ludic/runtime/native/bignum.ludic

# ============================================================================
# bignum.ludic — arbitrary-precision integers (`BigInt.*`) and exact base-10
# decimals (`Decimal.*`), written in Ludic.
#
# The thing a tycoon or idle game must never get wrong: money that adds up
# exactly, and counters that grow past what a 32- or 64-bit int can hold. Both
# types are EXACT, so they are deterministic — the same guarantee the rest of
# the runtime gives, with no binary floating point anywhere.
#
#   BigInt — a sign-magnitude integer of unbounded size. Limbs are base 1e9
#            (nine decimal digits each), little-endian, in a `words` buffer, so
#            printing is just per-limb decimal with zero-padding. Multiplication
#            accumulates in i64 (`long`) so a limb-by-limb product never
#            overflows. add / sub / mul / pow / compare / divide-by-int.
#   Decimal — a BigInt mantissa plus a decimal `scale` (digits after the point),
#            so a value is mantissa x 10^-scale. add / sub align scales and stay
#            exact; mul adds scales; `rescale` truncates toward zero. Perfect for
#            prices, balances and taxes on values like 0.10 that binary can't
#            represent.
#
# ludicc splices this file into any program that mentions `BigInt.*` or
# `Decimal.*` (like the regex/value runtimes); it is a self-contained fragment
# (only compiler intrinsics), so it works in a plain `program { entry }` tool as
# well as a game. The namespaces (emit_call.ludic) alias each method to the
# matching `bigint_*` / `decimal_*` function below.
# ============================================================================

const BN_BASE: int = 1000000000    # 1e9 — nine decimal digits per limb

# a sign-magnitude big integer. sign is -1 / 0 / +1 (0 only for the value zero);
# limbs are base-1e9, little-endian; n is the number of significant limbs.
property BigNum { sign: int = 0, n: int = 0, limbs: words }

# allocate a BigNum with room for `cap` limbs (at least one), all zeroed
function bn_alloc(cap: int) -> BigNum {
  var c = cap
  if c < 1 { c = 1 }
  let b = new BigNum
  b.sign = 0;  b.n = 0;  b.limbs = words(c)
  var i = 0
  while i < c { b.limbs[i] = 0;  i += 1 }
  return b
}

# drop high zero limbs; a magnitude of zero forces sign 0
function bn_norm(b: BigNum) -> BigNum {
  var k = b.n
  while k > 0 and b.limbs[k - 1] == 0 { k -= 1 }
  b.n = k
  if k == 0 { b.sign = 0 }
  return b
}

function bigint_zero() -> BigNum { return bn_alloc(1) }

# an int -> BigNum (widened through i64 so INT_MIN's magnitude is representable)
function bigint_from(value: int) -> BigNum {
  if value == 0 { return bigint_zero() }
  var sign = 1
  var v: long = value
  if v < 0 { sign = -1;  v = -v }
  let b = bn_alloc(3)
  var i = 0
  while v > 0 {
    let q: long = v / BN_BASE
    let lo: int = v - q * BN_BASE
    b.limbs[i] = lo
    v = q
    i += 1
  }
  b.n = i;  b.sign = sign
  return bn_norm(b)
}

# decimal text (optionally signed) -> BigNum. Assumes valid digits.
function bigint_from_str(text: pointer) -> BigNum {
  let e = len(text)
  var i = 0
  var sign = 1
  if e > 0 and text[0] == '-' { sign = -1;  i = 1 }    # '-'
  if e > 0 and text[0] == '+' { i = 1 }                # '+'
  while i < e - 1 and text[i] == '0' { i += 1 }     # skip leading zeros
  let ndig = e - i
  if ndig <= 0 { return bigint_zero() }
  let nlimb = (ndig + 8) / 9
  let b = bn_alloc(nlimb)
  var li = 0
  var pos = e
  while pos > i {
    var start = pos - 9
    if start < i { start = i }
    var v = 0
    var k = start
    while k < pos { v = v * 10 + (text[k] - 48);  k += 1 }
    b.limbs[li] = v;  li += 1
    pos = start
  }
  b.n = nlimb;  b.sign = sign
  return bn_norm(b)
}

# compare magnitudes: -1 / 0 / 1
function bn_ucmp(a: BigNum, b: BigNum) -> int {
  if a.n != b.n { if a.n > b.n { return 1 };  return -1 }
  var i = a.n - 1
  while i >= 0 {
    if a.limbs[i] != b.limbs[i] { if a.limbs[i] > b.limbs[i] { return 1 };  return -1 }
    i -= 1
  }
  return 0
}

# magnitude add (ignores signs)
function bn_uadd(a: BigNum, b: BigNum) -> BigNum {
  var m = a.n
  if b.n > m { m = b.n }
  let r = bn_alloc(m + 1)
  var carry = 0
  var i = 0
  while i < m {
    var s = carry
    if i < a.n { s += a.limbs[i] }
    if i < b.n { s += b.limbs[i] }
    if s >= BN_BASE { r.limbs[i] = s - BN_BASE;  carry = 1 } else { r.limbs[i] = s;  carry = 0 }
    i += 1
  }
  r.limbs[m] = carry
  r.n = m + 1
  return bn_norm(r)
}

# magnitude subtract, assuming |a| >= |b|
function bn_usub(a: BigNum, b: BigNum) -> BigNum {
  let r = bn_alloc(a.n)
  var borrow = 0
  var i = 0
  while i < a.n {
    var s = a.limbs[i] - borrow
    if i < b.n { s -= b.limbs[i] }
    if s < 0 { s += BN_BASE;  borrow = 1 } else { borrow = 0 }
    r.limbs[i] = s
    i += 1
  }
  r.n = a.n
  return bn_norm(r)
}

function bigint_neg(a: BigNum) -> BigNum {
  let r = bn_alloc(a.n)
  var i = 0
  while i < a.n { r.limbs[i] = a.limbs[i];  i += 1 }
  r.n = a.n;  r.sign = -a.sign
  return r
}

function bigint_add(a: BigNum, b: BigNum) -> BigNum {
  if a.sign == 0 { return b }
  if b.sign == 0 { return a }
  if a.sign == b.sign {
    let r = bn_uadd(a, b);  r.sign = a.sign
    return bn_norm(r)
  }
  let c = bn_ucmp(a, b)
  if c == 0 { return bigint_zero() }
  if c > 0 { let r = bn_usub(a, b);  r.sign = a.sign;  return bn_norm(r) }
  let r = bn_usub(b, a);  r.sign = b.sign
  return bn_norm(r)
}

function bigint_sub(a: BigNum, b: BigNum) -> BigNum { return bigint_add(a, bigint_neg(b)) }

function bigint_mul(a: BigNum, b: BigNum) -> BigNum {
  if a.sign == 0 or b.sign == 0 { return bigint_zero() }
  let r = bn_alloc(a.n + b.n)
  var i = 0
  while i < a.n {
    var carry: long = 0
    let ai: long = a.limbs[i]
    var j = 0
    while j < b.n {
      let bj: long = b.limbs[j]
      let cur: long = r.limbs[i + j]
      let t: long = cur + ai * bj + carry
      let q: long = t / BN_BASE
      let lo: int = t - q * BN_BASE
      r.limbs[i + j] = lo
      carry = q
      j += 1
    }
    var k = i + b.n
    while carry > 0 {
      let cur2: long = r.limbs[k]
      let t2: long = cur2 + carry
      let q2: long = t2 / BN_BASE
      let lo2: int = t2 - q2 * BN_BASE
      r.limbs[k] = lo2
      carry = q2
      k += 1
    }
    i += 1
  }
  r.n = a.n + b.n;  r.sign = a.sign * b.sign
  return bn_norm(r)
}

# a^exp for exp >= 0, by binary exponentiation
function bigint_pow(a: BigNum, exp: int) -> BigNum {
  var result = bigint_from(1)
  var base = a
  var e = exp
  while e > 0 {
    if e - (e / 2) * 2 == 1 { result = bigint_mul(result, base) }
    base = bigint_mul(base, base)
    e /= 2
  }
  return result
}

# floor-toward-zero divide by a nonzero int; quotient is a BigNum
function bigint_div_int(a: BigNum, d: int) -> BigNum {
  var dd = d
  var dsign = 1
  if d < 0 { dsign = -1;  dd = -d }
  let r = bn_alloc(a.n)
  var rem: long = 0
  let ddl: long = dd
  var i = a.n - 1
  while i >= 0 {
    let cur: long = rem * BN_BASE + a.limbs[i]
    let q: long = cur / ddl
    rem = cur - q * ddl
    r.limbs[i] = q
    i -= 1
  }
  r.n = a.n;  r.sign = a.sign * dsign
  return bn_norm(r)
}

# remainder of dividing by a nonzero int; carries the sign of `a`
function bigint_mod_int(a: BigNum, d: int) -> int {
  var dd = d
  if d < 0 { dd = -d }
  let ddl: long = dd
  var rem: long = 0
  var i = a.n - 1
  while i >= 0 {
    let cur: long = rem * BN_BASE + a.limbs[i]
    rem = cur - (cur / ddl) * ddl
    i -= 1
  }
  let r: int = rem
  return r * a.sign
}

function bigint_cmp(a: BigNum, b: BigNum) -> int {
  if a.sign != b.sign { if a.sign > b.sign { return 1 };  return -1 }
  if a.sign == 0 { return 0 }
  let c = bn_ucmp(a, b)
  if a.sign > 0 { return c }
  return -c
}

function bigint_eq(a: BigNum, b: BigNum) -> bool { return bigint_cmp(a, b) == 0 }
function bigint_is_zero(a: BigNum) -> bool { return a.sign == 0 }

# the value as a plain int when it fits in 32 bits, else clamped to the extreme
function bigint_to_int(a: BigNum) -> int {
  if a.sign == 0 { return 0 }
  var v: long = 0
  var i = a.n - 1
  while i >= 0 { v = v * BN_BASE + a.limbs[i];  i -= 1 }
  if a.sign < 0 { v = -v }
  let hi: long = 2147483647
  let lo: long = -hi - 1
  if v > hi { let r: int = hi;  return r }
  if v < lo { return -2147483647 - 1 }
  let r: int = v
  return r
}

# a limb (< 1e9) as exactly nine digits, left-padded with zeros
function bn_pad9(x: int) -> string {
  var s = string(x)
  var pad = 9 - len(s)
  var out = ""
  while pad > 0 { out += "0";  pad -= 1 }
  return out + s
}

function bigint_str(a: BigNum) -> string {
  if a.sign == 0 { return "0" }
  var out = ""
  if a.sign < 0 { out = "-" }
  out += string(a.limbs[a.n - 1])
  var i = a.n - 2
  while i >= 0 { out += bn_pad9(a.limbs[i]);  i -= 1 }
  return out
}

# ---- Decimal: a BigNum mantissa scaled by 10^-scale ------------------------

property Dec { m: BigNum, scale: int = 0 }

function dec_make(m: BigNum, scale: int) -> Dec {
  let d = new Dec
  d.m = m;  d.scale = scale
  return d
}

function decimal_from(value: int) -> Dec { return dec_make(bigint_from(value), 0) }

# "[-]int[.frac]" -> Dec, scale = number of fractional digits
function decimal_from_str(text: pointer) -> Dec {
  let e = len(text)
  var dot = -1
  var i = 0
  while i < e { if text[i] == '.' { dot = i };  i += 1 }   # '.'
  if dot < 0 { return dec_make(bigint_from_str(text), 0) }
  let intp = text[0..dot]
  let fracp = text[dot + 1..e]
  let scale = len(fracp)
  return dec_make(bigint_from_str(intp + fracp), scale)
}

# mantissa of `d` scaled up to `newscale` (newscale >= d.scale), exact
function dec_scaled(d: Dec, newscale: int) -> BigNum {
  let diff = newscale - d.scale
  if diff <= 0 { return d.m }
  return bigint_mul(d.m, bigint_pow(bigint_from(10), diff))
}

function decimal_add(a: Dec, b: Dec) -> Dec {
  var sc = a.scale
  if b.scale > sc { sc = b.scale }
  return dec_make(bigint_add(dec_scaled(a, sc), dec_scaled(b, sc)), sc)
}

function decimal_sub(a: Dec, b: Dec) -> Dec {
  var sc = a.scale
  if b.scale > sc { sc = b.scale }
  return dec_make(bigint_sub(dec_scaled(a, sc), dec_scaled(b, sc)), sc)
}

function decimal_mul(a: Dec, b: Dec) -> Dec { return dec_make(bigint_mul(a.m, b.m), a.scale + b.scale) }
function decimal_neg(a: Dec) -> Dec { return dec_make(bigint_neg(a.m), a.scale) }

function decimal_cmp(a: Dec, b: Dec) -> int {
  var sc = a.scale
  if b.scale > sc { sc = b.scale }
  return bigint_cmp(dec_scaled(a, sc), dec_scaled(b, sc))
}

function decimal_eq(a: Dec, b: Dec) -> bool { return decimal_cmp(a, b) == 0 }
function decimal_scale(d: Dec) -> int { return d.scale }

# change the number of fractional digits: scaling up is exact, scaling down
# truncates toward zero (drop the extra low digits)
function decimal_rescale(d: Dec, places: int) -> Dec {
  if places >= d.scale { return dec_make(dec_scaled(d, places), places) }
  var diff = d.scale - places
  var m = d.m
  var i = 0
  while i < diff { m = bigint_div_int(m, 10);  i += 1 }
  return dec_make(m, places)
}

function decimal_str(d: Dec) -> string {
  var neg = ""
  var m = d.m
  if m.sign < 0 { neg = "-";  m = bigint_neg(m) }
  var digits = bigint_str(m)
  if d.scale == 0 { return neg + digits }
  var nd = len(digits)
  var pad = d.scale + 1 - nd
  while pad > 0 { digits = "0" + digits;  pad -= 1;  nd += 1 }
  let cut = nd - d.scale
  let ip = digits[0..cut]
  let fp = digits[cut..nd]
  return neg + ip + "." + fp
}