simple-statistics/src/unique_count_sorted.js

Simple Statistics

/**
 * For a sorted input, counting the number of unique values
 * is possible in constant time and constant memory. This is
 * a simple implementation of the algorithm.
 *
 * Values are compared with `===`, so objects and non-primitive objects
 * are not handled in any special way.
 *
 * @param {Array<*>} x an array of any kind of value
 * @returns {number} count of unique values
 * @example
 * uniqueCountSorted([1, 2, 3]); // => 3
 * uniqueCountSorted([1, 1, 1]); // => 1
 */
function uniqueCountSorted(x) {
    let uniqueValueCount = 0;
    let lastSeenValue;
    for (let i = 0; i < x.length; i++) {
        if (i === 0 || x[i] !== lastSeenValue) {
            lastSeenValue = x[i];
            uniqueValueCount++;
        }
    }
    return uniqueValueCount;
}

export default uniqueCountSorted;