Let a part take its height from its wires

Stop bending wires to meet a fixed pitch

A block with several terminals on one side has a fixed pitch. A CNOT's rails are 18 units apart, and an IC spreads its pins evenly over its body. Qubits and buses are seldom 18 units apart, so every wire used to bend to meet the part on the way in and again on the way out.

fit=wires reverses that. On a part declared with it, each terminal takes the height of the terminal it is wired to. The part is centred between them, and a block's body grows to hold them. Hence the wires arrive straight.

prepare:D0 "|0\rangle" at (80, 100) #blue
prepare:AN "|0\rangle" at (80, 220) #blue
prepare:D1 "|1\rangle" at (80, 340) #blue
cnot:PA at (380, 160) #purple [fit=wires]
cnot:PB at (580, 280) #purple [fit=wires]
port:O0 "d_0" at (800, 100) #emerald
measure:MP "parity" at (800, 220) #emerald
port:O1 "d_1" at (800, 340) #emerald

D0.out -> PA.in1 #blue [ortho]
AN.out -> PA.in2 #blue [ortho]
PA.out1 -> O0.in #purple [ortho]
D1.out -> PB.in1 #blue [ortho]
PA.out2 -> PB.in2 #purple [ortho]
PB.out1 -> O1.in #purple [ortho]
PB.out2 -> MP.in #purple [ortho]

Both CNOTs write into the ancilla AN, so its measurement gives the parity of d_0 and d_1. A document that never writes fit=wires compiles to exactly the bytes it did in 1.0.

Rails on gates, pins on blocks

Two families accept the option:

A quantum gate's rails may come in any order. A CNOT whose control sits below its target needs nothing special, and its two vertical terminals still face away from each other. A block's pins keep the order in which they were declared, from top to bottom, and its body never shrinks below its usual height.

prepare:X0 "|0\rangle" at (80, 100) #blue
prepare:X1 "|0\rangle" at (80, 200) #blue
prepare:X2 "|0\rangle" at (80, 300) #blue
prepare:YQ "|1\rangle" at (80, 400) #blue
hadamard:H0 at (230, 100) #cyan
hadamard:H1 at (230, 200) #cyan
hadamard:H2 at (230, 300) #cyan
hadamard:HY at (230, 400) #cyan
ic:OR "U_f" at (500, 250) #purple [left="a,b,c,y" right="ao,bo,co,yo" fit=wires]
measure:M0 at (800, 100) #emerald
measure:M1 at (800, 200) #emerald
measure:M2 at (800, 300) #emerald

X0.out -> H0.in #blue [ortho]
X1.out -> H1.in #blue [ortho]
X2.out -> H2.in #blue [ortho]
YQ.out -> HY.in #blue [ortho]
H0.out -> OR.a #cyan [ortho]
H1.out -> OR.b #cyan [ortho]
H2.out -> OR.c #cyan [ortho]
HY.out -> OR.y #cyan [ortho]
OR.ao -> M0.in #purple [ortho]
OR.bo -> M1.in #purple [ortho]
OR.co -> M2.in #purple [ortho]

The unwired yo pin is spaced from its wired neighbour at the block's usual pitch. A part with nothing wired to the terminals that fit keeps its usual shape, so fit=wires can be written on a part before its wires exist. The Hadamards and measurements here carry no labels, since each symbol already says what it is.

Settled left to right, never by source order

Fitting is arithmetic and involves no search. Its rules are short:

• Fitted parts settle from left to right, then from top to bottom, then by ID. Each one reads only the parts already settled, so a chain of gates fits from its first gate onwards. In the parity figure, PB reads the rail that PA has already fitted.
• A terminal wired to several parts follows the nearest one, then the higher one, then the first by name.
• The centre of the part is derived. It sits halfway between its outermost wired terminals, so the y written in the declaration matters only while nothing is wired.
• Placement runs first. A part placed relative to a fitted one is measured from its unfitted shape. Its neighbours therefore need room for it to grow, or they can be placed with at (x, y).

Reordering the lines of a document never changes its drawing.

prepare:TG "|+\rangle" at (90, 120) #blue
prepare:CT "|1\rangle" at (90, 280) #blue
cnot:CR at (400, 200) #purple [fit=wires]
port:TO "target" at (720, 120) #emerald
port:CO "control" at (720, 280) #emerald

CT.out -> CR.in1 #blue [ortho]
TG.out -> CR.in2 #blue [ortho]
CR.out1 -> CO.in #purple [ortho]
CR.out2 -> TO.in #purple [ortho]

The compiled part records where its terminals landed, as offsets from its centre: rails on a fitted gate and rows on a fitted block. Markup in full mode carries data-fit="wires", and parseSchematicSvg reads it back as [fit=wires].

An impossible fit says what would work

Two terminals must arrive at least 16 units apart, and a block's terminals must arrive in the order its pins are declared. When they do not, the diagnostic names the terminal and gives a height that would work:

Fitting is meant for parts that span several wires. A part with one terminal on each side is lined up by that terminal. An alignment written at PORT places the part so that its own terminal sits level with the reference, where a plain alignment would use the part's centre. The placement page explains it in full.

port:IN "IN" at (90, 190) #blue
capacitor:CC "100 nF" right-of IN by 110 #cyan
nand:GN "NAND" right-of CC by 110 aligned-y with CC.out at in1 #purple [inputs=2]
port:OUTN "Q" right-of GN by 110 #emerald

IN.out -> CC.in #blue [ortho]
CC.out -> GN.in1 #cyan [ortho]
GN.out1 -> OUTN.in #purple [ortho]