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src/core/DiffEngine.js

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+export class DiffEngine {
+  build(oldText, newText) {
+    const a = (oldText ?? "").replace(/\r\n/g, "\n").split("\n");
+    const b = (newText ?? "").replace(/\r\n/g, "\n").split("\n");
+    const lcs = this.#lcsTable(a, b);
+    const ops = [];
+
+    let i = 0;
+    let j = 0;
+    while (i < a.length && j < b.length) {
+      if (a[i] === b[j]) {
+        ops.push({ kind: "equal", oldLine: a[i], newLine: b[j], oldIndex: i, newIndex: j });
+        i += 1;
+        j += 1;
+      } else if (lcs[i + 1][j] >= lcs[i][j + 1]) {
+        ops.push({ kind: "remove", oldLine: a[i], oldIndex: i, newIndex: j });
+        i += 1;
+      } else {
+        ops.push({ kind: "add", newLine: b[j], oldIndex: i, newIndex: j });
+        j += 1;
+      }
+    }
+
+    while (i < a.length) {
+      ops.push({ kind: "remove", oldLine: a[i], oldIndex: i, newIndex: j });
+      i += 1;
+    }
+    while (j < b.length) {
+      ops.push({ kind: "add", newLine: b[j], oldIndex: i, newIndex: j });
+      j += 1;
+    }
+
+    return ops.map((op, index) => ({ ...op, id: index }));
+  }
+
+  #lcsTable(a, b) {
+    const rows = a.length + 1;
+    const cols = b.length + 1;
+    const table = Array.from({ length: rows }, () => Array(cols).fill(0));
+    for (let i = rows - 2; i >= 0; i -= 1) {
+      for (let j = cols - 2; j >= 0; j -= 1) {
+        if (a[i] === b[j]) table[i][j] = table[i + 1][j + 1] + 1;
+        else table[i][j] = Math.max(table[i + 1][j], table[i][j + 1]);
+      }
+    }
+    return table;
+  }
+}