Skip to content

feat(mol): add support for fixed-size Union #174

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
Hanssen0 merged 2 commits into from
Aug 18, 2025
Merged

feat(mol): add support for fixed-size Union #174

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
7661c59
feat(mol): add support for fixed-size Union
phroi Aug 18, 2025
0af4257
feat(mol): clarify fixed-size Union TypeDoc
phroi Aug 18, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
63 changes: 50 additions & 13 deletions packages/core/src/molecule/codec.ts
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -459,34 +459,71 @@ type UnionDecoded<
: never;

/**
* Union is a dynamic-size type.
* Serializing a union has two steps:
* - Serialize an item type id in bytes as a 32 bit unsigned integer in little-endian. The item type id is the index of the inner items, and it's starting at 0.
* - Serialize the inner item.
* @param codecLayout the union item record
* @param fields the custom item type id record
* Constructs a union codec that can serialize and deserialize values tagged with a type identifier.
*
* If all variants have the same fixed size, the resulting union codec is fixed-size (header + payload).
* Otherwise, it falls back to a dynamic-size codec.
*
* Serialization format:
* 1. 4-byte little-endian unsigned integer for the variant index.
* 2. Encoded bytes of the selected variant.
*
* @typeParam T
* A record mapping variant names to codecs.
* @param codecLayout
* An object whose keys are variant names and values are codecs for each variant.
* @param fields
* Optional mapping from variant names to custom numeric IDs. If omitted, the index
* of each variant in `codecLayout` is used as its ID.
*
*
* @example
* // without custom id
* union({ cafe: Uint8, bee: Uint8 })
* // with custom id
* union({ cafe: Uint8, bee: Uint8 }, { cafe: 0xcafe, bee: 0xbee })
* // Dynamic union without custom numeric IDs
* union({ cafe: Uint8, bee: Uint16 })
*
* // Dynamic union with custom numeric IDs
* union({ cafe: Uint8, bee: Uint16 }, { cafe: 0xcafe, bee: 0xbee })
*
* // Fixed-size union without custom numeric IDs
* const PaddedUint8 = struct({ data : u8, padding : u8 })
* union({ cafe: PaddedUint8, bee: Uint16 });
*
* // Fixed-size union with custom numeric IDs
* union({ cafe: PaddedUint8, bee: Uint16 }, { cafe: 0xcafe, bee: 0xbee })
*/

export function union<T extends Record<string, CodecLike<any, any>>>(
codecLayout: T,
fields?: Record<keyof T, number | undefined | null>,
): Codec<UnionEncodable<T>, UnionDecoded<T>> {
const keys = Object.keys(codecLayout);
const entries = Object.entries(codecLayout);

// Determine if all variants have a fixed and equal byteLength.
let byteLength: number | undefined;
if (entries.length > 0) {
const firstLen = entries[0][1].byteLength;
if (
firstLen !== undefined &&
entries.every(([, { byteLength: len }]) => len === firstLen)
) {
// Add 4 bytes for the type header
byteLength = firstLen + 4;
}
}

return Codec.from({
byteLength,
encode({ type, value }) {
const typeStr = type.toString();
const codec = codecLayout[typeStr];
if (!codec) {
throw new Error(
`union: invalid type, expected ${keys.toString()}, but got ${typeStr}`,
`union: invalid type, expected ${entries.map((e) => e[0]).toString()}, but got ${typeStr}`,
);
}
const fieldId = fields ? (fields[typeStr] ?? -1) : keys.indexOf(typeStr);
const fieldId = fields
? (fields[typeStr] ?? -1)
: entries.findIndex((e) => e[0] === typeStr);
if (fieldId < 0) {
throw new Error(`union: invalid field id ${fieldId} of ${typeStr}`);
}
Comment on lines +499 to 529

@gemini-code-assist gemini-code-assist Bot Aug 18, 2025

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

high

The current implementation for looking up field IDs in encode can be inefficient as it performs a search on every call (entries.findIndex). For better performance, you can pre-compute a key -> id mapping once when the codec is created. This provides an O(1) lookup on average.

This change also allows for upfront validation of custom field IDs (e.g., checking for duplicates) and sets up an id -> key map that can be used to optimize the decode function as well.

  const entries = Object.entries(codecLayout);
  const keys = entries.map(([key]) => key);

  const keyToIdMap: Record<string, number> = {};
  const idToKeyMap: Record<number, string> = {};

  if (fields) {
    for (const key of keys) {
      const id = fields[key];
      if (id != null) {
        if (idToKeyMap[id]) {
          throw new Error(`union: duplicate field id ${id} for key ${key}`);
        }
        keyToIdMap[key] = id;
        idToKeyMap[id] = key;
      }
    }
  } else {
    entries.forEach(([key], index) => {
      keyToIdMap[key] = index;
      idToKeyMap[index] = key;
    });
  }

  // Determine if all variants have a fixed and equal byteLength.
  let byteLength: number | undefined;
  if (entries.length > 0) {
    const firstLen = entries[0][1].byteLength;
    if (
      firstLen !== undefined &&
      entries.every(([, { byteLength: len }]) => len === firstLen)
    ) {
      // Add 4 bytes for the type header
      byteLength = firstLen + 4;
    }
  }

  return Codec.from({
    byteLength,
    encode({ type, value }) {
      const typeStr = type.toString();
      const codec = codecLayout[typeStr];
      if (!codec) {
        throw new Error(
          `union: invalid type, expected ${keys.toString()}, but got ${typeStr}`,
        );
      }
      const fieldId = keyToIdMap[typeStr];
      if (fieldId === undefined) {
        throw new Error(`union: invalid type ${typeStr}`);
      }

@Hanssen0 Hanssen0 Aug 18, 2025

Copy link
Copy Markdown
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

@gemini-code-assist We can have a new PR for performance issue

@gemini-code-assist gemini-code-assist Bot Aug 18, 2025

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Understood. Deferring the performance improvement to a new PR is a reasonable approach. The suggestion was primarily aimed at improving the efficiency of encode by pre-computing the key -> id mapping and enabling upfront validation of custom field IDs, which would lead to O(1) lookups. Feel free to address it in a separate PR at your convenience.

Expand Down