# Reverse lookup (/en/docs/go/reverse) SolarDatesByBazi and ReverseChart - the functions and types for recovering candidate birth dates from BaZi pillars or chart features. Recover candidate birth dates from four BaZi pillars or from chart features. All computation runs in the wasm core (pruned enumeration + full re-charting, zero divergence from forward charting); the Go side is a typed wrapper. Concepts, how pillars follow the Config boundaries, and the multi-solution / truncation semantics are on the [reverse lookup guide](/en/docs/guide/guides/reverse). ```go cands, err := iztro.SolarDatesByBazi( iztro.Pillar{iztro.StemGeng, iztro.BranchChen}, iztro.Pillar{iztro.StemJia, iztro.BranchShen}, iztro.Pillar{iztro.StemBing, iztro.BranchWu}, iztro.Pillar{iztro.StemGeng, iztro.BranchYin}, 1900, 2100, nil) ``` Stems, branches, classes and stars all take language-independent keys (the `StemGeng`, `BranchChen`, `ClassWood3rd`, `StarZiweiMaj` constants and family). Both entry points have `Context` variants; `ctx` cancels the wait for a wasm instance. ## Types [#types] ### Pillar [#pillar] ```go type Pillar [2]string ``` One pillar: \[stem key, branch key]. The `RawDates.ChineseDate.YearlyKeys` family on an astrolabe converts directly: `iztro.Pillar(cd.YearlyKeys)`. ### BirthCandidate [#birthcandidate] One candidate birth moment, ready to hand to [`BySolar`](/en/docs/go/astro). | Field | Type | Meaning | | ----------- | -------- | ------------------------------------------------------ | | `SolarDate` | `string` | solar date, `YYYY-M-D` | | `TimeIndex` | `uint8` | hour index 0–12 (0 = early Zi hour, 12 = late Zi hour) | ### StarPosition [#starposition] A star and the branch of the palace it sits in: the atomic condition of a feature lookup. | Field | Type | Meaning | | -------- | -------- | -------------------------------------------------------------------------- | | `Star` | `string` | star key (natal chart stars only; horoscope-scope flow stars are rejected) | | `Branch` | `string` | branch key of its palace | ### ReverseCriteria [#reversecriteria] The condition set of a feature lookup. Zero values mean "not stated": an empty string leaves that condition unset, a zero `YearRange` takes `[1900, 2100]`, a `Limit` of 0 takes the core default (512), and a `nil` `FixLeap` takes the core default `true` (it is a `*bool`; switch it off explicitly with `iztro.Bool(false)`). Every condition is optional, but at least one must be given. | Field | Type | Meaning | | ------------------- | ---------------- | -------------------------------------------------------------------------------------------------- | | `SoulBranch` | `string` | soul palace branch key, empty = unconstrained | | `BodyBranch` | `string` | body palace branch key, empty = unconstrained | | `FiveElementsClass` | `string` | five elements class key, empty = unconstrained | | `Stars` | `[]StarPosition` | star placements, all of which must hold | | `Mutagens` | `[4]string` | star key carrying each birth-year mutagen \[Lu, Quan, Ke, Ji]; empty = unconstrained | | `YearRange` | `[2]int` | inclusive solar year range, within 1583–9999 | | `FixLeap` | `*bool` | leap month correction, same meaning as the charting parameter; `nil` takes the core default `true` | | `Limit` | `int` | candidate cap | ### ReverseResult [#reverseresult] | Field | Type | Meaning | | ------------ | ------------------ | ------------------------------------------------------------------------------------------ | | `Candidates` | `[]BirthCandidate` | the birth candidates satisfying every condition | | `Truncated` | `bool` | whether the search stopped early at the candidate cap; later solutions were never searched | *** ## SolarDatesByBazi [#solardatesbybazi] Recover solar birth dates from four BaZi pillars. ```go func SolarDatesByBazi(yearly, monthly, daily, hourly Pillar, startYear, endYear int, config *Config) ([]BirthCandidate, error) func SolarDatesByBaziContext(ctx context.Context, yearly, monthly, daily, hourly Pillar, startYear, endYear int, config *Config) ([]BirthCandidate, error) ``` The pillars are interpreted under the boundary readings of `config` (`YearDivide` for the year pillar, `HoroscopeDivide` for the month pillar, `DayDivide` for the late Zi hour) — the same semantics as the `RawDates.ChineseDate` a charted astrolabe reports, so reversing any chart's pillars always includes that chart's birth moment. A set of pillars recurs roughly every 60 years within the range; an hour branch of Zi may yield two candidates on adjacent days because of the early/late Zi hour split. **Example** ```go a, _ := iztro.BySolar("2000-8-16", 2, iztro.GenderFemale, true, iztro.LanguageEnUS, nil) cd := a.RawDates.ChineseDate cands, err := iztro.SolarDatesByBazi( iztro.Pillar(cd.YearlyKeys), iztro.Pillar(cd.MonthlyKeys), iztro.Pillar(cd.DailyKeys), iztro.Pillar(cd.HourlyKeys), 1900, 2100, nil) if err != nil { log.Fatal(err) } for _, c := range cands { fmt.Println(c.SolarDate, c.TimeIndex) } ``` **Output** ```text 1940-8-31 2 2000-8-16 2 2060-8-1 2 ``` **Errors** A pillar with mismatched stem/branch polarity (such as 甲丑 Jia-Chou — a yang stem on a yin branch), or a year range that is reversed or outside 1583–9999, returns an `ErrInvalidArgument`-class error (matchable with `errors.Is`). See [Error handling](/en/docs/go/errors). *** ## ReverseChart [#reversechart] Recover candidate birth dates from chart features. ```go func ReverseChart(criteria *ReverseCriteria, config *Config) (*ReverseResult, error) func ReverseChartContext(ctx context.Context, criteria *ReverseCriteria, config *Config) (*ReverseResult, error) ``` Judgement runs entirely under `config`: the mutagen table, the school and every boundary follow it, so charting a candidate with the same `config` is guaranteed to satisfy every condition. Chart layout does not depend on gender (gender only affects the direction the decadal horoscope advances), so the criteria carry no gender. **Example** ```go r, err := iztro.ReverseChart(&iztro.ReverseCriteria{ SoulBranch: iztro.BranchWu, FiveElementsClass: iztro.ClassWood3rd, Stars: []iztro.StarPosition{{Star: iztro.StarZiweiMaj, Branch: iztro.BranchWu}}, Mutagens: [4]string{iztro.StarTaiyangMaj, "", "", ""}, YearRange: [2]int{1998, 2002}, }, nil) if err != nil { log.Fatal(err) } fmt.Println(len(r.Candidates), r.Truncated) ``` **Output** ```text 39 false ``` **Errors** A `nil` criteria, empty criteria, a horoscope-scope flow star in `Stars`, or an invalid year range returns an `ErrInvalidArgument`-class error. Reaching `Limit` stops the search; later solutions never appear in the result. On `Truncated = true`, narrow `YearRange` or add conditions and query again.