Function _find_junction_points

SLiCAP/schematic/canvas.py:117–168

Return the set of snapped grid points where a junction dot is needed. Assumes through-wires have already been split at T-positions. Unified rule: a junction is required when the total number of connections at a grid point is >= 3, where each wire endpoint and each component pin

(wires, components)

def _find_junction_points(wires, components) -> set[tuple[int, int]]:
    """
    Return the set of snapped grid points where a junction dot is needed.
    Assumes through-wires have already been split at T-positions.

    Unified rule: a junction is required when the total number of connections
    at a grid point is >= 3, where each wire endpoint and each component pin
    counts as one connection.

    Examples (from the schematic wiring rules picture):
      - R1+R2 series (2 pins, 0 wire endpoints) → 2 connections → no junction
      - Wire+GND     (1 wire endpoint, 1 pin)   → 2 connections → no junction
      - R4+R5+R6     (3 pins, 0 wire endpoints) → 3 connections → junction ✓
      - R11+R12+wire (2 pins, 1 wire endpoint)  → 3 connections → junction ✓
      - T-connection (1 endpoint + wire through) → safety net rule → junction ✓
    """
    # Count wire endpoints per grid point
    endpoints: dict[tuple, int] = {}
    for wire in wires:
        if len(wire.points) < 2:
            continue
        for pt in (wire.points[0], wire.points[-1]):
            k = _pt_key(pt)
            endpoints[k] = endpoints.get(k, 0) + 1

    # Count component pins per grid point
    pin_counts: dict[tuple, int] = {}
    for comp in components:
        for pin_pt in comp.pin_scene_pos():
            k = _pt_key(pin_pt)
            pin_counts[k] = pin_counts.get(k, 0) + 1

    junctions: set[tuple[int, int]] = set()

    # Safety net: wire endpoint landing on the interior of another wire
    for wire in wires:
        if len(wire.points) < 2:
            continue
        for pt in (wire.points[0], wire.points[-1]):
            k = _pt_key(pt)
            for other in wires:
                if other is not wire and _on_wire_interior(k, other):
                    junctions.add(k)
                    break

    # Unified rule: junction when wire_endpoints + component_pins >= 3
    all_points = set(endpoints.keys()) | set(pin_counts.keys())
    for k in all_points:
        if endpoints.get(k, 0) + pin_counts.get(k, 0) >= 3:
            junctions.add(k)

    return junctions


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