Stop shifting timecodes to start at 0ms

This reverts commit 1dedfb1.
Such files exist in the wild (one example being a large set of old CR
rips, see https://redvice.org/2018/crunchyroll-83ms-delay/) and players
like mpv also respect this video delay (at least they do now, they might
not have when the linked commit was made).

Fixes TypesettingTools#21.
Fixes TypesettingTools#169.

libaegisub/common/vfr.cpp

@@ -45,13 +45,6 @@ void validate_timecodes(std::vector<int> const& timecodes) {
 		throw InvalidFramerate("Timecodes are all identical");
 }
 
-/// @brief Shift timecodes so that frame 0 starts at time 0
-/// @param timecodes List of timecodes to normalize
-void normalize_timecodes(std::vector<int> &timecodes) {
-	if (int front = timecodes.front())
-		boost::for_each(timecodes, [=](int &tc) { tc -= front; });
-}
-
 // A "start,end,fps" line in a v1 timecode file
 struct TimecodeRange {
 	int start;
@@ -153,9 +146,8 @@ Framerate::Framerate(int64_t numerator, int64_t denominator, bool drop)
 
 void Framerate::SetFromTimecodes() {
 	validate_timecodes(timecodes);
-	normalize_timecodes(timecodes);
 	denominator = default_denominator;
-	numerator = (timecodes.size() - 1) * denominator * 1000 / timecodes.back();
+	numerator = (timecodes.size() - 1) * denominator * 1000 / (timecodes.back() - timecodes.front());
 	last = (timecodes.size() - 1) * denominator * 1000;
 }
 
@@ -221,8 +213,8 @@ int Framerate::FrameAtTime(int ms, Time type) const {
 	if (type == END)
 		return FrameAtTime(ms - 1);
 
-	if (ms < 0)
-		return int((ms * numerator / denominator - 999) / 1000);
+	if (ms < timecodes.front())
+		return int(((ms - timecodes.front()) * numerator / denominator - 999) / 1000);
 
 	if (ms > timecodes.back())
 		return int((ms * numerator - numerator / 2 - last + numerator - 1) / denominator / 1000) + (int)timecodes.size() - 1;
@@ -245,7 +237,7 @@ int Framerate::TimeAtFrame(int frame, Time type) const {
 	}
 
 	if (frame < 0)
-		return (int)(frame * denominator * 1000 / numerator);
+		return (int)(frame * denominator * 1000 / numerator) + timecodes.front();
 
 	if (frame >= (signed)timecodes.size()) {
 		int64_t frames_past_end = frame - (int)timecodes.size() + 1;

tests/tests/vfr.cpp

@@ -394,24 +394,6 @@ TEST(lagi_vfr, load_v1_save_v2_ovr) {
 	EXPECT_TRUE(validate_save("data/vfr/in/v2_100_frames_30_with_override.txt", "data/vfr/out/v2_100_frames_30_with_override.txt"));
 }
 
-TEST(lagi_vfr, nonzero_start_time) {
-	Framerate fps;
-
-	ASSERT_NO_THROW(fps = Framerate({ 10, 20, 30, 40, 50 }));
-	EXPECT_EQ(0, fps.TimeAtFrame(0, EXACT));
-	EXPECT_EQ(10, fps.TimeAtFrame(1, EXACT));
-	EXPECT_EQ(20, fps.TimeAtFrame(2, EXACT));
-	EXPECT_EQ(30, fps.TimeAtFrame(3, EXACT));
-	EXPECT_EQ(40, fps.TimeAtFrame(4, EXACT));
-
-	ASSERT_NO_THROW(fps = Framerate({ -10, 20, 30, 40, 50 }));
-	EXPECT_EQ(0, fps.TimeAtFrame(0, EXACT));
-	EXPECT_EQ(30, fps.TimeAtFrame(1, EXACT));
-	EXPECT_EQ(40, fps.TimeAtFrame(2, EXACT));
-	EXPECT_EQ(50, fps.TimeAtFrame(3, EXACT));
-	EXPECT_EQ(60, fps.TimeAtFrame(4, EXACT));
-}
-
 TEST(lagi_vfr, rational_timebase) {
 	Framerate fps;