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lib/diff.js

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+/**
+ * @module diff
+ */
+
+/**
+ * A SimpleDiff describes a change on a String.
+ *
+ * @example
+ * console.log(a) // the old value
+ * console.log(b) // the updated value
+ * // Apply changes of diff (pseudocode)
+ * a.remove(diff.pos, diff.remove) // Remove `diff.remove` characters
+ * a.insert(diff.pos, diff.insert) // Insert `diff.insert`
+ * a === b // values match
+ *
+ * @typedef {Object} SimpleDiff
+ * @property {Number} pos The index where changes were applied
+ * @property {Number} remove The number of characters to delete starting
+ *                                  at `index`.
+ * @property {String} insert The new text to insert at `index` after applying
+ *                           `delete`
+ */
+
+/**
+ * Create a diff between two strings. This diff implementation is highly
+ * efficient, but not very sophisticated.
+ *
+ * @public
+ * @param {String} a The old version of the string
+ * @param {String} b The updated version of the string
+ * @return {SimpleDiff} The diff description.
+ */
+export const simpleDiff = (a, b) => {
+  let left = 0 // number of same characters counting from left
+  let right = 0 // number of same characters counting from right
+  while (left < a.length && left < b.length && a[left] === b[left]) {
+    left++
+  }
+  if (left !== a.length || left !== b.length) {
+    // Only check right if a !== b
+    while (right + left < a.length && right + left < b.length && a[a.length - right - 1] === b[b.length - right - 1]) {
+      right++
+    }
+  }
+  return {
+    pos: left, // TODO: rename to index (also in type above)
+    remove: a.length - left - right,
+    insert: b.slice(left, b.length - right)
+  }
+}