Fix ElevenLabs alignment chunk spacing

tests/test_elevenlabs_tts.py

@@ -0,0 +1,92 @@
+#
+# Copyright (c) 2024-2026, Daily
+#
+# SPDX-License-Identifier: BSD 2-Clause License
+#
+
+"""Tests for ElevenLabs TTS alignment handling."""
+
+from typing import Any
+
+from pipecat.services.elevenlabs.tts import (
+    _strip_utterance_leading_spaces,
+    calculate_word_times,
+)
+
+_WS_ALIGNMENT_KEYS = ("chars", "charStartTimesMs", "charDurationsMs")
+
+
+def _chunk(text: str) -> dict[str, list[Any]]:
+    chars = list(text)
+    return {
+        "chars": chars,
+        "charStartTimesMs": [i * 100 for i in range(len(chars))],
+        "charDurationsMs": [100 for _ in chars],
+    }
+
+
+def _words_from_chunks(chunks: list[dict[str, list[Any]]]) -> list[str]:
+    cumulative_time = 0.0
+    partial_word = ""
+    partial_word_start_time = 0.0
+    word_times = []
+    alignment_started = False
+
+    for chunk in chunks:
+        alignment = _strip_utterance_leading_spaces(
+            chunk,
+            _WS_ALIGNMENT_KEYS,
+            not alignment_started,
+        )
+        alignment_started = True
+        chunk_word_times, partial_word, partial_word_start_time = calculate_word_times(
+            alignment,
+            cumulative_time,
+            partial_word,
+            partial_word_start_time,
+        )
+        word_times.extend(chunk_word_times)
+
+        starts = alignment["charStartTimesMs"]
+        durations = alignment["charDurationsMs"]
+        if starts and durations:
+            cumulative_time += (starts[-1] + durations[-1]) / 1000.0
+
+    if partial_word:
+        word_times.append((partial_word, partial_word_start_time))
+
+    return [word for word, _ in word_times]
+
+
+def test_elevenlabs_flash_alignment_preserves_inter_word_chunk_space():
+    chunks = [
+        _chunk(" Why did the math book"),
+        _chunk(" look so sad? "),
+        _chunk(" Because it had too m"),
+        _chunk("any problems. "),
+    ]
+
+    assert _words_from_chunks(chunks) == [
+        "Why",
+        "did",
+        "the",
+        "math",
+        "book",
+        "look",
+        "so",
+        "sad?",
+        "Because",
+        "it",
+        "had",
+        "too",
+        "many",
+        "problems.",
+    ]
+
+
+def test_elevenlabs_alignment_strips_only_utterance_leading_spaces():
+    first = _strip_utterance_leading_spaces(_chunk("  Hello"), _WS_ALIGNMENT_KEYS, True)
+    subsequent = _strip_utterance_leading_spaces(_chunk(" world"), _WS_ALIGNMENT_KEYS, False)
+
+    assert first["chars"] == list("Hello")
+    assert subsequent["chars"] == list(" world")