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Merge pull request lightning#5 from ffranr/var-bytes-coef-fp
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blip-29: use variable-length bytes for fixed-point coefficient encoding
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@Roasbeef
Roasbeef authored Oct 17, 2024
2 parents 9746990 + 0576465 commit 579061b
Showing 1 changed file with 33 additions and 27 deletions.
60 changes: 33 additions & 27 deletions blip-tap.md
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Original file line number Diff line number Diff line change
Expand Up @@ -939,21 +939,21 @@ The `tap_rfq` message has a BOLT 01 wire message type integer of 52884, falling
within the custom message range. Its TLV stream consists of the following TLV
fields:

| TLV Type | Field Name | Field Type |
|----------|---------------------|------------|
| 0 | version | 1*byte |
| 2 | rfq_id | 32*byte |
| 4 | transfer_type | 1*byte |
| 6 | expiry | 8*byte |
| 9 | in_asset_id | 32*byte |
| 11 | in_asset_group_key | 33*byte |
| 13 | out_asset_id | 32*byte |
| 15 | out_asset_group_key | 33*byte |
| 16 | max_in_asset | BigSize |
| 19 | in_asset_to_btc | FixedPoint |
| 21 | out_asset_to_btc | FixedPoint |
| 23 | min_in_asset | BigSize |
| 25 | min_out_asset | BigSize |
| TLV Type | Field Name | Field Type |
|----------|---------------------|-------------------------------------------------|
| 0 | version | 1*byte |
| 2 | rfq_id | 32*byte |
| 4 | transfer_type | 1*byte |
| 6 | expiry | 8*byte |
| 9 | in_asset_id | 32*byte |
| 11 | in_asset_group_key | 33*byte |
| 13 | out_asset_id | 32*byte |
| 15 | out_asset_group_key | 33*byte |
| 16 | max_in_asset | BigSize |
| 19 | in_asset_to_btc | [FixedPoint](#Fixed-Point-Number-Serialization) |
| 21 | out_asset_to_btc | [FixedPoint](#Fixed-Point-Number-Serialization) |
| 23 | min_in_asset | BigSize |
| 25 | min_out_asset | BigSize |

where:

Expand Down Expand Up @@ -1087,14 +1087,14 @@ message has a BOLT 01 wire message type integer of 52885, which falls within the
custom message range. The associated TLV stream includes the following TLV
fields:

| TLV Type | Field Name | Field Type |
|----------|------------------|------------|
| 0 | version | 1*byte |
| 2 | rfq_id | 32*byte |
| 4 | expiry | 8*byte |
| 6 | rfq_sig | 64*byte |
| 8 | in_asset_to_btc | FixedPoint |
| 10 | out_asset_to_btc | FixedPoint |
| TLV Type | Field Name | Field Type |
|----------|------------------|-------------------------------------------------|
| 0 | version | 1*byte |
| 2 | rfq_id | 32*byte |
| 4 | expiry | 8*byte |
| 6 | rfq_sig | 64*byte |
| 8 | in_asset_to_btc | [FixedPoint](#Fixed-Point-Number-Serialization) |
| 10 | out_asset_to_btc | [FixedPoint](#Fixed-Point-Number-Serialization) |

where:

Expand Down Expand Up @@ -1228,8 +1228,14 @@ where:
specifying how many decimal places `F_c` should be divided by to obtain the
fractional representation.

When serialized into a wire message, the fixed-point number is represented as a
concatenation of its coefficient and scale fields: `<uint64><uint8>`.
##### Fixed-Point Number Serialization

When serialized into a wire message, the fixed-point field type `FixedPoint` is
represented as a concatenation of its scale and coefficient fields: a 1-byte
unsigned integer for the scale, followed by a variable-length byte sequence that
encodes the coefficient as a big-endian unsigned integer. The maximum
coefficient size is constrained by the maximum wire message size as defined in
BOLT 01.

##### Fixed-Point Number Operations

Expand Down Expand Up @@ -1455,8 +1461,8 @@ the invoice amount in milli-satoshis (msats), not in the TAP asset units that
the invoice creator wants to receive. This conversion rate is agreed upon during
the RFQ process.
For example, consider the case where a user wants to receive $100 over their USD
backed channel. An RFQ quote was agreed with a fractional conversion rate
For example, consider the case where a user wants to receive \$100 over their
USD backed channel. An RFQ quote was agreed with a fractional conversion rate
`asset_to_btc_rate`. The agreed quote was sufficient to support a volume of
$100.
Expand Down

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