1673. gopy long

What we are porting

Objects/longobject.c (~6500 lines). The most-used numeric type in CPython. Arbitrary precision via 30-bit digit arrays (15-bit on 32-bit builds, but gopy is 64-bit-only). Per-process cache of the small-int range (-5..256). Numeric-protocol slot table covering add, subtract, multiply, true/floor div, mod, pow, bitwise ops, shifts, negation, abs, conversion to/from float, hashing, repr, parsing from string with arbitrary base.

CPython 3.12+ stores small ints inline in a tagged-pointer style on free-threaded builds. gopy's GIL build matches the unboxed layout but does not use tagged pointers; the small-int cache is a plain [262]*Long array.

Go shape

// Long mirrors PyLongObject. Sign is encoded in size: positive
// magnitude in digits[], negative if size<0, zero if size==0.
type Long struct {
    Header
    digits []uint32  // 30 bits per digit, little-endian
    size   int       // signed; sign(size) == sign(value)
}

Use int64 fast path for values that fit; spill to digit array otherwise. CPython does the same with _PyLong_IsCompact.

Small-int cache

// smallInts[i] caches the int with value i-5 for i in [0..262).
// Mirrors _Py_SmallInts in longobject.c.
var smallInts [262]*Long

-5..256 are singletons. is comparison preserves identity: x = 1; y = 1; assert x is y.

Parsing

PyLong_FromString accepts any base in [2..36] plus base 0 (autodetect from 0x/0o/0b prefix or default to 10). Underscores allowed between digits per PEP 515.

File mapping

C source	Go target
Objects/longobject.c (struct + ctors)	objects/long.go
arithmetic	objects/long_arith.go
bitwise + shifts	objects/long_bitwise.go
parsing	objects/long_parse.go
hash, repr, format	objects/long_misc.go
small-int cache	objects/long_cache.go
clinic/longobject.c.h	folded into the above

Checklist

Status legend: [x] shipped, [ ] pending, [~] partial / scaffold, [n] deferred / not in scope this phase.

Files

• [~] objects/long.go: Long struct, FromInt64, FromUint64, AsInt64, IsZero, Sign. v0.2 placeholder shipped; full digit-array spill pending.
• objects/long_arith.go: Add, Sub, Mul, FloorDiv, TrueDiv, Mod, DivMod, Pow, Negative, Abs.
• objects/long_bitwise.go: And, Or, Xor, Invert, Lshift, Rshift.
• objects/long_parse.go: FromString(s, base), base-0 autodetect, underscore handling, leading/trailing whitespace rules.
• objects/long_misc.go: Hash (compress to Py_hash_t), Repr (decimal), Str, Format (__format__ with mini-language), RichCompare, Bool, Int, Float coercions.
• objects/long_cache.go: small-int cache initialiser, the Get(i) fast path.
• objects/long_test.go: per-op panel + small-int identity + hash parity.

Surface guarantees

• is identity holds for -5..256 across every constructor path.
• hash(int) matches CPython under PYTHONHASHSEED=0 for the full -2**128 .. 2**128 corpus from compat/hash_panel.txt.
• repr(int) matches CPython for arbitrary-precision values.
• int("0x1_0", 0) == 16 round-trips per PEP 515.
• int(2**1000) round-trips through digit-array path.
• int.__pow__(b, e, m) modular reduction matches CPython bit-for-bit.
• Division by zero raises ZeroDivisionError with the same text CPython uses.

Cross-references

• Header / refcount: 1671.
• Type slots: 1672.
• Hash key: 1607 / 1661.
• Format mini-language: 1660.

Out of scope

• bool subtype of int. Lives in 1675.
• SIMD-accelerated multiplication. Tracked under perf, not parity.
• Free-threaded tagged-pointer compaction. Lands with 1671 §free threading in v0.14.