feat(packages/stdlib): add trigram distance utill

Author: robonen

packages/stdlib/src/text/trigram-distance/index.ts

@@ -0,0 +1,49 @@
+export type Trigrams = Map<string, number>;
+
+/**
+ * Extracts trigrams from a text and returns a map of trigram to count
+ * 
+ * @param {string} text The text to extract trigrams
+ * @returns {Trigrams} A map of trigram to count
+ */
+export function trigramProfile(text: string): Trigrams {
+    text = '\n\n' + text + '\n\n';
+
+    const trigrams = new Map<string, number>();
+
+    for (let i = 0; i < text.length - 2; i++) {
+        const trigram = text.slice(i, i + 3);
+        const count = trigrams.get(trigram) ?? 0;
+        trigrams.set(trigram, count + 1);
+    }
+
+    return trigrams;
+}
+
+/**
+ * Calculates the trigram distance between two strings
+ * 
+ * @param {Trigrams} left First text trigram profile
+ * @param {Trigrams} right Second text trigram profile
+ * @returns {number} The trigram distance between the two strings
+ */
+export function trigramDistance(left: Trigrams, right: Trigrams) {
+    let distance = -4;
+    let total = -4;
+
+    for (const [trigram, left_count] of left) {
+        total += left_count;
+        const right_count = right.get(trigram) ?? 0;
+        distance += Math.abs(left_count - right_count);
+    }
+
+    for (const [trigram, right_count] of right) {
+        total += right_count;
+        const left_count = left.get(trigram) ?? 0;
+        distance += Math.abs(left_count - right_count);
+    }
+
+    if (distance < 0) return 0;
+
+    return distance / total;
+}