fix(graph): cycle overflow lane colors instead of collapsing to blue (#59)

When more lanes were active than the 12-color palette, pickColor fell
through its loop and always returned color 0, so every lane past the
12th rendered blue. Use a rotating fallback index so overflow lanes
keep cycling through the palette.

Author: the0807

src/git/__tests__/git-graph-builder.test.ts

@@ -264,6 +264,26 @@ describe('buildFullGraph color palette exhaustion', () => {
     const colors = graph.paths.map(p => p.color);
     expect(new Set(colors).size).toBeLessThan(colors.length);
   });
+
+  it('cycles overflow colors instead of collapsing them all to color 0', () => {
+    // Regression for #59: with more simultaneously-active lanes than palette
+    // colors, pickColor used to return 0 for every overflow lane, so all lanes
+    // past the 12th rendered the same (blue) color. An octopus merge with 14
+    // parents creates 14 lanes over a 12-color palette; each color should be
+    // reused at most twice instead of color 0 absorbing every overflow lane.
+    const parents = Array.from({ length: 14 }, (_, i) => `p${i + 1}`);
+    const commits = [
+      makeCommit('M', parents),
+      ...parents.map(p => makeCommit(p, [])),
+    ];
+    const graph = buildFullGraph(commits);
+    const counts = new Map<number, number>();
+    for (const path of graph.paths) {
+      counts.set(path.color, (counts.get(path.color) ?? 0) + 1);
+    }
+    const maxReuse = Math.max(...counts.values());
+    expect(maxReuse).toBeLessThanOrEqual(2);
+  });
 });
 
 describe('buildFullGraph upstream-based remote-only detection', () => {

src/git/git-graph-builder.ts

@@ -245,7 +245,7 @@ function buildPushedSet(commits: Commit[], hashIndex: Map<string, number>): Set<
   return pushed;
 }
 
-function pickColor(unsolved: PathHelper[]): number {
+function pickColor(unsolved: PathHelper[], overflow: { n: number }): number {
   // Track used colors in a bitmask (palette is < 32 colors) instead of allocating an
   // array + Set on every call. O(lanes), allocation-free. This runs once per new
   // branch head and per merge parent, so it adds up on graphs with many lanes.
@@ -257,7 +257,10 @@ function pickColor(unsolved: PathHelper[]): number {
   for (let i = 0; i < COLOR_PALETTE.length; i++) {
     if ((mask & (1 << i)) === 0) return i;
   }
-  return 0;
+  // All palette colors are in use by simultaneously-active lanes (more lanes
+  // than colors). Keep cycling through the palette instead of always returning
+  // 0, so overflow lanes stay visually distinct rather than all turning blue.
+  return overflow.n++ % COLOR_PALETTE.length;
 }
 
 // ── Main parse function (SourceGit CommitGraph.Parse port) ──
@@ -281,6 +284,9 @@ export function buildFullGraph(
 
   const unsolved: PathHelper[] = [];
   const ended: PathHelper[] = [];
+  // Rotating fallback index for pickColor: advances each time the palette is
+  // fully in use so overflow lanes cycle through colors instead of all reusing 0.
+  const colorOverflow = { n: 0 };
   // Track the rail (PathHelper) each dot sits on so we can backfill the dot's
   // pattern color after the loop. A rail's override may be set by a tip that
   // appears lower on the rail than commits already processed top-to-bottom;
@@ -363,7 +369,7 @@ export function buildFullGraph(
     if (major === null) {
       offsetX += UNIT_W;
       if (commit.parents.length > 0) {
-        major = new PathHelper(commit.parents[0], pickColor(unsolved), { x: offsetX, y: offsetY });
+        major = new PathHelper(commit.parents[0], pickColor(unsolved, colorOverflow), { x: offsetX, y: offsetY });
         unsolved.push(major);
         trackNext(major);
         result.paths.push(major.path);
@@ -408,7 +414,7 @@ export function buildFullGraph(
         } else {
           // New path for merge parent
           offsetX += UNIT_W;
-          const l = new PathHelper(parentHash, pickColor(unsolved), position, { x: offsetX, y: position.y + HALF_H });
+          const l = new PathHelper(parentHash, pickColor(unsolved, colorOverflow), position, { x: offsetX, y: position.y + HALF_H });
           unsolved.push(l);
           trackNext(l);
           result.paths.push(l.path);