Add variable-length string and new bytes field type

[Copilot]

• Explore codebase and understand existing patterns
• Make byteLength optional in the string field function in fields.ts
• Add bytes field function to fields.ts (fixed and variable-length, read-only, live Uint8Array view)
• Add tests in mod_test.ts for variable-length string and bytes field
• Run tests to validate changes (all 29 tests pass)
• Bump version to 0.17.0-pre in package.json and deno.json
• Add changelog entry for 0.17.0-pre in CHANGELOG.md

Original prompt

Summary

Add two new capabilities to the structview library:

1. Variable-length string field: The existing string(fieldOffset, byteLength) function requires a fixed byteLength. Add support for omitting byteLength, in which case the string extends from fieldOffset to the end of the struct's buffer. This makes byteLength optional and defaults to variable-length (rest-of-struct).

2. bytes field type: Add a new bytes() field function that returns a live Uint8Array view into the struct's underlying buffer. It should support:

   • Fixed-length: bytes(fieldOffset, byteLength) — returns a Uint8Array of exactly byteLength bytes starting at fieldOffset.
   • Variable-length: bytes(fieldOffset) — returns a Uint8Array from fieldOffset to the end of the struct. This is the default when byteLength is omitted.
   • The returned Uint8Array must be a live view (backed by the same ArrayBuffer), not a copy.
   • The bytes field should be read-only (no setter) — it returns a live view so mutations happen through the Uint8Array itself.

Implementation Details

Variable-length string

In fields.ts, modify the existing string function signature so byteLength is optional:

export function string(
  fieldOffset: number,
  byteLength?: number,
): StructPropertyDescriptor<string>

When byteLength is undefined, use structBytes(this, fieldOffset) (which already defaults end to dv.byteLength in core.ts).

bytes field

Add a new exported function in fields.ts:

export function bytes(
  fieldOffset: number,
  byteLength?: number,
): StructPropertyDescriptor<Uint8Array> & ReadOnlyAccessorDescriptor<Uint8Array>

• When byteLength is provided: return structBytes(this, fieldOffset, fieldOffset + byteLength)
• When byteLength is omitted: return structBytes(this, fieldOffset) (extends to end of struct)
• This should be a getter-only (read-only) descriptor since the Uint8Array is a live view.

Exports

Make sure bytes is exported from mod.ts (it already re-exports * from "./fields.ts", so just adding the export in fields.ts should suffice).

Tests

Add tests in mod_test.ts covering:

1. Variable-length string (no byteLength):

   • Reading a string that extends to the end of the struct
   • Writing a string that extends to the end of the struct
   • Trailing null trimming still works
   • Works correctly when the struct has other fields before it

2. Fixed-length bytes:

   • Returns a Uint8Array of the correct length
   • Returns a live view (modifications through the Uint8Array are reflected in the underlying buffer)
   • Is read-only (assigning to the property throws — use // @ts-expect-error pattern like existing tests)

3. Variable-length bytes (no byteLength):

   • Returns a Uint8Array extending from fieldOffset to the end of the struct
   • Returns a live view
   • Is read-only

4. Existing string tests continue to pass — the existing string(offset, length) usage must remain fully backward compatible.

Reference

• The structBytes helper in core.ts already supports optional end parameter (defaults to dv.byteLength), so variable-length behavior is straightforward.
• Follow the existing patterns in fields.ts for field function signatures and descriptor construction.
• Follow the existing test patterns in mod_test.ts (uses vitest test/expect and node:assert/strict).
• Make sure to import bytes in the test file alongside the existing field imports.
• Run deno x vitest run or npm x vitest run to verify tests pass.

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: Please add the ability to declare a variable-length string field, extending to the end of the struct. that should be the default if length is not specified. if we don't have a bytes field type (either fixed or variable length, returning a live Uint8Array), add that too

User: Please add the ability to declare a variable-length string field, extending to the end of the struct. that should be the default if length is not specified. if we don't have a bytes field type (either fixed or variable length, returning a live Uint8Array), add that too. make sure to add tests

Assistant:

I'll start by exploring the repository to understand its structure and current capabilities before creating the PR.

Assistant: I'd like to help you create a PR to add variable-length string and bytes field types, but I need to know which repository to target. Could you please provide the repository in owner/repo format?

User: rotu/structview. open a PR

Assistant:

Let me first explore the repository to understand its current structure and capabilities.

Assistant: Now I have a thorough understanding of the codebase. Let me create the PR with a detailed problem statement.

Assistant: I've kicked off the Copilot cod...

This pull request was created from Copilot chat.

[rotu]

Please do a pre-minor version bump and add a changelog entry.

[Copilot]

Please do a pre-minor version bump and add a changelog entry.

Done in f8b59f1 — bumped to 0.17.0 in package.json and deno.json, and added the changelog entry.

[rotu]

No, a prerelease version. e.g. 0.17.0-pre

[Copilot]

No, a prerelease version. e.g. 0.17.0-pre

Updated to 0.17.0-pre in 7549a42.