deep_time/dt/conversions.rs
1use crate::historical_utc::historical_utc_offset;
2use crate::{
3 Drift, Dt, LB_DEN, LB_NUM, LG_DEN, LG_NUM, Scale, TCG_TCB_REF_ATTOS_SINCE_J2000, TDB0_ATTOS,
4 TT_TAI_OFFSET,
5};
6
7impl Dt {
8 /// Converts this instant to its internally stored `target` scale and returns
9 /// the signed difference from the given epoch.
10 ///
11 /// This is a low-level `const fn` used internally by higher-level conversion
12 /// methods such as [`to_ymd`](Dt::to_ymd).
13 ///
14 /// ## Arguments
15 ///
16 /// - `epoch` — The reference epoch (e.g. [`Dt::UNIX_EPOCH`]) from which the
17 /// difference is calculated.
18 /// - `convert_epoch` — Whether to also convert the provided `epoch` to this
19 /// [`Dt`]'s `target` time scale.
20 ///
21 /// ## Returns
22 ///
23 /// A [`Dt`] representing the signed difference (seconds + attoseconds) between
24 /// this instant (after conversion to `to`) and the provided `epoch`.
25 ///
26 /// It can be interpreted as a timestamp when `epoch` is something like
27 /// [`Dt::UNIX_EPOCH`](../struct.Dt.html#associatedconstant.UNIX_EPOCH) (e.g. for
28 /// generating Unix timestamps via `.to_ms()` or `.to_sec()`).
29 ///
30 /// ## See also
31 ///
32 /// * [`Dt::to`](../struct.Dt.html#method.to).
33 /// * [`Dt::to_diff_raw`](../struct.Dt.html#method.to_diff_raw).
34 /// * [`Dt::from_diff_and_scale`](../struct.Dt.html#method.from_diff_and_scale).
35 ///
36 /// ## Examples
37 ///
38 /// ```rust
39 /// use deep_time::{Dt, Scale};
40 ///
41 /// let dt = Dt::from_ymd(2024, 6, 15, 12, 0, 0, 0, Scale::UTC);
42 /// let diff = dt.to_scale_and_diff(Dt::UNIX_EPOCH, true);
43 ///
44 /// // diff can be used as a Unix timestamp offset
45 /// let unix_ms = diff.to_ms();
46 /// assert!(unix_ms > 1_700_000_000_000);
47 /// ```
48 pub const fn to_scale_and_diff(&self, epoch: Dt, convert_epoch: bool) -> Dt {
49 if convert_epoch {
50 self.to(self.target).to_diff_raw(epoch.to(self.target))
51 } else {
52 self.to(self.target).to_diff_raw(epoch)
53 }
54 }
55
56 /// Creates a **TAI** [`Dt`] by adding a difference to an epoch and interpreting
57 /// the result on the given time scale.
58 ///
59 /// This is the inverse counterpart to
60 /// [`Dt::to_scale_and_diff`](../struct.Dt.html#method.to_scale_and_diff)
61 /// and is used by [`Dt::from_ymd`](../struct.Dt.html#method.from_ymd)
62 /// and related constructors.
63 ///
64 /// ## Arguments
65 ///
66 /// - `diff` — The signed difference (as a [`Dt`]) to add to the epoch.
67 /// - `epoch` — The reference epoch (commonly
68 /// [`Dt::UNIX_EPOCH`](../struct.Dt.html#associatedconstant.UNIX_EPOCH) or
69 /// [`Dt::ZERO`](../struct.Dt.html#associatedconstant.ZERO)).
70 /// - `current` — The time scale on which `diff` + `epoch` should be interpreted.
71 ///
72 /// ## Returns
73 ///
74 /// A [`Dt`] on the **TAI** scale representing the absolute instant
75 /// `epoch + diff` when interpreted on `current`.
76 ///
77 /// ## Notes
78 ///
79 /// - The input `diff` is treated as being on the `current` scale.
80 /// - The final result is always converted to TAI (the internal canonical representation).
81 ///
82 /// ## See also
83 ///
84 /// - [`Dt::to_scale_and_diff`](../struct.Dt.html#method.to_scale_and_diff)
85 /// - [`Dt::from_attos`](../struct.Dt.html#method.from_attos)
86 ///
87 /// ## Examples
88 ///
89 /// ```rust
90 /// use deep_time::{Dt, Scale};
91 ///
92 /// let diff = Dt::from_tai_sec(1_718_467_200); // ~2024-06-15
93 /// let dt = Dt::from_diff_and_scale(diff, Dt::UNIX_EPOCH, true);
94 ///
95 /// let ymd = dt.to_ymd();
96 /// assert_eq!(ymd.yr(), 2024);
97 /// assert_eq!(ymd.mo(), 6);
98 /// assert_eq!(ymd.day(), 15);
99 /// ```
100 pub const fn from_diff_and_scale(diff: Dt, epoch: Dt, convert_epoch: bool) -> Dt {
101 if convert_epoch {
102 Self::from_attos(
103 epoch
104 .to(diff.scale)
105 .to_attos()
106 .saturating_add(diff.to_attos()),
107 diff.scale,
108 )
109 } else {
110 Self::from_attos(epoch.to_attos().saturating_add(diff.to_attos()), diff.scale)
111 }
112 }
113
114 /// Converts the internal attos to be on the TAI time [`Scale`].
115 ///
116 /// ```rust
117 /// use deep_time::{Dt, Scale};
118 ///
119 /// let tai = Dt::from_ymd(2000, 1, 1, 12, 0, 0, 0, Scale::UTC);
120 /// let tt = tai.to(Scale::TT);
121 ///
122 /// assert_eq!(tt.scale, Scale::TT);
123 ///
124 /// let roundtrip = tt.to_tai();
125 ///
126 /// assert_eq!(tai.scale, Scale::TAI);
127 /// assert_eq!(roundtrip, tai);
128 /// ```
129 ///
130 /// See [`Dt::to`](../struct.Dt.html#method.to) for more info.
131 pub const fn to_tai(&self) -> Dt {
132 match self.scale {
133 // we're going utc -> tai, check if it's
134 // post 1972 using the leap seconds table
135 Scale::UTC | Scale::UtcHist | Scale::UtcSpice => match self.utc_to_tai() {
136 // leap seconds table returned an offset, so use that
137 Some(dt) => dt.with(Scale::TAI),
138 // leap seconds table returned None so it must be pre 1972
139 None => match self.scale {
140 Scale::UtcHist => match historical_utc_offset(&self) {
141 Some(offset) => self.add(Dt::span_f(offset)).with(Scale::TAI),
142 None => self.with(Scale::TAI),
143 },
144 Scale::UtcSpice => self.add_sec(9).with(Scale::TAI),
145 _ => self.with(Scale::TAI),
146 },
147 },
148 Scale::TAI => *self,
149 Scale::TT => Dt::new(
150 self.attos.saturating_sub(TT_TAI_OFFSET.to_attos()),
151 Scale::TAI,
152 self.target,
153 ),
154 Scale::GPS | Scale::QZSS | Scale::GST => Dt::new(
155 self.attos.saturating_add(Dt::SEC_19.to_attos()),
156 Scale::TAI,
157 self.target,
158 ),
159 Scale::BDT => Dt::new(
160 self.attos.saturating_add(Dt::SEC_33.to_attos()),
161 Scale::TAI,
162 self.target,
163 ),
164 Scale::TDB | Scale::ET => {
165 Self::tdb_to_tai(Dt::new(self.attos, Scale::TAI, self.target))
166 }
167 Scale::TCG => {
168 let tt = Self::tcg_to_tt(Dt::new(self.attos, Scale::TAI, self.target));
169 tt.sub(TT_TAI_OFFSET)
170 }
171 Scale::TCB => {
172 let tdb = Self::tcb_to_tdb(Dt::new(self.attos, Scale::TAI, self.target));
173 Self::tdb_to_tai(tdb)
174 }
175 Scale::LTC => {
176 let tt = Self::ltc_to_tt(Dt::new(self.attos, Scale::TAI, self.target));
177 tt.sub(TT_TAI_OFFSET)
178 }
179 Scale::TCL => Self::tcl_to_tai(Dt::new(self.attos, Scale::TAI, self.target)),
180 _ => Dt::new(self.attos, Scale::TAI, self.target),
181 }
182 }
183
184 /// Converts directly to `new` [`Scale`], without first converting to TAI.
185 ///
186 /// **Warning:**
187 ///
188 /// - This function should really only be used if the [`Dt`] is on the TAI
189 /// time scale, or if you really know what you're doing.
190 /// - For the normal time scale conversion function see
191 /// [`Dt::to`](../struct.Dt.html#method.to) which correctly first converts
192 /// to TAI before converting to the target scale.
193 pub const fn convert(&self, new: Scale) -> Dt {
194 match new {
195 Scale::TAI => self.to_tai(),
196 Scale::UTC | Scale::UtcHist | Scale::UtcSpice => match self.tai_to_utc() {
197 // leap seconds table returned an offset, so use that
198 Some(dt) => dt.with(new),
199 // leap seconds table returned None so it must be pre 1972
200 None => match self.scale {
201 Scale::UtcHist => match historical_utc_offset(&self) {
202 Some(offset) => self.sub(Dt::span_f(offset)).with(new),
203 None => self.with(new),
204 },
205 Scale::UtcSpice => self.add_sec(-9).with(new),
206 _ => self.with(new),
207 },
208 },
209 Scale::TT => self.add(TT_TAI_OFFSET).with(new),
210 Scale::GPS | Scale::QZSS | Scale::GST => {
211 self.add_attos(-Dt::SEC_19.to_attos()).with(new)
212 }
213 Scale::BDT => self.add_attos(-Dt::SEC_33.to_attos()).with(new),
214 Scale::TDB | Scale::ET => Self::tai_to_tdb(*self).with(new),
215 Scale::TCG => Self::tai_to_tcg(*self).with(new),
216 Scale::TCB => Self::tai_to_tcb(*self).with(new),
217 Scale::LTC => {
218 let tt = self.add(TT_TAI_OFFSET);
219 Self::tt_to_ltc(tt).with(new)
220 }
221 Scale::TCL => Self::tai_to_tcl(*self).with(new),
222 _ => *self,
223 }
224 }
225
226 /// Converts this instant to another time scale, going via TAI.
227 ///
228 /// Essentially when converting TT to TDB the internal process goes like TT
229 /// -> TAI -> TDB. It uses the [`Dt`]s `scale` field to determine what scale
230 /// to convert from to TAI, and then the `new` arg dictates the new time scale.
231 ///
232 /// - It is not necessary to do this if you just want to use such functions
233 /// as [`Dt::to_ymd`](../struct.Dt.html#method.to_ymd) as these internally
234 /// convert to the scale of the object's `target` field before output.
235 /// - If a TAI [`Dt`] was created using
236 /// [`Dt::from_ymd`](../struct.Dt.html#method.from_ymd) and the datetime
237 /// had 60 seconds, converting to UTC would lose that info. To round trip a
238 /// 60 second UTC datetime you need only set the
239 /// [`Dt::target`](../struct.Dt.html#method.target) [`Scale`] to `UTC` and
240 /// then call the desired output function, such as
241 /// [`Dt::to_ymd`](../struct.Dt.html#method.to_ymd).
242 /// - The internal `attos` field changes to be on the new time scale.
243 /// - The [`Dt`]s `target` field is ignored and left unchanged.
244 /// - The [`Dt`]s `scale` field is changed to the new [`Scale`].
245 ///
246 /// ## Returns
247 ///
248 /// - A [`Dt`] representing the same physical instant but on the `new` scale.
249 /// - The returned objects `scale` field has been changed to `new`.
250 ///
251 /// If `current == new`, this method returns `*self` without any computation.
252 ///
253 /// ## See also
254 ///
255 /// * [`Dt::to_tai`](../struct.Dt.html#method.to_tai)
256 /// * [`Dt::from_attos`](../struct.Dt.html#method.from_attos)
257 ///
258 /// ## Examples
259 ///
260 /// ```rust
261 /// use deep_time::{Dt, Scale};
262 ///
263 /// let tai = Dt::from_ymd(2024, 6, 15, 12, 0, 0, 0, Scale::UTC);
264 /// let tt = tai.to(Scale::TT);
265 /// let tdb = tt.to(Scale::TDB);
266 /// let roundtrip = tdb.to(Scale::TAI);
267 ///
268 /// let ymd = roundtrip.to_ymd();
269 ///
270 /// assert_eq!(ymd.yr(), 2024);
271 /// assert_eq!(ymd.mo(), 6);
272 /// assert_eq!(ymd.day(), 15);
273 /// assert_eq!(ymd.hr(), 12);
274 /// assert_eq!(ymd.min(), 0);
275 /// assert_eq!(ymd.sec(), 0);
276 /// assert_eq!(ymd.attos(), 0);
277 /// ```
278 #[inline]
279 pub const fn to(&self, new: Scale) -> Dt {
280 if matches!(self.scale, Scale::TAI) {
281 self.convert(new)
282 } else if !self.scale.eq(new) {
283 self.to_tai().convert(new)
284 } else {
285 *self
286 }
287 }
288
289 #[inline(always)]
290 pub(crate) const fn utc_to_tai(&self) -> Option<Dt> {
291 match self.leap_sec(true) {
292 Some(info) => Some(self.add_sec(info.offset as i128)),
293 None => None,
294 }
295 }
296
297 #[inline(always)]
298 pub(crate) const fn tai_to_utc(&self) -> Option<Dt> {
299 match self.leap_sec(false) {
300 Some(info) => Some(self.add_sec(-info.offset as i128)),
301 None => None,
302 }
303 }
304
305 #[inline]
306 pub(crate) const fn tai_to_tcg(tai: Dt) -> Dt {
307 let tt = tai.add(TT_TAI_OFFSET);
308 Self::tt_to_tcg(tt)
309 }
310
311 #[inline]
312 pub(crate) const fn tai_to_tcb(tai: Dt) -> Dt {
313 let tdb = Self::tai_to_tdb(tai);
314 Self::tdb_to_tcb(tdb)
315 }
316
317 /// Exact integer helper: elapsed attoseconds since the TCG/TCB reference epoch (1977-01-01.0 TAI),
318 /// using only the numerical value of the supplied `Dt` (scale is ignored).
319 #[inline]
320 pub(crate) const fn to_attos_since_tcg_tcb_epoch(numerical: Dt) -> i128 {
321 numerical.to_attos() - TCG_TCB_REF_ATTOS_SINCE_J2000
322 }
323
324 /// Exact fixed-point multiplication: `attos * num / den` (handles negative values safely, no overflow for library time range).
325 pub(crate) const fn mul_rate(attos: i128, num: i128, den: i128) -> i128 {
326 if attos == 0 {
327 return 0;
328 }
329 let sign = if attos < 0 { -1i128 } else { 1i128 };
330 let a = if attos < 0 { -attos } else { attos };
331 let q = a / den;
332 let r = a % den;
333 sign * (q * num + (r * num) / den)
334 }
335
336 #[inline]
337 pub(crate) const fn mul_lg(attos: i128) -> i128 {
338 Self::mul_rate(attos, LG_NUM, LG_DEN)
339 }
340
341 #[inline]
342 pub(crate) const fn mul_lb(attos: i128) -> i128 {
343 Self::mul_rate(attos, LB_NUM, LB_DEN)
344 }
345
346 pub(crate) const fn tt_to_tcg(tt: Dt) -> Dt {
347 let elapsed = Self::to_attos_since_tcg_tcb_epoch(tt);
348 let span_attos = Self::mul_lg(elapsed);
349 tt.add_attos(span_attos)
350 }
351
352 pub(crate) const fn tcg_to_tt(tcg: Dt) -> Dt {
353 let elapsed_cg = Self::to_attos_since_tcg_tcb_epoch(tcg);
354 let span_attos = Self::mul_rate(elapsed_cg, LG_NUM, LG_DEN + LG_NUM);
355 tcg.add_attos(-span_attos)
356 }
357
358 pub(crate) const fn tcb_to_tdb(tcb: Dt) -> Dt {
359 let elapsed_cg = Self::to_attos_since_tcg_tcb_epoch(tcb);
360 let span_attos = Self::mul_rate(elapsed_cg, LB_NUM, LB_DEN + LB_NUM);
361 tcb.add_attos(-span_attos).add_attos(-TDB0_ATTOS)
362 }
363
364 pub(crate) const fn tdb_to_tcb(tdb: Dt) -> Dt {
365 let elapsed = Self::to_attos_since_tcg_tcb_epoch(tdb);
366 let span_attos = Self::mul_lb(elapsed);
367 tdb.add_attos(span_attos).add_attos(TDB0_ATTOS)
368 }
369
370 /// Converts this instant to any other [`Scale`] while applying an exact quadratic relativistic
371 /// or clock-drift correction defined by a [`Drift`] model relative to a reference instant.
372 #[inline]
373 pub const fn convert_using_drift(self, reference: Dt, drift: Drift) -> Dt {
374 let span = self.to_diff_raw(reference);
375 let correction = drift.time_diff_after(&span);
376 self.add(correction)
377 }
378
379 /// Performs the inverse conversion of [`Dt::convert_using_drift`], recovering the original proper
380 /// time on the source clock scale.
381 ///
382 /// A fixed-point iteration (at most 16 steps) is used to solve the implicit equation. For the common
383 /// case of a pure constant offset the function returns immediately without iteration.
384 pub const fn convert_back_using_drift(self, reference: Dt, drift: Drift) -> Dt {
385 if drift.rate.is_zero() && drift.accel.is_zero() {
386 return self.sub(drift.constant);
387 }
388 let mut guess = self;
389 let mut i = 0u32;
390 while i < 16 {
391 let span = guess.to_diff_raw(reference);
392 let correction = drift.time_diff_after(&span);
393 guess = self.sub(correction);
394 i += 1;
395 }
396 guess
397 }
398}