// Mirrors ref/hare/strconv/decimal.ha (Hare in turn ports Go's
// lib/strconv/decimal.go). Pure integer arithmetic; no f32/f64
// references (#121 residual-guard SAFE).
//
// Spelling divergences from Hare (mechanical, ww-side parser shape):
//   - Hare `let a = X, b = Y;` → two single `let` statements
//     (ww parser doesn't accept comma-separated bindings).
//   - Hare `tbl[lo..]` open-ended slice → direct indexing
//     `tbl[lo + i]` at point-of-use (equivalent algorithm; no
//     allocation, no aliasing). ww `[lo:hi]` uses `:`; `..` form
//     is not parsed.
//   - Hare `0z`/`1z` size literals → ww has no `z` suffix; pre-bind
//     `let SZ_ZERO: size = (0u64: size);` etc. at function entry
//     ("hoisted size casts as local consts" — ww `T: type` casts
//     embedded inside expressions confuse the parser).
//   - Hare `~0u64` typed-suffix literal → ww parser rejects `~` on
//     typed-suffix; route via a named zero local + `~zero`.
//   - Hare `for (cond; afterthought)` 2-clause → ww 3-clause
//     `for (init; cond; post)` (when continue is used; the post
//     must run each iteration) or inline-the-afterthought in body
//     (when no continue exists in the loop).
//   - Hare `fn foo() T = if (cond) {...} else expr;` expression body
//     → ww requires a `{}` block body throughout.
//   - Hare bare `assert(cond)` builtin → `assert(cond, msg)`;
//     wwstage cgen has no `assert` intercept (deferred fold).
//   - In-file instances of the above hoist pattern: `i_sz` (line 93)
//     hoists a per-iteration size cast out of a for-loop comparison
//     (bullet 3 sub-case — the size-cast hoist applied inside a loop
//     body, not just at function entry); `lowbit_lit` (line 242)
//     decomposes Hare's `(nd > 0 && d.digits[nd - 1] & 1 != 0)` into
//     a stepwise boolean local to dodge ww parser precedence on mixed
//     `&` / `&&` / `!=` within a single expression.
//
// CGEN class closures consumed (post-prereqs):
//   - #131 (4acab6e) — `len(d.digits)` compile-time-folds cs==ww
//   - #134 (36bf603) — `d.digits[nd] >= 5u8` picks JAE (unsigned)
//   - #133 (3986818) — `d.digits[i] += 1u8` load-op-store BOTH
//     stages
//   - #135 (ade6840) — `(*d).digits[i]` read+write N_DOT-base addr
//
// Drew CGEN-SAFE invariants:
//   - #129: module-level decls here are integer-literal defs only.
//   - #128: digits is fundamental [800]u8, zero-init only.
//   - #121: zero float ops.
//   - Drew watch-item `*d = decimal{...};` reset (line 110 in Hare):
//     pointer-deref reset to composite-literal probed cs==ww
//     byte-id safe.

package strconv;

import os;

// ref/hare/strconv/decimal.ha:5.
def maxshift: u8 = 60u8;

// ref/hare/strconv/decimal.ha:6.
def decimal_point_range: u16 = 2047u16;

// ref/hare/strconv/decimal.ha:8-26. Field layout 1:1. The 800-digit
// bound covers subnormal doubles (min exp -1074, max mantissa 4e16
// → at most 767 digits; 800 leaves headroom).
export type decimal = struct {
	digits: [800]u8,
	nd: size,
	dp: i32,
	negative: bool,
	truncated: bool,
};

// ref/hare/strconv/decimal.ha:29-33. Strip trailing zeros.
fn trim(d: *decimal) void = {
	let SZ_ZERO: size = (0u64: size);
	let SZ_ONE:  size = (1u64: size);
	for (d.nd > SZ_ZERO && d.digits[d.nd - SZ_ONE] == 0u8) {
		d.nd -= SZ_ONE;
	};
};

// ref/hare/strconv/decimal.ha:35-55. Compute the digit-count
// increase for a left-shift `shift` (consults left_shift_table +
// pow5_table from stof_data.ww, bb6f840). Uses `continue` so the
// loop stays in 3-clause form for byte-id-correct post-increment.
fn leftshift_newdigits(d: *decimal, shift: u32) u32 = {
	shift &= 63u32;
	let x_a: u32 = (left_shift_table[shift]: u32);
	let x_b: u32 = (left_shift_table[shift + 1u32]: u32);
	let nn:  u32 = x_a >> 11u32;
	let pow5_a: u32 = 0x7FFu32 & x_a;
	let pow5_b: u32 = 0x7FFu32 & x_b;
	let n: u32 = pow5_b - pow5_a;
	for (let i: u32 = 0u32; i < n; i += 1u32) {
		let i_sz: size = (i: size);
		if (i_sz >= d.nd) {
			return nn - 1u32;
		} else if (d.digits[i] == pow5_table[pow5_a + i]) {
			continue;
		} else if (d.digits[i] < pow5_table[pow5_a + i]) {
			return nn - 1u32;
		} else {
			return nn;
		};
	};
	return nn;
};

// ref/hare/strconv/decimal.ha:57-91. Shift `d` left by k bits.
fn leftshift(d: *decimal, k: u32) void = {
	let SZ_ONE:   size = (1u64: size);
	let SZ_BOUND: size = (len(d.digits): size);
	let kU64: u64 = (k: u64);
	let MAXSHIFT_U32: u32 = (maxshift: u32);
	assert(k <= MAXSHIFT_U32, "strconv.leftshift: k > maxshift");
	if (d.nd == (0u64: size)) { return; };
	let nn: u32 = leftshift_newdigits(d, k);
	let r: int = (d.nd: int) - 1;
	let w: size = (r: size) + (nn: size);
	let n: u64 = 0u64;
	for (r >= 0) {
		n += (d.digits[r]: u64) << kU64;
		let quo: u64 = n / 10u64;
		let rem: u64 = n - 10u64 * quo;
		if (w < SZ_BOUND) {
			d.digits[w] = (rem: u8);
		} else if (rem != 0u64) {
			d.truncated = true;
		};
		n = quo;
		r -= 1;
		w -= SZ_ONE;
	};
	for (n > 0u64) {
		let quo: u64 = n / 10u64;
		let rem: u64 = n - 10u64 * quo;
		if (w < SZ_BOUND) {
			d.digits[w] = (rem: u8);
		} else if (rem != 0u64) {
			d.truncated = true;
		};
		n = quo;
		w -= SZ_ONE;
	};
	d.nd += (nn: size);
	if (d.nd > SZ_BOUND) {
		d.nd = SZ_BOUND;
	};
	d.dp += (nn: i32);
	trim(d);
};

// ref/hare/strconv/decimal.ha:93-134. Shift `d` right by k bits.
// Two outer Hare 2-clause loops (`for (cond; r += 1)`) are inlined
// as `for (cond) { ... r += SZ_ONE; }` since neither uses continue.
fn rightshift(d: *decimal, k: u32) void = {
	let SZ_ZERO:  size = (0u64: size);
	let SZ_ONE:   size = (1u64: size);
	let SZ_BOUND: size = (len(d.digits): size);
	let kU64: u64 = (k: u64);
	let r: size = SZ_ZERO;
	let w: size = SZ_ZERO;
	let n: u64 = 0u64;
	for ((n >> kU64) == 0u64) {
		if (r >= d.nd) {
			if (n == 0u64) {
				d.nd = SZ_ZERO;
				return;
			};
			for ((n >> kU64) == 0u64) {
				n *= 10u64;
				r += SZ_ONE;
			};
			break;
		};
		n = n * 10u64 + (d.digits[r]: u64);
		r += SZ_ONE;
	};
	d.dp -= (r: i32) - 1;
	if (d.dp < -(decimal_point_range: i32)) {
		// Drew-watch-item: pointer-deref reset to composite
		// literal — probed cs==ww byte-id safe in pre-flight.
		*d = decimal { ... };
		return;
	};
	let mask: u64 = (1u64 << kU64) - 1u64;
	for (r < d.nd) {
		let dig: u64 = n >> kU64;
		n &= mask;
		d.digits[w] = (dig: u8);
		w += SZ_ONE;
		n = n * 10u64 + (d.digits[r]: u64);
		r += SZ_ONE;
	};
	for (n > 0u64) {
		let dig: u64 = n >> kU64;
		n &= mask;
		if (w < SZ_BOUND) {
			d.digits[w] = (dig: u8);
			w += SZ_ONE;
		} else if (dig > 0u64) {
			d.truncated = true;
		};
		n *= 10u64;
	};
	d.nd = w;
	trim(d);
};

// ref/hare/strconv/decimal.ha:138-153. Shift right (k < 0) or left
// (k > 0). Hardware shifts cap at 60 bits without losing top
// digits, so break large shifts into maxshift-sized chunks.
fn decimal_shift(d: *decimal, k: int) void = {
	let MAXSHIFT_INT: int = (maxshift: int);
	let MAXSHIFT_U32: u32 = (maxshift: u32);
	if (d.nd == (0u64: size)) { return; };
	if (k > 0) {
		for (k > MAXSHIFT_INT) {
			leftshift(d, MAXSHIFT_U32);
			k -= MAXSHIFT_INT;
		};
		leftshift(d, (k: u32));
	} else if (k < 0) {
		for (k < -MAXSHIFT_INT) {
			rightshift(d, MAXSHIFT_U32);
			k += MAXSHIFT_INT;
		};
		rightshift(d, ((-k): u32));
	};
};

// ref/hare/strconv/decimal.ha:155-160. Banker's rounding decision:
// at the exact half (digit==5, no more digits) round to even (the
// preceding digit's low bit decides); past-half rounds up; below-
// half rounds down. Hare's expression-bodied `if` re-shaped as a
// block per ww parser.
fn should_round_up(d: *decimal, nd: uint) bool = {
	let nd_sz:  size = (nd: size);
	let SZ_ONE: size = (1u64: size);
	let U_ONE:  uint = (1u32: uint);
	let U_ZERO: uint = (0u32: uint);
	if (nd_sz < d.nd) {
		if (d.digits[nd] == 5u8 && (nd_sz + SZ_ONE) == d.nd) {
			let lowbit_lit: bool = false;
			if (nd > U_ZERO) {
				if ((d.digits[nd - U_ONE] & 1u8) != 0u8) {
					lowbit_lit = true;
				};
			};
			return d.truncated || lowbit_lit;
		} else {
			return d.digits[nd] >= 5u8;
		};
	};
	return false;
};

// ref/hare/strconv/decimal.ha:162-166. Round to `nd` digits.
fn round(d: *decimal, nd: uint) void = {
	if ((nd: size) >= d.nd) { return; };
	if (should_round_up(d, nd)) {
		roundup(d, nd);
	} else {
		rounddown(d, nd);
	};
};

// ref/hare/strconv/decimal.ha:168-172. Truncate to `nd` digits.
fn rounddown(d: *decimal, nd: uint) void = {
	if ((nd: size) >= d.nd) { return; };
	d.nd = (nd: size);
	trim(d);
};

// ref/hare/strconv/decimal.ha:174-186. Round up to `nd` digits;
// propagate carry. If all 9s, the result is a single 1 with the
// decimal point advanced.
fn roundup(d: *decimal, nd: uint) void = {
	let SZ_ONE: size = (1u64: size);
	if ((nd: size) >= d.nd) { return; };
	for (let i: int = (nd: int) - 1; i >= 0; i -= 1) {
		if (d.digits[i] < 9u8) {
			d.digits[i] += 1u8;
			d.nd = (i: size) + SZ_ONE;
			return;
		};
	};
	d.digits[0] = 1u8;
	d.nd = SZ_ONE;
	d.dp += 1;
};

// ref/hare/strconv/decimal.ha:188-202. Read `d` as the integer
// rounded to `d.dp` digits. Returns 0 if `d.dp <= 0`; returns
// ~0u64 if `d.dp > 18` (exceeds u64 range). Hare's two 2-clause
// loops (`for (cond; i += 1)`) are inlined per the spelling
// divergence at file top.
fn decimal_round(d: *decimal) u64 = {
	let SZ_ZERO: size = (0u64: size);
	let SZ_ONE:  size = (1u64: size);
	if (d.nd == SZ_ZERO || d.dp < 0) { return 0u64; };
	if (d.dp > 18) {
		// Hare's `~0u64` doesn't parse on a typed-suffix literal
		// in ww; route via a named zero.
		let zero: u64 = 0u64;
		return ~zero;
	};
	let dp_sz: size = ((d.dp: uint): size);
	let i: size = SZ_ZERO;
	let n: u64 = 0u64;
	for (i < dp_sz && i < d.nd) {
		n = n * 10u64 + (d.digits[i]: u64);
		i += SZ_ONE;
	};
	for (i < dp_sz) {
		n *= 10u64;
		i += SZ_ONE;
	};
	if (should_round_up(d, (d.dp: uint))) {
		n += 1u64;
	};
	return n;
};