1 files changed, 122 insertions, 0 deletions
diff --git a/src/core/telemetry/reducer.ts b/src/core/telemetry/reducer.ts
new file mode 100644
index 0000000..4083231
--- /dev/null
+++ b/src/core/telemetry/reducer.ts
@@ -0,0 +1,122 @@
+import type { AgentEvent, StepId, Usage } from "@dispatch/wire";
+import type { StepMetrics, TelemetryState, TurnMetrics } from "./types";
+
+/** The initial empty telemetry state. */
+export function initialState(): TelemetryState {
+	return { turns: new Map() };
+}
+
+function mergeStep(existing: StepMetrics, patch: StepMetrics): StepMetrics {
+	const merged: StepMetrics = { ...existing };
+	if (patch.ttftMs !== undefined) (merged as { ttftMs?: number }).ttftMs = patch.ttftMs;
+	if (patch.decodeMs !== undefined) (merged as { decodeMs?: number }).decodeMs = patch.decodeMs;
+	if (patch.genTotalMs !== undefined)
+		(merged as { genTotalMs?: number }).genTotalMs = patch.genTotalMs;
+	if (patch.usage !== undefined) {
+		(merged as { usage?: Usage }).usage = { ...existing.usage, ...patch.usage };
+	}
+	if (patch.toolDurationMs !== undefined) {
+		(merged as { toolDurationMs?: number }).toolDurationMs =
+			(existing.toolDurationMs ?? 0) + patch.toolDurationMs;
+	}
+	return merged;
+}
+
+function upsertStep(
+	steps: readonly StepMetrics[],
+	stepId: StepId,
+	patch: StepMetrics,
+): readonly StepMetrics[] {
+	const idx = steps.findIndex((s) => s.stepId === stepId);
+	if (idx === -1) {
+		return [...steps, patch];
+	}
+	return [...steps.slice(0, idx), mergeStep(steps[idx]!, patch), ...steps.slice(idx + 1)];
+}
+
+function setTurn(
+	turns: ReadonlyMap<string, TurnMetrics>,
+	turnId: string,
+	turn: TurnMetrics,
+): ReadonlyMap<string, TurnMetrics> {
+	const next = new Map(turns);
+	next.set(turnId, turn);
+	return next;
+}
+
+/**
+ * Fold one live AgentEvent into the telemetry state.
+ *
+ * - `turn-start` records the active turnId.
+ * - `step-complete` creates/updates the step's timing metrics.
+ * - `usage` merges token counts into the step (joined by `stepId`).
+ * - `tool-result` accumulates `durationMs` into the step.
+ * - `done` records turn-level wall-clock + token totals.
+ * - All other event types are no-ops (content events belong to the transcript).
+ *
+ * Pure: input → output, no DOM, no side effects.
+ */
+export function foldMetricEvent(state: TelemetryState, event: AgentEvent): TelemetryState {
+	switch (event.type) {
+		case "turn-start": {
+			return {
+				...state,
+				turns: setTurn(state.turns, event.turnId, { steps: [] }),
+			};
+		}
+
+		case "step-complete": {
+			const turnId = event.turnId;
+			const existing = state.turns.get(turnId);
+			const patch: StepMetrics = { stepId: event.stepId };
+			if (event.ttftMs !== undefined) (patch as { ttftMs?: number }).ttftMs = event.ttftMs;
+			if (event.decodeMs !== undefined) (patch as { decodeMs?: number }).decodeMs = event.decodeMs;
+			if (event.genTotalMs !== undefined)
+				(patch as { genTotalMs?: number }).genTotalMs = event.genTotalMs;
+			const steps =
+				existing !== undefined ? upsertStep(existing.steps, event.stepId, patch) : [patch];
+			return {
+				...state,
+				turns: setTurn(state.turns, turnId, { ...existing, steps } as TurnMetrics),
+			};
+		}
+
+		case "usage": {
+			if (event.stepId === undefined) return state;
+			const turnId = event.turnId;
+			const existing = state.turns.get(turnId);
+			const patch: StepMetrics = { stepId: event.stepId, usage: event.usage };
+			const steps =
+				existing !== undefined ? upsertStep(existing.steps, event.stepId, patch) : [patch];
+			return {
+				...state,
+				turns: setTurn(state.turns, turnId, { ...existing, steps } as TurnMetrics),
+			};
+		}
+
+		case "tool-result": {
+			if (event.durationMs === undefined) return state;
+			const turnId = event.turnId;
+			const existing = state.turns.get(turnId);
+			if (existing === undefined) return state;
+			const patch: StepMetrics = { stepId: event.stepId, toolDurationMs: event.durationMs };
+			const steps = upsertStep(existing.steps, event.stepId, patch);
+			return { ...state, turns: setTurn(state.turns, turnId, { ...existing, steps }) };
+		}
+
+		case "done": {
+			const turnId = event.turnId;
+			const existing = state.turns.get(turnId);
+			const updated: TurnMetrics = {
+				...(existing ?? { steps: [] }),
+			};
+			if (event.durationMs !== undefined)
+				(updated as { wallMs?: number }).wallMs = event.durationMs;
+			if (event.usage !== undefined) (updated as { doneUsage?: Usage }).doneUsage = event.usage;
+			return { ...state, turns: setTurn(state.turns, turnId, updated) };
+		}
+
+		default:
+			return state;
+	}
+}