F3 SLICE-2: AttributionTracker span algebra (INV-6/INV-7) (#6)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/attributionTracker.ts

@@ -0,0 +1,85 @@
+/**
+ * AttributionTracker — pure span algebra over offset ranges (spec §6.2/§6.5).
+ * Char-honest (INV-7): attributes exactly the characters each author produced —
+ * split character-precisely, coalesce only adjacent same-author/same-turn
+ * spans. vscode-free; the controller converts vscode change events to TextEdit.
+ */
+import type { TextEdit } from "./anchorer";
+import type { Provenance } from "./model";
+
+export interface LiveSpan {
+  id: string;
+  /** half-open [start, end) character offsets in the live document. */
+  start: number;
+  end: number;
+  author: Provenance;
+  turnId?: string;
+  /** ISO-8601; survives splits/merges (earliest wins on merge). */
+  createdAt: string;
+}
+
+/** Injected effects so the algebra stays pure and deterministic in tests. */
+export interface TrackerCtx {
+  newId: () => string;
+  now: () => string;
+  turnId?: string;
+}
+
+function sameAuthor(a: LiveSpan, b: LiveSpan): boolean {
+  return JSON.stringify(a.author) === JSON.stringify(b.author) && a.turnId === b.turnId;
+}
+
+/** Merge adjacent same-author/same-turn spans; drop empties; sort by start. */
+export function coalesce(spans: LiveSpan[]): LiveSpan[] {
+  const sorted = spans.filter((s) => s.end > s.start).sort((a, b) => a.start - b.start);
+  const out: LiveSpan[] = [];
+  for (const s of sorted) {
+    const last = out[out.length - 1];
+    if (last && last.end === s.start && sameAuthor(last, s)) {
+      last.end = s.end;
+      if (s.createdAt < last.createdAt) last.createdAt = s.createdAt;
+    } else {
+      out.push({ ...s });
+    }
+  }
+  return out;
+}
+
+/**
+ * Apply one document edit (the half-open range [start,end) replaced by
+ * newLength chars) authored by `author`. Existing spans shift/split/clip;
+ * inserted chars become a new span of `author` (INV-7).
+ */
+export function applyChange(
+  spans: LiveSpan[],
+  edit: TextEdit,
+  author: Provenance,
+  ctx: TrackerCtx,
+): LiveSpan[] {
+  const delta = edit.newLength - (edit.end - edit.start);
+  const out: LiveSpan[] = [];
+  for (const s of spans) {
+    if (s.end <= edit.start) {
+      out.push({ ...s });
+    } else if (s.start >= edit.end) {
+      out.push({ ...s, start: s.start + delta, end: s.end + delta });
+    } else {
+      const left = s.start < edit.start ? { ...s, end: edit.start } : null;
+      const right =
+        s.end > edit.end ? { ...s, start: edit.end + delta, end: s.end + delta } : null;
+      if (left) out.push(left);
+      if (right) out.push(left ? { ...right, id: ctx.newId() } : right);
+    }
+  }
+  if (edit.newLength > 0) {
+    out.push({
+      id: ctx.newId(),
+      start: edit.start,
+      end: edit.start + edit.newLength,
+      author,
+      turnId: ctx.turnId,
+      createdAt: ctx.now(),
+    });
+  }
+  return coalesce(out);
+}