/* The Sørensen–Dice coefficient is a statistic used to gauge the similarity of two samples.

* Applied to strings, it can give you a value between 0 and 1 (included) which tells you how similar they are.

* Dice coefficient is calculated by comparing the bigrams of both strings,

* a bigram is a substring of the string of length 2.

* read more: https://en.wikipedia.org/wiki/S%C3%B8rensen%E2%80%93Dice_coefficient

*/

// Time complexity: O(m + n), m and n being the sizes of string A and string B

// Find the bistrings of a string and return a hashmap (key => bistring, value => count)

function mapBigrams(string) {

const bigrams = new Map()

for (let i = 0; i < string.length - 1; i++) {

const bigram = string.substring(i, i + 2)

const count = bigrams.get(bigram)

bigrams.set(bigram, (count || 0) + 1)

}

return bigrams

}

// Calculate the number of common bigrams between a map of bigrams and a string

function countCommonBigrams(bigrams, string) {

let count = 0

for (let i = 0; i < string.length - 1; i++) {

const bigram = string.substring(i, i + 2)

if (bigrams.has(bigram)) count++

}

return count

}

// Calculate Dice coeff of 2 strings

function diceCoefficient(stringA, stringB) {

if (stringA === stringB) return 1

else if (stringA.length < 2 || stringB.length < 2) return 0

const bigramsA = mapBigrams(stringA)

const lengthA = stringA.length - 1

const lengthB = stringB.length - 1

let dice = (2 * countCommonBigrams(bigramsA, stringB)) / (lengthA + lengthB)

// cut 0.xxxxxx to 0.xx for simplicity

dice = Math.floor(dice * 100) / 100

return dice

}

export { diceCoefficient }