Dice

String/DiceCoefficient.js

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+/* 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 stings,
+ * 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
+
+  console.log('Dice coefficient of', stringA, 'and', stringB, 'is', dice)
+
+  return dice
+}
+export { diceCoefficient }

String/test/DiceCoefficient.test.js

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+import { diceCoefficient } from '../DiceCoefficient'
+
+describe('diceCoefficient', () => {
+  it('should calculate edit distance between two strings', () => {
+    // equal strings return 1 (max possible value)
+    expect(diceCoefficient('abc', 'abc')).toBe(1)
+    expect(diceCoefficient('', '')).toBe(1)
+
+    // string length needs to be atleast 2 (unless equal)
+    expect(diceCoefficient('a', '')).toBe(0)
+    expect(diceCoefficient('', 'a')).toBe(0)
+
+    expect(diceCoefficient('skate', 'ate')).toBe(0.66)
+
+    expect(diceCoefficient('money', 'honey')).toBe(0.75)
+
+    expect(diceCoefficient('love', 'hate')).toBe(0)
+
+    expect(diceCoefficient('skilled', 'killed')).toBe(0.9)
+  })
+})