// Get list item of index i
function get(i) {
// Validation
if(
// i is a falsy value except 0
(!i && i !== 0)
// is not a number
|| (typeof i !== 'number')
// is... not a number
|| Number.isNaN(i)
) return;
//TODO: Cache limiting. (Removed after unexpected behaviour)
// Initializing first item if it doesn't exist
if(!cache[0]) cache[0] = start;
// Create returnable object
const obj = {
next: () => this.get(i+1),
previous: () => this.get(i - 1),
[Symbol.iterator]: () => ({
next: () => ({
done: false,
value: this.get(i+1)
})
}),
toString: () => {
const val = get(i).value;
return ( 'InfiniteLinkedListItem [ ..., '
+ ((typeof val === 'object' && val !== null)
? JSON.stringify(val, null, 2)
: val.toString())
+ ', ... ]' )
}
}
// If index were to be infinity, value and index are infinity
if(i === Infinity) {
obj.value = Infinity;
obj.index = Infinity;
return obj;
}
// If index exists in cache, return the value
if(i in cache) {
obj.value = cache[i];
obj.index = i;
return obj;
}
// If i doesn't exist in cache
if(!(i in cache)) {
if(cache.length <= i && (cache.length - 1) in cache)
while (cache.length <= i)
cache[cache.length] = next(cache[cache.length - 1]);
}
obj.value = cache[i];
obj.index = i;
return obj;
}
const take = (from, to) => {
const arr = [];
let source, target;
// "from" number of elements
if(!to) { source = 0; target = from; }
// "target" is the end index!
else { source = from; target = to + 1 };
for(let i = source; i < target; i ++) {
arr.push(get(i));
};
return arr;
};
// Clear cache manually.
clearCache = () => (cache = [], undefined);
top = function () { return this.get(0) };
end = function () { return this.get(Infinity) };
const returns = {
get,
take,
top,
first: top,
end,
last: end,
clearCache,
[Symbol.iterator]: () => ({
next: () => ({
done: false,
value: get(j++)
})
}),
toString: () => 'InfiniteLinkedList [ ' + take(0, 10).map(x => (' ' + x.value.toString())) + ' ... ]'
};
return returns;
}
}
module.exports = infiniteList;