@oh-my-pi/hashline

A compact, line-anchored patch language and applier.

Hashline is a diff format designed for LLM-driven file edits. It binds every hunk to a file-content hash so stale anchors are rejected before they corrupt code, and it abstracts over the filesystem so the same patcher works on disk, in memory, over the network, or against any custom backend.

Quick start

import {
	Filesystem,
	InMemoryFilesystem,
	InMemorySnapshotStore,
	Patcher,
	Patch,
} from "@oh-my-pi/hashline";

const fs = new InMemoryFilesystem();
const snapshots = new InMemorySnapshotStore();
const before = `const greeting = "hi";\nexport { greeting };\n`;
await fs.writeText("hello.ts", before);

const tag = snapshots.recordContiguous("hello.ts", 1, before.split("\n"), { fullText: before });
const patcher = new Patcher({ fs, snapshots });
const patch = Patch.parse(String.raw`¶hello.ts#${tag}
@@ 1..1 @@
+const greeting = "hello";`);
const result = await patcher.apply(patch);

console.log(result.sections[0].op); // "update"
console.log(await fs.readText("hello.ts"));

Format

See src/prompt.md for the user-facing description and src/grammar.lark for the formal grammar.

Each file section starts with ¶PATH#TAG. The tag is a 3-hex opaque pointer into the SnapshotStore that minted it; it is not content-derived and is not meaningful outside that store. The patcher protects against stale anchors by resolving the tag, verifying the recorded snapshot lines against live file content, and refusing or attempting session-aware recovery on mismatch.

Inside a section:

• @@ A..B @@ — open a hunk on lines A..B (use @@ A,A @@ for a single line; bare @@ A @@ is also accepted).
• @@ BOF @@ / @@ EOF @@ — virtual hunks at the beginning/end of file.
• +TEXT — literal body row (use + alone for a blank line).
• &A..B — repeat original file lines A..B inline (&A for one line).
• Empty body — delete the selected range.

Abstractions

Filesystem

Read and write text by path. The default implementations:

• InMemoryFilesystem — backed by a Map. Tests, sandboxes.
• NodeFilesystem — disk-backed via Bun.file/Bun.write. Default for CLIs.

Subclass Filesystem to wire hashline into any storage: VFS, S3, an LSP text-document protocol, a Git tree, anything.

SnapshotStore

Required. Hashline tags are opaque store pointers, so Patcher must receive the store that minted them. Recovery replays edits against the cached pre-edit snapshot and 3-way-merges onto current content when the live file diverged.

Patcher

The orchestration class. Reads, normalizes line endings + BOM, applies edits, restores line endings, and writes via the configured Filesystem. Multi-section patches are preflighted up front so a partial batch never lands.