Choose an account layout

Fixed and compact accounts use the same field representations. They differ in where bounded capacity lives.

Fixed accounts reserve capacity in the account

A fixed account always occupies Schema::SIZE bytes.

A container reserves its full capacity wherever it appears, so a smaller value never shrinks the representation.

String<32> occupies its one-byte prefix plus all 32 payload bytes, and Vec<u64, 8> occupies its two-byte prefix plus space for all eight elements.

Option<T> also reserves its tag and the complete representation of T, even when the value is absent.

This layout is the better default when the maximum allocation is small or the program does not need reallocations. A field offset never moves, so each access is direct after validation.

Fixed accounts support recursively bounded combinations. For example, Vec<String<16>, 4>, Option<Vec<u64, 8>>, and Vec<Option<String<8>>, 4> all have one compile-time size.

Read Fixed accounts for construction and read semantics.

Compact accounts reserve capacity in the schema

A compact account has a fixed header followed by active tail bytes. The schema still declares a maximum for each string and vector, but the allocation does not reserve every maximum. A five-byte String<64> tail occupies five payload bytes. Its length metadata lives in the header.

Compact fields save rent when active data is much smaller than its bound. The cost is a resize lifecycle and moving later tails when an earlier tail changes size. Accessors may also need to calculate the positions of preceding tails.

Read Compact accounts for the accepted field grammar and the update lifecycle.

The layouts keep the same field bytes

The container encodings do not change between layouts:

• Integers use little-endian bytes.
• Booleans use one byte with values 0 and 1.
• Strings store a length prefix and UTF-8 bytes.
• Vectors store an element count and fixed-size element bytes.
• Options store a zero or one tag and an optional payload.

The compact layout moves string and vector payloads out of their header fields. It does not introduce another value encoding.