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trampoline: _allocate_fee_budget_among_route: followup comment

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SomberNight
2026-04-24 14:41:48 +00:00
parent 06fd088992
commit b483e0d163
1 changed files with 6 additions and 5 deletions
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electrum/trampoline.py
+6 -5
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@@ -193,6 +193,7 @@ def _allocate_fee_budget_among_route(
The constraint total_fee(x) <= usable_budget_msat gives
x = (usable_budget_msat - total_fee_const) / total_fee_coeff
which we floor to an integer msat.
(note: the code is equivalent to the above description but has been simplified a bit)
"""
placeholder_edges = [e for e in route[1:] if e.fee_base_msat == PLACEHOLDER_FEE]
known_edges = [e for e in route[1:] if e.fee_base_msat != PLACEHOLDER_FEE]
@@ -205,17 +206,17 @@ def _allocate_fee_budget_among_route(
budget_const = amt_in_const - amount_msat_for_dest
budget_remaining = Fraction(usable_budget_msat) - budget_const
coeff = Fraction(0)
amt_in_coeff = Fraction(0)
for edge in reversed(route[1:]): # known_edges AND placeholder_edges
if edge.fee_base_msat == PLACEHOLDER_FEE:
coeff += Fraction(1)
amt_in_coeff += Fraction(1)
else: # for a known-edge, allocate same small fee for each placeholder-edge later in path
coeff += coeff * edge.fee_proportional_millionths / 1_000_000
amt_in_coeff += amt_in_coeff * edge.fee_proportional_millionths / 1_000_000
if budget_remaining <= 0 or coeff <= 0:
if budget_remaining <= 0 or amt_in_coeff <= 0:
placeholder_fee = 0
else:
placeholder_fee_exact = budget_remaining / coeff
placeholder_fee_exact = budget_remaining / amt_in_coeff
placeholder_fee = placeholder_fee_exact.numerator // placeholder_fee_exact.denominator # floor
_logger.debug(f"_allocate_fee_along_route: {placeholder_fee=}, placeholders={len(placeholder_edges)}")
for edge in placeholder_edges: