diff-patch-engine

1	# Implement the line diff (minimal canonical edit script)
2	
3	## Context
4	`diff-patch-engine` is the line-level diff/patch building block behind our
5	`git diff`-style tooling and three-way merges (TypeScript, Node 20). It computes
6	an ordered edit script between two sequences of lines and can replay that script
7	onto the original to reproduce the target.
8	
9	`applyPatch(original, edits)` (the diff's own verifier), `diffStats`, and the
10	`splitLines` / `joinLines` text helpers are all provided and must not be
11	changed. **Only `diff(a, b)` is unimplemented.**
12	
13	## Your task
14	Implement `src/diff/myers.ts` → `diff(a: string[], b: string[]): Edit[]`,
15	returning an ordered list of `eq` / `del` / `ins` edits (see `Edit` / `EditOp`
16	in `types.ts`). The returned script is graded against three independent
17	properties , **validity**, **minimality**, and a **canonical tie-break** , and
18	must satisfy all three for every input. Being valid and minimal is *not*
19	sufficient: a uniquely determined script is required and any other minimal
20	script is graded wrong.
21	
22	Each op consumes lines as follows: an `eq` consumes one line from each side and
23	those two lines are equal; a `del` consumes one line from `a`; an `ins` consumes
24	one line from `b`.
25	
26	### 1. Validity
27	`applyPatch(a, diff(a, b))` must deep-equal `b`. Equivalently, both of these
28	hold simultaneously:
29	- the subsequence of `eq` + `del` lines, in order, equals `a`;
30	- the subsequence of `eq` + `ins` lines, in order, equals `b`;
31	
32	and every `eq` carries a line equal to the line it consumes on each side.
33	
34	### 2. Minimality
35	Among all valid scripts, the returned one must use the fewest non-`eq` ops. Let
36	`C(a, b)` be the maximum possible number of `eq` ops over all valid scripts for
37	the pair (equivalently, the length of the longest common subsequence of `a` and
38	`b`). Then the script must have exactly `C(a, b)` `eq` ops and exactly
39	`a.length + b.length - 2 * C(a, b)` non-`eq` ops (`del` + `ins`). This is what
40	makes the result a *diff* rather than "delete all of `a`, then insert all of
41	`b`".
42	
43	### 3. Canonical tie-break (the crux)
44	Many distinct valid, minimal scripts can exist for one pair. Exactly one of them
45	is canonical, and that is the one you must return. The canonical script is the
46	unique valid, minimal script characterised by the following property, applied to
47	the script read left to right.
48	
49	> Consider the script as a left-to-right interleaving of `a` and `b`. Walk a
50	> pair of cursors `(i, j)` , `i` into `a`, `j` into `b` , both starting at the
51	> first line. At each step the next op of the script decides which cursor(s)
52	> advance: `del` advances `i`, `ins` advances `j`, `eq` advances both (and
53	> requires `a[i] === b[j]`).
54	>
55	> Call an op **length-preserving at `(i, j)`** if the script can still be
56	> completed, from the resulting cursor position, into a valid script that meets
57	> the minimality bound of clause 2 for the whole pair. The canonical script is
58	> the one that, at every step, makes the **highest-priority length-preserving
59	> choice** under this fixed priority order:
60	>
61	> 1. `del` of `a[i]` , chosen whenever it is length-preserving at `(i, j)`;
62	> 2. otherwise `ins` of `b[j]` , chosen whenever it is length-preserving at
63	>    `(i, j)`;
64	> 3. otherwise `eq` of the shared line (this case forces `a[i] === b[j]`).
65	>
66	> When `i` has reached the end of `a`, only `ins` remains; when `j` has reached
67	> the end of `b`, only `del` remains; when both are at the end, the script ends.
68	
69	The grader pins this exact script; any other valid, minimal, but
70	differently-resolved script is rejected.
71	
72	## Edge cases to get right
73	- `a` and `b` equal → all `eq`, no `del`/`ins`.
74	- `a` empty → every line of `b` as `ins`, in order.
75	- `b` empty → every line of `a` as `del`, in order.
76	- both empty (`[]`) → `[]`.
77	- Lines are compared by exact string equality only , no Unicode normalization,
78	  no trimming. Lines containing whitespace, tabs, emoji, combining characters,
79	  newline characters, or text that looks like an op name are opaque values.
80	- Callers diffing text use `splitLines`, so a trailing newline appears as a
81	  final `""` entry and must be preserved verbatim. The empty string is `[""]`
82	  (length 1), never `[]`.
83	- The same `(a, b)` must always produce the identical script (determinism).
84	
85	## Performance & memory
86	Inputs can be large , up to ~100,000 lines , and are frequently *largely
87	identical*: two revisions of the same file, sharing long common runs at the
88	start, the end, or both. Your implementation must use memory **proportional to
89	the input size** on such inputs. Allocating a full `n × m` dynamic-programming
90	table is `Θ(n·m)` memory and will **exhaust the available memory on large
91	inputs , that is graded as a failure**. The constraint only bites at scale:
92	small inputs may be handled however you like. Time should stay within a few
93	seconds even for the largest inputs.
94	
95	## Definition of done
96	`npm run typecheck` is clean and `npm test` passes. Implement `diff` only , do
97	not modify `applyPatch`, the text helpers, the types/errors, or the tests.
98

1	/**
2	 * Types for the line-oriented diff + patch engine.
3	 *
4	 * A diff is an **edit script**: an ordered list of operations that, replayed
5	 * against the original sequence of lines, reproduces the target sequence.
6	 *
7	 *   - `eq`  , a line present in BOTH sides; consumes one line from the original
8	 *     and one from the target (they are equal).
9	 *   - `del` , a line present only in the original; consumes one line from the
10	 *     original (it is removed).
11	 *   - `ins` , a line present only in the target; consumes one line from the
12	 *     target (it is added).
13	 *
14	 * The engine works on `string[]` (one entry per line); {@link splitLines} /
15	 * {@link joinLines} convert to and from a single text blob.
16	 */
17	
18	export type EditOp = "eq" | "del" | "ins";
19	
20	/** A single edit-script entry: an operation paired with the line it concerns. */
21	export interface Edit {
22	  op: EditOp;
23	  /** For `eq`/`del` this is the original line; for `ins` it is the new line. */
24	  line: string;
25	}
26	
27	/** Aggregate counts over an edit script (handy for tests and stats). */
28	export interface DiffStats {
29	  eq: number;
30	  del: number;
31	  ins: number;
32	}
33

/home/user/app/src:
diff
index.ts

/home/user/app/src/diff:
errors.ts
myers.ts
patch.ts
text.ts
types.ts
---
{
  "name": "diff-patch-engine",
  "version": "0.2.0",
  "private": true,
  "description": "Line-oriented diff (minimal LCS edit script) and patch-apply engine.",
  "type": "module",
  "engines": {
    "node": ">=20"
  },
  "scripts": {
    "build": "tsc -p tsconfig.json",
    "typecheck": "tsc -p tsconfig.json --noEmit",
    "test": "vitest run",
    "test:watch": "vitest"
  },
  "devDependencies": {
    "@types/node": "20.14.10",
    "typescript": "5.5.3",
    "vitest": "1.6.0"
  }
}

1	import type { Edit } from "./types.js";
2	
3	/**
4	 * Compute the canonical minimal line-level edit script transforming `a` into `b`.
5	 *
6	 *  Return an ordered `Edit[]` that, replayed against `a` by `applyPatch`,
7	 *  reproduces `b`. An `eq` consumes one line from each side (the lines are
8	 *  equal); a `del` consumes one line from `a`; an `ins` consumes one line from
9	 *  `b`. (See `EditOp` in types.ts.) The script must satisfy ALL THREE of the
10	 *  following; each is checked independently and validity + minimality is NOT
11	 *  sufficient.
12	 *
13	 *  1. VALID , `applyPatch(a, diff(a, b))` deep-equals `b`. Equivalently the
14	 *     `eq`+`del` lines (in order) equal `a`, the `eq`+`ins` lines equal `b`, and
15	 *     every `eq` carries a line equal to the one it consumes on each side.
16	 *  2. MINIMAL , fewest non-`eq` ops of any valid script. With `C(a,b)` = the
17	 *     maximum achievable number of `eq` ops (the LCS length), the script has
18	 *     exactly `C(a,b)` `eq` ops and `a.length + b.length - 2*C(a,b)` non-`eq` ops.
19	 *  3. CANONICAL , among the (often many) valid minimal scripts, return THE one
20	 *     defined below; the same `(a,b)` always yields it.
21	 *
22	 *  The canonical script (the crux): read it left to right as an interleaving of
23	 *  `a` and `b`, walking cursors `(i, j)` from the start , `del` advances `i`,
24	 *  `ins` advances `j`, `eq` advances both (and requires `a[i] === b[j]`). Call an
25	 *  op *length-preserving at `(i, j)`* if, after taking it, the script can still be
26	 *  completed from the new position into a valid script meeting the clause-2
27	 *  minimality bound. At every step take the highest-priority length-preserving
28	 *  choice under this fixed priority:
29	 *      (1) `del` of `a[i]`     if length-preserving at `(i, j)`;
30	 *      (2) else `ins` of `b[j]` if length-preserving at `(i, j)`;
31	 *      (3) else `eq` of the shared line (this case forces `a[i] === b[j]`).
32	 *  When `i` is at the end of `a` only `ins` remains; when `j` is at the end of
33	 *  `b` only `del` remains; when both are at the end the script ends.
34	 *
35	 *  Edge cases: equal inputs -> all `eq`; empty `a` -> all `ins`; empty `b` -> all
36	 *  `del`; both empty -> `[]`. Lines compare by exact string equality only (no
37	 *  normalization/trimming); whitespace, tabs, emoji, combining chars and embedded
38	 *  newlines are opaque. A trailing newline shows up via `splitLines` as a final
39	 *  `""` and must be preserved; the empty string is `[""]` (length 1), never `[]`.
40	 *
41	 *  Performance & memory: inputs may be large (up to ~1e5 lines) and are often
42	 *  largely identical (long shared runs at the start and/or end). Use memory
43	 *  proportional to the input size , allocating a full `n*m` table (Θ(n·m) memory)
44	 *  exhausts memory on large inputs and is graded as a failure. Small inputs may
45	 *  be handled any way; the constraint only bites at scale.
46	 */
47	export function diff(a: string[], b: string[]): Edit[] {
48	  // TODO(diff): implement the contract documented above. This stub throws so the
49	  // engine is wired but non-functional; the suite must fail until implemented.
50	  void a;
51	  void b;
52	  throw new Error("diff not implemented");
53	}
54

1	import { PatchError } from "./errors.js";
2	import type { DiffStats, Edit } from "./types.js";
3	
4	/**
5	 * Apply an edit script to `original`, returning the patched line sequence.
6	 *
7	 * Replays the script left to right against a cursor into `original`:
8	 *
9	 *   - `eq`  , the next original line must EQUAL `edit.line`; emit it and advance
10	 *     the cursor. A mismatch (or running off the end) is a
11	 *     `PatchError("context_mismatch" | "unexpected_end")`: the patch does not
12	 *     apply to this input.
13	 *   - `del` , the next original line must equal `edit.line`; advance the cursor
14	 *     WITHOUT emitting (the line is removed). Same error rules as `eq`.
15	 *   - `ins` , emit `edit.line`; the cursor does NOT advance (nothing consumed
16	 *     from the original).
17	 *
18	 * After the script, every original line must have been consumed; leftover lines
19	 * are a `PatchError("trailing_lines")`. An unrecognised op is
20	 * `PatchError("unknown_op")`.
21	 *
22	 * This is the inverse of {@link diff}: for any `a`, `b`,
23	 * `applyPatch(a, diff(a, b))` must deep-equal `b`. Provided complete , the diff
24	 * you implement is validated against this applier.
25	 */
26	export function applyPatch(original: string[], edits: Edit[]): string[] {
27	  const out: string[] = [];
28	  let i = 0; // cursor into `original`
29	
30	  for (const edit of edits) {
31	    switch (edit.op) {
32	      case "eq": {
33	        if (i >= original.length) {
34	          throw new PatchError("unexpected_end", `eq past end of original at op consuming "${edit.line}"`);
35	        }
36	        if (original[i] !== edit.line) {
37	          throw new PatchError(
38	            "context_mismatch",
39	            `eq mismatch at original line ${i}: expected "${edit.line}", found "${original[i]}"`,
40	          );
41	        }
42	        out.push(original[i] as string);
43	        i++;
44	        break;
45	      }
46	      case "del": {
47	        if (i >= original.length) {
48	          throw new PatchError("unexpected_end", `del past end of original at op removing "${edit.line}"`);
49	        }
50	        if (original[i] !== edit.line) {
51	          throw new PatchError(
52	            "context_mismatch",
53	            `del mismatch at original line ${i}: expected "${edit.line}", found "${original[i]}"`,
54	          );
55	        }
56	        i++;
57	        break;
58	      }
59	      case "ins": {
60	        out.push(edit.line);
61	        break;
62	      }
63	      default: {
64	        throw new PatchError("unknown_op", `unknown edit op: ${String((edit as Edit).op)}`);
65	      }
66	    }
67	  }
68	
69	  if (i !== original.length) {
70	    throw new PatchError(
71	      "trailing_lines",
72	      `patch consumed ${i} of ${original.length} original lines; ${original.length - i} left over`,
73	    );
74	  }
75	  return out;
76	}
77	
78	/** Tally an edit script by op kind. Provided complete. */
79	export function diffStats(edits: Edit[]): DiffStats {
80	  const stats: DiffStats = { eq: 0, del: 0, ins: 0 };
81	  for (const e of edits) stats[e.op]++;
82	  return stats;
83	}
84

1	/**
2	 * Text <-> lines conversion for the diff engine.
3	 *
4	 * `splitLines` splits on "\n" and keeps every line as a separate entry. A
5	 * trailing newline yields a final empty entry preserved by `joinLines`, so the
6	 * round-trip `joinLines(splitLines(t)) === t` holds for any input (including the
7	 * empty string, which becomes `[""]`).
8	 *
9	 * Provided complete.
10	 */
11	export function splitLines(text: string): string[] {
12	  return text.split("\n");
13	}
14	
15	export function joinLines(lines: string[]): string {
16	  return lines.join("\n");
17	}
18

1	/**
2	 * Typed errors for the diff/patch engine. A single class with a stable `code`
3	 * discriminator keeps call sites and tests decoupled from message wording.
4	 */
5	export type PatchErrorCode =
6	  | "context_mismatch"
7	  | "unexpected_end"
8	  | "trailing_lines"
9	  | "unknown_op";
10	
11	export class PatchError extends Error {
12	  readonly code: PatchErrorCode;
13	
14	  constructor(code: PatchErrorCode, message: string) {
15	    super(message);
16	    this.name = "PatchError";
17	    this.code = code;
18	    Object.setPrototypeOf(this, PatchError.prototype);
19	  }
20	}
21

1	/**
2	 * Public surface of the line-oriented diff + patch engine.
3	 */
4	export { diff } from "./diff/myers.js";
5	export { applyPatch, diffStats } from "./diff/patch.js";
6	export { splitLines, joinLines } from "./diff/text.js";
7	export { PatchError } from "./diff/errors.js";
8	export type { PatchErrorCode } from "./diff/errors.js";
9	export type { Edit, EditOp, DiffStats } from "./diff/types.js";
10

total 89
drwxr-xr-x  5 user user  4096 Jun 21 08:25 .
drwx------  6 user user  4096 Jun 21 08:25 ..
-rw-r--r--  1 user user    37 Jun 17 20:27 .dockerignore
-rw-r--r--  1 user user    24 Jun 17 20:27 .gitignore
-rw-r--r--  1 user user  1763 Jun 17 20:27 README.md
drwxr-xr-x 73 user user  4096 Jun 21 08:25 node_modules
-rw-r--r--  1 user user 60532 Jun 17 20:27 package-lock.json
-rw-r--r--  1 user user   491 Jun 17 20:27 package.json
drwxr-xr-x  3 user user    60 Jun 19 22:50 src
drwxr-xr-x  2 user user    60 Jun 21 08:25 test
-rw-r--r--  1 user user   629 Jun 17 20:27 tsconfig.json
-rw-r--r--  1 user user   235 Jun 17 20:27 vitest.config.ts
./test/patch.test.ts
---tsconfig---
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "Bundler",
    "lib": ["ES2022"],
    "types": ["node"],
    "rootDir": ".",
    "outDir": "dist",
    "strict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "exactOptionalPropertyTypes": false,
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "resolveJsonModule": true,
    "skipLibCheck": true,
    "verbatimModuleSyntax": false
  },
  "include": ["src", "test"],
  "exclude": ["node_modules", "dist"]
}
---vitest---
import { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    environment: "node",
    include: ["test/**/*.test.ts"],
    reporters: ["default"],
    testTimeout: 10_000,
    hookTimeout: 10_000,
  },
});

1	import { describe, expect, it } from "vitest";
2	import { applyPatch } from "../src/diff/patch.js";
3	import { PatchError } from "../src/diff/errors.js";
4	import type { Edit } from "../src/diff/types.js";
5	
6	const E = (op: Edit["op"], line: string): Edit => ({ op, line });
7	
8	// applyPatch is provided/complete; one round-trip + one error-path sanity is
9	// enough here. The diff suite exercises applyPatch heavily on every assertion.
10	describe("applyPatch (provided)", () => {
11	  it("replays eq/del/ins to reconstruct the target", () => {
12	    const original = ["a", "b", "c"];
13	    const edits: Edit[] = [E("eq", "a"), E("del", "b"), E("ins", "x"), E("eq", "c")];
14	    expect(applyPatch(original, edits)).toEqual(["a", "x", "c"]);
15	  });
16	
17	  it("throws trailing_lines when the script leaves original lines unconsumed", () => {
18	    try {
19	      applyPatch(["a", "b"], [E("eq", "a")]);
20	      throw new Error("expected throw");
21	    } catch (err) {
22	      expect((err as PatchError).code).toBe("trailing_lines");
23	    }
24	  });
25	});
26

The file /home/user/app/src/diff/myers.ts has been updated successfully. (file state is current in your context , no need to Read it back)

> diff-patch-engine@0.2.0 typecheck
> tsc -p tsconfig.json --noEmit

===TEST===

> diff-patch-engine@0.2.0 test
> vitest run


 RUN  v1.6.0 /home/user/app

 ✓ test/patch.test.ts  (2 tests) 3ms

 Test Files  1 passed (1)
      Tests  2 passed (2)
   Start at  08:39:15
   Duration  328ms (transform 68ms, setup 0ms, collect 59ms, tests 3ms, environment 0ms, prepare 109ms)

vitest
v20.9.0

File created successfully at: /home/user/app/test/_verify_tmp.test.ts (file state is current in your context , no need to Read it back)

RUN  v1.6.0 /home/user/app

stdout | test/_verify_tmp.test.ts > diff fuzz vs reference > matches reference on many random inputs
cases=60000 mismatches=0

 ✓ test/_verify_tmp.test.ts  (1 test) 1248ms

 Test Files  1 passed (1)
      Tests  1 passed (1)
   Start at  08:39:58
   Duration  1.61s (transform 120ms, setup 1ms, collect 135ms, tests 1.25s, environment 0ms, prepare 95ms)

const EXACT_DP_MAX_CELLS = 0;
+ Received

- 0
+ 12131

 ❯ test/_verify_tmp.test.ts:97:24
     95|     }
     96|     console.log(`cases=${cases} mismatches=${mismatches}`);
     97|     expect(mismatches).toBe(0);
       |                        ^
     98|   });
     99| });

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[1/1]⎯

 Test Files  1 failed (1)
      Tests  1 failed (1)
   Start at  08:40:06
   Duration  1.56s (transform 78ms, setup 0ms, collect 99ms, tests 1.23s, environment 0ms, prepare 92ms)

144	}
145	
146	/**
147	 * Myers' O(ND) greedy LCS over `a[0..N)`, `b[0..M)`, returning a minimal (but not
148	 * necessarily canonical) edit script. Forward search records per-distance
149	 * contour snapshots (O(D^2) total) which are then back-tracked into the script.
150	 */
151	function myers(a: string[], b: string[], N: number, M: number): Edit[] {
152	  const MAX = N + M;
153	  const vOff = MAX;
154	  const V = new Int32Array(2 * MAX + 1);
155	  V[vOff + 1] = 0;
156	
157	  const snapshots: Int32Array[] = [];
158	  let foundD = -1;
159	
160	  outer: for (let d = 0; d <= MAX; d++) {
161	    for (let k = -d; k <= d; k += 2) {
162	      let x: number;
163	      if (k === -d || (k !== d && V[vOff + k - 1] < V[vOff + k + 1])) {
164	        x = V[vOff + k + 1]; // move down (insertion)
165	      } else {
166	        x = V[vOff + k - 1] + 1; // move right (deletion)
167	      }
168	      let y = x - k;
169	      while (x < N && y < M && a[x] === b[y]) {
170	        x++;
171	        y++;
172	      }
173	      V[vOff + k] = x;
174	      if (x >= N && y >= M) {
175	        foundD = d;
176	        break outer;
177	      }
178	    }
179	    // Snapshot the level-d contour (diagonals -d..d) for back-tracking.
180	    snapshots.push(V.slice(vOff - d, vOff + d + 1));
181	  }
182	
183	  // Back-track from (N, M) to (0, 0), emitting edits in reverse.
184	  const rev: Edit[] = [];
185	  let x = N;
186	  let y = M;
187	  for (let d = foundD; d > 0; d--) {
188	    const prev = snapshots[d - 1] as Int32Array; // covers diagonals -(d-1)..(d-1)
189	    const pOff = d - 1;
190	    const k = x - y;
191	
192	    let prevK: number;
193	    if (k === -d || (k !== d && prev[pOff + k - 1] < prev[pOff + k + 1])) {
194	      prevK = k + 1; // came via a down move (insertion)
195	    } else {
196	      prevK = k - 1; // came via a right move (deletion)
197	    }
198	
199	    const prevX = prev[pOff + prevK] as number;
200	    const prevY = prevX - prevK;
201	
202	    // Step end (just after the single non-diagonal move from the predecessor).
203	    const stepX = prevK === k + 1 ? prevX : prevX + 1;
204	    const stepY = prevK === k + 1 ? prevY + 1 : prevY;
205	
206	    // Diagonal snake (eq runs) back to the step end.
207	    while (x > stepX && y > stepY) {
208	      x--;
209	      y--;
210	      rev.push({ op: "eq", line: a[x] as string });
211	    }
212	
213	    // The single non-diagonal edit.
214	    if (prevK === k + 1) {
215	      rev.push({ op: "ins", line: b[prevY] as string });
216	    } else {
217	      rev.push({ op: "del", line: a[prevX] as string });
218	    }
219	    x = prevX;
220	    y = prevY;
221	  }
222	
223	  // The leading snake (distance 0) back to the origin.
224	  while (x > 0 && y > 0) {
225	    x--;
226	    y--;
227	    rev.push({ op: "eq", line: a[x] as string });
228	  }
229	  while (x > 0) {
230	    x--;
231	    rev.push({ op: "del", line: a[x] as string });
232	  }
233	  while (y > 0) {
234	    y--;
235	    rev.push({ op: "ins", line: b[y] as string });
236	  }
237	
238	  rev.reverse();
239	  return rev;
240	}
241	
242	/**
243	 * Normalise a minimal script toward the canonical ordering (del > ins > eq).
244	 * Three value-preserving adjacent rewrites, applied as a single insertion-style
245	 * left-bubble pass:
246	 *   - `del` moves left past an immediately preceding `ins` (priority del>ins);
247	 *   - `del` moves left past a preceding `eq` whose line equals it (delete the
248	 *     earlier of two equal lines);
249	 *   - `ins` moves left past a preceding `eq` whose line equals it (insert before
250	 *     matching the later equal line).
251	 * Each swap preserves both the `eq`+`del` (==a) and `eq`+`ins` (==b) subsequences
252	 * and the eq count, so validity and minimality are retained. For inputs whose LCS
253	 * matching is uniquely determined this yields exactly the canonical script.
254	 */
255	function canonicalize(s: Edit[]): void {
256	  for (let k = 0; k < s.length; k++) {
257	    const op = s[k]!.op;
258	    if (op === "del") {
259	      let p = k;
260	      while (p > 0) {
261	        const prev = s[p - 1]!;
262	        if (prev.op === "ins" || (prev.op === "eq" && prev.line === s[p]!.line)) {
263	          const tmp = s[p - 1]!;
264	          s[p - 1] = s[p]!;
265	          s[p] = tmp;
266	          p--;
267	        } else break;
268	      }
269	    } else if (op === "ins") {
270	      let p = k;
271	      while (p > 0) {
272	        const prev = s[p - 1]!;
273	        if (prev.op === "eq" && prev.line === s[p]!.line) {
274	          const tmp = s[p - 1]!;
275	          s[p - 1] = s[p]!;
276	          s[p] = tmp;
277	          p--;
278	        } else break;
279	      }
280	    }
281	  }
282	}
283

The file /home/user/app/src/diff/myers.ts has been updated successfully. (file state is current in your context , no need to Read it back)

The file /home/user/app/src/diff/myers.ts has been updated successfully. (file state is current in your context , no need to Read it back)

> diff-patch-engine@0.2.0 typecheck
> tsc -p tsconfig.json --noEmit

===FUZZ===

 RUN  v1.6.0 /home/user/app

stdout | test/_verify_tmp.test.ts > diff fuzz vs reference > matches reference on many random inputs
cases=60000 mismatches=0

 ✓ test/_verify_tmp.test.ts  (1 test) 1252ms

 Test Files  1 passed (1)
      Tests  1 passed (1)
   Start at  08:43:47
   Duration  1.61s (transform 121ms, setup 0ms, collect 115ms, tests 1.25s, environment 0ms, prepare 102ms)

const EXACT_DP_MAX_CELLS = 8_000_000;

The file /home/user/app/test/_verify_tmp.test.ts has been updated successfully. (file state is current in your context , no need to Read it back)

RUN  v1.6.0 /home/user/app

 ❯ test/_verify_tmp.test.ts  (6 tests | 1 failed) 1531ms
   ❯ test/_verify_tmp.test.ts > large path (forced > threshold) , exactness + perf > matches reference on a large duplicate-heavy core
     → expected 86905 to be greater than 8000000
stdout | test/_verify_tmp.test.ts > large path (forced > threshold) , exactness + perf > 100k largely-identical lines is fast and low-memory
100k diff: 22ms, ~6.8MB heap delta, edits=100051


⎯⎯⎯⎯⎯⎯⎯ Failed Tests 1 ⎯⎯⎯⎯⎯⎯⎯

 FAIL  test/_verify_tmp.test.ts > large path (forced > threshold) , exactness + perf > matches reference on a large duplicate-heavy core
AssertionError: expected 86905 to be greater than 8000000
 ❯ test/_verify_tmp.test.ts:108:33
    106|     const a = randArr(rng, 3000, 3);
    107|     const b = randArr(rng, 3000, 3);
    108|     expect(a.length * b.length).toBeGreaterThan(8_000_000);
       |                                 ^
    109|     const got = diff(a, b);
    110|     expect(applyPatch(a, got)).toEqual(b);

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[1/1]⎯

 Test Files  1 failed (1)
      Tests  1 failed | 5 passed (6)
   Start at  08:44:22
   Duration  1.88s (transform 120ms, setup 0ms, collect 123ms, tests 1.53s, environment 0ms, prepare 100ms)

The file /home/user/app/test/_verify_tmp.test.ts has been updated successfully. (file state is current in your context , no need to Read it back)

RUN  v1.6.0 /home/user/app

 ✓ test/_verify_tmp.test.ts  (6 tests) 1811ms
stdout | test/_verify_tmp.test.ts > large path (forced > threshold) , exactness + perf > 100k largely-identical lines is fast and low-memory
100k diff: 34ms, ~2.1MB heap delta, edits=100051


 Test Files  1 passed (1)
      Tests  6 passed (6)
   Start at  08:44:43
   Duration  2.20s (transform 133ms, setup 0ms, collect 132ms, tests 1.81s, environment 0ms, prepare 101ms)