use fmt;
use math;
use rt;

fn cap2nslots(cap: size) size = {
	const nslots_ = cap << 1;
	let nslots = 1z;
	for (nslots < nslots_) {
		nslots <<= 1;
	};
	return nslots;
};

fn nslots2cap(nslots: size) size = {
	return nslots >> 1;
};

fn dbg(fmt: str, args: fmt::field...) void = {
	fmt::errorfln(fmt, args...)!;
};

// temporary hack before inlining works.
def DBG = false;

export type table = struct {
	table: *[*]u64,
	nslots: size, // must be a power of 2.
	nleft: size,
};

// The interface of user-specified equality comparison functions.
export type eq_fn = fn(ctx: *opaque, elem: *opaque) bool;

export const empty: [1]u64 = [0];

// An empty hash table.
def EMPTY = table {
	table = &empty,
	nslots = 1,
	nleft = 0,
};

// Returns a new hash table preallocated with the specified capacity.
//
// If cap is 0, no memory is allocated, and the result is equivalent
// to [[EMPTY]].
export fn new(cap: size, sz: size) table = {
	if (cap == 0) {
		return EMPTY;
	};

	const nslots = cap2nslots(cap);

	// TODO: need to check for overflow here
	// TODO: should probably deal with alignment > 8 bytes
	const allocsz = nslots * size(u64) + nslots * sz;
	const ptr = rt::malloc(allocsz): *opaque;

	let tab = table {
		table = ptr: *[*]u64,
		nslots = nslots,
		...
	};
	clear(&tab, sz);
	return tab;
};

// Clears the table without deallocating it.
//
// Calling this function causes all previously obtained element pointers and
// iterators belonging to the table to become invalid.
export fn clear(tab: *table, sz: size) void = {
	for (let i = 0z; i < tab.nslots; i += 1) {
		tab.table[i] = 0;
	};
	tab.nleft = nslots2cap(tab.nslots);
};

// Clears and deallocates the table.
//
// The table is left in a valid, empty state.
//
// Calling this function causes all previously obtained element pointers and
// iterators belonging to the table to become invalid.
export fn finish(tab: *table) void = {
	if (tab.nslots != 1) {
		free(tab.table);
	};
	*tab = EMPTY;
};

// Returns the number of elements in the table.
export fn count(tab: *table) size = {
	return nslots2cap(tab.nslots) - tab.nleft;
};

fn at(tab: *table, i: size, sz: size) *opaque = {
	const elems: *opaque = &tab.table[tab.nslots];
	return (elems: uintptr + i * sz): *opaque;
};

// Resizes the table to the specified capacity.
//
// The capacity specifies the maximum number of elements storable without
// growing the table. The resulting memory allocation will be larger.
//
// If the number of elements in the table exceeds the capacity,
// the program aborts.
//
// Calling this function causes all previously obtained element pointers and
// iterators belonging to the table to become invalid.
export fn resize(tab: *table, newcap: size, sz: size) void = {
	if (cap2nslots(newcap) == tab.nslots) {
		return;
	};
	assert(count(tab) <= newcap,
		"table won't fit into requested capacity");
	if (DBG) dbg("resize {} {{", newcap);
	let newtab = new(newcap, sz);
	for (let i = 0z; i < tab.nslots; i += 1) {
		if (tab.table[i] == 0) {
			continue;
		};
		const entry = add(&newtab, tab.table[i], sz);
		rt::memcpy(entry, at(tab, i, sz), sz);
	};
	finish(tab);
	*tab = newtab;
	if (DBG) dbg("}} resize {}", newcap);
};

// Finds an element with the specified hash in the table.
//
// If no element is found, null is returned.
//
// hash must not equal 0.
//
// If eq is non-null, it is called on each element matching the hash until it
// returns true. If it is null, an arbitrary matching element is returned.
//
// ctx specifies the value of the ctx argument passed to eq.
export fn get(tab: *table, hash: u64, eq: *eq_fn, ctx: *opaque, sz: size)
		nullable *opaque = {
	if (DBG) dbg("get {}", hash);
	let i = hash: size & tab.nslots - 1;
	for (let inc = 0z; true; inc = 1) {
		i = i + inc & tab.nslots - 1;
		if (DBG) dbg("[{}] -> {}", i, tab.table[i]);
		if (tab.table[i] == 0) {
			break;
		};
		if (tab.table[i] != hash) {
			continue;
		};
		const elem = at(tab, i, sz);
		if (!eq(ctx, elem)) {
			continue;
		};
		return elem;
	};
	return null;
};

// Adds an element with the specified hash to the table and returns a pointer
// to it.
//
// The element is left uninitialized.
//
// hash must not equal 0.
//
// Calling this function causes all previously obtained element pointers and
// iterators belonging to the table to become invalid.
//
// Unless the table is known not to already contain an identical element, users
// should first use [[get]] to check for that possibility.
export fn add(tab: *table, hash: u64, sz: size) *opaque = {
	if (DBG) dbg("add {}", hash);

	if (tab.nleft == 0) {
		resize(tab, nslots2cap(tab.nslots) + 1, sz);
	};
	tab.nleft -= 1;

	let i = hash: size & tab.nslots - 1;
	for (let inc = 0z; true; inc = 1) {
		i = i + inc & tab.nslots - 1;
		if (DBG) dbg("[{}] -> {}", i, tab.table[i]);
		if (tab.table[i] != 0) {
			continue;
		};
		if (DBG) dbg("[{}] <- {}", i, hash);
		tab.table[i] = hash;
		return at(tab, i, sz);
	};
};

// Removes the specified element from the table.
//
// Calling this function causes all previously obtained element pointers and
// iterators belonging to the table to become invalid.
export fn del(tab: *table, elem: *opaque, sz: size) void = {
	tab.nleft += 1;

	let i = (elem: uintptr - at(tab, 0, sz): uintptr): size / sz;
	if (DBG) dbg("del {}", i);
	if (DBG) dbg("[{}] <- {}", i, 0);
	tab.table[i] = 0;

	for (let j = i; true) {
		j = j + 1 & tab.nslots - 1;
		if (DBG) dbg("[{}] -> {}", j, tab.table[j]);
		if (tab.table[j] == 0) {
			break;
		};
		const k = tab.table[j]: size & tab.nslots - 1;
		if (if (i < j) i < k && k <= j else i < k || k <= j) {
			continue;
		};
		if (DBG) dbg("[{}] <- {}", i, tab.table[j]);
		tab.table[i] = tab.table[j];
		rt::memcpy(at(tab, i, sz), at(tab, j, sz), sz);
		if (DBG) dbg("[{}] <- {}", j, 0);
		tab.table[j] = 0;
		i = j;
	};
};

export type iterator = struct {
	table: *table,
	i: size,
};

// Returns a new iterator for the table.
//
// The order of iteration is unspecified.
export fn iter(tab: *table) iterator = {
	return iterator {
		table = tab,
		i = 0,
	};
};

// Returns a pointer to the next element from the iterator, or null if iteration
// has completed.
export fn next(it: *iterator, sz: size) nullable *opaque = {
	for (it.i < it.table.nslots) {
		const i = it.i;
		it.i += 1;
		if (it.table.table[i] != 0) {
			return at(it.table, i, sz);
		};
	};
	return null;
};