# Web data format v1

## Invariants

- Supported civil years are 1950 through 17000, inclusive.
- Civil dates use the proleptic Gregorian calendar and are represented by
  integer `year`, `month`, and `day` values.  Consumers must not parse them with
  JavaScript `Date` or `<input type="date">`.
- Display timezone and lunar day boundary are both UTC+08:00.
- Huangli longitude is 120 degrees east.  Civil time, lunar day boundary,
  Huangli day boundary, and true solar time remain separate concepts.
- Daily state is sampled at local 12:00:00.  Exact solar-term and four-phase
  events are roots calculated by the C++ core, never noon phase labels.
- `sec` is seconds since local midnight, rounded to the nearest second.  A
  rounded value that crosses midnight is carried into the adjacent civil day.
- `null` means the algorithm defines the field but no value/contact exists.
  Missing keys are used only where the schema says a field is optional.

## Directory layout

```text
v1/
  manifest.json
  schema.json
  dict.json
  calendar/<year>/<month>.json[.br|.gz]
  eclipse/<year>.json[.br|.gz]
  astro/<year>/<month>.json[.br|.gz]
  index/years.json
  checksums/sha256.txt
  failed_years.json
  .state/<year>.json
```

`.state` is resumability metadata and need not be deployed.  Brotli files use
the `.json.br` suffix and gzip files use `.json.gz`.

## Calendar month

`v`, `y`, and `m` are schema version, civil year, and civil month.  `meta`
records timezone, lunar boundary, Huangli longitude, noon sample second,
ephemeris selection, and event rounding.

`days` is the light month-grid array:

- `d`: civil day of month.
- `w`: weekday, Sunday=0 through Saturday=6.
- `l`: `[lunar_year, lunar_month, lunar_day, leap_flag]`.
- `fest`: stable festival codes.
- `terms`: solar-term codes occurring on the civil day.
- `phases`: exact `new_moon`, `fst_qtr`, `full_moon`, or `lst_qtr` roots.
- `solar_eclipses` / `lunar_eclipses`: `[type_code, event_id]` pairs.

`events` contains the month's solar terms and four exact lunar phases.  Each
entry has stable `kind`/`code`, local `civil`, `day`, `sec`, `jd_tdb`,
`jd_utc`, and the source program's UTC/local strings.  JD and source strings
are retained even though the front end normally formats `sec`.

`detail` parallels `days` and retains:

- the full noon sample (`jd_utc`, `jd_tdb`, UTC/local source strings);
- compact lunar date and 60-cycle indexes for year/month/day;
- year zodiac code;
- daily lunar geometry: solar/lunar apparent longitude and rates,
  elongation, illumination, waxing state, phase-state code, and lunar mansion;
- festival, exact-event, eclipse, and astronomy references;
- all four Huangli profiles as lossless `common + profiles` data.

The daily `moon.phase_name` is a sampled eight-sector state.  It must not be
used as an exact four-phase event.

## Dictionary reconstruction

`dict.json.domains` is an array-per-domain keyed by the integer code emitted by
the C++ core.  Null array positions are unused codes.  `event_names` maps stable
string codes to labels.  Huangli label domains carry an `@<profile_code>`
suffix because the repository intentionally emits profile-specific wording
(for example, the same duty-god code may render differently).

The following source redundancies are omitted from day records and restored
without information loss:

- a ganzhi index restores text, stem=`index % 10`, and branch=`index % 12`;
- lunar month/day labels restore from the lunar dictionaries; the complete
  source label is `农历{year}年{month_label}{day_label}`;
- paired Huangli text/code fields restore from the profile-specific domain;
- god/activity text arrays restore from their code arrays;
- each hour triple `[slot_index, ganzhi_index, is_bad]` restores slot text,
  ganzhi object, luck label, and boolean.  All 13 source slots are preserved;
- `common` is merged with the named profile diff, then dictionary expansion is
  applied.  The generator compares the result to the original profile object
  before writing any year.

Bit masks remain present because they are useful for fast filtering.  Raw
strings without a stable source code (for example Pengzu text or a composed
clash string) are retained rather than assigned order-dependent Web codes.

## Festivals

Festivals and legal holiday arrangements are distinct datasets.  v1 generates
only rule-based festivals:

- Gregorian: New Year's Day, Labour Day, National Day.
- Non-leap lunar months: Spring Festival, Lantern Festival, Dragon Boat,
  Qixi, Mid-Autumn, Double Ninth, Laba, and northern Xiaonian.
- Derived: New Year's Eve when the following civil day is non-leap lunar 1/1.
- Solar-term based: Qingming on the exact `J3` local civil day.

Levels are 0 (stored but hidden by default), 1 (normal month display), and 2
(emphasized).  No future statutory holiday or make-up-work policy is inferred.

## Huangli profiles and boundaries

Profiles are `folk`, `ziping`, `purple`, and `xieji`.  Each profile retains its
source rule profile, year boundary, month boundary, leap-month mode, day
boundary, ganzhi nodes, 建除, duty god/tag, clashes, harmonies, Pengzu, nayin,
five elements, fetal god, meridian, direction fields, lunar mansions, gods,
activities, Yi/Ji level/rule, equation/true-solar time, and all 13 hour slots.

Profile defaults come from `make_hli_rule_set`; the generator does not collapse
their distinct boundaries.  UTC+08 civil midnight is not substituted for a
profile's Hour-23/Hour-0 Huangli boundary.

## Eclipses

The month grid references an eclipse on the UTC+08 civil date containing the
algorithm's maximum eclipse (`MAX`).  Contact points may cross dates.  The
annual eclipse record retains the complete original nested C++ JSON: all
available contacts, magnitudes, durations, distances, gamma, geometry,
libration, contact metadata, delta-T, and solar Besselian coefficients.
Nonexistent contacts are JSON `null`; NaN is forbidden.

Solar `MAX` uses the standard greatest-eclipse definition: the instant when
the lunar shadow axis has minimum distance from Earth's geocentre, equivalently
the minimum of `x^2 + y^2` in the Besselian fundamental plane.  The final
Besselian Newton refinement makes repeated solutions independent of the
new-moon seed (the 2023 and 3462 regression cases agree within 0.00005 second).
The Web serializer canonicalizes the outer event time to the complete nested
`max` node, so the two representations are byte-for-byte consistent in JD.

## Astronomy

Astronomy events live outside the calendar payload.  Geocentric conjunction,
opposition, quadrature, greatest elongation, and stationary events are emitted
by default.  Transit and occultation events require an explicit observer and
the generator records that observer in each shard.  Algorithm-produced
`detail` (separation, elongation, or state) is retained.

## Solar-term count at far-future civil-year boundaries

Solar terms remain a continuous 24-code astronomical cycle, but a UTC+08
Gregorian civil year is only a time slice of that cycle.  In the far future,
calendar/tropical-year drift can place two occurrences of one code in a civil
year (25 total) and none in the following year (23 total).  Validation accepts
23..25 per civil year while still requiring exact code succession and an
astronomically plausible 13..17 day interval; it does not treat 24 as an
unconditional per-file invariant.

## Time scales and future uncertainty

The supplied code treats TDB and TT as equal at its current precision.  For
1972..2026 it converts with the built-in leap-second table; 1970..<1972 uses
the legacy model; outside 1970..2026 it uses the repository's `delta53`/EM53
quadratic-periodic extrapolation.  Far-future UTC+08 civil dates are therefore
model projections, not claims about future observed UTC.  JD TDB is retained
as the stable dynamical coordinate.

## Intentionally omitted representations

Only values exactly and cheaply reconstructible from retained codes are
omitted.  Debug temporaries and internal caches have no Web semantics and are
not serialized.  No calculated astronomy, calendar, Huangli, eclipse, or
festival information is removed merely because the default UI hides it.