Amount and STO helpers

AMOUNT_TO_DROPS

#define AMOUNT_TO_DROPS(amount_buffer)\
     (((amount_buffer)[0] >> 7) ? -2 : ( /* decode 8-byte big-endian XAH drops */ ))

Decode the 8-byte serialized native XAH Amount payload into an int64_t drop count. Expression. Returns the drop value on success, or -2 if the high bit of byte 0 is set — which in Xahau Amount encoding means the value is not native XAH (it is an IOU/MPT), so there are no drops to extract. The macro masks off the format/sign flag bits in byte 0 and assembles the remaining bytes big-endian.

uint8_t amt[48];                       // Amount field from otxn_field/sto_subfield
otxn_field(SBUF(amt), sfAmount);
int64_t drops = AMOUNT_TO_DROPS(amt);  // >=0 for XAH, -2 if the amount is an IOU
if (drops < 0)
    NOPE("expected native XAH");

Caveats:

• Pass the 8-byte Amount payload (for XAH the serialized amount is 8 bytes; an IOU amount is 48 bytes and this macro will detect the IOU flag and return -2). This is a raw bit decode, not the float_* path — use the float APIs when you need to do arithmetic on IOU amounts.
• The mask constant in the definition is written 0xb00111111; it reads like an intended binary literal 0b00111111 (i.e. 0x3F, clearing the top two flag bits). For all valid XAH drop amounts the two encodings produce the same result, but if you are adapting this macro for unusual inputs, verify the masking against the Amount serialization first. Unverified — needs confirmation whether the literal is intentional or a latent typo.

SUB_OFFSET / SUB_LENGTH

#define SUB_OFFSET(x) ((int32_t)(x >> 32))
#define SUB_LENGTH(x) ((int32_t)(x & 0xFFFFFFFFULL))

Unpack the int64_t returned by sto_subfield, sto_subarray and the slot subfield/subarray APIs, which encode a result as (offset << 32) | length. Expressions. SUB_OFFSET gives the byte offset relative to the buffer you passed in; SUB_LENGTH gives the field's byte length. Full usage and the offset-is-relative caveat are documented once, under sto_subfield — this section only names the macros so callers know where they come from.

int64_t r = sto_subfield(SBUF(sto), sfAmount);
if (r < 0)
    NOPE("no amount field");
uint8_t* amount = sto + SUB_OFFSET(r);   // SUB_LENGTH(r) bytes