feat: shift+drag to cut wires and create bus connectors

Hold Shift and drag across wires to create a BUS gate that groups
them together. The cut line shows a live preview with wire count.
BUS gates are pass-through (each input maps to its output) and
render as a thin cyan bar with ports on each side.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/bus.js

@@ -0,0 +1,161 @@
+// Bus system — shift+drag to cut wires and create bus connectors
+import { state } from './state.js';
+import { getOutputPorts, getInputPorts, getComponentWidth, getComponentHeight, evaluateAll } from './gates.js';
+
+/**
+ * Sample a cubic bezier curve into discrete line segments.
+ * Returns array of {x, y} points.
+ */
+function sampleBezier(p0x, p0y, p1x, p1y, p2x, p2y, p3x, p3y, steps = 24) {
+    const points = [];
+    for (let i = 0; i <= steps; i++) {
+        const t = i / steps;
+        const mt = 1 - t;
+        const mt2 = mt * mt, mt3 = mt2 * mt;
+        const t2 = t * t, t3 = t2 * t;
+        points.push({
+            x: mt3 * p0x + 3 * mt2 * t * p1x + 3 * mt * t2 * p2x + t3 * p3x,
+            y: mt3 * p0y + 3 * mt2 * t * p1y + 3 * mt * t2 * p2y + t3 * p3y
+        });
+    }
+    return points;
+}
+
+/**
+ * Test intersection between two line segments.
+ * Returns { x, y, t } if they intersect, null otherwise.
+ * t is the parameter along the first segment (0..1).
+ */
+function segmentIntersect(ax1, ay1, ax2, ay2, bx1, by1, bx2, by2) {
+    const dx1 = ax2 - ax1, dy1 = ay2 - ay1;
+    const dx2 = bx2 - bx1, dy2 = by2 - by1;
+    const d = dx1 * dy2 - dy1 * dx2;
+    if (Math.abs(d) < 1e-10) return null;
+
+    const t = ((bx1 - ax1) * dy2 - (by1 - ay1) * dx2) / d;
+    const u = ((bx1 - ax1) * dy1 - (by1 - ay1) * dx1) / d;
+
+    if (t >= 0 && t <= 1 && u >= 0 && u <= 1) {
+        return {
+            x: ax1 + t * dx1,
+            y: ay1 + t * dy1,
+            t // position along the CUT line (for sorting)
+        };
+    }
+    return null;
+}
+
+/**
+ * Get the bezier control points for a connection, matching drawConnection in renderer.js.
+ */
+function getConnectionBezier(conn) {
+    const fromGate = state.gates.find(g => g.id === conn.from);
+    const toGate = state.gates.find(g => g.id === conn.to);
+    if (!fromGate || !toGate) return null;
+
+    const fromPort = getOutputPorts(fromGate)[conn.fromPort];
+    const toPort = getInputPorts(toGate)[conn.toPort];
+    if (!fromPort || !toPort) return null;
+
+    const midX = (fromPort.x + toPort.x) / 2;
+    return {
+        p0x: fromPort.x, p0y: fromPort.y,
+        p1x: midX, p1y: fromPort.y,
+        p2x: midX, p2y: toPort.y,
+        p3x: toPort.x, p3y: toPort.y
+    };
+}
+
+/**
+ * Find all connections that intersect a cut line.
+ * Returns array of { conn, hitPoint } sorted by position along the cut line.
+ */
+export function findIntersectingConnections(startX, startY, endX, endY) {
+    const results = [];
+
+    for (const conn of state.connections) {
+        const bez = getConnectionBezier(conn);
+        if (!bez) continue;
+
+        const points = sampleBezier(
+            bez.p0x, bez.p0y, bez.p1x, bez.p1y,
+            bez.p2x, bez.p2y, bez.p3x, bez.p3y
+        );
+
+        // Test each bezier segment against the cut line
+        for (let i = 0; i < points.length - 1; i++) {
+            const hit = segmentIntersect(
+                startX, startY, endX, endY,
+                points[i].x, points[i].y, points[i + 1].x, points[i + 1].y
+            );
+            if (hit) {
+                results.push({ conn, hitPoint: hit });
+                break; // one hit per connection is enough
+            }
+        }
+    }
+
+    // Sort by position along the cut line (t parameter) so bus ports are ordered visually
+    results.sort((a, b) => a.hitPoint.t - b.hitPoint.t);
+    return results;
+}
+
+/**
+ * Create a BUS gate from a cut line and rewire intersecting connections through it.
+ */
+export function createBusFromCut() {
+    const cut = state.busCutting;
+    if (!cut) return;
+
+    const hits = findIntersectingConnections(cut.startX, cut.startY, cut.endX, cut.endY);
+    if (hits.length === 0) {
+        console.log('[bus] no connections intersected');
+        return;
+    }
+
+    const n = hits.length;
+    console.log(`[bus] cut intersected ${n} connection(s)`);
+
+    // Calculate bus position: centroid of all hit points
+    const avgX = hits.reduce((s, h) => s + h.hitPoint.x, 0) / n;
+    const avgY = hits.reduce((s, h) => s + h.hitPoint.y, 0) / n;
+
+    // Create BUS gate
+    const busType = `BUS:${n}`;
+    const busGate = {
+        id: state.nextId++,
+        type: busType,
+        x: avgX - 15, // half of bus width (30)
+        y: avgY - Math.max(40, (n + 1) * 22) / 2,
+        value: 0,
+        outputValues: new Array(n).fill(0)
+    };
+    state.gates.push(busGate);
+
+    // Rewire: for each intersected connection, split through the bus
+    hits.forEach((hit, i) => {
+        const origConn = hit.conn;
+
+        // Remove the original connection
+        state.connections = state.connections.filter(c => c !== origConn);
+
+        // Source → BUS input port i
+        state.connections.push({
+            from: origConn.from,
+            fromPort: origConn.fromPort,
+            to: busGate.id,
+            toPort: i
+        });
+
+        // BUS output port i → original destination
+        state.connections.push({
+            from: busGate.id,
+            fromPort: i,
+            to: origConn.to,
+            toPort: origConn.toPort
+        });
+    });
+
+    console.log(`[bus] created BUS#${busGate.id} with ${n} channels`);
+    evaluateAll();
+}

js/constants.js

@@ -7,7 +7,8 @@ export const PORT_R = 7;
 export const GATE_COLORS = {
     AND: '#00e599', OR: '#3388ff', NOT: '#ff6644',
     NAND: '#e5cc00', NOR: '#cc44ff', XOR: '#ff44aa',
-    INPUT: '#3388ff', CLOCK: '#ff44aa', OUTPUT: '#ff8833'
+    INPUT: '#3388ff', CLOCK: '#ff44aa', OUTPUT: '#ff8833',
+    BUS: '#44ddff'
 };
 
 export const SIGNAL_COLORS = [

js/events.js

@@ -10,6 +10,7 @@ import { getLevel } from './levels.js';
 import { saveState, loadState, exportAsFile, importFromFile } from './saveLoad.js';
 import { enterComponentEditor, editComponentBlueprint, exitComponentEditor, updateComponentButtons, setResizeCallback } from './components.js';
 import { getExampleList, loadExample } from './examples.js';
+import { createBusFromCut } from './bus.js';
 
 const PAN_SPEED = 40;
 
@@ -31,6 +32,14 @@ export function initEvents() {
 
         // Convert to world coords for gate/port detection
         const world = screenToWorld(e.offsetX, e.offsetY);
+
+        // Update bus cut line endpoint
+        if (state.busCutting) {
+            state.busCutting.endX = world.x;
+            state.busCutting.endY = world.y;
+            return;
+        }
+
         state.hoveredPort = findPortAt(world.x, world.y);
         state.hoveredGate = state.hoveredPort ? state.hoveredPort.gate : findGateAt(world.x, world.y);
 
@@ -65,6 +74,19 @@ export function initEvents() {
         dragStartPos = { x: e.offsetX, y: e.offsetY };
         dragMoved = false;
 
+        // Shift+click on empty space → start bus cut
+        if (e.shiftKey && !state.placingGate) {
+            const port = findPortAt(world.x, world.y);
+            const gate = findGateAt(world.x, world.y);
+            if (!port && !gate) {
+                state.busCutting = {
+                    startX: world.x, startY: world.y,
+                    endX: world.x, endY: world.y
+                };
+                return;
+            }
+        }
+
         // Placing a new gate
         if (state.placingGate) {
             let w = GATE_W, h = GATE_H;
@@ -132,6 +154,13 @@ export function initEvents() {
     });
 
     canvas.addEventListener('mouseup', e => {
+        // Finish bus cut
+        if (state.busCutting) {
+            createBusFromCut();
+            state.busCutting = null;
+            return;
+        }
+
         // Toggle INPUT/CLOCK only on click (no drag movement)
         if (state.dragging && !dragMoved) {
             const gate = state.dragging;

js/gates.js

@@ -4,13 +4,14 @@ import { state } from './state.js';
 import { recordSample, setEvaluateAll } from './waveform.js';
 import { evaluateComponent } from './components.js';
 
-// Wrappers that handle component types
+// Wrappers that handle component and BUS types
 export function gateInputCount(type) {
     if (type.startsWith('COMPONENT:')) {
         const componentId = type.substring(10);
         const component = state.customComponents?.[componentId];
         return component ? component.inputCount : 0;
     }
+    if (type.startsWith('BUS:')) return parseInt(type.substring(4)) || 0;
     return baseGateInputCount(type);
 }
 
@@ -20,10 +21,12 @@ export function gateOutputCount(type) {
         const component = state.customComponents?.[componentId];
         return component ? component.outputCount : 0;
     }
+    if (type.startsWith('BUS:')) return parseInt(type.substring(4)) || 0;
     return baseGateOutputCount(type);
 }
 
 export function getComponentWidth(gate) {
+    if (gate.type.startsWith('BUS:')) return 30;
     if (gate.type.startsWith('COMPONENT:')) {
         const count = Math.max(gate.component?.inputCount || 1, gate.component?.outputCount || 1);
         return Math.max(120, (count + 1) * 25);
@@ -32,6 +35,10 @@ export function getComponentWidth(gate) {
 }
 
 export function getComponentHeight(gate) {
+    if (gate.type.startsWith('BUS:')) {
+        const n = parseInt(gate.type.substring(4)) || 1;
+        return Math.max(40, (n + 1) * 22);
+    }
     if (gate.type.startsWith('COMPONENT:')) {
         const count = Math.max(gate.component?.inputCount || 1, gate.component?.outputCount || 1);
         return Math.max(60, (count + 1) * 25);
@@ -42,8 +49,8 @@ export function getComponentHeight(gate) {
 export function getInputPorts(gate) {
     const count = gateInputCount(gate.type);
     const ports = [];
-    const isComponent = gate.type.startsWith('COMPONENT:');
-    const gateHeight = isComponent ? getComponentHeight(gate) : GATE_H;
+    const isDynamic = gate.type.startsWith('COMPONENT:') || gate.type.startsWith('BUS:');
+    const gateHeight = isDynamic ? getComponentHeight(gate) : GATE_H;
 
     for (let i = 0; i < count; i++) {
         const spacing = gateHeight / (count + 1);
@@ -55,9 +62,9 @@ export function getInputPorts(gate) {
 export function getOutputPorts(gate) {
     const count = gateOutputCount(gate.type);
     const ports = [];
-    const isComponent = gate.type.startsWith('COMPONENT:');
-    const gateWidth = isComponent ? getComponentWidth(gate) : GATE_W;
-    const gateHeight = isComponent ? getComponentHeight(gate) : GATE_H;
+    const isDynamic = gate.type.startsWith('COMPONENT:') || gate.type.startsWith('BUS:');
+    const gateWidth = isDynamic ? getComponentWidth(gate) : GATE_W;
+    const gateHeight = isDynamic ? getComponentHeight(gate) : GATE_H;
 
     for (let i = 0; i < count; i++) {
         const spacing = gateHeight / (count + 1);
@@ -105,6 +112,12 @@ function computeGate(gate) {
         return outputs[0] || 0;
     }
 
+    // BUS: pass-through, each input maps to corresponding output
+    if (gate.type.startsWith('BUS:')) {
+        gate.outputValues = [...inputs];
+        return inputs[0] || 0;
+    }
+
     switch (gate.type) {
         case 'AND':    return (inputs[0] && inputs[1]) ? 1 : 0;
         case 'OR':     return (inputs[0] || inputs[1]) ? 1 : 0;
@@ -168,8 +181,9 @@ setEvaluateAll(evaluateAll);
 
 export function findGateAt(x, y) {
     return state.gates.find(g => {
-        const w = g.type.startsWith('COMPONENT:') ? getComponentWidth(g) : GATE_W;
-        const h = g.type.startsWith('COMPONENT:') ? getComponentHeight(g) : GATE_H;
+        const isDynamic = g.type.startsWith('COMPONENT:') || g.type.startsWith('BUS:');
+        const w = isDynamic ? getComponentWidth(g) : GATE_W;
+        const h = isDynamic ? getComponentHeight(g) : GATE_H;
         return x >= g.x && x <= g.x + w && y >= g.y && y <= g.y + h;
     });
 }

js/renderer.js

@@ -32,10 +32,9 @@ export function screenToWorld(sx, sy) {
 }
 
 function drawGate(gate) {
-    // Component gates have different rendering
-    if (gate.type.startsWith('COMPONENT:')) {
-        return drawComponentGate(gate);
-    }
+    // Special gate types have different rendering
+    if (gate.type.startsWith('BUS:')) return drawBusGate(gate);
+    if (gate.type.startsWith('COMPONENT:')) return drawComponentGate(gate);
 
     const color = GATE_COLORS[gate.type];
     const isHovered = state.hoveredGate === gate;
@@ -127,6 +126,80 @@ function drawGate(gate) {
     });
 }
 
+function drawBusGate(gate) {
+    const isHovered = state.hoveredGate === gate;
+    const w = getComponentWidth(gate);  // 30
+    const h = getComponentHeight(gate);
+    const color = '#44ddff';
+    const n = parseInt(gate.type.substring(4)) || 1;
+    const hasActive = gate.outputValues?.some(v => v === 1);
+
+    if (hasActive) {
+        ctx.shadowColor = color;
+        ctx.shadowBlur = 12 * state.zoom;
+    }
+
+    // Main bus bar
+    ctx.fillStyle = hasActive ? color + '22' : '#14141e';
+    ctx.strokeStyle = isHovered ? '#fff' : color;
+    ctx.lineWidth = isHovered ? 2.5 : 1.5;
+    ctx.beginPath();
+    ctx.roundRect(gate.x, gate.y, w, h, 4);
+    ctx.fill();
+    ctx.stroke();
+    ctx.shadowBlur = 0;
+
+    // Thick center line (bus bar visual)
+    ctx.strokeStyle = isHovered ? '#fff' : color;
+    ctx.lineWidth = 3;
+    ctx.beginPath();
+    ctx.moveTo(gate.x + w / 2, gate.y + 6);
+    ctx.lineTo(gate.x + w / 2, gate.y + h - 6);
+    ctx.stroke();
+
+    // Bus size label
+    ctx.font = 'bold 9px monospace';
+    ctx.fillStyle = color;
+    ctx.textAlign = 'center';
+    ctx.textBaseline = 'middle';
+    ctx.fillText(`${n}`, gate.x + w / 2, gate.y + h + 10);
+
+    // Input ports (left)
+    getInputPorts(gate).forEach(p => {
+        const isPortHovered = state.hoveredPort &&
+            state.hoveredPort.gate === gate &&
+            state.hoveredPort.index === p.index &&
+            state.hoveredPort.type === 'input';
+        const conn = state.connections.find(c => c.to === gate.id && c.toPort === p.index);
+        const portActive = conn ? state.gates.find(g => g.id === conn.from)?.value : 0;
+
+        ctx.beginPath();
+        ctx.arc(p.x, p.y, PORT_R - 1, 0, Math.PI * 2);
+        ctx.fillStyle = isPortHovered ? '#fff' : (portActive ? '#00ff88' : '#1a1a2e');
+        ctx.fill();
+        ctx.strokeStyle = isPortHovered ? '#fff' : '#555';
+        ctx.lineWidth = 1.5;
+        ctx.stroke();
+    });
+
+    // Output ports (right)
+    getOutputPorts(gate).forEach(p => {
+        const isPortHovered = state.hoveredPort &&
+            state.hoveredPort.gate === gate &&
+            state.hoveredPort.index === p.index &&
+            state.hoveredPort.type === 'output';
+        const portVal = gate.outputValues ? (gate.outputValues[p.index] || 0) : 0;
+
+        ctx.beginPath();
+        ctx.arc(p.x, p.y, PORT_R - 1, 0, Math.PI * 2);
+        ctx.fillStyle = isPortHovered ? '#fff' : (portVal ? '#00ff88' : '#1a1a2e');
+        ctx.fill();
+        ctx.strokeStyle = isPortHovered ? '#fff' : '#555';
+        ctx.lineWidth = 1.5;
+        ctx.stroke();
+    });
+}
+
 function drawComponentGate(gate) {
     const isHovered = state.hoveredGate === gate;
     const isActive = gate.value === 1;
@@ -305,6 +378,86 @@ function drawConnectingWire() {
     ctx.setLineDash([]);
 }
 
+function drawBusCutLine() {
+    if (!state.busCutting) return;
+    const cut = state.busCutting;
+
+    // Dashed cyan line showing the cut
+    ctx.beginPath();
+    ctx.moveTo(cut.startX, cut.startY);
+    ctx.lineTo(cut.endX, cut.endY);
+    ctx.strokeStyle = '#44ddff';
+    ctx.lineWidth = 2.5;
+    ctx.setLineDash([8, 5]);
+    ctx.stroke();
+    ctx.setLineDash([]);
+
+    // Small circles at endpoints
+    ctx.beginPath();
+    ctx.arc(cut.startX, cut.startY, 4, 0, Math.PI * 2);
+    ctx.fillStyle = '#44ddff';
+    ctx.fill();
+    ctx.beginPath();
+    ctx.arc(cut.endX, cut.endY, 4, 0, Math.PI * 2);
+    ctx.fill();
+
+    // Highlight intersecting wires
+    // Import is circular so we compute inline: sample bezier + test intersection
+    const hits = countCutIntersections(cut);
+    if (hits > 0) {
+        ctx.save();
+        ctx.resetTransform();
+        ctx.fillStyle = '#44ddff';
+        ctx.font = 'bold 12px "Segoe UI", system-ui, sans-serif';
+        ctx.textAlign = 'left';
+        ctx.fillText(`✂ ${hits} wire${hits > 1 ? 's' : ''}`, 10, canvas.height - 10);
+        ctx.restore();
+    }
+}
+
+/**
+ * Quick inline intersection count for preview (avoids circular import from bus.js)
+ */
+function countCutIntersections(cut) {
+    let count = 0;
+    for (const conn of state.connections) {
+        const fromGate = state.gates.find(g => g.id === conn.from);
+        const toGate = state.gates.find(g => g.id === conn.to);
+        if (!fromGate || !toGate) continue;
+        const fp = getOutputPorts(fromGate)[conn.fromPort];
+        const tp = getInputPorts(toGate)[conn.toPort];
+        if (!fp || !tp) continue;
+
+        const midX = (fp.x + tp.x) / 2;
+        // Sample bezier at 16 points
+        for (let i = 0; i < 16; i++) {
+            const t1 = i / 16, t2 = (i + 1) / 16;
+            const bx1 = bezAt(fp.x, midX, midX, tp.x, t1);
+            const by1 = bezAt(fp.y, fp.y, tp.y, tp.y, t1);
+            const bx2 = bezAt(fp.x, midX, midX, tp.x, t2);
+            const by2 = bezAt(fp.y, fp.y, tp.y, tp.y, t2);
+            if (segsHit(cut.startX, cut.startY, cut.endX, cut.endY, bx1, by1, bx2, by2)) {
+                count++;
+                break;
+            }
+        }
+    }
+    return count;
+}
+
+function bezAt(p0, p1, p2, p3, t) {
+    const mt = 1 - t;
+    return mt * mt * mt * p0 + 3 * mt * mt * t * p1 + 3 * mt * t * t * p2 + t * t * t * p3;
+}
+
+function segsHit(ax1, ay1, ax2, ay2, bx1, by1, bx2, by2) {
+    const d = (ax2 - ax1) * (by2 - by1) - (ay2 - ay1) * (bx2 - bx1);
+    if (Math.abs(d) < 1e-10) return false;
+    const t = ((bx1 - ax1) * (by2 - by1) - (by1 - ay1) * (bx2 - bx1)) / d;
+    const u = ((bx1 - ax1) * (ay2 - ay1) - (by1 - ay1) * (ax2 - ax1)) / d;
+    return t >= 0 && t <= 1 && u >= 0 && u <= 1;
+}
+
 function drawPlacingGhost() {
     if (!state.placingGate) return;
     ctx.globalAlpha = 0.5;
@@ -356,6 +509,7 @@ function draw() {
     state.connections.forEach(drawConnection);
     state.gates.forEach(drawGate);
     drawConnectingWire();
+    drawBusCutLine();
     drawPlacingGhost();
 
     ctx.restore();

js/state.js

@@ -42,5 +42,8 @@ export const state = {
     componentEditorActive: false,
     savedMainCircuit: null,  // { gates, connections, nextId } saved before entering editor
     componentEditorName: '',
-    editingComponentId: null  // ID of component being edited (null = new component)
+    editingComponentId: null,  // ID of component being edited (null = new component)
+
+    // Bus cutting (shift+drag)
+    busCutting: null  // { startX, startY, endX, endY } in world coords, or null
 };