const debug = @import("./debug.zig");

const io = @import("./io.zig");

const math = @import("./math.zig");

///
/// Hash type used by tables and their associated structures.
///
pub const Hash = u64;

///
/// Returns a table type of `Key`-`Value` pairs implementing a hash-only approach to key-value storage.
///
/// Entries are hashed using the `keyer` and collisions are resolved by looking for another empty space nearby. This
/// repeats until the load factor exceeds the implementation-defined load maximum, at which point the table will rehash
/// itself to acquire more capacity.
///
pub fn Hashed(comptime Key: type, comptime Value: type, comptime keyer: Keyer(Key)) type {
	const hash_info = @typeInfo(Hash).Int;
	const load_max = 0.75;
	const growth_factor = 0.6;

	return struct {
		count: usize = 0,
		table: []?Entry = &.{},

		///
		/// Key-value pair bundling.
		///
		pub const Entry = struct {
			key: Key,
			value: Value,

			///
			/// Attempts to write `self` into `entry_table`, returning `true` if no identical entry already existed,
			/// otherwise `false`.
			///
			/// Note that this does not modify the memory pointed to by `entry_table` in any way, meaning that
			/// completely filled entry tables cannot perform the write at all and will invoke safety-checked behavior.
			///
			fn write_into(self: Entry, entry_table: []?Entry) bool {
				const hash_max = math.min(math.max_int(hash_info), entry_table.len);
				var hashed_key = math.wrap(keyer.hasher(self.key), math.min_int(hash_info), hash_max);
				var iterations = @as(usize, 0);

				while (true) : (iterations += 1) {
					debug.assert(iterations < entry_table.len);

					const table_entry = &(entry_table[hashed_key] orelse {
						entry_table[hashed_key] = .{
							.key = self.key,
							.value = self.value,
						};

						return true;
					});

					if (keyer.comparer(table_entry.key, self.key) == 0) {
						return false;
					}

					hashed_key = (hashed_key +% 1) % hash_max;
				}
			}
		};

		///
		/// Table type.
		///
		const Self = @This();

		///
		/// Attempts to write the `key`-`value` pair into `self`, using `allocator` as the memory allocation strategy,
		/// and overwriting any value stored with a matching `key` and returning it if one existed.
		///
		/// The function returns [AllocationError] instead if `allocator` cannot commit the memory required to grow the
		/// entry table of `self` when necessary.
		///
		/// *Note* `allocator` must reference the same allocation strategy as the one originally used to initialize
		/// `self`.
		///
		pub fn assign(self: *Self, allocator: io.Allocator, key: Key, value: Value) io.AllocationError!?Entry {
			if (self.calculate_load_factor() >= load_max) {
				const growth_size = @intToFloat(f64, math.max(1, self.table.len)) * growth_factor;

				if (growth_size > math.max_int(@typeInfo(usize).Int)) {
					return error.OutOfMemory;
				}

				try self.rehash(allocator, @floatToInt(usize, growth_size));
			}

			debug.assert(self.table.len > self.count);

			{
				const hash_max = math.min(math.max_int(hash_info), self.table.len);
				var hashed_key = math.wrap(keyer.hasher(key), math.min_int(hash_info), hash_max);

				while (true) {
					const entry = &(self.table[hashed_key] orelse {
						self.table[hashed_key] = .{
							.key = key,
							.value = value,
						};

						return null;
					});

					if (keyer.comparer(entry.key, key) == 0) {
						const original_entry = entry.*;

						entry.* = .{
							.key = key,
							.value = value,
						};

						return original_entry;
					}

					hashed_key = (hashed_key +% 1) % hash_max;
				}
			}

			return false;
		}

		///
		/// Returns the calculated load factor of `self` at the moment.
		///
		pub fn calculate_load_factor(self: Self) f32 {
			return if (self.table.len == 0) 1 else @intToFloat(f32, self.count) / @intToFloat(f32, self.table.len);
		}

		///
		/// Clears all entries from `self`, resetting the count to `0`.
		///
		/// To clean up memory allocations made by the stack and deinitialize it, see [deinit] instead.
		///
		pub fn clear(self: *Self) void {
			for (self.table) |*entry| {
				entry.* = null;
			}

			self.count = 0;
		}

		///
		/// Deinitializes `self` and sets it to an invalid state, freeing all memory allocated by `allocator`.
		///
		/// To clear all items from the table while preserving the current capacity, see [clear] instead.
		///
		/// *Note* `allocator` must reference the same allocation strategy as the one originally used to initialize
		/// `self`.
		///
		pub fn deinit(self: *Self, allocator: io.Allocator) void {
			if (self.table.len == 0) {
				return;
			}

			io.deallocate(allocator, self.table);

			self.table = &.{};
			self.count = 0;
		}

		///
		/// Attempts to write the `key`-`value` pair into `self`, using `allocator` as the memory allocation strategy,
		/// if no value already exists with a matching `key`, returning `true` if it was inserted, otherwise `false`.
		///
		/// The function returns [AllocationError] instead if `allocator` cannot commit the memory required to grow the
		/// entry table of `self` when necessary.
		///
		/// *Note* `allocator` must reference the same allocation strategy as the one originally used to initialize
		/// `self`.
		///
		pub fn insert(self: *Self, allocator: io.Allocator, key: Key, value: Value) io.AllocationError!bool {
			if (self.calculate_load_factor() >= load_max) {
				const growth_amount = @intToFloat(f64, self.table.len) * growth_factor;
				const min_size = 1;

				try self.rehash(allocator, self.table.len + math.max(min_size, @floatToInt(usize, growth_amount)));
			}

			debug.assert(self.table.len > self.count);

			defer self.count += 1;

			return (Entry{
				.key = key,
				.value = value,
			}).write_into(self.table);
		}

		///
		/// Attempts to find an entry in `self` matching `key`, returning it or `null` if no matching entry was found.
		///
		pub fn lookup(self: Self, key: Key) ?Value {
			if (self.count == 0) {
				return null;
			}

			const hash_max = math.min(math.max_int(hash_info), self.table.len);
			var hashed_key = math.wrap(keyer.hasher(key), math.min_int(hash_info), hash_max);
			var iterations = @as(usize, 0);

			while (iterations < self.count) : (iterations += 1) {
				const entry = &(self.table[hashed_key] orelse return null);

				if (keyer.comparer(entry.key, key) == 0) {
					return entry.value;
				}

				hashed_key = (hashed_key +% 1) % hash_max;
			}

			return null;
		}

		///
		/// Attempts to reallocate and regenerate the table capacity in `self` using `allocator` to be equal to or
		/// greater than `requested_range`, returning [io.AllocationError] if `allocator` cannot commit the memory
		/// required for the table capacity size.
		///
		/// *Note* `allocator` must reference the same allocation strategy as the one originally used to initialize
		/// `self`.
		///
		pub fn rehash(self: *Self, allocator: io.Allocator, requested_range: usize) io.AllocationError!void {
			const old_table = self.table;

			self.table = try io.allocate_many(allocator, math.max(requested_range, self.count), ?Entry);

			errdefer {
				io.deallocate(allocator, self.table);

				self.table = old_table;
			}

			for (self.table) |*entry| {
				entry.* = null;
			}

			if (old_table.len != 0)
			{
				for (old_table) |maybe_entry| {
					if (maybe_entry) |entry| {
						debug.assert(entry.write_into(self.table));
					}
				}

				io.deallocate(allocator, old_table);
			}
		}
	};
}

///
/// Returns a function group for defining table keying operations performable on `Key`.
///
pub fn Keyer(comptime Key: type) type {
	return struct {
		hasher: fn (key: Key) Hash,
		comparer: fn (key_a: Key, key_b: Key) isize,
	};
}

///
/// A standard [Keyer] for `[]const u8` types that provides general-purpose string keying.
///
pub const string_keyer = Keyer([]const u8){
	.hasher = hash_string,
	.comparer = io.compare,
};

///
/// Returns a general-purpose, non-cryptographically safe hash value for `string`.
///
pub fn hash_string(string: []const u8) Hash {
	var hash_code = @as(Hash, 5381);

	for (string) |byte| {
		hash_code = ((hash_code << 5) + hash_code) + byte;
	}

	return hash_code;
}