exact-pi

Exact finite decimal prefixes of π to arbitrary length — 1,000,000 digits in under 30 seconds, 10,000,000 in under 40 minutes, single-threaded — with zero runtime dependencies, built on native bigint, and cross-verified by three mathematically independent methods discovered across three different centuries.

Why "the exact value" means "an exact prefix"

π is irrational (Lambert, 1761) and transcendental (Lindemann, 1882). Its decimal expansion neither terminates nor repeats, so "the last decimal of π" does not exist — not as a limitation of hardware or algorithms, but as a theorem. What is computable is the exact truncated prefix: for any requested n, every one of the n digits returned is a true digit of π.

Install

npm install exact-pi

Node ≥ 20, ESM. Ships compiled JavaScript plus full TypeScript declarations; zero runtime dependencies.

Use as a library

import {
  computePiDigits,   // (digits, engine?) => "3.14159..." exact truncated prefix
  piHexDigits,       // (count) => hex digits of pi after the point
  verifyPiDigits,    // (digits) => cross-engine + BBP verification report
  certifyPiDigits,   // (digits) => reproducible SHA-256 correctness certificate
  bbpHexDigits,      // (position, count) => hex digits at arbitrary position
} from "exact-pi";

computePiDigits(100);
// "3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679"

const cert = certifyPiDigits(10_000);
cert.verification.allPassed; // true — Chudnovsky ≡ Machin, BBP spot-checks

Use as a CLI

npx exact-pi              # 10,000 digits + verification certificate
npx exact-pi 100000       # any digit count
npx exact-pi 50 --print   # print all digits

Develop from source

git clone https://github.com/AlbatrossMicrosystems/exact-pi && cd exact-pi
npm install          # dev tooling only (tsx, typescript)
npm test             # 12 tests, all engines cross-checked
npm run demo -- 100000
npm run bench        # timing table up to 10^6 digits

How it works

Primary engine — Chudnovsky with binary splitting

The Chudnovsky brothers' series (1988) delivers ~14.18 decimal digits per term and underpins every π world record since 1989:

1/π = 12 · Σ  (−1)^k (6k)! (13591409 + 545140134k)
      k≥0    ───────────────────────────────────────
             (3k)! (k!)³ · 640320^(3k + 3/2)

Naive term-by-term summation costs O(n²) bigint work. This package instead evaluates the series by binary splitting: the partial sum over [a, b) is represented by three integers P, Q, T combined recursively from half-ranges, so every multiplication happens between balanced, similar-sized integers — the same strategy as y-cruncher and GMP. Combined with a Newton-iteration integer square root for √10005, the whole computation stays in exact integer arithmetic; floating point never touches a digit.

Truncation, not rounding — with an ambiguity-tested guard band

Reference digit tables are truncated. Rounding is a trap: at 100 places π continues ...170679|8…, so half-up rounding corrupts the final digit to ...170680, which is not a digit of π (the same trap exists at 50 places: ...937510|58…).

computePiDigits computes with a guard band and truncates. If the guard band ever evaluates to all 0s or all 9s — the only case in which the floor at the boundary could be ambiguous — the guard doubles and the computation reruns. The returned prefix is therefore unconditionally exact, not probabilistically exact.

Independent verification — three formulas, three centuries

Engine	Year	Mathematics	Role
Chudnovsky + binary splitting	1988	Ramanujan-type hypergeometric series	primary computation
Machin fixed-point	1706	π/4 = 4·arctan(1/5) − arctan(1/239), Taylor series	full digit-for-digit decimal cross-check
Bailey–Borwein–Plouffe	1995	base-16 digit extraction at arbitrary position	spot-checks hex digits without computing predecessors

certifyPiDigits(n) emits a compact JSON certificate — digit count, head/tail, SHA-256 of the full prefix, verification results — so anyone can reproduce and confirm a computation without shipping megabytes of digits.

Performance

Measured on Node 24, Intel Core Ultra 7 155H, single thread (npm run bench):

Digits	Time
1,000	0.2 ms
10,000	5.6 ms
100,000	218 ms
1,000,000	26.7 s
10,000,000	2,316 s (38.6 min)

The practical ceiling of this code on stock V8 is the engine's 2³⁰-bit BigInt cap (~3×10⁸ decimal digits), not the algorithm.

External validation

• All 1,000,000 digits match Wikimedia Commons' published million-digit reference (E. Andersson).
• All 10,000,000 digits match an independent published 10M-digit reference file; the first 7.45M digits additionally match an unrelated 2005 computation (QPI, Chudnovsky-based) that predates JavaScript BigInt entirely.
• Deep BBP hex probes at positions 1,000,000 / 4,000,000 / 8,300,000 after the hexadecimal point agree with the decimal-derived expansion.

Full methodology, hashes, and reproduction commands: see verification/REPORT.md.

Citing

See CITATION.cff. A DOI-stamped archival snapshot of each release is deposited on Zenodo.

License

MIT — see LICENSE.