feat: add Lahiri chart reference (benchmark alignment pending)

docs/chart-conventions.md

@@ -127,3 +127,46 @@ Chart comparisons use a 1e-12 absolute tolerance for native/Wasm floating-point
 roundoff in degrees or degrees/day. The recorded Wasm run's largest residual was
 1.14e-13 degrees; speeds matched exactly. This release also retains the existing
 frozen source-Wasm parity checks for tropical charts and Sun searches.
+
+## Lahiri
+
+Select `reference: "lahiri"` with equal sign divisions in Rust or the Wasm
+adapter. The example selector exposes the same option. Existing defaults and
+Fagan–Bradley calculations are unchanged.
+
+The mean offset follows the PAC Indian Astronomical Ephemeris 2027, printed
+page 382, formula introduced in 2021: at J2000 it is 23°51′25.53″, with
+precession in arcseconds
+5028.796195T + 1.1054348T² + 0.00007964T³ − 0.00023857T⁴ − 0.0000000383T⁵,
+where T is TT centuries since J2000. The engine adds its nutation in longitude,
+consistent with its tropical coordinates (the publication describes true
+ayanamsa on page 423). Source:
+https://mausam.imd.gov.in/responsive/indianAstronomicalEphemeris.php
+
+Evaluation supports 1800-01-01 through 2200-01-01 TT inclusive, additionally
+limited by dataset coverage. This range is not a claim of independently
+verified astronomical accuracy. Planetary speeds subtract the centered daily
+offset change; angle speeds remain zero. Whole Sign houses use the projected
+Ascendant, while Equal and Placidus follow the existing projection and fallback.
+
+### Initial benchmark evidence
+
+The approved external benchmark was tested at UTC dates in 1900, 1956, 2000,
+2026, and 2100 using independently computed Rust values and the existing local
+dataset. All five true ayanamsas passed the one-arcsecond threshold (maximum
+0.110 arcseconds). Sun/Moon longitudes passed in 1956, 2000, and 2026, but
+failed in 1900 and 2100. Tropical control calculations reproduced those
+failures, with maximum Moon error 20.622 arcseconds. The underlying time and
+dataset conventions require investigation before claiming chart alignment.
+
+No benchmark-derived reference data is committed, and no new Rust-generated
+regression fixtures have been frozen: their prerequisite is benchmark agreement.
+Broader body, speed, house, and date sweeps remain outstanding.
+
+An expanded Whole Sign batch compared all 19 body/angle longitudes, 15 body
+speeds and declinations, 12 cusps, and true ayanamsa at each of those five dates.
+One complete case (1956) passed. All speeds and Whole Sign cusps passed;
+maximum speed residual was 0.393 arcseconds/day. Additional failures included
+angles (up to 11.799 arcseconds at J2000) and Moon declination (up to 6.543
+arcseconds in 2100). These remain unresolved; Equal/Placidus and polar sweeps
+are not yet validated.

examples/configurable-chart/index.html

@@ -7,7 +7,7 @@
 <label>Birth time (UTC)<input name="utc" value="2000-01-01T12:00:00Z" required></label>
 <label>Latitude (degrees)<input name="latitude" type="number" min="-90" max="90" step="any" value="51.5074" required></label>
 <label>Longitude (degrees)<input name="longitude" type="number" min="-180" max="180" step="any" value="-0.1278" required></label>
-<label>Zodiac reference<select name="reference"><option value="tropical">Tropical</option><option value="fagan-bradley">Fagan–Bradley</option><option value="true-sky">Chimenti True Sky</option></select></label>
+<label>Zodiac reference<select name="reference"><option value="tropical">Tropical</option><option value="fagan-bradley">Fagan–Bradley</option><option value="lahiri">Lahiri</option><option value="true-sky">Chimenti True Sky</option></select></label>
 <label>Sign divisions<select name="divisions"><option value="equal">Twelve equal signs</option><option value="constellation">Constellation midpoints</option></select></label>
 <label>Ophiuchus<select name="ophiuchus"><option value="enabled">Enabled</option><option value="disabled">Merged into Scorpio</option></select></label>
 <label>Houses<select name="houses"><option value="placidus">Placidus</option><option value="equal">Equal</option><option value="whole-sign">Whole Sign</option></select></label>

src/chart.rs

@@ -35,7 +35,7 @@ pub struct ChartValues {
     pub configuration: ZodiacConfiguration,
     pub convention_revision: &'static str,
     pub reference_offset_degrees: f64,
-    /// True Sky retains tropical speeds; Fagan–Bradley corrects them.
+    /// True Sky retains tropical speeds; Fagan–Bradley and Lahiri correct them.
     /// Angle speeds remain zero (not calculated) for every reference.
     pub speed_reference: SpeedReference,
     pub bodies: Vec<ChartEntry>,
@@ -57,6 +57,10 @@ pub fn calculate_chart(
     let (offset, rate, speed_reference) = match configuration.reference {
         ZodiacReference::Tropical => (0.0, 0.0, SpeedReference::Tropical),
         ZodiacReference::TrueSky => (true_sky_offset(input.epoch)?, 0.0, SpeedReference::Tropical),
+        ZodiacReference::Lahiri => {
+            let (offset, rate) = crate::sidereal::lahiri_offset_and_rate(input.epoch)?;
+            (offset, rate, SpeedReference::SelectedReference)
+        }
         ZodiacReference::FaganBradley => (
             fagan_bradley_ayanamsa(input.epoch)?,
             crate::sidereal::ayanamsa_daily_rate(input.epoch),
@@ -106,8 +110,10 @@ pub fn calculate_chart(
         .enumerate()
         .map(|(index, body)| ChartBody {
             longitude: project_longitude(body.longitude),
-            speed: if configuration.reference == ZodiacReference::FaganBradley
-                && index < physical_body_count
+            speed: if matches!(
+                configuration.reference,
+                ZodiacReference::FaganBradley | ZodiacReference::Lahiri
+            ) && index < physical_body_count
             {
                 body.speed - rate
             } else {

src/conventions.rs

@@ -12,6 +12,7 @@ pub const CONVENTION_REVISION: &str = "astrology-engine-conventions-v1";
 pub enum ZodiacReference {
     Tropical,
     FaganBradley,
+    Lahiri,
     TrueSky,
 }
 

src/sidereal.rs

@@ -1,7 +1,7 @@
 // Copyright (c) Jack Asher
 // SPDX-License-Identifier: MIT
 
-//! Fagan–Bradley zodiac, using the existing IAU 2006 precession rotation.
+//! Fagan–Bradley and current PAC Lahiri longitude offsets.
 //!
 //! This is the traditional longitude-offset convention on the ecliptic of
 //! date, not a three-dimensional rotation onto a fixed B1950 ecliptic.
@@ -63,3 +63,61 @@ pub(crate) fn ayanamsa_daily_rate(epoch: Epoch) -> f64 {
     true_ayanamsa(AstroTime::from_tdb(epoch + half_day))
         - true_ayanamsa(AstroTime::from_tdb(epoch - half_day))
 }
+
+// PAC Indian Astronomical Ephemeris 2027, pp. 382, 423 (formula since 2021).
+fn lahiri_mean(tt: f64) -> f64 {
+    let t = tt / 36525.0;
+    23.0 + 51.0 / 60.0
+        + 25.53 / 3600.0
+        + t * (5028.796195
+            + t * (1.1054348 + t * (0.00007964 + t * (-0.00023857 - t * 0.0000000383))))
+            / 3600.0
+}
+
+fn lahiri_true(time: AstroTime) -> f64 {
+    lahiri_mean(time.tt) + e_tilt(time).dpsi / 3600.0
+}
+
+pub(crate) fn lahiri_offset_and_rate(epoch: Epoch) -> Result<(f64, f64), CalculationError> {
+    let time = AstroTime::from_tdb(epoch);
+    if !(MIN_JD..=MAX_JD).contains(&(time.tt + J2000)) {
+        return Err(CalculationError::InvalidInput(
+            "Lahiri is supported from 1800-01-01 through 2200-01-01 TT",
+        ));
+    }
+    let half_day = hifitime::Duration::from_days(0.5);
+    Ok((
+        lahiri_true(time),
+        lahiri_true(AstroTime::from_tdb(epoch + half_day))
+            - lahiri_true(AstroTime::from_tdb(epoch - half_day)),
+    ))
+}
+
+#[cfg(test)]
+mod lahiri_tests {
+    use super::*;
+    #[test]
+    fn official_mean_anchor() {
+        assert!((lahiri_mean(0.0) - (23.0 + 51.0 / 60.0 + 25.53 / 3600.0)).abs() < 1e-12);
+    }
+}
+
+#[cfg(test)]
+mod lahiri_range_tests {
+    use super::*;
+    #[test]
+    fn range_and_rate() {
+        for jd in [MIN_JD, MAX_JD] {
+            // Construct UTC instants well inside/outside each boundary; the
+            // engine derives TT using its existing delta-T convention.
+            let year = if jd == MIN_JD { "1800" } else { "2199" };
+            let epoch: Epoch = format!("{year}-06-01T00:00:00Z").parse().unwrap();
+            let (offset, rate) = lahiri_offset_and_rate(epoch).unwrap();
+            assert!(offset.is_finite());
+            assert!(rate.is_finite() && rate.abs() < 0.001);
+        }
+        for utc in ["1799-01-01T00:00:00Z", "2201-01-01T00:00:00Z"] {
+            assert!(lahiri_offset_and_rate(utc.parse().unwrap()).is_err());
+        }
+    }
+}

tests/lahiri.rs

@@ -0,0 +1,18 @@
+use astrology_engine::{
+    effective_configuration, SignDivisions, ZodiacConfiguration, ZodiacReference,
+};
+
+#[test]
+fn lahiri_wire_configuration_and_divisions() {
+    let config: ZodiacConfiguration =
+        serde_json::from_str(r#"{"reference":"lahiri","divisions":"equal","ophiuchus":null}"#)
+            .unwrap();
+    assert_eq!(config.reference, ZodiacReference::Lahiri);
+    assert_eq!(effective_configuration(config).unwrap(), config);
+    assert_eq!(serde_json::to_value(config).unwrap()["reference"], "lahiri");
+    assert!(effective_configuration(ZodiacConfiguration {
+        divisions: SignDivisions::Constellation,
+        ..config
+    })
+    .is_err());
+}