Units

Units

Overview

lob stores trajectory quantities internally in feet, seconds and pounds, but the public API speaks the shooter's units. Internally source/eng_units.hpp provides StrongT<E,U,T> — a zero-cost wrapper that makes mismatched units fail to compile. Conversion helpers are exposed as C and C++ free functions.

Strong types (internal)

StrongT is templated on an enum category E (Length, Speed, Angle …), a specific unit U, and a representation T. Arithmetic preserves the category; same-category unit conversions (e.g. InchT → FeetT, MoaT → RadiansT) change only the unit via the supplied operator StrongT<E, Other, T>; cross-category operations are ill-formed. Helpers like IsNaN(), Value(), U32(), U16() and Inverse() are provided. The type is constexpr where possible.

The strong types eliminate unit mixups inside the integrator and builder, then disappear at the ABI boundary — LobContext fields are plain double.

Conversion helpers

Most conversions are exact multiplications by scale factors from source/eng_units.hpp (convert:: namespace); LobMoaToInch and LobInchToMoa also depend on range_ft and Celsius-to-Fahrenheit includes an additive offset. Selected helpers (include/lob/lob.h):

• Angle: LobMoaToMil, LobMoaToDeg, LobMoaToIphy, LobMoaToInch(moa, range_ft), LobInchToMoa, LobInchToMil, LobInchToDeg, and the Mil/Deg variants.
• Length: LobMToYd, LobYdToFt, LobMToFt, LobFtToIn, LobMmToIn, LobCmToIn and inverses.
• Pressure: LobPaToInHg, LobMbarToInHg, LobPsiToInHg.
• Mass: LobLbsToGrain, LobGToGrain, LobKgToGrain, LobKgSqMToPmsi.
• Speed: LobFpsToMps, LobMpsToFps, LobKphToMph, LobKnToMph.
• Time: LobMsToS, LobUsToS, LobSToMs, LobSToUs.
• Temperature: LobDegCToDegF.

Each has a lob:: inline wrapper (MoaToMil, MToYd, …) in lob.hpp.

Roots

Angle conversion constants are rooted at kPi (source/eng_units.hpp):

kRadiansPerDegree = π/180
kMoaPerRadian     = 60 / kRadiansPerDegree
kMilPerRadian     = 1000
kIphyPerMoa       = 1.047

Length is rooted at feet: kInchPerFoot = 12, kFeetPerYard = 3, kMeterPerFoot = 0.3048. Mass at kGrainsPerLb = 7000 and kLbsPerSlug = 32.17405 (standard gravity). Time and temperature use the usual SI/FPS scales.

Limitations

• The helpers are pure arithmetic; they do not validate ranges.
• The strong-type system stops at the public ABI. Callers passing raw double to the C API must get the units right themselves — prefer the lob::Builder setters which name the unit (e.g. DiameterInch, MassGrains).