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fix(gp2rs): write beat times at 6-decimal precision so imported tempo matches Guitar Pro (#819)

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* fix(gp2rs): write beat times at 6-decimal precision

The editor/timeline derives per-bar BPM from beat spans
(bpm = beats*60/span), which amplifies rounding: at millisecond
(3-decimal) precision a constant-tempo GP import (e.g. 140 BPM) shows a
spurious per-bar "tempo drift" of ~0.05-0.7 BPM because most bar lengths
don't land on a ms boundary (worse for fast/odd meters). gp2rs computes
these beat times exactly from the GP tempo map, so the only precision
loss is the ebeat/startBeat format string. Writing them at 6 decimals
(microseconds) makes the derived tempo match GP's authored value.

Verified on GP5 imports (Highway to Hell 116, Equivalence 140, Living
After Midnight 138): the derived per-bar BPM collapses from two drifting
values to the single authored constant.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013JgxKh99UAeQqmhzSc73tv
Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>

* test(gp2rs): compare ebeat times by value, not string

The 6-decimal beat-time write makes _assert_ebeats' exact-string compare fail
("0.500" vs "0.500000"). These tests only assert spacing, so parse both sides
to float — precision-agnostic, no need to rewrite every parametrized list.

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

---------

Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Byron Gamatos <xasiklas@gmail.com>
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ChrisBeWithYou 2026-07-10 06:12:10 -05:00 committed by GitHub
parent 751209b80e
commit f1bae9774c
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2 changed files with 14 additions and 4 deletions
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13
lib/gp2rs.py
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@ -1114,7 +1114,7 @@ def _build_xml(
ET.SubElement(root, "arrangement").text = arrangement
ET.SubElement(root, "offset").text = f"{audio_offset:.3f}"
ET.SubElement(root, "songLength").text = f"{song_length:.3f}"
ET.SubElement(root, "startBeat").text = f"{beats[0].time:.3f}" if beats else "0.000"
ET.SubElement(root, "startBeat").text = f"{beats[0].time:.6f}" if beats else "0.000000"
ET.SubElement(root, "averageTempo").text = str(tempo)
ET.SubElement(root, "artistName").text = artist
ET.SubElement(root, "albumName").text = album
@ -1139,10 +1139,17 @@ def _build_xml(
tuning_el.set(f"string{i}", str(tuning[i] if i < len(tuning) else 0))
ET.SubElement(root, "capo").text = "0"
# Ebeats
# Ebeats — write beat times at MICROSECOND (6-decimal) precision, not
# millisecond (3-decimal). The editor/timeline DERIVES per-bar BPM from beat
# spans (bpm = beats·60/span), which amplifies any rounding: at 3 decimals a
# constant-tempo GP (e.g. 140) shows a spurious ±0.05–0.7 BPM per-bar drift
# (worse for fast/odd meters) because most bar lengths don't land on a ms
# boundary. gp2rs computes these times exactly from the GP tempo map, so the
# only loss is this format string — 6 decimals makes the derived tempo match
# GP's authored value. (Everything else stays at :.3f; only beats drive tempo.)
ebeats = ET.SubElement(root, "ebeats", count=str(len(beats)))
for b in beats:
ET.SubElement(ebeats, "ebeat", time=f"{b.time:.3f}", measure=str(b.measure))
ET.SubElement(ebeats, "ebeat", time=f"{b.time:.6f}", measure=str(b.measure))
# Sections
sections_el = ET.SubElement(root, "sections", count=str(len(sections)))

5
tests/test_gp2rs.py
View File

@ -121,7 +121,10 @@ def _converter_ebeats(converter, numerator, denominator, tempo_changes=None):
def _assert_ebeats(converter, numerator, denominator, expected_times, tempo_changes=None):
ebeats = _converter_ebeats(converter, numerator, denominator, tempo_changes)
assert [ebeat.get("time") for ebeat in ebeats] == expected_times
# Compare by value, not string: beat times are written at 6-decimal
# (microsecond) precision so the derived per-bar tempo matches the authored
# GP value, but these tests only care about the spacing, not the format.
assert [float(ebeat.get("time")) for ebeat in ebeats] == [float(t) for t in expected_times]
assert [ebeat.get("measure") for ebeat in ebeats] == [
"1",
*["-1"] * (len(expected_times) - 1),